Question

calculate the change in entropy in this reaction: 3C2H2 (g) ---> C6H6 (g). I cannot find the stardard entropy for C6H6

Answer 1

Entropy of a reaction refers to the positional probabilities for each reactant. An atom in gas phase has more options for position than the same atom in a solid phase. This is why gases have more entropy than solids.

In reactions, the positional probabilities must be compared for all the reactants to the products produced.

If the reaction involves only gases, the entropy is related to the total number of moles on either side of the reaction.

so before we calculate the entropy change, we can predict whether the change in entropy is positive or negative.

3 C2H2 (g) -------> C6H6 (g)

In this reaction , the reactant side contains 3 moles where the product side has only one mole produced. So the change in entropy will be negative.

Standard molar entropy of C6H6 (g) = +49.08 J mol-1 K-1

Standard molar entropy of C2H2 (g) = +200.9 J mol-1 K-1

Entropy change in the reaction = S⁰ C6H6 - (3* S⁰ C2H2 )

= 49.08 - (3 * 200.9)

= - 553.62 J mol-1 K-1