Question

For a reaction ΔH = - 30kJ and ΔS = - 45 JK^-1, at what temperature reaction changes from spontaneous to non-spontaneous?

Options

• 666.6 K

• 375.0 K

• 675.0 K

• 777.0 K

Answer 1

666.6 K

Explanation:

To find the temperature at which a reaction changes from spontaneous to non-spontaneous, we use the Gibbs free energy formula: 

ΔG = ΔH − TΔS

A reaction is: 

• Spontaneous when ΔG < 0
• Non-spontaneous when ΔG > 0
• At equilibrium (transition point) when ΔG = 0

Step 1: Set ΔG = 0 to find the transition temperature

0 = ΔH − TΔS ⇒ T = `(DeltaH)/(DeltaS)`

Step 2: Plug in values

ΔH = −30kJ = −30000J

ΔS = −45J K⁻¹

`T = (-30000)/(-45) = (30000)/45 = 666.6` K