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:

ΔG??= ΔHreaction??- TΔSsystem?

• The units of ΔG??are in kJ mol-1
• The units of ΔHreaction??are in kJ mol-1
• The units of T are in K
• The units of ΔSsystem??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?
  • From ΔH??and ΔS??values
  • 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??= ΔHreaction??- TΔSsystem?

= +135 - (298 x 0.344)

=?+32.49 kJ mol-1

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

Answer

ΔG??= ΣΔGproducts??- ΣΔGreactants?

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

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

ΔG??=?-1300 kJ?mol-1