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20 Chapter 2
Reaction 4: Freezing of water, i.e. H20(1) --+ H,O,,).

At constant pressure, w = -PA V = -p( Vfinal - Vinitial). But Vinitial is
approximately equal to Vfinal, from Chapter 1. Hence, A V = 0. There-
fore, w M 0 i.e. no work is done. Note that water does in fact expand
slightly on freezing, which is very unusual. This is why water pipes
burst in winter.
Heats of Reaction
The standard molar enthalpy of formation, AH;, is defined as the
enthalpy change when 1 mole of a substance is formed from its
free elements in their standard states (i.e. a solid, liquid or gas at 1
bar pressure and 25 "C = 298 K). For example, for the reaction
C(,) + 02(g) + CO,,,, AH;, the standard molar enthalpy of
formation of carbon dioxide gas is equal to -394 kJ mol-', i.e.
AH;, = AH; = -394 kJ mol-'.
The enthalpy of combustion, AH, is defined as the change in
enthalpy when one mole of a substance is burnt in excess oxygen gas
at 1 bar pressure and 0 "C = 273 K. For example, for the combustion
of propane, C3Hg(l) + so,(,) --+ 3co2(,, + 4H20(1), AH,,, = AH, =
- 2220 kJ mol- * (rxn = reaction).
THE FIRST LAW OF THERMODYNAMICS
The First Law of Thermodynamics is the law of conservation of
energy, i.e. energy cannot be created or destroyed, but is converted
from one form to another. Expressed in an alternative way, the First
Law of Thermodynamics states that the total energy of the universe is
constant, i.e. AUuni,,,, = 0.

STATE FUNCTIONS
When a certain property of a system (such as U, the internal energy) is
independent of how that system attains the state that exhibits such a
property, the property is deemed to be a state function, i.e. it does not
matter which path is followed when a system changes from its initial
state to its final state; all that is relevant is the value of such a function
in its final state. This concept forms the basis of Hess's Law.

HESS'S LAW

Defiition of Hess's Law: Hess's Law states that a reaction enthalpy
is the sum of the enthalpies of any path into which the reaction may
be divided at the same temperature and pressure.