Edexcel A Level Chemistry:复习笔记5.4.3 Gibbs Free Energy

Feasibility of Reactions


Gibbs free energy

  • The feasibility of a reaction is determined by two factors
    • The enthalpy and 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 ΔHreactionꝋare 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–1by dividing by 1000)


  • For a reaction to be feasible, ΔGꝋ must be equal or less than zero

Gibbs Free Energy Calculations

Calculating ΔGΘ

  • This can be done by using the Gibbs equation, using enthalpy change, ΔHꝋ, and entropy change, ΔSꝋ, values

Worked Example

ΔGꝋfrom ΔHꝋand ΔSꝋvalues

Calculate the free energy change for the following reaction:

2NaHCO3 (s) → Na2CO3 (s) + H2O (l) + CO2 (g)

  • ΔHꝋ = +135 kJ mol-1
  • ΔSꝋ = +344 J K-1 mol-1


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

  • Rearranging the Gibbs equation allows you to determine the temperature at which a non-spontaneous reaction become feasible

ΔGꝋ = ΔHreactionꝋ - TΔSsystemꝋ

  • Remember, for a reaction to be feasible ΔGΘꝋmust be zero or negative

0 = ΔHꝋ - TΔSꝋ

ΔHꝋ = TΔSꝋ

T = ΔHꝋ ÷ ΔSꝋ

Worked Example

At what temperature will the reduction of aluminium oxide with carbon become spontaneous?

Al2O3(s) + 3C(s)→ 2Al(s) + 3CO(g)

ΔHꝋ = +1336 kJ mol-1

ΔSꝋ = +581 J K-1 mol-1


    • If ΔGꝋ = 0 then T = ΔHꝋ ÷ ΔSꝋ
    • Covert ΔSꝋ to kJ K-1 mol-1 by dividing by 1000
    • T = 1336 ÷ (581/1000)
    • T = 2299 K

Temperature and Gibbs free energy 

  • When ΔGꝋ is negative, the reaction is spontaneous / feasible and likely to occur
  • When ΔGꝋ is positive, the reaction is not spontaneous / feasible and unlikely to occur
  • We can also look at the the values for enthalpy change, ΔH, and entropy change, ΔS
    • Depending on the value for ΔH and ΔS we can determine whether the reaction is spontaneous at a given temperature (T)


Summary for temperature and Gibbs free energy