Neutralization Reactions and Types of Salts

A neutralization reaction is the reaction of an acid with a base to form a salt and water, and the salt can be neutral, acidic or basic in solution depending on the strengths of the acid and base from which it comes. In brief: HCl + NaOH → NaCl + H₂O gives a neutral salt solution (pH 7), the weak acid acetic acid with NaOH gives sodium acetate (basic, pH about 8.9 at 0.1 mol/L), and the weak base ammonia with HCl gives ammonium chloride (acidic, pH about 5.1).

Neutralization does not always give a solution with a pH of 7. This article describes the reaction, shows the four combinations of strong and weak acids and bases, classifies salts into normal, acid and basic salts, explains the heat of neutralization, works through quantitative examples, lists uses and gives some history.

What happens in neutralization?

An acid and a base react to form a salt and water: acid + base → salt + water. In ionic terms the hydrogen ion of the acid reacts with the hydroxide ion of the base. At the equivalence point, the amounts of acid and base are in exact stoichiometric proportion. Neutralization refers to this stoichiometry, even if the resulting solution is not neutral.

What is the pH after neutralization?

The reason is hydrolysis, covered in the next article: ions from a weak acid or base react with water.

What types of salts are there?

• Normal salts form when all replaceable hydrogen ions are neutralized: Na₂SO₄, K₃PO₄.
• Acid salts contain unreplaced hydrogen: NaHSO₄, NaHCO₃, KH₂PO₄. They form with partial neutralization of polyprotic acids, e.g., H₂SO₄ + NaOH → NaHSO₄ + H₂O.
• Basic salts contain hydroxide or oxide: Mg(OH)Cl, Pb(OH)NO₃, malachite Cu₂(OH)₂CO₃.
• Double salts contain two cations: alum, KAl(SO₄)₂·12H₂O.

How much heat is released?

For a strong acid with a strong base, ΔH ≈ −57 kJ per mole of water formed, because the net reaction H⁺ + OH⁻ → H₂O is always the same. Weak acids release less heat since part of the energy is needed to ionize them: for acetic acid it is about −52 kJ/mol, but hydrofluoric acid gives about −68 kJ/mol because of strong hydration of fluoride.

Worked examples

1. Write the reaction. Sulfuric acid with potassium hydroxide: H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O (normal salt). With 1 mol of KOH: H₂SO₄ + KOH → KHSO₄ + H₂O (acid salt).

2. Volume needed. 100.0 mL of 0.250 mol/L H₂SO₄ contains 0.0250 mol and needs 0.0500 mol of KOH, which is 50.0 mL of 1.00 mol/L KOH.

3. Mass of salt. The K₂SO₄ formed in example 2: 0.0250 mol × 174.26 g/mol = 4.36 g.
4. Antacid. A tablet of 750 mg CaCO₃ (7.49 mmol) neutralizes 15.0 mmol of HCl: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
5. Predict the character. Sodium nitrate: neutral (strong acid, strong base). Ammonium sulfate: acidic (strong acid, weak base). Potassium cyanide: basic (weak acid HCN, strong base).
6. Temperature rise. 50.0 mL of 1.00 mol/L HNO₃ and 50.0 mL of 1.00 mol/L KOH release 0.0500 × 57.1 = 2.86 kJ. In 100 g of solution, ΔT = 2,855 ÷ (100 × 4.18) = 6.8 °C.

Where is neutralization used?

• Medicine: antacids; treatment of acid or alkali burns with plenty of water, not with a counter-acid or base.
• Agriculture: liming acidic soils with CaCO₃ or Ca(OH)₂.
• Industry: treating acidic waste water and removing SO₂ from flue gases with lime.
• Making salts: fertilizers such as ammonium nitrate and ammonium sulfate.

How was neutralization studied?

Robert Boyle noted that acids and alkalis destroy each other’s properties. Jeremias Richter showed in the 1790s that fixed masses of acid and base neutralize each other, and Julius Thomsen found in the 1850s and later that the heat of neutralization of strong acids is nearly constant.

Which mistakes are common?

Mistake 1: Expecting pH 7 at the equivalence point every time

The pH depends on the salt formed.

Mistake 2: Neutralizing a chemical burn with the opposite chemical

The heat released can worsen the injury. Rinse with water.

Mistake 3: Ignoring the mole ratio for polyprotic acids

H₂SO₄ needs two moles of NaOH per mole.

Mistake 4: Calling every salt “table salt”

Table salt is sodium chloride. Other salts differ widely in their properties.