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

What happens when an acid such as HCl is mixed with a base such as NaOH:

$\ce{HCl (aq) + NaOH (aq) → NaCl (aq) + H_2O (l)}$

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, 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:

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

Removing the spectator ions we get the net ionic equation:

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

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.

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

Acids and Bases React with Metals

Acids react with most metals to form a salt and hydrogen gas. As discussed in Chapter 7, metals that are more active than acids can undergo a single diplacement reaction.

For example, zinc metal reacts with hydrochloric acid producing zinc chloride and hydrogen gas.

$\ce{Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)}$

Bases react with certain metals like zinc or aluminum for example to also produce hydrogen gas. Sodium hydroxide reacts with zinc and water to form sodium zincate and hydrogen gas.

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

Contributors

• Peggy Lawson (Oxbow Prairie Heights School). Funded by Saskatchewan Educational Technology Consortium.

• Henry Agnew (UC Davis)