Question 16.2: PREDICTING THE SIGN OF ΔS FOR A GAS-PHASE REACTION Consider ...
PREDICTING THE SIGN OF ΔS FOR A GAS-PHASE REACTION
Consider the gas-phase reaction of A_{2} molecules (red) with B atoms (blue):
(a) Write a balanced equation for the reaction.
(b) Predict the sign of ΔS for the reaction.
STRATEGY
To determine the stoichiometry of the reaction, count the number of reactant A_{2} molecules and B atoms and the number of product AB molecules. To predict the sign of the entropy change, see if the reaction increases or decreases the number of gaseous particles.
(a) In this reaction, 3 A_{2} molecules and 6 B atoms are consumed and 6 AB molecules are formed (3 A_{2} + 6 B → 6 AB). Dividing by 3 to reduce the coefficients to their smallest whole number values gives the balanced equation A_{2}(g) + 2 B(g) → 2 AB(g).
(b) Because the reaction decreases the number of gaseous particles from 3 mol to 2 mol, the entropy change is negative.