ΔG from ΔH and ΔS

Worked example

A reaction has ΔH = 95.0 kJ mol⁻¹ and ΔS = 121 J K⁻¹ mol⁻¹. Calculate ΔG at 274 °C.

1. 1. Gibbs energy from ΔH and ΔS: solve for ΔG

   $\Delta G = \Delta H - T\Delta S$

   = 28.79 kJ mol⁻¹

Answer: ΔG = 28.8 kJ mol⁻¹

Method

ΔG = ΔH − TΔS. ΔH is usually in kJ mol⁻¹ and ΔS in J K⁻¹ mol⁻¹, so put them in the same units before combining, and use T in kelvin.

Formulas used

• Gibbs energy from ΔH and ΔS

  $\Delta G = \Delta H - T\Delta S$

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