7.17: Acids-Bases Reactions- Neutralization

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Neutralization reactions

Acids and bases have another property: they react with each other to make water and an ionic compound called a salt. A salt, in chemistry, is any ionic compound made by combining an acid with a base. A reaction between an acid and a base is called a neutralization reaction and can be represented as:

acid + base → H₂O + salt

Example 1:

HCl_(aq) + NaOH_(aq) → H₂O_(l) + NaCl_(aq)

Neutralization reactions are an example of irreversible double-replacement reactions, like the ones we studied in CHE 103. The stoichiometry of the balanced neutralization equation depends on the number of H⁺ ions in the acid and the number of OH⁻ ions in the base.

Example 2:

Write the balanced chemical equation for the neutralization reaction between H₂SO₄ and KOH. What is the name of the salt that is formed?

Solution

The general reaction is as follows:

H₂SO_4(aq) + KOH_(aq) → H₂O_(l) + salt

Because the acid has two H⁺ ions in its formula, we need two OH⁻ ions to react with it, making two H₂O molecules as product. The remaining ions, K⁺ and SO₄²⁻, make the salt potassium sulfate (K₂SO₄). The balanced chemical reaction is as follows:

H₂SO_4(aq) + 2KOH_(aq) → 2H₂O_(l) + K₂SO_4(aq)

Exercise

Write the balanced chemical equation for the neutralization reaction between HCl and Mg(OH)₂. What is the name of the salt that is formed?

Answer: 2HCl_(aq)+ Mg(OH)_2(aq) → 2H₂O_(l) + MgCl_2(aq); magnesium chloride

Neutralization reactions and net ionic equations

In CHE 103, we examined how ionic compounds dissociate in water. In this chapter, we also learned that acids can also undergo dissociation in water.

Thus, sodium hydroxide, NaOH, is dissolved in water, it ionizes to form the sodium cation and the hydroxide anion (HO^–).

NaOH (aq) → Na⁺ (aq) + HO^– (aq)

When hydrochloric acid is dissolved in water, it ionizes to form the chloride anion and H^₊:

HCl (aq) → H⁺ (aq) + Cl^– (aq)

When NaCl is dissolved in water, it also ionizes to form the chloride anion and sodium cation:

NaCl (aq) → Na⁺ (aq) + Cl^– (aq)

Therefore, the neutralization reaction between HCl and NaOH can be written in its total ionic form:

HCl(aq) + NaOH(aq) → H₂O(l) + NaCl(aq) molecular equation

Na⁺ (aq) + HO^– (aq) + H⁺ (aq) + Cl^– (aq) → H₂O(l) + Na⁺ (aq) + Cl^– (aq) total ionic equation

If we cancel the ions that do not change from reactants to products, we are left with the net ionic equation:

HO^– (aq) + H⁺ (aq) → H₂O(l) net ionic equation

We have written this equation using a single arrow because sodium hydroxide is a strong base and is essentially 100% ionized in solution. The hydroxide anion (HO^–) reacts with the hydronium ion (H₃O⁺) to form two moles of water, as shown in the equation given below.

HCl (aq) + NaOH (aq) → H₂O + NaCl (aq)

Thus, we can see that the net reaction in a neutralization is the formation of water from H+ and HO- ions. This is the case no matter the nature of the acid or the base.

Exercise

Write a balanced molecular equation, a total ionic equation, and a net ionic equation for the reaction between HNO₃ acid and Ca(OH)₂

Answer

Molecular equation: 2HNO₃(aq) + Ca(OH)₂(aq) → 2H₂O(l) + Ca(NO₃)₂(aq)

Total ionic equation: 2H⁺(aq) + 2NO₃^– (aq) + Ca²⁺ (aq) + 2HO^– (aq) → 2 H₂O(l) + Ca²⁺ (aq) + 2NO₃^– (aq)

Net ionic equation: 2H⁺(aq) + 2HO^– (aq) → 2 H₂O(l)

Using neutralization reactions to obtain salts

As described above, salts can be obtained by neutralization reactions. Since salts are ionic compounds, their boiling points are much higher than water. So after the neutralization reaction is performed using equimolar quantities of acid and base, the water can be evaporated, resulting in solid salts.

Therefore, we can use our discussion above to decide on a combination of acid and base that will result in a specific salt. For example, what acid and base could result in the formation of NaCl by neutralization?

NaCl can be dissociated into cation and anion:

NaCl (aq) → Na⁺ (aq) + Cl^– (aq)

As we have seen above, the cation always derives from the hydroxide (base) in the neutralization reaction. To reform this hydroxide, we need to add HO^–to the cation. In other words:

Na⁺ (aq) + HO^– (aq) → NaOH (aq)

The anion always derives from the acid in the neutralization reaction. To reform this acid, we need to add H⁺to the anion. In other words:

Cl^–(aq) + H⁺ (aq) → HCl (aq)

Therefore the acid and base that result in the formation of sodium chloride by neutralization reaction are:

HCl (aq) + NaOH (aq) → H₂O + NaCl (aq)

Exercise \(\PageIndex{1}\)

For each of the following, write a balanced neutralization equation:
1. The reaction of calcium hydroxide with hydrochloric acid.
2. The reaction of sodium hydroxide with sulfuric acid (both ionizations).
3. The reaction of barium hydroxide with nitric acid.
Write a balanced neutralization equation for the reaction of calcium hydroxide with sulfuric acid
What acid and base would you use to obtain the following salts by neutralization? Write a balanced neutralization reaction in each case

a. KBr

b. NaNO₃

c. NH₄Cl

d. MgSO4

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