Thermodynamics ❮ 123DH° = S DHf° products - S DHf° reactants
DG° = S DGf° products - S DGf° reactants
DG° = DH° - TDS°
= - RT In K = - 2.303 RT log K
= - n F E°
DG = DG° + RT ln Q = DG° + 2.303 RT log Q
q = mcDT=
∆
∆C
H
p TGas constant, R = 8.31 J mol–1 K–1Calorimetry
Calorimetry is the laboratory technique used to measure the heat released or absorbed
during a chemical or physical change. The quantity of heat absorbed or released during a
chemical or physical change is represented as q and is proportional to the change in tem-
perature of the system being studied. This system has what is called a heat capacity (Cp),
which is the quantity of heat needed to change the temperature 1 K. It has the form:
Cp = heat capacity = q/DT
Heat capacity most commonly has units of J/K. The specific heat capacity (or specific
heat) (c) is the quantity of heat needed to raise the temperature of 1 g of a substance 1 K:
c = q/(m × DT) or q = cmDT,
where m is the mass of the substance.
The specific heat capacity commonly has units of J/g.K. Because of the original defi-
nition of the calorie, the specific heat capacity of water is 4.184 J/g.K. If the specific heat
capacity, the mass, and the change of temperature are all known, the amount of energy
absorbed can easily be calculated.
Another related quantity is the molar heat capacity (C), the amount of heat needed
to change the temperature of 1 mol of a substance by 1 K.
Calorimetry involves the use of a laboratory instrument called a calorimeter. Two types
of calorimeter, a simple coffee-cup calorimeter and a more sophisticated bomb calorimeter,
are shown in Figure 9.1, on the next page. In both, a process is carried out with known
amounts of substances and the change in temperature is measured.
The coffee-cup calorimeter can be used to measure the heat changes in reactions or
processes that are open to the atmosphere: qp, constant-pressure reactions. These might
be reactions that occur in open beakers and the like. This type of calorimeter is also com-
monly used to measure the specific heats of solids. A known mass of solid is heated to a
certain temperature and then is added to the calorimeter containing a known mass of water
at a known temperature. The final temperature is then measured allowing us to calculate the
DT. We know that the heat lost by the solid (the system) is equal to the heat gained by the
surroundings (the water and calorimeter, although for simple coffee-cup calorimetry the heat
gained by the calorimeter is small and is ignored):- qsolid = qwater