Question

For each reaction predict if ∆S is positive or negative briefly state your reason.

a) N2 (g) + 3 H2 (g) → 2 NH3 (g)

b) 2 LiOH (aq) + CO2 (g) → Li2CO3 (aq) + H2O (l)

c) H2 (g) + Cl2 (g) → 2 HCl (g)

d) Ba2+ (aq) + SO42- (aq) → BaSO4 (s)

Answer 1

Entropy is a measure of chaos; therefore, the more chaotic it becomes, the more entropy increases.

In general:

Entropy increases with the number of particles/moles in the system:

1 mol of AB has less entropy than 1 mol of A and 1 mol of B; due to higher amount of microstates

Entropy increases as the volume of particles increases. This can be compared with physical states

Volume of Solid < Volume of Liquid << Volume of Gas

Therefore, Gases have higher entropy than liquids and solids. Liquids have higher entropy than solids.

As T increases, the entropy increases, by definition, since it is a state function which depends on Temperature.

Mixture typically increases Entropy, so ordered states get higher amount of microstates.

Example could be Gas A + Gas B separated by a membrane, then eventually mixing A + B, microstates increases, meaning that entropy increases

a) N2 (g) + 3 H2 (g) → 2 NH3 (g)

1+3 = 4; goes to 2 moles, there is DECREASE in chaos/entropy

b) 2 LiOH (aq) + CO2 (g) → Li2CO3 (aq) + H2O (l)

gas forms liquid, therefore DECREASE in chaos/entropy

c) H2 (g) + Cl2 (g) → 2 HCl (g)

2 mol of gas = 2 mol of gas --> no change in entropy du to gases

then, HCl is much mor complex than 2 molcules of H2 and Cl2, therefore, expct small incrase in entropy

d) Ba2+ (aq) + SO42- (aq) → BaSO4 (s)

ions in solution form an ordered BaSO4 crystal systm, there must be an increase in entropy