1. Energy and Enthalpy.
A system can exchange energy with its surroundings either by transferring heat or by
doing work. Using q to represent transferred heat and w = - P ΔV, the total energy change
of a system, ΔU, can be represented as
ΔU = q + w = q - P ΔV
where q has a positive sign if the system gains heat and a negative sign if the system loses
it. Rearrange this equation to give the amount of heat transferred. Then, think of two
ways in which a chemical reaction might be carried out: a reaction might be carried out in
a closed container with a constant volume, or carried out in an open flask that keeps the
pressure constant and allows the volume of the system to change freely. Write out the
two equations that would describe these two scenarios in terms of heat transfer and/or PV
work. Lastly, the enthalpy (H) of a system is described by the quantity U + PV. What is
the equation that relates heat to enthalpy at constant pressure?
q = ΔU + P ΔV
At constant volume; ΔV = 0
= ΔU + P ΔV
At constant pressure
= ΔU + P ΔV = ΔH
2. Fill in the blanks.
When reactions are performed in an open container, such as a coffee-cup calorimeter, the
pressure (= atmospheric pressure) does not change and q
= ΔH; in a closed container,
such as a bomb calorimeter, the volume (= volume of the bomb) does not change and q
= ΔU. Remember that even in an open container, unless a gas is involved in the reaction,
volume changes are usually negligible and ΔH ≅ ΔU.
3. What are extensive and intensive properties and some examples of each?
Intensive properties are values that do not depend on the sample size, such as
temperature and melting point. Thus, a small ice cube might have the same
temperature as a massive iceberg. Extensive properties, like length and volume,
have values that do depend on the sample size. An ice cube is much smaller than an
4. Suppose the enthalpy change when carbon monoxide is burned with oxygen to carbon
dioxide is given by the reaction
(g) ➝ CO(g)
C(s,gr) + ½ O
ΔH = -110.5 kJ
at 25°C. What is the internal energy of this reaction?
ΔU = ΔH – PΔV