Lesson 1 Calorimetry
A calorimeter is a device used to measure heat of reaction.
The reactants are enclosed in a vessel surrounded by a known
amount of water. This amount of water is enclosed in a well,
insulated vessel where heat loss is minimised. The reactants
are allowed to react and at the end of the reaction the
temperature change of the water is measured.
Calorimeters can be expensive or cheap and easy to construct
such as the polystyrene cup calorimeter shown on the right.
Measuring the temperature change of the known body of
water should be enough to calculate the amount of energy
produced or absorbed by the reaction according to the
formula E = 4.18 J/Co/g X mass X ΔT, where ΔT is the change
in temperature and mass is the mass of water. This, however,
assumes that all the energy given out or absorbed by the reaction goes into or comes from the water.
This is not the case as energy is used to heat the physical parts of the calorimeter and also escapes
into the environment from poorly insulated containers, so not 100% of the energy involved can be
measured.
In order to accurately determine the energy lost or absorbed by the reaction we must first calibrate
the calorimeter. This is done by delivering a known amount of energy, most often by passing a
known electric current at a given voltage for a period of time measured in seconds. A known amount
of energy can also be delivered by burning an accurately known mass of fuel whose molar heat of
combustion is accurately known. The change in temperature of water is then measured and the
calibration factor (Cf) determined.
Cf = Energy / ΔT
The formula used to determine the electrical energy delivered is given by
E = VIt (Joules)
Example 1
A calorimeter is calibrated by passing a current of 1.20 A
at 6.00 volts through the heating coil for 90.0 seconds. The
temperature of the water rose by 12.2 oC.
a) Calculate the Cf of the calorimeter.
Step 1 find the amount of energy delivered>
=> E = 6.00 X 1.20 X 90.0 = 648 J
Step 2 Calculate Cf = 648 / 12.2 = 53.1J/Co
b) Magnesium metal was placed into a calorimeter containing 100 mL of 1.00 M HCl solution at
25.0oC. Calculate the amount of energy released in kJ if the final temperature of the water
reached 29.6oC.
=> E = Cf X ΔT
=> E = 0.0531 kJ/Co X (29.6 – 25.0) = 0.244 kJ
