Gibbs Free Energy 18.4 – 18.5

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Gibbs Free Energy A new thermodynamic quantity in terms of H and S that is directly related to spontaneity.  G =  H - T  S

The free energy refers to the formation of one mole of compound. Write reaction for the formation: NaCl CHCl 3

Example 18.4 p772. What’s  G for N 2(g) + 3H 2(g)  2NH 3(g) ? Using  H and  S

Exercise 18.6 p772 Calculate  G CH 4(g) + 2O 2(g)  CO 2(g) + 2H 2 O (g)

Solving for  G  G o =  n  G o f (products) -  n  G o f (reactants)  Elements are always zero as with  H.

Example 18.5 p 773 Calc  G for C 2 H 5 OH (l) + 3O 2(g)  2CO 2(g) + 3H 2 O (g)

exercise 18.7 p 773 Calc  G CaCO 3(s)  CaO (s) + CO 2(g)

GoGoGoGo If < -10 kJ, spontaneous If > 10 kJ, non-spontaneous If between –10 &10 kJ, at equilibrium

Example 18.6 p 775 Calc  H o &  G o for 2 KClO 3(s) + 2 KCl (s)  3 O 2(g) Discuss enthalpy change and spontaneity.

exercise 18.8 Which are spontaneous as written? C (graphite) + 2H 2(g)  CH 4(g) 2H 2(g) + O 2(g)  2H 2 O (l) 4HCN (g) + 5O 2(g)  2H 2 O (l) + 4CO 2(g) + 2N 2(g) Ag + (aq) + I - (aq)  AgI (s) Mostly reactants or products or both?

Interpretation of Free Energy Chemical reactions are used to produce useful work – Burn gasoline to make car move – Reaction in battery produce electricity to drive motors Not all reactions produce useful work Maximum work is obtained when no entropy is produced. The free-energy change is the maximum energy available, or free, to do useful work

W max =  G Maximum work is an idealization – Some entropy is created During spontaneous reactions, the free energy decreases as equilibrium is reached. – The energy created is used During nonspontaneous reactions, the free energy increases as equilibrium is reached. – Have to put energy in to make reaction occur.

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