AQA A Level Chemistry复习笔记5.1.7 Gibbs Free Energy

Gibbs free energy

• As we have seen in the previous sections, the feasibility of a reaction is determined by two factors, the enthalpy change and the entropy change
• The two factors come together in a fundamental thermodynamic concept called the Gibbs free energy (G)
• The Gibbs equation is:

ΔGꝋ = ΔHreactionꝋ - TΔSsystemꝋ

• The units of ΔGꝋ are in kJ mol-1
• The units of ΔHreactionꝋ are in kJ mol-1
• The units of T are in K
• The units of ΔSsystemꝋ are in J K-1 mol-1(and must therefore be converted to kJ K-1 mol-1 by dividing by 1000)

Calculating ΔGꝋ

• There are two ways you can calculate the value of ΔGꝋ
  • From ΔHꝋ and ΔSꝋ values
  • From ΔGꝋ  values of all the substances present

   

Answer

Step 1: Convert the entropy value in kilojoules

ΔSꝋ = +344 J K-1 mol-1  ÷ 1000 = +0.344 kJ K-1 mol-1

Step 2: Substitute the terms into the Gibbs Equation

ΔGꝋ = ΔHreactionꝋ - TΔSsystemꝋ

= +135 - (298 x 0.344)

= +32.49 kJ mol-1

The temperature is 298 K since standard values are quoted in the question

Answer

ΔGꝋ = ΣΔGproductsꝋ - ΣΔGreactantsꝋ

ΔGꝋ = [(2 x CO2 ) + (3 x H2O )] - [(C2H5OH) + (3 x O2)]

ΔGꝋ = [(2 x -394 ) + (3 x -229 )] - [-175 + 0]

ΔGꝋ = -1300 kJ mol-1