15.4 Spontaneity

1
15.4 Spontaneity

• 15.4.1 Predict whether a reaction or process
  will be spontaneous by using the sign of ?Go
• 15.4.2 Calculate ?Go for a reaction using the
  equation, ?Go ?Ho - T?So and by using values of
  the standard free energy change of formation,
  ?Gof
• 15.4.3 Predict the effect of a change in
  temperature on the spontaneity of a reaction
  using standard entropy and enthalpy changes and
  the equation, ?Go ?Ho - T?So

2
Spontaneity and Entropy

• All real processes occur spontaneously in the
  direction that increases the entropy of the
  universe (system surroundings)
• For a spontaneous process the sum of entropy
  changes must be positive
• ?Suniv ?Ssys ?Ssurr gt O

3
Spontaneity

• The final state is more probable than the initial
  state
• Final entropy of the universe is greater than the
  initial entropy of the universe
• ?S entropy of the system
• Heat is gained or lost to the surroundings
• Exothermic reactions increase the entropy of the
  surroundings, but by what magnitude?

4
Magnitude of ?So

• Is - ?Ho/T, where T is absolute temperature.
• Spontaneous change if
• Final state has a lower enthalpy than the initial
  state (?Ho is negative)
• AND
• Final state is more disordered than the initial
  state (?So is positive)

5
Gibbs Free Energy change, ?G

• ?G ?Ho - T?So
• ?Ho standard enthalpy change
• ?So standard entropy change of the system
• T temperature in Kelvin

6

• ?G is NEGATIVE when ?Suniverse is positive and
  the change is spontaneous.
• Whether a reaction actually occurs spontaneous
• The influence of the ?So is always dominant at
  high temperatures where ?Ho does not vary much
• ?G is equal to the amount of energy from that
  system that is available to do useful work.
• ? E (internal energy) q w

7
?Ho ?So ?G spontaneity
, endo , random Depends on T Spontaneous at high temps when T?SogtHo
, endo -, ordered Always Never spontaneous
-, exo , random Always - Always spontaneous
-, exo -, ordered Depends on T Spontaneous at low temps when T?SoltHo
8
(No Transcript)
9
Graphing
Temperature (K) Gibb's Free Energy(kJ/mol)
200 3000
220 3400
240 3800
260 4200
280 4600
300 5000
320 5400
340 5800
360 6200

• Place ?G on the y axis and T on the x axis.
• Calculate the slope of the line of best fit. This
  value represents the entropy change for the
  reaction.
• Calculate the y intercept on the graph This value
  is the ?H of the reaction.
• Write the Gibb's Free Energy equation for this
  reaction

10
1. Slope 20 kJ/ K mol 2. y intercept
1000 kJ/mol3. G 1000 - (200)(20)
11
Calculations using ?G ?Ho - T?So
12
Answer
This reaction is Non-spontaneous at this
temperature, what temperature would we need so
that it would be spontaneous, assuming ?S and ? H
are independent of temp?
13
Calculations using ?Gfo (standard temp and
pressure)
14

• http//www.saskschools.ca/curr_content/chem30/modu
  les/module3/lesson6/gibbs.htm
• Good site with practice problems about
  spontaneity, entropy and Gibbs free energy