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THINK ABOUT IT

There are many advertisements on television about products that are designed to treat “acid indigestion.” Some of these substances claim to neutralize extra acid that may have built up in your stomach.

What happens when acids and bases are mixed?

To answer this question, you will explore

EXPLORING THE TOPIC

Neutralization Reactions

Pure water is neutral because it has equal numbers of H⁺ and OH⁻ ions.

In pure water at 25 °C, [H⁺] and [OH⁻] are both 1.0 × 10⁻⁷ mol/L. But if an acid or a base is added to water, it upsets the balance of ions in solution. Acidic solutions have an excess of H⁺ and basic solutions have an excess of OH⁻.

One way to reduce the toxicity of an acidic or basic solution is by dilution. If you add enough water, you can reduce the concentration of the acid or base. Extremely dilute acids and bases have a pH of 7 at 25 °C because this is the pH of pure water.

However, there is another way you can reduce the toxicity of an acidic or a basic solution. To counteract the effects of an acid in solution, you can add a base. Likewise, to counteract the effects of a base in solution, you can add an acid.

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The acid and base react in solution and neutralize each other. The chemical equation for the reaction between an acid and base is shown below.

Notice that this reaction is a double exchange reaction. The H⁺ and Na⁺ cations exchange anions. The result is the production of a salt, NaCl, and water. The excess H⁺ from the acid combines with OH⁻ from the added base to form H₂O. The reaction is referred to as a neutralization reaction.

So, one way to make an acidic solution safe is by adding a base. Likewise, you can make a basic solution safe by adding an acid.

Not all neutralization reactions produce neutral solutions. For example, if you use an acidic solution that is very concentrated and a basic solution that is not very concentrated, there will be leftover H⁺ ions after mixing. There will not be enough OH⁻ ions to neutralize all of the H⁺ ions. But some neutralization will have taken place. As a result of mixing, the solution will be closer to neutral than either of the starting solutions.

Predicting the Products of Neutralization Reactions

Several neutralization reactions are shown. Notice the patterns in the products of the reactions.

HCl(aq) + KOH(aq) → KCl(aq) + H₂O(l)

HNO₃(aq) + NaOH(aq) → NaNO₃(aq) + H₂O(l)

H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)

2HCl + Mg(OH)₂(aq) → MgCl₂(aq) + 2H₂O(l)

H₂SO₄(aq) + Ca(OH)₂(aq) → CaSO₄(aq) + 2H₂O(l)

Each reaction results in the production of an ionic compound (a salt) plus water. The salt consists of the cation of the base and the anion of the acid. For example, the cation in potassium hydroxide, KOH, is potassium, K⁺. The anion in nitric acid, HNO, is nitrate, NO₃⁻. The K⁺ and NO₃⁻ produce the ionic compound, potassium nitrate, KNO₃, which is a salt.

Some acids transfer more than one H⁺ ion. For example, each mole of sulfuric acid, H₂SO₄, transfers two moles of H⁺ ions. This is because there are two hydrogen ions in solution for every one sulfate anion, SO₄²⁻. Likewise, each mole of calcium hydroxide, Ca(OH)₂, transfers two moles of OH⁻ ions. There are two hydroxide ions in each formula unit of calcium hydroxide to balance the charge on the calcium cations, Ca²⁺.

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STOMACH ACID RELIEF

A common over-the-counter remedy for excess stomach acid uses a neutralization reaction. Milk of magnesia is a white mixture containing magnesium hydroxide, Mg(OH)₂. The mixture looks like milk because magnesium hydroxide, a white solid, does not dissolve completely in water. Instead, the white solid is suspended in the liquid.

Magnesium hydroxide is a base. It neutralizes stomach acid, HCl, to produce a salt and water as shown by this reaction.

2HCl(aq) + Mg(OH)₂(aq) → MgCl₂(aq) + 2H₂O(l)

stomach acid + milk of magnesia → magnesium chloride + water

LESSON SUMMARY

What happens when acids and bases are mixed?

A neutralization reaction is a reaction in which an acid and a base react in aqueous solution to produce an ionic compound (a salt) and water. The pH approaches 7 because the H⁺ from the acid and the OH⁻ from the base combine to form H₂O. It is relatively easy to predict the products of a neutralization reaction. The salt that forms is made from the cation of the base and the anion of the acid.