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JAMB Chemistry 2019 Paper

Question : 27

Total: 80

_2

_4

^{-1}

_2

_6

^{-1}

. Calculate the heat evolved in the process:

Solution:

Concept:

Heat of hydrogenation is calculated from the enthalpy change of formation using

\Delta H = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})

Explanation:

The process is the hydrogenation of ethene to ethane:

\mathrm{C_2H_4(g) + H_2(g) \rightarrow C_2H_6(g)}

The heat of formation of

\mathrm{H_2(g)}

is zero.

Substitute the given values:

\Delta H = \Delta H_f(\mathrm{C_2H_6}) - [\Delta H_f(\mathrm{C_2H_4}) + \Delta H_f(\mathrm{H_2})]

\Delta H = -82 - [50 + 0] = -132\ \mathrm{kJ}

The negative sign indicates heat is evolved.

Answer:

-132\ \mathrm{kJ}

Correct option: B

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