# 14.4: 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

What 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:

$\ce{H^{+} (aq) + Cl^{-} (aq) + Na^{+} (aq) + OH^{-} (aq) → Na^{+} (aq) + Cl^{-} (aq) + H_2O (l)}\nonumber$

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. We will not go into details here, but generally if a strong acid is mixed with a weak base there 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 for example to produce hydrogen gas. For exanple, sodium hydroxide reacts with zinc and water to form sodium zincate and hydrogen gas.

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