Standard Energy Worksheet
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4Worksheet 19 – Standard Free Energy
rd
According to the 3
Law of Thermodynamics, the spontaneity of a reaction
depends on the entropy change of the universe. We defined a new function,
Gibbs’ Free Energy, G, which reflects !S
. The change in free energy
universe
during a chemical process is given by
o
o
o
!G
= !H
- T!S
< 0 for a spontaneous process
Whether or not a reaction will proceed, under standard conditions, at given T
depends on whether it is endothermic or exothermic, and whether the entropy of
the system increases or decreases during the reaction.
o
When !G
< 0, the reaction, as written, will be spontaneous.
o
When !G
> 0, the reverse reaction will be spontaneous.
o
o
Using only the signs of !H
and !S
, predict the signs and temperature
dependence for problems 1 and 2.
1.
CH
OH (l) + 3/2 O
(g) ! CO
(g) + 2 H
O (g)
3
2
2
2
Methanol (CH
OH) is used as rocket fuel. Look at the reactants and
3
o
products an predict the sign of !S
.
O
!S
( > or < ) 0
o
Predict the sign of !H
knowing that this is a combustion reaction.
o
!H
( > or < ) 0
o
Is the sign of !G
temperature depend for this reaction?
o
o
o
Predict the signs of !G
, !H
and !S
2.
for:
o
a) the vaporization of water above 100
C
o
b) the vaporization of water at 100
C
o
o
c) Does !H
or !S
favor the vaporization process?
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