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_2C=CH_2(g) + Br_2(g) → BrCH_2CH_2Br(l)
(b) 2 C_2H_6(g) + 7 O_2(g) → 4 CO_2(g) + 6 H_2O(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°.