<h2 data-l10n-args='{"chapter":"4.2","title":"Classifying Chemical Reactions"}' data-l10n-id="textbook-view-header-no-publishing-chapter">Classifying Chemical Reactions</h2>
<p>In this lesson and the next few, we will be looking at how we classify <a data-lesson="representing-chemical-change" data-subject="chemistry" data-tutorial="physical-and-chemical-change" href="https://app.ulearngo.com/chemistry/physical-and-chemical-change/representing-chemical-change">chemical</a> reactions. By the end of these lessons, you should be able to:</p>
<ul>
<li>Define three common types of chemical reactions (precipitation, acid-base, and oxidation-reduction)</li>
<li>Classify chemical reactions as one of these three types given appropriate descriptions or chemical equations</li>
<li>Identify common <a data-lesson="what-are-acids-and-bases" data-subject="chemistry" data-tutorial="acid-base-redox-reactions" href="https://app.ulearngo.com/chemistry/acid-base-redox-reactions/what-are-acids-and-bases">acids</a> and bases</li>
<li>Predict the solubility of common inorganic compounds by using solubility rules</li>
<li>Compute the oxidation states for <a data-lesson="elements" data-subject="chemistry" data-tutorial="classification-of-matter" href="https://app.ulearngo.com/chemistry/classification-of-matter/elements">elements</a> in compounds</li>
</ul>
<p>Humans interact with one another in various and complex ways, and we classify these interactions according to common patterns of behavior. When two humans exchange information, we say they are communicating. When they exchange blows with their fists or feet, we say they are fighting.</p>
<p>Faced with a wide range of varied interactions between chemical substances, scientists have likewise found it convenient (or even necessary) to classify chemical interactions by identifying common patterns of reactivity. This lesson and the next few will provide an introduction to three of the most prevalent types of chemical reactions: <strong>precipitation</strong>, <strong>acid-base</strong>, and <strong>oxidation-reduction</strong>.</p>
<h2 data-type="title">Precipitation Reactions and Solubility Rules</h2>
<p id="fs-idp140132618169728">A <strong>precipitation reaction</strong> is one in which dissolved substances react to form one (or more) solid products. Many reactions of this type involve the exchange of ions between ionic compounds in aqueous solution and are sometimes referred to as <em data-effect="italics">double displacement</em>, <em data-effect="italics">double replacement</em>, or <em data-effect="italics">metathesis</em> reactions.</p>
<p>These reactions are common in nature and are responsible for the formation of coral reefs in ocean waters and kidney stones in animals. They are used widely in industry for production of a <a data-lesson="atomic-and-atomic-mass-number" data-subject="chemistry" data-tutorial="chemistry-essential-ideas" href="https://app.ulearngo.com/chemistry/chemistry-essential-ideas/atomic-and-atomic-mass-number">number</a> of commodity and specialty chemicals. Precipitation reactions also play a central role in many chemical analysis techniques, including spot tests used to identify metal ions and <em data-effect="italics">gravimetric methods</em> for determining the composition of matter (see <a href="https://app.ulearngo.com/lesson/composition-of-matter">Chemistry 101</a>).</p>
<p id="fs-idp140132617792992">The extent to which a substance may be dissolved in water, or any solvent, is quantitatively expressed as its <strong>solubility</strong>, defined as the maximum <a data-lesson="equilibrium-calculations" data-subject="chemistry" data-tutorial="basic-equilibrium-concepts" href="https://app.ulearngo.com/chemistry/basic-equilibrium-concepts/equilibrium-calculations">concentration</a> of a substance that can be achieved under specified conditions. Substances with relatively large solubilities are said to be <strong>soluble</strong>.</p>
<p>A substance will <strong>precipitate</strong> when solution conditions are such that its concentration exceeds its solubility. Substances with relatively low solubilities are said to be <strong>insoluble</strong>, and these are the substances that readily precipitate from solution.</p>
<p>More information on these important concepts is provided in a later tutorial that covers solutions. For purposes of predicting the identities of <a data-lesson="the-solid-state-of-matter" data-subject="chemistry" data-tutorial="liquids-and-solids" href="https://app.ulearngo.com/chemistry/liquids-and-solids/the-solid-state-of-matter">solids</a> formed by precipitation reactions, one may simply refer to patterns of solubility that have been observed for many ionic compounds (see table below).</p>
<table class="span-all" id="fs-idp140132617697568" summary="This table has two columns and two rows. The cell that is in the first column and first row contains the following information: “Soluble compounds contain, (bullet point) group 1 metal cations (L I superscript positive sign, N a superscript positive sign, K superscript positive sign, R b superscript positive sign, C s superscript positive sign and ammonium ion (N H subscript 4 superscript positive sign); (bullet point) the halide ions C l superscript negative sign, B r superscript negative sign, and I superscript negative sign; (bullet point) the acetate C subscript 2 H subscript 3 O subscript 2 superscript negative sign, bicarbonate, H C O subscript 3 superscript negative sign, nitrate, N O subscript 3 superscript negative sign, and chlorate, C l O subscript 3 superscript negative sign ions. (bullet point) the sulfate, S O subscript 4 superscript negative sign ion.” The cell that is in the top row, second column, contains the following information: “Exceptions to these solubility rules include (bullet point) halides of A g superscript positive sign, H g subscript 2 superscript 2 positive sign, and P B superscript 2 positive sign; (bullet point) sulfates of A g superscript positive sign, B a superscript 2 positive sign, C a superscript 2 positive sign, H g subscript 2 superscript 2 positive sign, P b superscript 2 positive sign and S r superscript 2 positive sign.” The next cell on the bottom row, first column, contains the following information: “Insoluble compounds contain (bullet point) carbonate C O subscript 3 superscript 2 negative sign, chromate, C r O subscript 3 superscript 2 negative sign, phosphate, P O subscript 4 superscript 3 negative sign, and sulfide, S superscript 2 negative sign ions; (bullet point) hydroxide ion, O H superscript negative sign.” The last cell, in the last row, second column, contains the following information: “Exceptions to these insolubility rules include (bullet point) compounds of these anions with group 1 metal cations and ammonium ion; (bullet point) hydroxides of group 1 metal cations and B a superscript 2 positive sign.">
<thead>
<tr>
<th colspan="3" data-align="center" scope="col">
<h3>Solubilities of Common Ionic Compounds in Water</h3>
</th>
</tr>
</thead>
<tbody>
<tr valign="top">
<td data-align="left"><strong data-effect="bold">Soluble compounds</strong> contain

<ul data-bullet-style="bullet" id="fs-idp140132617715536">
<li>group 1 metal cations (\(\mathrm{Li^+},\) \(\mathrm{Na^+},\) \(\mathrm{K^+},\) \(\mathrm{Rb^+},\) and \(\mathrm{Cs^+}\)) and ammonium ion \((\mathrm{NH_4^+})\)</li>
<li>the halide ions (\(\mathrm{Cl^−},\) \(\mathrm{Br^−},\) and \(\mathrm{I^−}\))</li>
<li>the acetate \((\mathrm{C_2H_3O_2^−}), \) bicarbonate \((\mathrm{HCO_3^−})\) nitrate \((\mathrm{NO_3^−}),\) and chlorate \((\mathrm{ClO_3^−})\) ions</li>
<li>the sulfate \((\mathrm{SO_4^{2−}})\) ion</li>
</ul>
</td>
<td data-align="left"><strong data-effect="bold">Exceptions</strong> to these solubility rules include

<ul data-bullet-style="bullet" id="fs-idp40035120">
<li>halides of \(\mathrm{Ag^+},\) \(\mathrm{Hg^{2+}_2},\) and \(\mathrm{Pb^{2+}}\)</li>
<li>sulfates of \(\mathrm{Ag^+},\) \(\mathrm{Ba^{2+}},\) \(\mathrm{Ca^{2+}},\) \(\mathrm{Hg_2^{2+}},\) \(\mathrm{Pb^{2+}},\) and \(\mathrm{Sr^{2+}}\)</li>
</ul>
</td>
</tr>
<tr valign="top">
<td data-align="left"><strong data-effect="bold">Insoluble compounds</strong> contain

<ul data-bullet-style="bullet" id="fs-idp34201600">
<li>carbonate \((\mathrm{CO_3^{2−}}),\) chromate \((\mathrm{CrO_4^{2−}}),\) phosphate \((\mathrm{PO_4^{3−}}),\) and sulfide \((\mathrm{S^{2−}})\) ions</li>
<li>hydroxide ion \((\mathrm{OH^−})\)</li>
</ul>
</td>
<td data-align="left"><strong data-effect="bold">Exceptions</strong> to these insolubility rules include

<ul data-bullet-style="bullet" id="fs-idp180890064">
<li>compounds of these anions with group 1 metal cations and ammonium ion</li>
<li>hydroxides of group 1 metal cations and \(\mathrm{Ba^{2+}}\)</li>
</ul>
</td>
</tr>
</tbody>
</table>
<p id="fs-idm63476864">A vivid example of precipitation is observed when solutions of potassium iodide and lead nitrate are mixed, resulting in the formation of solid lead iodide:</p>
<h4>\(\mathrm{2KI}(aq) + \mathrm{Pb(NO_3)_2}(aq) \longrightarrow \mathrm{PbI_2}(s) + \mathrm{2KNO_3}(aq)\)</h4>
<p id="fs-idp24544224">This observation is consistent with the solubility guidelines: The only insoluble compound among all those involved is lead iodide, one of the exceptions to the general solubility of iodide salts.</p>
<p id="fs-idp30939280">The net ionic equation representing this reaction is:</p>
<h4>\(\mathrm{Pb}^{2+}(aq) + \mathrm{2I}^−(aq) \longrightarrow \mathrm{PbI_2}(s)\)</h4>
<p>Lead iodide is a bright yellow solid that was formerly used as an artist’s pigment known as iodine yellow (<a href="#attachment_96432">see image below</a>). The properties of pure PbI<sub>2</sub> crystals make them useful for fabrication of X-ray and gamma ray detectors.</p>
<div class="wp-caption alignnone" id="attachment_96432" style="width: 590px"><a href="https://data.ulearngo.com/assets/c8f219da-caaa-4264-a7dc-1c78e3050cc9"><img alt="lead-iodide" class="size-full wp-image-96432" height="646" src="https://data.ulearngo.com/assets/c8f219da-caaa-4264-a7dc-1c78e3050cc9" width="590"/></a><p class="wp-caption-text">A precipitate of PbI2 forms when solutions containing Pb<sup>2+</sup> and I<sup>−</sup> are mixed. Image credit: Der Kreole/Wikimedia Commons</p></div>
<p>The solubility guidelines in the table above may be used to predict whether a precipitation reaction will occur when solutions of soluble ionic compounds are mixed together. One merely needs to identify all the ions present in the solution and then consider if possible cation/anion pairing could result in an insoluble compound.</p>
<p>For example, mixing solutions of <a data-lesson="isolation-of-silver" data-subject="chemistry" data-tutorial="transition-metals" href="https://app.ulearngo.com/chemistry/transition-metals/isolation-of-silver">silver</a> nitrate and <a data-lesson="redox-reactions" data-subject="chemistry" data-tutorial="reactions-in-aqueous-solution" href="https://app.ulearngo.com/chemistry/reactions-in-aqueous-solution/redox-reactions">sodium</a> <a data-lesson="role-of-fluoride-in-preventing-tooth-decay" data-subject="chemistry" data-tutorial="equilibria-other-reaction-classes" href="https://app.ulearngo.com/chemistry/equilibria-other-reaction-classes/role-of-fluoride-in-preventing-tooth-decay">fluoride</a> will yield a solution containing \(\mathrm{Ag^+},\) \(\mathrm{NO_3^−},\) \(\mathrm{Na^+},\) and \(\mathrm{F^−}\) ions. Aside from the two ionic compounds originally present in the solutions, AgNO<sub>3</sub> and NaF, two additional ionic compounds may be derived from this collection of ions: NaNO<sub>3</sub> and AgF.</p>
<p>The solubility guidelines indicate all nitrate salts are soluble but that AgF is one of the exceptions to the general solubility of fluoride salts. A precipitation reaction, therefore, is predicted to occur, as described by the following equations:</p>
<h4>\(\mathrm{NaF}(aq) + \mathrm{AgNO_3}(aq) \longrightarrow \mathrm{AgF}(s) + \mathrm{NaNO_3}(aq) \text{ (molecular)}\)</h4>
<h4>\(\mathrm{Ag^+}(aq) + \mathrm{F^−}(aq) \longrightarrow \mathrm{AgF}(s) \text{ (net ionic)}\)</h4>

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Precipitation Reactions

Chemistry generally explores the substances of which matter is composed, the investigation of their properties and reactions, and the use of such reactions to form new substances. 

Chemistry

Discover the fundamentals of chemical reactions, from balancing equations to reaction stoichiometry, and explore topics like acid-base and redox reactions in this tutorial, also uncover titration, gravimetric, and combustion analysis in quantitative chemical analysis.


Precipitation Reactions

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