Free energy and the temperature of feasibility

Overview

Condition on ΔG for a process to be feasible and what is true of ΔG for a system at equilibrium.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   State the equation that relates free energy change to enthalpy and entropy changes.

   Model answer

   The relationship is ΔG = ΔH − TΔS, where T is the absolute temperature in kelvin.

2. 2

   State the condition on ΔG for a process to be feasible.

   Model answer

   A process is feasible when the free energy change is negative.

3. 3

   Give the units of ΔG and of T in the free energy equation.

   Model answer

   ΔG is in kJ mol⁻¹ and T is in kelvin.

4. 4

   For a reaction ΔH is −92 kJ mol⁻¹ and ΔS is −201 J K⁻¹ mol⁻¹. Calculate ΔG at 298 K.

   Model answer

   ΔG is −92 − 298(−0.201) = −92 + 59.9 = −32.1 kJ mol⁻¹.

5. 5

   The decomposition of calcium carbonate has ΔH = +178 kJ mol⁻¹ and ΔS = +161 J K⁻¹ mol⁻¹. Calculate ΔG at 298 K.

   Model answer

   ΔG is 178 − 298(0.161) = 178 − 48 = +130 kJ mol⁻¹.

6. 6

   Using ΔH = +178 kJ mol⁻¹ and ΔS = +161 J K⁻¹ mol⁻¹, calculate the minimum temperature at which calcium carbonate decomposes.

   Model answer

   Setting ΔG to zero gives T = 178 000 ÷ 161 = 1106 K.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.