Calorimetry - Standard Heats of Formation

Calorimetry is an experimental method in which the heat of a reaction (the system) is determined by measuring temperature changes in the surroundings.  In the lab, heat changes in physical and chemical processes are measured with a calorimeter (a closed container designed specifically for these measurements).  The basics of calorimetry lie with specific heat and heat capacity, so let's examine them first.

Specific Heat and Heat Capacity
The specific heat (s) of a substance is the amount of heat (q) required to raise the temperature of one gram of that substance by one degree Celsius.  It has the units J/g  C

The heat capacity (C) of a substance is the amount of heat required to raise the temperature of a given quantity (m) of the substance by one degree Celsius. It has the units J/C

Specific heat is an intensive property, where as heat capacity is an extensive property.  The relationship between them is C = ms (s = specific heat of water [4.184 J/gC]; m = mass in grams; C = heat capacity).  Knowing the change in the sample's temperature will tell us the amount of heat (q) that has been gained or lost in the process.  The equations we use to calculate heat change are:

1. q = (m)(s)(Δt)
2. q = (C)(Δt)

Let's look at an example problem:

Constant-Volume Calorimetry

Heat of combustion is usually measured in a steel container called a constant-volume bomb calorimeter.  Because of the way it's built, we can assume that no heat (or mass) is lost to the surroundings during the time it takes to make the measurements.  Therefore we can call the bomb and the water surrounding it an isolated system.  To calculate qcal we need to know the heat capacity of the calorimeter (Ccal) and the temperature rise, that is,  qcal = Ccal Δt

Constant-Pressure Calorimetry

A simpler device is the constant-pressure calorimeter, which is used to determine the heat-changes for noncombustion reactions.  It can be made from something as crude as two styrofoam cups.  This device measures the heat effects of a variety of reactions, such as acid-base neutralization.  Because the pressure is constant, the heat change for the process (qrxn) is equal to the enthalpy change (ΔH) so we treat this calorimeter as an isolated system.

Let's do another example.  This one separates the B's from the C's.