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Calorimetry

Calorimetry is the field of science that deals with the measurement of the state of a body with respect to the thermal aspects in order to examine its physical and chemical changes. The changes could be physical such as melting, evaporation or could also be chemical such as burning, acid-base neutralization etc. Calorimetry is applied extensively in the fields of thermochemistry in calculating the enthalpy, stability, heat capacity etc.

Calorimeter

A calorimeter is a device used for heat measurements necessary for calorimetry. It mainly consists of a metallic vessel made of materials which are good conductors of electricity such as copper and aluminium etc. There is also a facility for stirring the contents of the vessel. This metallic vessel with a stirrer is kept in an insulating jacket to prevent heat loss to the environment. There is just one opening through which a thermometer can be inserted to measure the change in thermal properties inside. Let us discuss how exactly heat measurements are made. In the previous article, we discussed the specific heat capacity of substances.

Such measurements can be made easily with this. Say in a calorimeter a fixed amount of fuel is burned. The vessel is filled with water, and the fuel is burned, leading to the heating of water. Heat loss by the fuel is equal to the heat gained by the water. This is why it is important to insulate the calorimeter from the environment; to improve the accuracy of the experiment. This change in heat can be measured through the thermometer. Through such a measurement we can find out both the heat capacity of water and also the energy stored inside a fuel.

Types of Calorimeter

Different types of calorimeters are given below:

1. Adiabatic Calorimeters
2. Reaction Calorimeters
3. Bomb Calorimeters (Constant Volume Calorimeters)
4. Constant Pressure Calorimeters
5. Differential Scanning Calorimeters

Calorimeter Principle

When two bodies of different temperatures (preferably a solid and a liquid) are placed in physical contact with each other, the heat is transferred from the body with higher temperature to the body with lower temperature until thermal equilibrium is attained between them. The body at higher temperature releases heat while the body at lower temperature absorbs heat. The principle of calorimetry indicates the law of conservation energy, i.e. the total heat lost by the hot body is equal to the total heat gained by the cold body.

•                Heat Lost = Heat Gained

The heat transfer in a system is calculated using the formula,

•                    q= mcΔt
• Where
• q is the measure of heat transfer
• m is the mass of the body
• c is the specific heat of the body
• Δt is the change in the temperature

Calorimeter Problems

Let us look at the example below to understand how to calculate the heat transfer between two objects.

Example 1.1: A metal weighing 4.82 g was heated to 115.0 °C and put into 35 mL of water of temperature 28.7 °C. The metal and water were allowed to come to an equilibrium temperature, determined to be 34.5 °C. Assuming no heat was lost to the environment, calculate the specific heat of the metal. Consider the specific heat capacity of water as 4.186 joule/gram °C.

Solution:

First, let us calculate the heat absorbed by the water and use the value obtained to calculate the specific heat of the metal.

• The formula to find the heat absorbed by the water is given as
•            q=mcΔt
• Substituting the values in the equation, we get
• qabsorbed=(4.186J/g⋅∘C) × 35g × (5.8  ^oC)
•                   = 850J

Now, using this formula let us calculate the specific heat of the metal as follows:

850J=s×4.82g×80.5 ^oC

s=2.19J/G⋅ ^oC

The specific heat of the metal is 2.19J/G⋅∘C

The measurements obtained using the principle of calorimetry explain a lot of important phenomena in thermodynamics.