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class="mw-page-title-main">Molecularity</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. 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href="https://ca.wikipedia.org/wiki/Molecularitat" title="Molecularitat – Catalan" lang="ca" hreflang="ca" data-title="Molecularitat" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Molekularit%C3%A4t" title="Molekularität – German" lang="de" hreflang="de" data-title="Molekularität" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-et mw-list-item"><a href="https://et.wikipedia.org/wiki/Molekulaarsus" title="Molekulaarsus – Estonian" lang="et" hreflang="et" data-title="Molekulaarsus" data-language-autonym="Eesti" data-language-local-name="Estonian" class="interlanguage-link-target"><span>Eesti</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Molecularidad" title="Molecularidad – Spanish" lang="es" hreflang="es" data-title="Molecularidad" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D9%85%D9%88%D9%84%DA%A9%D9%88%D9%84%D8%A7%D8%B1%DB%8C%D8%AA%D9%87" title="مولکولاریته – Persian" lang="fa" hreflang="fa" data-title="مولکولاریته" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Mol%C3%A9cularit%C3%A9" title="Molécularité – French" lang="fr" hreflang="fr" data-title="Molécularité" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/Molekularitas" title="Molekularitas – Indonesian" lang="id" hreflang="id" data-title="Molekularitas" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Molecolarit%C3%A0" title="Molecolarità – Italian" lang="it" hreflang="it" data-title="Molecolarità" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/Molekularit%C3%A1s" title="Molekularitás – Hungarian" lang="hu" hreflang="hu" data-title="Molekularitás" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a href="https://ms.wikipedia.org/wiki/Kemolekularan" title="Kemolekularan – Malay" lang="ms" hreflang="ms" data-title="Kemolekularan" data-language-autonym="Bahasa Melayu" data-language-local-name="Malay" class="interlanguage-link-target"><span>Bahasa Melayu</span></a></li><li class="interlanguage-link interwiki-nl mw-list-item"><a href="https://nl.wikipedia.org/wiki/Moleculariteit" title="Moleculariteit – Dutch" lang="nl" hreflang="nl" data-title="Moleculariteit" data-language-autonym="Nederlands" data-language-local-name="Dutch" class="interlanguage-link-target"><span>Nederlands</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E5%88%86%E5%AD%90%E5%BA%A6" title="分子度 – Japanese" lang="ja" hreflang="ja" data-title="分子度" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-no mw-list-item"><a href="https://no.wikipedia.org/wiki/Molekylaritet" title="Molekylaritet – Norwegian Bokmål" lang="nb" hreflang="nb" data-title="Molekylaritet" data-language-autonym="Norsk bokmål" data-language-local-name="Norwegian Bokmål" class="interlanguage-link-target"><span>Norsk bokmål</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/Molecularidade" title="Molecularidade – Portuguese" lang="pt" hreflang="pt" data-title="Molecularidade" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-sk mw-list-item"><a href="https://sk.wikipedia.org/wiki/Molekulovos%C5%A5" title="Molekulovosť – Slovak" lang="sk" hreflang="sk" data-title="Molekulovosť" data-language-autonym="Slovenčina" data-language-local-name="Slovak" class="interlanguage-link-target"><span>Slovenčina</span></a></li><li class="interlanguage-link interwiki-ta mw-list-item"><a href="https://ta.wikipedia.org/wiki/%E0%AE%B5%E0%AE%BF%E0%AE%A9%E0%AF%88_%E0%AE%AE%E0%AF%82%E0%AE%B2%E0%AE%95%E0%AF%8D%E0%AE%95%E0%AF%82%E0%AE%B1%E0%AF%81_%E0%AE%8E%E0%AE%A3%E0%AF%8D" title="வினை மூலக்கூறு எண் – Tamil" lang="ta" hreflang="ta" data-title="வினை மூலக்கூறு எண்" data-language-autonym="தமிழ்" data-language-local-name="Tamil" class="interlanguage-link-target"><span>தமிழ்</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/%D0%9C%D0%BE%D0%BB%D0%B5%D0%BA%D1%83%D0%BB%D1%8F%D1%80%D0%BD%D1%96%D1%81%D1%82%D1%8C_%D1%80%D0%B5%D0%B0%D0%BA%D1%86%D1%96%D1%97" title="Молекулярність реакції – Ukrainian" lang="uk" hreflang="uk" data-title="Молекулярність реакції" data-language-autonym="Українська" data-language-local-name="Ukrainian" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-zh mw-list-item"><a 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<div class="mw-indicators"> </div> <div id="siteSub" class="noprint">From Wikipedia, the free encyclopedia</div> </div> <div id="contentSub"><div id="mw-content-subtitle"></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Number of molecules that participate in a single-step reaction</div><style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">Not to be confused with <a href="/wiki/Molarity" class="mw-redirect" title="Molarity">Molarity</a>.</div> <p>In <a href="/wiki/Chemistry" title="Chemistry">chemistry</a>, <b>molecularity</b> is the number of molecules that come together to react in an <a href="/wiki/Elementary_reaction" title="Elementary reaction">elementary (single-step) reaction</a><sup id="cite_ref-Atkins_1-0" class="reference"><a href="#cite_note-Atkins-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> and is equal to the sum of <a href="/wiki/Stoichiometry" title="Stoichiometry">stoichiometric coefficients</a> of <a href="/wiki/Reactant" class="mw-redirect" title="Reactant">reactants</a> in the elementary reaction with effective collision (<a href="/wiki/Activation_energy" title="Activation energy">sufficient energy</a>) and correct orientation.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> Depending on how many molecules come together, a reaction can be unimolecular, bimolecular or even trimolecular. </p><p>The kinetic order of any elementary reaction or reaction step is <i>equal</i> to its molecularity, and the <a href="/wiki/Rate_equation" title="Rate equation">rate equation</a> of an elementary reaction can therefore be determined by inspection, from the molecularity.<sup id="cite_ref-Atkins_1-1" class="reference"><a href="#cite_note-Atkins-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p>The kinetic order of a complex (multistep) reaction, however, is not necessarily equal to the number of molecules involved. The concept of molecularity is only useful to describe elementary reactions or steps. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Unimolecular_reactions">Unimolecular reactions</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Molecularity&action=edit&section=1" title="Edit section: Unimolecular reactions"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In a unimolecular reaction, a single molecule rearranges atoms, forming different molecules.<sup id="cite_ref-Atkins_1-2" class="reference"><a href="#cite_note-Atkins-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> This is illustrated by the equation </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\ce {A -> P,}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mtext>A</mtext> <mo stretchy="false">⟶</mo> <mtext>P</mtext> <mo>,</mo> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\ce {A -> P,}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/9255f7bf4b242b1e7dc1702602f2190020a287d2" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:9.069ex; height:2.509ex;" alt="{\displaystyle {\ce {A -> P,}}}"></span></dd></dl> <p>where <span class="nowrap">⁠<span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\rm {P}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="normal">P</mi> </mrow> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\rm {P}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/11f12099c106e1a884a2d2c51a407c6b9f22d42e" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.583ex; height:2.176ex;" alt="{\displaystyle {\rm {P}}}"></span>⁠</span> refers to chemical <a href="/wiki/Product_(chemistry)" title="Product (chemistry)">product(s)</a>. The reaction or <a href="/wiki/Reaction_step" title="Reaction step">reaction step</a> is an <a href="/wiki/Isomerization" title="Isomerization">isomerization</a> if there is only one product molecule, or a <a href="/wiki/Dissociation_(chemistry)" title="Dissociation (chemistry)">dissociation</a> if there is more than one product molecule. </p><p>In either case, the rate of the reaction or step is described by the <a href="/wiki/Rate_equation" title="Rate equation">first order rate law</a> </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\frac {d\left[{\ce {A}}\right]}{dt}}=-k_{r}\left[{\ce {A}}\right],}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mfrac> <mrow> <mi>d</mi> <mrow> <mo>[</mo> <mrow class="MJX-TeXAtom-ORD"> <mtext>A</mtext> </mrow> <mo>]</mo> </mrow> </mrow> <mrow> <mi>d</mi> <mi>t</mi> </mrow> </mfrac> </mrow> <mo>=</mo> <mo>−</mo> <msub> <mi>k</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>r</mi> </mrow> </msub> <mrow> <mo>[</mo> <mrow class="MJX-TeXAtom-ORD"> <mtext>A</mtext> </mrow> <mo>]</mo> </mrow> <mo>,</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\frac {d\left[{\ce {A}}\right]}{dt}}=-k_{r}\left[{\ce {A}}\right],}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/92482ab62adf787ba05f981c8a1e2ed67e7da3e1" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.005ex; width:17.025ex; height:5.843ex;" alt="{\displaystyle {\frac {d\left[{\ce {A}}\right]}{dt}}=-k_{r}\left[{\ce {A}}\right],}"></span></dd></dl> <p>where <span class="nowrap">⁠<span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle [{\rm {A]}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo stretchy="false">[</mo> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="normal">A</mi> <mo stretchy="false">]</mo> </mrow> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle [{\rm {A]}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/6fdf5b161b5bc659e3286aaf42973c24fef2b86d" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:3.037ex; height:2.843ex;" alt="{\displaystyle [{\rm {A]}}}"></span>⁠</span> is the <a href="/wiki/Concentration" title="Concentration">concentration</a> of <a href="/wiki/Chemical_species" title="Chemical species">species</a> A, <span class="nowrap">⁠<span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle t}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>t</mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle t}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/65658b7b223af9e1acc877d848888ecdb4466560" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:0.84ex; height:2.009ex;" alt="{\displaystyle t}"></span>⁠</span> is time, and <span class="nowrap">⁠<span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle k_{r}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>k</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>r</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle k_{r}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/58d7e5bb94a98612867e7ad219e44b5499bb7ebe" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:2.185ex; height:2.509ex;" alt="{\displaystyle k_{r}}"></span>⁠</span> is the <a href="/wiki/Reaction_rate_constant" title="Reaction rate constant">reaction rate constant</a>. </p><p>As can be deduced from the rate law equation, the number of A molecules that decay is proportional to the number of A molecules available. An example of a unimolecular reaction, is the <a href="/wiki/Isomerization" title="Isomerization">isomerization</a> of <a href="/wiki/Cyclopropane" title="Cyclopropane">cyclopropane</a> to propene: </p> <figure class="mw-default-size mw-halign-center" typeof="mw:File"><a href="/wiki/File:Izomerization_of_cyclopropane.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/c/c6/Izomerization_of_cyclopropane.jpg" decoding="async" width="372" height="102" class="mw-file-element" data-file-width="372" data-file-height="102" /></a><figcaption></figcaption></figure> <p>Unimolecular reactions can be explained by the <a href="/wiki/Lindemann_mechanism" title="Lindemann mechanism">Lindemann-Hinshelwood</a> mechanism. </p> <div class="mw-heading mw-heading2"><h2 id="Bimolecular_reactions">Bimolecular reactions</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Molecularity&action=edit&section=2" title="Edit section: Bimolecular reactions"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In a bimolecular reaction, two molecules collide and exchange energy, atoms or groups of atoms.<sup id="cite_ref-Atkins_1-3" class="reference"><a href="#cite_note-Atkins-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p>This can be described by the equation </p><p><span class="mwe-math-element"><span class="mwe-math-mathml-display mwe-math-mathml-a11y" style="display: none;"><math display="block" xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\ce {A + B -> P}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mtext>A</mtext> <mo>+</mo> <mtext>B</mtext> <mo stretchy="false">⟶</mo> <mtext>P</mtext> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\ce {A + B -> P}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/bac8f00b70039a5952d2d0812f43e74248f3302b" class="mwe-math-fallback-image-display mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.505ex; width:12.908ex; height:2.343ex;" alt="{\displaystyle {\ce {A + B -> P}}}"></span> </p><p>which corresponds to the second order rate law: <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\frac {d[{\ce {A}}]}{dt}}=-k_{r}{\ce {[A][B]}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mfrac> <mrow> <mi>d</mi> <mo stretchy="false">[</mo> <mrow class="MJX-TeXAtom-ORD"> <mtext>A</mtext> </mrow> <mo stretchy="false">]</mo> </mrow> <mrow> <mi>d</mi> <mi>t</mi> </mrow> </mfrac> </mrow> <mo>=</mo> <mo>−</mo> <msub> <mi>k</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>r</mi> </mrow> </msub> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">[</mo> <mtext>A</mtext> <mo stretchy="false">]</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">[</mo> <mtext>B</mtext> <mo stretchy="false">]</mo> </mrow> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\frac {d[{\ce {A}}]}{dt}}=-k_{r}{\ce {[A][B]}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/95260098718d0110a8e98749163fca51a7625577" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.005ex; width:18.156ex; height:5.843ex;" alt="{\displaystyle {\frac {d[{\ce {A}}]}{dt}}=-k_{r}{\ce {[A][B]}}}"></span>. </p><p>Here, the rate of the reaction is proportional to the rate at which the reactants come together. An example of a bimolecular reaction is the <a href="/wiki/SN2" class="mw-redirect" title="SN2">S<sub>N</sub>2</a>-type <a href="/wiki/Nucleophilic_substitution" title="Nucleophilic substitution">nucleophilic substitution</a> of <a href="/wiki/Methyl_bromide" class="mw-redirect" title="Methyl bromide">methyl bromide</a> by <a href="/wiki/Hydroxide_ion" class="mw-redirect" title="Hydroxide ion">hydroxide ion</a>:<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> </p><p><span class="mwe-math-element"><span class="mwe-math-mathml-display mwe-math-mathml-a11y" style="display: none;"><math display="block" xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\ce {CH3Br + OH^- -> CH3OH + Br^-}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <msubsup> <mtext>CH</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>3</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <mtext>Br</mtext> <mo>+</mo> <msup> <mtext>OH</mtext> <mrow class="MJX-TeXAtom-ORD"> <mo>−</mo> </mrow> </msup> <mo stretchy="false">⟶</mo> <msubsup> <mtext>CH</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>3</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <mtext>OH</mtext> <mo>+</mo> <msup> <mtext>Br</mtext> <mrow class="MJX-TeXAtom-ORD"> <mo>−</mo> </mrow> </msup> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\ce {CH3Br + OH^- -> CH3OH + Br^-}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/6c592f8c62f29c0aef4d272a2060e8d9131be9d9" class="mwe-math-fallback-image-display mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:34.966ex; height:3.176ex;" alt="{\displaystyle {\ce {CH3Br + OH^- -> CH3OH + Br^-}}}"></span> </p> <div class="mw-heading mw-heading2"><h2 id="Termolecular_reactions">Termolecular reactions</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Molecularity&action=edit&section=3" title="Edit section: Termolecular reactions"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A termolecular<sup id="cite_ref-Steinfeld_4-0" class="reference"><a href="#cite_note-Steinfeld-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> (or trimolecular)<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> reaction in <a href="/wiki/Solution_(chemistry)" title="Solution (chemistry)">solutions</a> or gas mixtures involves three reactants simultaneously <a href="/wiki/Collision_frequency" title="Collision frequency">colliding</a>, with appropriate orientation and sufficient energy.<sup id="cite_ref-Steinfeld_4-1" class="reference"><a href="#cite_note-Steinfeld-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> However the term <i>trimolecular</i> is also used to refer to three body association reactions of the type: </p><p><span class="mwe-math-element"><span class="mwe-math-mathml-display mwe-math-mathml-a11y" style="display: none;"><math display="block" xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\ce {A + B ->[{\ce {M}}] C}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mtext>A</mtext> <mo>+</mo> <mtext>B</mtext> <mrow class="MJX-TeXAtom-REL"> <mover> <mo>→</mo> <mpadded width="+0.611em" lspace="0.278em" voffset=".15em"> <mrow class="MJX-TeXAtom-ORD"> <mtext>M</mtext> </mrow> </mpadded> </mover> </mrow> <mtext>C</mtext> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\ce {A + B ->[{\ce {M}}] C}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/054c891df55fd8304d9ff829d30eacf575dadccf" class="mwe-math-fallback-image-display mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.505ex; margin-top: -0.329ex; width:12.028ex; height:3.843ex;" alt="{\displaystyle {\ce {A + B ->[{\ce {M}}] C}}}"></span> </p><p>Where the M over the arrow denotes that to conserve <a href="/wiki/Energy" title="Energy">energy</a> and <a href="/wiki/Momentum" title="Momentum">momentum</a> a second reaction with a third body is required. After the initial bimolecular collision of A and B an energetically excited <a href="/wiki/Reaction_intermediate" title="Reaction intermediate">reaction intermediate</a> is formed, then, it collides with a M body, in a second bimolecular reaction, transferring the excess energy to it.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> </p><p>The reaction can be explained as two consecutive reactions: </p><p><span class="mwe-math-element"><span class="mwe-math-mathml-display mwe-math-mathml-a11y" style="display: none;"><math display="block" xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\ce {A + B -> AB}}^{*}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msup> <mrow class="MJX-TeXAtom-ORD"> <mtext>A</mtext> <mo>+</mo> <mtext>B</mtext> <mo stretchy="false">⟶</mo> <mtext>AB</mtext> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mo>∗</mo> </mrow> </msup> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\ce {A + B -> AB}}^{*}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/67e4d5a6fa40733172a19114ebfafb241a2b7df6" class="mwe-math-fallback-image-display mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.505ex; width:15.768ex; height:2.509ex;" alt="{\displaystyle {\ce {A + B -> AB}}^{*}}"></span> <span class="mwe-math-element"><span class="mwe-math-mathml-display mwe-math-mathml-a11y" style="display: none;"><math display="block" xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\ce {AB}}^{*}{\ce {+ M -> C + M}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msup> <mrow class="MJX-TeXAtom-ORD"> <mtext>AB</mtext> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mo>∗</mo> </mrow> </msup> <mrow class="MJX-TeXAtom-ORD"> <mo>+</mo> <mtext>M</mtext> <mo stretchy="false">⟶</mo> <mtext>C</mtext> <mo>+</mo> <mtext>M</mtext> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\ce {AB}}^{*}{\ce {+ M -> C + M}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/869b584c33132de69ba189d910d5ae6914f50ec1" class="mwe-math-fallback-image-display mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.505ex; width:20.127ex; height:2.509ex;" alt="{\displaystyle {\ce {AB}}^{*}{\ce {+ M -> C + M}}}"></span> </p><p>These reactions frequently have a pressure and temperature dependence region of transition between second and third order kinetics.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> </p><p>Catalytic reactions are often three-component, but in practice a complex of the starting materials is first formed and the rate-determining step is the reaction of this complex into products, not an adventitious collision between the two species and the catalyst. For example, in hydrogenation with a metal catalyst, molecular dihydrogen first dissociates onto the metal surface into hydrogen atoms bound to the surface, and it is these monatomic hydrogens that react with the starting material, also previously adsorbed onto the surface. </p><p>Reactions of higher molecularity are not observed due to very small probability of simultaneous interaction between 4 or more molecules.<sup id="cite_ref-Carr_9-0" class="reference"><a href="#cite_note-Carr-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Steinfeld_4-2" class="reference"><a href="#cite_note-Steinfeld-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Difference_between_molecularity_and_order_of_reaction">Difference between molecularity and order of reaction</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Molecularity&action=edit&section=4" title="Edit section: Difference between molecularity and order of reaction"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>It is important to distinguish molecularity from <a href="/wiki/Order_of_reaction" class="mw-redirect" title="Order of reaction">order of reaction</a>. The order of reaction is an empirical quantity determined by experiment from the rate law of the reaction. It is the sum of the exponents in the rate law equation.<sup id="cite_ref-Rogers_10-0" class="reference"><a href="#cite_note-Rogers-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> Molecularity, on the other hand, is deduced from the mechanism of an elementary reaction, and is used only in context of an elementary reaction. It is the number of molecules taking part in this reaction. </p><p>This difference can be illustrated on the reaction between <a href="/wiki/Nitric_oxide" title="Nitric oxide">nitric oxide</a> and hydrogen:<sup id="cite_ref-Laidler_11-0" class="reference"><a href="#cite_note-Laidler-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> </p><p><span class="mwe-math-element"><span class="mwe-math-mathml-display mwe-math-mathml-a11y" style="display: none;"><math display="block" xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\ce {2NO + 2H2 -> N2 + 2H2O,}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> <mspace width="thinmathspace" /> <mtext>NO</mtext> <mo>+</mo> <mn>2</mn> <mspace width="thinmathspace" /> <msubsup> <mtext>H</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <mo stretchy="false">⟶</mo> <msubsup> <mtext>N</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <mo>+</mo> <mn>2</mn> <mspace width="thinmathspace" /> <msubsup> <mtext>H</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <mtext>O</mtext> <mo>,</mo> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\ce {2NO + 2H2 -> N2 + 2H2O,}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/801b3d641dad8d95d7f720f512576d053abfa13b" class="mwe-math-fallback-image-display mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:29.824ex; height:2.843ex;" alt="{\displaystyle {\ce {2NO + 2H2 -> N2 + 2H2O,}}}"></span> </p><p>where the observed rate law is <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle v=k{\ce {[NO]^2[H2]}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>v</mi> <mo>=</mo> <mi>k</mi> <mrow class="MJX-TeXAtom-ORD"> <msup> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">[</mo> <mtext>NO</mtext> <mo stretchy="false">]</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msup> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">[</mo> <msubsup> <mtext>H</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <mo stretchy="false">]</mo> </mrow> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle v=k{\ce {[NO]^2[H2]}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/72ed0525d38b0d0b832d1c056fbd833ac0d85b74" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:15.427ex; height:3.509ex;" alt="{\displaystyle v=k{\ce {[NO]^2[H2]}}}"></span>, so that the reaction is <i>third order</i>. Since the order does <i>not</i> equal the sum of reactant stoichiometric coefficients, the reaction must involve more than one step. The proposed two-step mechanism<sup id="cite_ref-Laidler_11-1" class="reference"><a href="#cite_note-Laidler-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> has a rate-limiting first step whose molecularity corresponds to the overall order of 3: </p><p>Slow: <span class="mwe-math-element"><span class="mwe-math-mathml-display mwe-math-mathml-a11y" style="display: none;"><math display="block" xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\ce {2 NO + H2 -> N2 + H2O2}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> <mspace width="thinmathspace" /> <mtext>NO</mtext> <mo>+</mo> <msubsup> <mtext>H</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <mo stretchy="false">⟶</mo> <msubsup> <mtext>N</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <mo>+</mo> <msubsup> <mtext>H</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <msubsup> <mtext>O</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\ce {2 NO + H2 -> N2 + H2O2}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/59b0a6a2851982a8a3e74d64fc040bed64d641b5" class="mwe-math-fallback-image-display mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:27.132ex; height:2.843ex;" alt="{\displaystyle {\ce {2 NO + H2 -> N2 + H2O2}}}"></span> Fast: <span class="mwe-math-element"><span class="mwe-math-mathml-display mwe-math-mathml-a11y" style="display: none;"><math display="block" xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\ce {H2O2 + H2 -> 2H2O}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <msubsup> <mtext>H</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <msubsup> <mtext>O</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <mo>+</mo> <msubsup> <mtext>H</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <mo stretchy="false">⟶</mo> <mn>2</mn> <mspace width="thinmathspace" /> <msubsup> <mtext>H</mtext> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mspace width="0pt" height="0pt" depth=".2em" /> </mrow> </msubsup> <mtext>O</mtext> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\ce {H2O2 + H2 -> 2H2O}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/2d233d6cfad618fdead6cf58fbab56c5af88682c" class="mwe-math-fallback-image-display mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:22.548ex; height:2.843ex;" alt="{\displaystyle {\ce {H2O2 + H2 -> 2H2O}}}"></span> </p><p>On the other hand, the molecularity of this reaction is undefined, because it involves a mechanism of more than one step. However, we can consider the molecularity of the individual elementary reactions that make up this mechanism: the first step is trimolecular because it involves three reactant molecules, while the second step is bimolecular because it involves two reactant molecules. </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Molecularity&action=edit&section=5" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Reaction_rate" title="Reaction rate">Reaction rate</a></li> <li><a href="/wiki/Dissociation_(chemistry)" title="Dissociation (chemistry)">Dissociation (chemistry)</a></li> <li><a href="/wiki/Lindemann_mechanism" title="Lindemann mechanism">Lindemann mechanism</a></li> <li><a href="/wiki/Crossed_molecular_beam" title="Crossed molecular beam">Crossed molecular beam</a></li> <li><a href="/wiki/Cage_effect" title="Cage effect">Cage effect</a></li> <li><a href="/wiki/Reaction_progress_kinetic_analysis" title="Reaction progress kinetic analysis">Reaction progress kinetic analysis</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Molecularity&action=edit&section=6" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-Atkins-1"><span class="mw-cite-backlink">^ <a href="#cite_ref-Atkins_1-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Atkins_1-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Atkins_1-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-Atkins_1-3"><sup><i><b>d</b></i></sup></a></span> <span class="reference-text">Atkins, P.; de Paula, J. Physical Chemistry. Oxford University Press, 2014</span> </li> <li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text">Temkin, O. N. State-of-the-Art in the Theory of Kinetics of Complex Reactions. In Homogeneous Catalysis with Metal Complexes: Kinetic Aspects and Mechanisms, John Wiley and Sons, ltd, 2012</span> </li> <li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text">Morrison R.T. and Boyd R.N. <i>Organic Chemistry</i> (4th ed., Allyn and Bacon 1983) p.215 <style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-205-05838-8" title="Special:BookSources/0-205-05838-8">0-205-05838-8</a></span> </li> <li id="cite_note-Steinfeld-4"><span class="mw-cite-backlink">^ <a href="#cite_ref-Steinfeld_4-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Steinfeld_4-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Steinfeld_4-2"><sup><i><b>c</b></i></sup></a></span> <span class="reference-text">J.I. Steinfeld, J.S. Francisco and W.L. Hase <i>Chemical Kinetics and Dynamics</i> (2nd ed., Prentice Hall 1999) p.5, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-13-737123-3" title="Special:BookSources/0-13-737123-3">0-13-737123-3</a></span> </li> <li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="https://goldbook.iupac.org/html/M/M03989.html">IUPAC Gold Book: Molecularity</a></span> </li> <li id="cite_note-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-6">^</a></b></span> <span class="reference-text">One textbook which mentions both <i>termolecular</i> and <i>trimolecular</i> as alternative names is J.W. Moore and <a href="/wiki/Ralph_Pearson" title="Ralph Pearson">R.G. Pearson</a>, <i>Kinetics and Mechanism</i> (3rd ed., John Wiley 1981) p.17, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-471-03558-0" title="Special:BookSources/0-471-03558-0">0-471-03558-0</a></span> </li> <li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text">Text discussing <a href="/wiki/Rate_constant" class="mw-redirect" title="Rate constant">rate constants</a> for termolecular reactions <a rel="nofollow" class="external autonumber" href="https://jpldataeval.jpl.nasa.gov/pdf/Jpl15_Sectn2_TermolecRxs.pdf">[1]</a></span> </li> <li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text"><a href="/wiki/International_Union_of_Pure_and_Applied_Chemistry" title="International Union of Pure and Applied Chemistry">IUPAC</a> definition of <i>Troe expression</i>, a semiempirical expression for the rate constant of termolecular reactions <a rel="nofollow" class="external autonumber" href="http://goldbook.iupac.org/T06517.html">[2]</a></span> </li> <li id="cite_note-Carr-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-Carr_9-0">^</a></b></span> <span class="reference-text">Carr, R. W. Chemical Kinetics. In Encyclopedia of Applied Physics. WILEY-VCH Verlag GmbH & Co KGaA, 2003</span> </li> <li id="cite_note-Rogers-10"><span class="mw-cite-backlink"><b><a href="#cite_ref-Rogers_10-0">^</a></b></span> <span class="reference-text">Rogers, D. W. Chemical Kinetics. In Concise Physical Chemistry, John Wiley and Sons, Inc. 2010.</span> </li> <li id="cite_note-Laidler-11"><span class="mw-cite-backlink">^ <a href="#cite_ref-Laidler_11-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Laidler_11-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><a href="/wiki/Keith_J._Laidler" title="Keith J. Laidler">Keith J. Laidler</a>, <i>Chemical Kinetics</i> (3rd ed., Harper & Row 1987), p.277 <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-06-043862-2" title="Special:BookSources/0-06-043862-2">0-06-043862-2</a></span> </li> </ol></div></div> <div class="navbox-styles"><style data-mw-deduplicate="TemplateStyles:r1129693374">.mw-parser-output .hlist dl,.mw-parser-output .hlist ol,.mw-parser-output .hlist ul{margin:0;padding:0}.mw-parser-output .hlist dd,.mw-parser-output .hlist dt,.mw-parser-output .hlist li{margin:0;display:inline}.mw-parser-output .hlist.inline,.mw-parser-output .hlist.inline dl,.mw-parser-output .hlist.inline ol,.mw-parser-output .hlist.inline ul,.mw-parser-output .hlist dl dl,.mw-parser-output .hlist dl ol,.mw-parser-output .hlist dl ul,.mw-parser-output .hlist ol dl,.mw-parser-output .hlist ol ol,.mw-parser-output .hlist ol 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.navbox{display:none!important}}</style></div><div role="navigation" class="navbox" aria-labelledby="Basic_reaction_mechanisms" style="padding:3px"><table class="nowraplinks mw-collapsible autocollapse navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="2"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><style data-mw-deduplicate="TemplateStyles:r1239400231">.mw-parser-output .navbar{display:inline;font-size:88%;font-weight:normal}.mw-parser-output .navbar-collapse{float:left;text-align:left}.mw-parser-output .navbar-boxtext{word-spacing:0}.mw-parser-output .navbar ul{display:inline-block;white-space:nowrap;line-height:inherit}.mw-parser-output .navbar-brackets::before{margin-right:-0.125em;content:"[ "}.mw-parser-output .navbar-brackets::after{margin-left:-0.125em;content:" ]"}.mw-parser-output .navbar li{word-spacing:-0.125em}.mw-parser-output .navbar a>span,.mw-parser-output .navbar a>abbr{text-decoration:inherit}.mw-parser-output .navbar-mini abbr{font-variant:small-caps;border-bottom:none;text-decoration:none;cursor:inherit}.mw-parser-output .navbar-ct-full{font-size:114%;margin:0 7em}.mw-parser-output .navbar-ct-mini{font-size:114%;margin:0 4em}html.skin-theme-clientpref-night .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}@media(prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}}@media print{.mw-parser-output .navbar{display:none!important}}</style><div class="navbar plainlinks hlist navbar-mini"><ul><li class="nv-view"><a href="/wiki/Template:Reaction_mechanisms" title="Template:Reaction mechanisms"><abbr title="View this template">v</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:Reaction_mechanisms" title="Template talk:Reaction mechanisms"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:Reaction_mechanisms" title="Special:EditPage/Template:Reaction mechanisms"><abbr title="Edit this template">e</abbr></a></li></ul></div><div id="Basic_reaction_mechanisms" style="font-size:114%;margin:0 4em">Basic <a href="/wiki/Reaction_mechanism" title="Reaction mechanism">reaction mechanisms</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Nucleophilic_substitution" title="Nucleophilic substitution">Nucleophilic substitutions</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/SN1_reaction" title="SN1 reaction">Unimolecular nucleophilic substitution</a> (S<sub>N</sub>1)</li> <li><a href="/wiki/SN2_reaction" title="SN2 reaction">Bimolecular nucleophilic substitution</a> (S<sub>N</sub>2)</li> <li><a href="/wiki/Nucleophilic_aromatic_substitution" title="Nucleophilic aromatic substitution">Nucleophilic aromatic substitution</a> (S<sub>N</sub>Ar)</li> <li><a href="/wiki/SNi" title="SNi">Nucleophilic internal substitution</a> (S<sub>N</sub>i)</li> <li><a href="/wiki/Nucleophilic_acyl_substitution" class="mw-redirect" title="Nucleophilic acyl substitution">Nucleophilic acyl substitution</a> (S<sub>N</sub>Acyl)</li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Electrophilic_substitution" title="Electrophilic substitution">Electrophilic substitutions</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Electrophilic_aromatic_substitution" title="Electrophilic aromatic substitution">Electrophilic aromatic substitution</a> (S<sub>E</sub>Ar)</li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Elimination_reaction" title="Elimination reaction">Elimination reactions</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/E1_elimination" class="mw-redirect" title="E1 elimination">Unimolecular elimination</a> (E1)</li> <li><a href="/wiki/E1cB-elimination_reaction" title="E1cB-elimination reaction">E1cB-elimination</a></li> <li><a href="/wiki/E2_elimination" class="mw-redirect" title="E2 elimination">Bimolecular elimination</a> (E2)</li> <li><a href="/wiki/Ei_mechanism" title="Ei mechanism">E<sub>i</sub> elimination</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Addition_reaction" title="Addition reaction">Addition reactions</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Electrophilic_addition" title="Electrophilic addition">Electrophilic addition</a> (A<sub>E</sub>)</li> <li><a href="/wiki/Nucleophilic_addition" title="Nucleophilic addition">Nucleophilic addition</a> (A<sub>N</sub>)</li> <li><a href="/wiki/Free-radical_addition" title="Free-radical addition">Free-radical addition</a></li> <li><a href="/wiki/Cycloaddition" title="Cycloaddition">Cycloaddition</a></li> <li><a href="/wiki/Oxidative_addition" title="Oxidative addition">Oxidative addition</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Unimolecular reactions</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Intramolecular_reaction" title="Intramolecular reaction">Intramolecular reaction</a></li> <li><a href="/wiki/Isomerization" title="Isomerization">Isomerization</a></li> <li><a href="/wiki/Photodissociation" title="Photodissociation">Photodissociation</a></li> <li><a href="/wiki/Lindemann%E2%80%93Hinshelwood_mechanism" class="mw-redirect" title="Lindemann–Hinshelwood mechanism">Lindemann–Hinshelwood mechanism</a></li> <li><a href="/wiki/RRKM_theory" title="RRKM theory">RRKM theory</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Electron_transfer" title="Electron transfer">Electron/Proton transfer</a> reactions</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Redox" title="Redox">Redox</a></li> <li><a href="/wiki/Harpoon_reaction" title="Harpoon reaction">Harpoon reaction</a></li> <li><a href="/wiki/Grotthuss_mechanism" title="Grotthuss mechanism">Grotthuss mechanism</a></li> <li><a href="/wiki/Marcus_theory" title="Marcus theory">Marcus theory</a></li> <li><a href="/wiki/Inner_sphere_electron_transfer" title="Inner sphere electron transfer">Inner sphere electron transfer</a></li> <li><a href="/wiki/Outer_sphere_electron_transfer" title="Outer sphere electron transfer">Outer sphere electron transfer</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Medium effects</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Solvent_effects" title="Solvent effects">Solvent effects</a></li> <li><a href="/wiki/Cage_effect" title="Cage effect">Cage effect</a></li> <li><a href="/wiki/Matrix_isolation" title="Matrix isolation">Matrix isolation</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Related topics</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Elementary_reaction" title="Elementary reaction">Elementary reaction</a></li> <li><a href="/wiki/Reaction_dynamics" title="Reaction dynamics">Reaction dynamics</a></li> <li><a href="/wiki/Reactive_intermediate" title="Reactive intermediate">Reactive intermediate</a></li> <li><a href="/wiki/Radical_(chemistry)" title="Radical (chemistry)">Radical (chemistry)</a></li> <li><a class="mw-selflink selflink">Molecularity</a></li> <li><a href="/wiki/Stereochemistry" title="Stereochemistry">Stereochemistry</a></li> <li><a href="/wiki/Catalysis" title="Catalysis">Catalysis</a></li> <li><a href="/wiki/Collision_theory" title="Collision theory">Collision theory</a></li> <li><a href="/wiki/Arrow_pushing" title="Arrow pushing">Arrow pushing</a></li> <li><a href="/wiki/Potential_energy_surface" title="Potential energy surface">Potential energy surface</a></li> <li><a href="/wiki/More_O%27Ferrall%E2%80%93Jencks_plot" title="More O'Ferrall–Jencks plot">More O'Ferrall–Jencks plot</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Chemical_kinetics" title="Chemical kinetics">Chemical kinetics</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Rate_equation" title="Rate equation">Rate equation</a></li> <li><a href="/wiki/Equilibrium_constant" title="Equilibrium constant">Equilibrium constant</a></li> <li><a href="/wiki/Rate-determining_step" title="Rate-determining step">Rate-determining step</a></li> <li><a href="/wiki/Reaction_coordinate" title="Reaction coordinate">Reaction coordinate</a></li> <li><a href="/wiki/Energy_profile_(chemistry)" title="Energy profile (chemistry)">Energy profile (chemistry)</a></li> <li><a href="/wiki/Transition_state_theory" title="Transition state theory">Transition state theory</a></li> <li><a href="/wiki/Activation_energy" title="Activation energy">Activation energy</a></li> <li><a href="/wiki/Activated_complex" title="Activated complex">Activated complex</a></li> <li><a href="/wiki/Arrhenius_equation" title="Arrhenius equation">Arrhenius equation</a></li> <li><a href="/wiki/Eyring_equation" title="Eyring equation">Eyring equation</a></li> <li><a href="/wiki/Michaelis%E2%80%93Menten_kinetics" title="Michaelis–Menten kinetics">Michaelis–Menten kinetics</a></li> <li><a href="/wiki/Diffusion-controlled_reaction" title="Diffusion-controlled reaction">Diffusion-controlled reaction</a></li></ul> </div></td></tr></tbody></table></div>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?useformat=desktop&type=1x1&usesul3=0" alt="" width="1" height="1" style="border: none; position: absolute;"></noscript> <div class="printfooter" data-nosnippet="">Retrieved from "<a dir="ltr" href="https://en.wikipedia.org/w/index.php?title=Molecularity&oldid=1178142124">https://en.wikipedia.org/w/index.php?title=Molecularity&oldid=1178142124</a>"</div></div> <div id="catlinks" class="catlinks" data-mw="interface"><div id="mw-normal-catlinks" class="mw-normal-catlinks"><a href="/wiki/Help:Category" title="Help:Category">Category</a>: <ul><li><a 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