Fig. Calorimetry - Heat Capacity Calculation - Screenshot from CHEMIX School.

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Calorimetry

Calorimetry is the measurement of heat exchanged during physical or chemical processes. A calorimeter is an insulated container used to measure temperature changes so that the amount of heat transferred during a reaction or physical change can be calculated.
When heat is released or absorbed during a reaction, the temperature of the surroundings changes. By measuring this temperature change and knowing the heat capacities of the materials involved, the heat transferred can be determined.

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Heat Capacity of the Calorimeter

The calorimeter itself absorbs or releases some heat during an experiment. The amount of heat required to raise the temperature of the calorimeter by one degree is called the heat capacity of the calorimeter.

The heat absorbed by the calorimeter can be calculated using the equation:

q = C_cal × ΔT

where
q = heat absorbed by the calorimeter
C_cal = calorimeter heat capacity
ΔT = temperature change

Before performing experiments, the calorimeter constant is usually determined using a calibration experiment. This allows the heat absorbed by the calorimeter to be included in later calculations so the measured heat from reactions is accurate.

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Enthalpy of a Chemical Reaction

The enthalpy change of a chemical reaction (ΔH) represents the heat released or absorbed at constant pressure.

In calorimetry experiments, the heat released by the reaction is equal in magnitude but opposite in sign to the heat absorbed by the solution and the calorimeter.

q_reaction = −(q_solution + q_calorimeter)

If the reaction releases heat, it is exothermic and ΔH is negative.
If the reaction absorbs heat, it is endothermic and ΔH is positive.

The enthalpy change per mole of reaction can be calculated by dividing the measured heat by the number of moles of reactant involved.

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Specific Heat Capacity

Specific heat capacity is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius.

The equation used is:

q = m × c × ΔT

where
q = heat absorbed or released
m = mass of the substance
c = specific heat capacity
ΔT = temperature change

Different substances have different specific heat capacities. For example, water has a relatively high specific heat capacity, meaning it requires a large amount of heat to change its temperature.

Specific heat capacity is important in calorimetry because it allows us to determine how much heat is absorbed by a substance when its temperature changes.

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Identifying an Unknown Metal

Calorimetry can be used to determine the specific heat capacity of a metal, which can then be compared with known values to identify the metal.

In a typical experiment:

1. A sample of metal is heated in boiling water.

2. The hot metal is placed into a calorimeter containing water at a known temperature.

3. The metal transfers heat to the water until both reach the same final temperature.

Because energy is conserved:

heat lost by metal = heat gained by water + heat gained by calorimeter

Using the heat equation (q = mcΔT), the specific heat capacity of the metal can be calculated.

Once the specific heat capacity is determined, it can be compared with known values for metals such as aluminum, copper, iron, or lead to identify the unknown sample.