Without doing a calculation, predict whether the entropy change will be positive or negative when the following reaction occurs in the direction it is written.

a. CO2 (g) + 2H2 (g) → CH3OH (l)

b. Br2 (l) + H2 (g) → 2HBr (g)

c. CH3OH (l) + 3/2O2 (g) → CO2 (g) + 2H2O (g)

d. 2NH3 (g) → N2 (g) + 3H2 (g)

CO2 (g) + 2H2 (g) → CH3OH (l) negative change in entropy

Br2 (l) + H2 (g) → 2HBr (g) positive change in entropy

CH3OH (l) + 3/2O2 (g) → CO2 (g) + 2H2O (g) positive change in entropy

2NH3 (g) → N2 (g) + 3H2 (g) positive change in entropy

Explanation:

The change in entropy of a system is said to be positive when there is an increase in the degree of disorderliness of a system. Recall that entropy is defined as the degree of disorderliness in a system.

When gaseous products are formed in a reaction, the more the increase in entropy of the system. Reactions that yield more gaseous products have a very positive increase in entropy.

Hence all reactions in which there was a positive change in entropy yielded gaseous products.