Question 8.11: PREDICTING THE SIGN OF ΔS FOR A REACTION Predict whether ΔS°...
PREDICTING THE SIGN OF ΔS FOR A REACTION
Predict whether ΔS° is likely to be positive or negative for each of the following reactions:
(a) H_{2}C=CH_{2}(g) + Br_{2}(g) → BrCH_{2}CH_{2}Br(l)
(b) 2 C_{2}H_{6}(g) + 7 O_{2}(g) → 4 CO_{2}(g) + 6 H_{2}O(g)
STRATEGY
Look at each reaction, and try to decide whether molecular randomness increases or decreases. Reactions that increase the number of gaseous molecules generally have a positive ΔS, while reactions that decrease the number of gaseous molecules have a negative ΔS.
(a) The amount of molecular randomness in the system decreases when 2 mol of gaseous reactants combine to give 1 mol of liquid product, so the reaction has a negative ΔS°.
(b) The amount of molecular randomness in the system increases when 9 mol of gaseous reactants give 10 mol of gaseous products, so the reaction has a positive ΔS°.