How can entropy changes be calculated from standard molar entropies?

Entropy changes can be calculated from standard molar entropies by subtracting the entropy of reactants from that of the products.

In more detail, the standard molar entropy is a measure of the randomness or disorder of a substance at a standard state, typically at 25 degrees Celsius and 1 atmosphere of pressure. It is denoted by S° and its units are joules per mole per kelvin (J mol-1 K-1). The larger the standard molar entropy, the more disordered the substance.

To calculate the entropy change for a chemical reaction, you need to know the standard molar entropies of all the reactants and products involved in the reaction. The entropy change (ΔS°) for a reaction is calculated using the formula ΔS° = ΣS°(products) - ΣS°(reactants), where ΣS°(products) is the sum of the standard molar entropies of the products and ΣS°(reactants) is the sum of the standard molar entropies of the reactants.

For example, consider the reaction of hydrogen gas with iodine gas to form hydrogen iodide gas: H2(g) + I2(g) → 2HI(g). The standard molar entropies of H2, I2, and HI are 130.7, 116.1, and 206.4 J mol-1 K-1, respectively. The entropy change for this reaction would be calculated as follows: ΔS° = [2(206.4)] - [(130.7) + (116.1)] = 165.9 J mol-1 K-1.

It's important to note that a positive entropy change indicates an increase in disorder during the reaction, which is often the case for reactions where gases are produced. Conversely, a negative entropy change indicates a decrease in disorder, which is often the case for reactions where gases are consumed.

In summary, calculating entropy changes from standard molar entropies involves subtracting the sum of the standard molar entropies of the reactants from the sum of the standard molar entropies of the products. This gives a measure of the change in disorder that occurs during the reaction.