Unit 9: Topic 2 - Absolute Entropy and Entropy Change

Question 1

How can we calculate the entropy change in a reaction?

Answer 1

Given the absolute entropies of each of the reactants and products in a reaction, we can calculate the change in entropy by subtracting the total absolute entropy of the reactants from the total absolute entropy of the products.

Question 2

The absolute entropies of NH3, H2, and N2 are 192.5 J/mol K, 130.6 J/mol K, and 191.5 J/mol K, respectively. For the reaction N2(g) + 3H2(g) <–> 2NH3(g), what is the change in entropy?

Answer 2

We have to subtract the absolute entropies of the reactants [1]N2 and [3]H2 from the absolute entropy of the product [2]NH3. Thus, we get 2(192.5) - [191.5 + 3(130.6)] = -198.3 J/mol K. The negative sign indicates a decrease in entropy, which makes sense because we begin with 4 moles of gas and end with only 2 moles of gas.