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Thermodynamics I 19
From physics, work = force x distance (w = F x d) and pres-
sure = force/area (p = F/A), as defined in Chapter 1. Therefore,
force = pressure x area, i.e. w = (pA)d. But since volume =
length x breadth x height = area x height, this means w =PAY,
where A V is the change in volume, i.e. Vfinal - Vinitial. Since w is - ve,
then w = -pAV.
But, AU = q + w. Therefore, q = AU - w = AU - (-pAV) =
AU+pAV. At constant pressure, p, for a system with internal
energy, U and volume V, the enthalpy H(= qp) is defined as
H= U +pV. More specifically, AH, the change in enthalpy, is
defined as AH = AU +pAV. This will be discussed further in
Chapter 3.
I Change in Enthalpy AH = AU +PA V I

Under standard state conditions (i.e. 1 bar pressure and 25°C = 298
K), AH is written as AH".

Some Examples of Reactions

Consider the following four reactions at constant
pressure. In each case, answer the following questions: (a) Is work
done by the system on the surroundings or by the surroundings on
the system? (b) What is the sign of w in each case, i.e. is w +ve, is
w -ve or is w = O?

Reaction I: H20(,) + H,O(,).
At constant pressure, w = -pAV = -p( Vfinal - Vinitial). But Vinitia, >
Vfinal, from Chapter 1. Hence, AV is -ve. Therefore, w is +ve, i.e.
work is done by the surroundings on the system.

Reaction 2: 2NaN03(,) + HEAT + 2NaN02(,) + 02(g).
At constant pressure, w = -PAY = -p( Vfinal - Vinitial). But Vfinal >
Vinitial, from Chapter 1. Hence, A V is + ve, and therefore, w is - ve,
i.e. work is done by the system on the surroundings.
Reaction 3: 2N~05(~) 2N204(,) + 02(g).
-+
At constant pressure, w = -pAv,RT, from the equation of state of
an ideal gas, Chapter 1. Av,, is the change in the coefficients of gas
reagents = C[v(Gaseous products)] - C[v(Gaseous reactants)] =
(2 + 1) - (2) = (3) - (2) = + 1. Hence, w is -ve, i.e. work is
done by the system on the surroundings.