Spontaneity of Reaction

1
Chapter 17

• Spontaneity of Reaction
• Thermodynamic functions of enthalpy, entropy, and
  Gibbs free energy.

2
Chapter 17

• Spontaneous reactions examples -
• CH4(g) 2O2(g) --gt CO2(g) 2 H2O(l)
• H2O(s) --gt H2O(l) at 25oC.

3
Spontaneity Factors

• 1.) Energy factor at 25oC, 1 atm, exothermic
  reactions are ordinarily spontaneous (nature
  tends toward stability)

4
Spontaneity Factors

• 2.) Randomness factor other things being equal,
  systems tend move from a more ordered to a more
  random state.

5
Spontaneity Factors

• Nature tends toward disorder (the entropy of the
  universe is increasing).

6
Entropy Changes

• ?S S products - S reactants measure of change
  in order (randomness)
• Ex. Solid --gt liquid ?S positive

7
Entropy Changes

• ?S is usually positive for a reaction in which
  the number of moles of gas increases.

8
Entropy Changes

• Determine if the entropy of the following
  reactions will be positive or negative.
• 2 SO3(g)--gt2 SO2(g) O2(g)
• N2(g) 3 H2(g) --gt 2 NH3(g)
• first reaction positive, second negative.

9
Entropy Changes

• ?So calculations (?S at 1atm, 25oC - see chart).
• ?So ?So products - ?So reactants.
• ?S is positive for elements and compounds.

10
Entropy Changes

• Reactions/processes for which ?So is positive
  tend to be spontaneous, at least at high
  temperatures

11
Entropy Changes

• H2O(s)--gtH2O(l),?So gt 0
• H2O(l)--gtH2O(g),?So gt 0
• Fe2O3(s)3H2(g)--gt 2Fe(s) 3 H2O(g), ?So gt 0.
• All are endothermic, but become spontaneous at
  high temps.

12
Free Energy Changes

• ?Go represents the useful energy available for
  work. Negative energy available from the
  reaction to do work.
• Positive energy needed to get the reaction to
  occur.

13
Free Energy Changes

• ?Go is a state function and is dependent only on
  the initial states of the reactants and not the
  pathway of the reaction.

14
Free Energy Changes

• ?Go ??o - T?So
• Note that ?G, like ?S, is dependent on pressure
  and concentration. Unlike ?Ho and ?So, Go is
  strongly temperature dependent.

15
Free Energy Changes

• If ?Go lt 0, reaction is spontaneous at standard
  conditions (1 M conc., 1 atm P).
• If Go gt 0, reaction is nonspontaneous at standard
  conditions.

16
Free Energy Changes

• If Go 0, reaction is at equilibrium at standard
  conditions.

17
Free Energy Changes

• Effect of ?Ho and ?So on spontaneity
• If ?Ho gt 0, ?So lt 0, and ?Go gt 0 at all temps?
• If ?Ho lt 0, ?So gt 0, and ?Go lt 0 at all temps?

18
Free Energy Changes

• If ?Ho gt 0, ?So gt 0, and ?Go gt 0 at low temps?
  High temps?
• If ?Ho lt 0, ?So lt 0, and ?Go lt 0 at low temps?
  High temps?

19
Free Energy Changes

• Calculation of ?Go from ?So and ?Ho.
• Fe2O3(s)3H2(g)--gt 2Fe(s) 3 H2O(g). What is ?Go
  at 25oC? 500oC?
• 56.6kJ, -10.6 kJ.

20
Free Energy Changes

• At what temp does the reduction of iron (III)
  oxide by hydrogen become spontaneous at 1 atm?
• T 96.8/0.1387 698 K.

21
Free Energy Changes

• Calculation ?Go at 25oC from, ?Gof
• ?Go (? ?Gof products) -
• (? ? Gof reactants).

22
Free Energy Changes

• Calculation ?G from ?Go
• ?G ?Go RT ln Q.
• R 0.00831 kJ/K

23
Free Energy Changes

• Given that ?Go lead (II) chloride ionizing in
  solution is 27.3 kJ and that the concentration of
  Pb2 and Cl- are each 0.0010 M,calculate ?G of
  this reaction (at 25oC).
• -24.0 kJ

24
Free Energy Changes

• Relationship of ?G and K (Keq).
• ?Go -RT ln K.
• If K gt 1, ?Go lt 0, reaction is spontaneous at
  standard conditions.

25
Free Energy Changes

• Relationship of ?G and K (Keq).
• ?Go -RT ln K.
• If K lt 1, ?Go gt 0, reaction is nonspontaneous at
  standard conditions.

26
Free Energy Changes

• Relationship of ?G and K (Keq).
• ?Go -RT ln K.
• If K 1, ?Go 0, reaction is at equilibrium at
  standard conditions.