Gibbs Free EnergyGibbs Free Energy

18.4 – 18.5

Homework Issues?Homework Issues?

Gibbs Free EnergyGibbs Free Energy

A new thermodynamic quantity in terms of H and S that is directly related to spontaneity.

G = H - TS

The free energy refers to the The free energy refers to the formation of one mole of compound. formation of one mole of compound.

Write reaction for the formation:NaCl

CHCl3

Example 18.4 p772. Example 18.4 p772. What’s What’s G for NG for N

2(g)2(g) + 3H + 3H2(g)2(g) 2NH 2NH

3(g)3(g)??

Using Using H and H and SS

Exercise 18.6 p772 Calculate Exercise 18.6 p772 Calculate G G

CHCH4(g) 4(g) + 2O+ 2O2(g)2(g) CO CO2(g)2(g)+ 2H+ 2H22OO(g)(g)

Solving for Solving for GG

Go = nGof (products)

-nGof (reactants)

       Elements are always zero as withH.

Example 18.5 p 773 Calc Example 18.5 p 773 Calc G for G for CC22HH55OHOH

(l)(l) + 3O + 3O2(g)2(g) 2CO 2CO

2(g)2(g) + 3H + 3H22OO(g)(g)

exercise 18.7 p 773 Calc exercise 18.7 p 773 Calc GGCaCOCaCO3(s)3(s) CaO CaO(s)(s) + CO + CO2(g)2(g)

GGoo

If < -10 kJ, spontaneousIf > 10 kJ, non-spontaneousIf between –10 &10 kJ, at equilibrium

Example 18.6 p 775Example 18.6 p 775Calc Calc HHoo & & GGoo for 2 KClO for 2 KClO

3(s)3(s) + 2 KCl + 2 KCl(s)(s) 3 O 3 O

2(g)2(g)

Discuss enthalpy change and spontaneity.Discuss enthalpy change and spontaneity.

exercise 18.8 Which areexercise 18.8 Which arespontaneous as written?spontaneous as written?

C(graphite) + 2H2(g) CH4(g)

2H2(g) + O2(g) 2H2O(l)

4HCN(g) + 5O2(g) 2H2O(l)+ 4CO2(g) + 2N2(g)

Ag+(aq) + I-

(aq) AgI(s)

Mostly reactants or products or both?

Interpretation of Free EnergyInterpretation of Free EnergyChemical reactions are used to produce

useful work– Burn gasoline to make car move– Reaction in battery produce electricity to drive

motorsNot all reactions produce useful workMaximum work is obtained when no

entropy is produced.The free-energy change is the maximum

energy available, or free, to do useful work

WWmaxmax = = GG

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.

Homework 37 - 49 odds