Gibbs free energy

• The feasibility of a reaction is determined by two factors, the?enthalpy change?and the??[popover id="F7rfknVfbfcSJVkN" label='entropy?change']
• The two factors come together in a fundamental thermodynamic concept called the?Gibbs free energy (G)
• The Gibbs equation is:

ΔG^??= ΔH_reaction^??–?TΔS_system^?

• • The units of ΔG^??are in kJ mol^–1
  • The units of ΔH_reaction^??are in kJ mol^–1
  • The units of?T?are in K
  • The units of ΔS_system^??are in J K^-1?mol^–1(and must therefore be converted to kJ K^–1?mol^–1?by dividing by 1000)

Calculating ΔG^?

• There are two ways you can calculate the value of ΔG^?
  1. From the Gibbs equation, using enthalpy change, ΔH^?, and entropy change, ΔS^?, values
  2. From ΔG^??values of all the substances present

Answer:

Step 1:?Convert the entropy value in kilojoules

• • • ΔS^??= +344 J K^-1?mol^-1??÷ 1000 = +0.344 kJ K^-1?mol^-1?

Step 2:?Substitute the terms into the Gibbs Equation

• • • ΔG^??= ΔH_reaction^??– TΔS_system^?
      • = +135 – (298 x 0.344)
      • =?+32.49 kJ mol^-1?

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

Answer:

• • This can be calculated in the same way as you complete enthalpy calculations
  • • ΔG^??= [(2 x CO_2?) + (3 x H₂O )] – [(C₂H₅OH) + (3 x O₂)]
    • ΔG^??= [(2 x -394) + (3 x -229 )] – [-175 + 0]
    • ΔG^??=?-1300 kJ?mol^-1?ΔG^??= ΣΔG_products^??– ΣΔG_reactants^?
  • This can also be done by drawing a Hess cycle - find the way that is best for you