Chemical Reactions And Equations Worksheet With Answer Key - The University Of Sydney Page 3
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40CHEM1101
2014-N-10
November 2014
1
• The Second Law states that all observable processes must involve a net increase in
entropy. When liquid water freezes into ice at 0 °C, the entropy of the water
decreases. Since the freezing of water is certainly observable, the processes must still
satisfy the Second Law. Provide a brief explanation of how this is so.
The Second Law requires that there is a net increase in the entropy of universe:
Δ
S = Δ
S + Δ
S > 0
universe
system
surroundings
When water freezes, Δ
S < 0. However, freezing is an exothermic process:
system
heat is given out to the surroundings. The heat gain in the surroundings is equal
and opposite to the heat lost in the system: q
= -Δ
H. This increases the
surr
freezing
entropy in the surroundings:
Δ
S = -Δ
H / T
surroundings
freezing
Overall:
Δ
S = Δ
S - Δ
H / T
universe
system
freezing
As long as the second term is larger than the first, Δ
S > 0. This is true at
universe
low temperatures.
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