Reaction Feasibility

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Reaction Feasibility

• Will a specific
• reaction occur ?

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How can we predict if a reaction is feasible ?
NaHCO3 HCl ? NaCl H2O CO2
?H is ve (endothermic)
Entropy increases
We know this reaction is feasible
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How can we predict if a reaction is feasible ?
?G ?H - T ?S
?G free energy ( kJmol-1)
?H enthalpy change ( kJmol-1)
T temperature (K)
?S entropy change ( kJK-1mol-1)
?S must be converted from JK-1mol-1 (/1000)
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How can we predict if a reaction is feasible ?
?G ?H - T ?S
A reaction is feasible if ?G is negative ?G lt 0
A reaction becomes feasible when ?G 0
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Calculate ?G for the formation of CO2 from
graphite at 298K, given ?Hf -393.5kJmol-1 ?S
is 3.3 JK-1mol-1
?G ?H - T ?S
(-393.5) (298 x 3.3/1000)
?G -394.5 kJmol-1
Feasible reaction ?G -ve
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Is the decomposition of limestone spontaneous at
standard temperature? ?S 165 JK-1mol-1 ?H
178 kJmol-1
?G ?H - T ?S
(178) (298 x 165/1000)
?G 128.83 kJmol-1
Not feasible ?G ve
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Feasible ? Spontaneous
Thermodynamics tells us nothing about the speed a
reaction happens
It only tells us that it can happen
C O2 ? CO2
?G -394.5 kJmol-1
EA is large so reaction very slow at 298K
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Calculate ?G for the rusting of Fe at 298K, given
?Hf -825 kJmol-1 ?S is -272 JK-1mol-1
2Fe(s) 3/2O2(g) ? Fe2O3 (s)
?G ?H - T ?S
(-825) (298 x (-272/1000)
?G -744 kJmol-1
Feasible reaction ?G -ve
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At what temperature does this reaction cease to
be feasible? ?Hf -825 kJmol-1 ?S is -272
JK-1mol-1
?G ?H - T ?S
Reaction becomes feasible when ?G 0
?H T ?S
?H T ?S
3033K
T -825/-0.272
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Free energy Equilibrium
?G ?H - T ?S
?G 0 for a system at equilibrium
Changes of state are systems at equilibria
?S ?H T
?H T ?S
?H T ?S
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Calculate the entropy change when ice melts given
that ?Hfusion (melting) 6.0 kJmol-1
?G ?H - T ?S
?G 0 for a change of state
?H T ?S
?S ?H T
?S 6/273
0.0219 kJK-1mol-1
0.0219 x 1000 JK-1mol-1
21.9 JK-1mol-1