Question

Sodium carbonate can be made by heating sodium bicarbonate:
         2NaHCO₃(s)--> Na₂CO₃(s) + CO₂(g) + H₂O(g)

Given that ΔH° = 128.9 kJ/mol and ΔG° = 33.1 kJ/mol at 25°C, What is the value of ΔS°

Answer in J/mol K to 3 sig figs.

Answer 1

The standard Gibbs free energy ΔG° is related to standard enthalpy change ΔH° and standard entropy change ΔS° as

ΔG° = ΔH°-TΔS°

Where T is the temperature in K

Given that ΔH° = 128.9 kJ/mol

ΔG° = 33.1 kJ/mol

T = 25 C = 273+25 K = 298 K

33.1 kJ/mol = 128.9 kJ/mol - 298K*ΔS°

33.1 kJ/mol -128.9 kJ/mol = - 298K*ΔS°

-95.8 kJ/mol =  - 298K*ΔS°

ΔS° = -95.8 kJ/mol/- 298K

= -95.8*1000 J/mol/- 298K

= 321.476 J/mol-K

Rounding off to 3 significant figures we have ΔS° = 321 J/mole-K