JAMB 2022 Chemistry · Question 12

The reactions below represent neutralization reaction, in which of them is the value of ΔH highest?

Options

A.CH $_3$ CH $_2$ COOH + KOH → CH $_3$ CH $_2$ COOK + H $_2$ O
B.NH $_4$ OH + HCL → NH $_4$ + H $_2$ O
C.NaOH + HCL → NaCL + H $_2$ O
D.CH $_3$ COOH + NaOH → CH $_3$ COONa + H $_2$ O

Answer: C. NaOH + HCL → NaCL + H $_2$ O

Explanation written with AI and checked against the answer key

1. Analyze the question

The question asks you to identify which of the given neutralization reactions has the highest enthalpy of neutralization ( $\Delta H$ ). In chemistry, the enthalpy of neutralization is the heat energy released when one mole of water is formed from the reaction of an acid and a base.

2. Explain the concept

The standard enthalpy of neutralization for a strong acid reacting with a strong base is a constant value of approximately -57.3 kJ/mol. This is because both the strong acid and the strong base dissociate completely into ions in water. The reaction is essentially just the combination of hydrogen ions ( $H^+$ ) and hydroxide ions ( $OH^-$ ) to form water:

H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}

However, when a weak acid or a weak base is involved, some of the heat released is consumed to complete the dissociation of the weak electrolyte. Consequently, the net heat released is less than -57.3 kJ/mol.

3. Show the solution

Let's analyze the nature of the reactants in each option:

Option A: $CH_3CH_2COOH$ (Propanoic acid) is a weak acid, and $KOH$ is a strong base. The heat of neutralization will be less than -57.3 kJ/mol.
Option B: $NH_4OH$ (Ammonium hydroxide) is a weak base, and $HCl$ is a strong acid. The heat of neutralization will be less than -57.3 kJ/mol.
Option C: $NaOH$ is a strong base, and $HCl$ is a strong acid. This reaction involves the complete neutralization of strong species, resulting in the maximum standard enthalpy of approximately -57.3 kJ/mol.
Option D: $CH_3COOH$ (Ethanoic acid) is a weak acid, and $NaOH$ is a strong base. The heat of neutralization will be less than -57.3 kJ/mol.

Comparing these, Option C represents the reaction between a strong acid and a strong base, which yields the highest amount of energy released (the highest absolute value of $\Delta H$ ).

4. Identify the answer

The correct option is C.

5. Tip

Always remember: Strong + Strong = Maximum Heat. If you see a weak acid or a weak base in the equation, the heat of neutralization will always be lower than the standard -57.3 kJ/mol because energy is "wasted" on breaking the bonds of the undissociated weak acid or base.

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