11.5: Reactions of Acids and Bases
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Learning Objectives
- Write acid-base neutralization reactions.
- Write reactions of acids with metals.
- Write reactions of bases with metals.
Neutralization Reactions
The reaction that happens when an acid, such as \(\ce{HCl}\), is mixed with a base, such as \(\ce{NaOH}\):
\[\ce{HCl (aq) + NaOH (aq) → NaCl (aq) + H_2O (l)}\nonumber \]
When an acid and a base are combined, water and a salt are the products. Salts are ionic compounds containing a positive ion other than \(\ce{H^{+}}\) and a negative ion other than the hydroxide ion, \(\ce{OH^{-}}\). Double displacement reactions of this type are called neutralization reactions . We can write an expanded version of this equation, with aqueous substances written in their longer form:
\[\ce{H^{+} (aq) + Cl^{-} (aq) + Na^{+} (aq) + OH^{-} (aq) → Na^{+} (aq) + Cl^{-} (aq) + H_2O (l)}\nonumber \]
After removing the spectator ions, we get the net ionic equation:
\[\ce{H^{+} (aq) + OH^{-} (aq) → H_2O (l)}\nonumber \]
When a strong acid and a strong base are combined in the proper amounts—when \([\ce{H^{+}}]\) equals \([\ce{OH^{-}}\)]\)—a neutral solution results in which pH = 7. The acid and base have neutralized each other, and the acidic and basic properties are no longer present.
Salt solutions do not always have a pH of 7, however. Through a process known as hydrolysis , the ions produced when an acid and base combine may react with the water molecules to produce a solution that is slightly acidic or basic. As a general concept, if a strong acid is mixed with a weak base, the resulting solution will be slightly acidic. If a strong base is mixed with a weak acid, the solution will be slightly basic.
Example \(\PageIndex{1}\): Propionic Acid + Calcium Hydroxide
Calcium propionate is used to inhibit the growth of molds in foods, tobacco, and some medicines. Write a balanced chemical equation for the reaction of aqueous propionic acid (CH 3 CH 2 CO 2 H) with aqueous calcium hydroxide [Ca(OH) 2 ].
Solution
| Steps | Reaction |
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Write the unbalanced equation. This is a double displacement reaction, so the cations and anions swap to create the water and the salt. |
CH 3 CH 2 CO 2 H(aq) + Ca(OH) 2 (aq)→(CH 3 CH 2 CO 2 ) 2 Ca(aq) + H 2 O(l) |
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Balance the equation. Because there are two OH − ions in the formula for Ca(OH) 2 , we need two moles of propionic acid, CH 3 CH 2 CO 2 H , to provide H + ions. |
2 CH 3 CH 2 CO 2 H(aq) + Ca(OH) 2 (aq)→(CH 3 CH 2 CO 2 ) 2 Ca(aq) + 2 H 2 O(l) |
Exercise \(\PageIndex{1}\)
Write a balanced chemical equation for the reaction of solid barium hydroxide with dilute acetic acid.
- Answer
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\[\ce{Ba(OH)2(s) + 2CH3CO2H (aq)→Ba(CH3CO2)2 (aq) + 2H2O(l)} \nonumber \nonumber \]
Acids and Bases React with Metals
Acids react with most metals to form a salt and hydrogen gas. As discussed previously, metals that are more active than acids can undergo a single displacement reaction . For example, zinc metal reacts with hydrochloric acid, producing zinc chloride and hydrogen gas.
\[\ce{Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)}\nonumber \]
Bases also react with certain metals, like zinc or aluminum, to produce hydrogen gas. For example, sodium hydroxide reacts with zinc and water to form sodium zincate and hydrogen gas.
\[\ce{Zn(s) + 2NaOH (aq) + 2H2O(l) → Na2Zn(OH)4(aq) + H2 (g)}.\nonumber \]