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JAMB Chemistry 2023 Paper

Question : 9

Total: 82

Which of the following is an example of an endothermic reaction?

Combustion of propane (C $_3$ H $_8$ ) to produce carbon dioxide (CO $_2$ ) and water (H $_2$ O).
Burning of methane (CH $_4$ ) in the presence of oxygen
Formation of table salt (NaCl) from sodium (Na) and chlorine (Cl $_2$ ).
Decomposition of hydrogen peroxide (H $_2$ O $_2$ ) into water (H $_2$ O) and oxygen (O $_2$ ).

Solution:

Concept:

An endothermic reaction absorbs heat from its surroundings, so it has a positive enthalpy change,

\Delta H > 0

Explanation:

Combustion of propane, burning of methane, and formation of sodium chloride all release heat, so they are exothermic.

The decomposition of hydrogen peroxide,

2\mathrm{H_2O_2(l)} \to 2\mathrm{H_2O(l)} + \mathrm{O_2(g)}

, actually releases heat, so

\Delta H < 0

The energy needed to break bonds in

\mathrm{H_2O_2}

is only part of the activation process; it does not make the overall reaction endothermic.

Therefore, none of the listed reactions is truly endothermic.

Answer:

None of the given options is an endothermic reaction.

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