# 14.5: Reactions of Acids and Bases

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

Solution

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.

$\ce{Ba(OH)2(s) + 2CH3CO2H (aq)→Ba(CH3CO2)2 (aq) + 2H2O(l)} \nonumber\nonumber$

## 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.

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