Chemistry Practice Problems
Thermochemistry: Enthalpy change and the First Law of
Thermodynamics
Solutions:*
1. The first law of thermodynamics provides that DE= q + w. At constant pressure, q
= DH and
w = –PDV, where DV = the change in the volume of gas.
To determine q, determine the heat released for the quantities given. We must
determine the limiting reactant.
For H2:
HBr mol 0.6
H mol 1
HBr mol 2
H mol 0.3
2
2
=×
For Br2:
HBr mol 0.1
Br mol 1
HBr mol 2
Br g 8.159
Br mol 1
Br g .80
22
2
2
=××
The Br2 is the limiting reactant. The amount of heat given off when 1.0 mol HBr is
produced is determined as follows:
kJ 36
HBr mol 2
kJ 4.72
Br mol 0.1
2
−=
−
×
Note: look at the thermochemical equation to see how the conversion factor was
obtained.
To determine w, determine the final an initial volume. This isn’t according to the
balanced equation, but according to the actual amounts of gas in the reaction chamber.
To determine this, we know we have one mole of HBr gas, but there is also some of the
unreacted H2 gas present in the chamber as well. To determine how many moles of H2
reacted, we can use the amount of HBr that is produced and use the balanced equation
as follows.
2
2
H mol 50.0
HBr mol 2
H mol 1
HBr mol 0.1 =×
