which of the following pairs of substances has the highest heat of neutralization?

Analyze the question

The question asks which acid-base pair releases the most heat when it neutralizes. Heat of neutralization is the heat change when one mole of water is formed from an acid and a base.

Explain the concept

• A strong acid and a strong base are fully ionized in solution.
• Their neutralization reduces to one simple reaction:

H⁺(aq) + OH⁻(aq) → H₂O(l) ΔH ≈ −57.3 kJ/mol

• A weak acid or weak base is only partly ionized. Some of the energy released when H₂O forms is used up to ionize the weak electrolyte completely. This is the energy of dissociation, and it is absorbed (endothermic).
• So the net heat released is smaller for any pair involving a weak acid or weak base.

Show the solution

| Pair | Heat of neutralization (approx.) | |---|---| | Strong acid + strong base (HCl + NaOH) | −57.3 kJ/mol (highest) | | Weak acid + strong base (CH₃COOH + NaOH) | about −55 kJ/mol | | Strong acid + weak base (HCl + NH₃) | about −52 kJ/mol | | Weak acid + weak base (CH₃COOH + NH₃) | about −50 kJ/mol (lowest) |

The more weakly ionized the reactants are, the lower the magnitude of the heat of neutralization. Strong acid with strong base has no ionization energy to "pay", so it gives the highest value.

Answer: A. Strong acid and strong base.

Exam tip: Remember "strong + strong = about 57 kJ/mol, and always the biggest." Any weak partner "steals" some heat for ionization, so the value drops.