14.5: 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.

Video: Equimolar (~0.01 M) and equivolume solutions of \(\ce{HCl}\) and \(\ce{NaOH}\) are combined to make salt water. https://youtu.be/TS-I9KrUjB0

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 (CH₃CH₂CO₂H) with aqueous calcium hydroxide [Ca(OH)₂].

Solution

Solutions to Example 14.5.1
Steps	Reaction
Write the unbalanced equation. This is a double displacement reaction, so the cations and anions swap to create the water and the salt.	CH₃CH₂CO₂H(aq) + Ca(OH)₂(aq)→(CH₃CH₂CO₂)₂Ca(aq) + H₂O(l)
Balance the equation. Because there are two OH⁻ ions in the formula for Ca(OH)₂, we need two moles of propionic acid, CH₃CH₂CO₂H, to provide H⁺ ions.	2CH₃CH₂CO₂H(aq) + Ca(OH)₂(aq)→(CH₃CH₂CO₂)₂Ca(aq) +2H₂O(l)

Exercise \(\PageIndex{1}\)

Write a balanced chemical equation for the reaction of solid barium hydroxide with dilute acetic acid.

Answer: \[\ce{Ba(OH)2(s) + 2CH3CO2H (aq)→Ba(CH3CO2)2 (aq) + 2H2O(l)} \nonumber \nonumber \]

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.

\[\ce{Zn(s) + 2NaOH (aq) + 2H2O(l) → Na2Zn(OH)4(aq) + H2 (g)}.\nonumber \]