Benzene can be reduced to cyclohexane when it is treated with hydrogen in the pr

Question

Benzene can be reduced to cyclohexane when it is treated with hydrogen in the presence of a nickel catalyst under conditions of high temperature and pressure. Predict the signs of H° and S° for this process. Benzene can be reduced to cyclohexane when it is treated with hydrogen in the presence of a nickel catalyst under conditions of high temperature and pressure. Predict the signs of H° and S° for this process.

A. H° > 0, S° < 0

B. H° > 0, S° > 0

C. H° < 0, S° < 0

D. H° < 0, S° > 0

E. H° > 0, S° = 0

Explanation / Answer

The given chemical reaction is

C6H6 + 3 H2 -> C6H12

The reaction is initiated at high temperature and pressure.

This indicates that the formation of cyclohexane (6 C-H bonds)from 3 C=C bonds and 3 C-C bonds.

The formation of cyclohexane from benzene is an exothermic reaction.

C-H bond energy is 414kJ/mol

C=C bond energy is 724kJ/mol

C-C bond energy is 368kJ/mol

H-H bond energy is 435 kj/mol

Hence, deltaH° is negative.

Entropy is the measure of randomness.

For this reaction, degree of randomness is decreasing due to the formation of stable compound cyclohexane. Hence, deltaS° is negative.

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