H2g O2g H2Ol

1
Example

• H2(g) ½ O2(g) H2O(l)
• DS0 163 J K 1 mol1
• DH0 285 kJ mol1
• Decrease in entropy of system DS0reaction
  DS0system lt 0
• Exothermic heat to the surroundings
• Reaction is spontaneous DSsystem
  DSsurroundings gt 0
• Energy released by the reaction randomizes the
  surroundings

• Spontaneity depends on entropy and enthalpy

2
Gibbs Free Energy
G H TS

• Because a spontaneous reaction maximizes
  Suniverse, it also minimizes G

Spontaneous reactions minimize free energy
Free Energy is the driving force for chemical
reactions It is a combination of enthalpic and
entropic effects
3
Understanding Free Energy Minimization

• Pinball machine, being bounced.
• Balls in pockets molecules in different energy
  states
• Very gentle bouncing (low temperature) most
  balls in lowest pockets

• Higher temperature some balls in higher pockets
  (increasing S)

4

• Now put many more states at higher energy (system
  with high specific heat).
• Except at very low T, more balls in upper states

• This is high-entropy system.
• Low T enthalpy dominates (most balls in lower
  states)
• High T entropy dominates.
• Exactly models G H TS
• low T, G ? H
• high T, G ? TS

5
Equilibrium Between a Solid and its Vapour

• (eg, ice water vapour below 0 C)
• solid ? gas
• H low high
• S low high

Low T more solid hence lower
enthalpy (solid has lower enthalpy than gas)
for low T, G ? H and hence enthalpy
wins High T more gas hence higher
entropy (more disorder in a gas) for high
T, G ? TS and hence entropy wins
6

• Standard free energy of formation at 298K and 1
  atm
• All components are in their standard states
• eg 1 M solutions etc

DG0 S DG0f (products) S DG0f (reactants)
DG0T DH0 TDS0
DH0 kJ mol1 and DS0 J K1 mol1
Condition for spontaneous change DG -ve
7
Spontaneous?
No
No
Yes
Yes
8
Summary

• DG ve Reaction not spontaneous
• DG 0 System at equilibrium
• DG -ve Reaction spontaneous

• Some reactions become spontaneous when the
  temperature is raised
• DG0T DH0 TDS0

cf fig 20.11
9
Endothermic Reaction With Ba(OH)2

• Ba(OH)2.8H2O (s) 2NH4SCN (s) ?
• Ba(SCN)2 (s) 2NH3 (g) 10 H2O (l)
• 3 moles solid ? 13 moles
  solid,liquid and gas
• goes spontaneously because reactants are low S
  (solids), products are high S (liquid, dissolved
  ions, NH3(g) ).
• DG for this is dominated by DS, so it goes to an
  enthalpically unfavourable state (high H, ie
  takes in heat) in order to minimize overall DG.

10
Summary

• DG DH TDS

Table 20.1