Calorimetry

We have common sense notions of heat and temperature. The universe comprises matter and energy, which are made up of molecules. The molecules’ back-and-forth movement or invariable motion creates a form of energy known as thermal energy or, in simplified terms, known as heat. Temperature is a relative measure or indication of hotness or coldness. Heat is the form of energy transferred between multiple systems or a system and its surroundings by temperature difference. Temperature and heat energy are directly proportional to each other. So, this means that the more the amount of heat energy, the more the body’s temperature. An object’s temperature is measured before and after heat transfer to evaluate the loss and gain of thermal energy. Hence, this temperature difference ascertains the heat change of a body.

What is calorimetry?

It is a way of measuring the amount of heat that is either released or absorbed during any chemical reaction that takes place. It helps you determine whether the reaction is exothermic or endothermic in nature. It is associated with a change in states i.e physical changes or transitions in phases under specified conditions.

The principle of calorimetry

A system is defined as an isolated one if no exchange or transfer of heat occurs between the system and its respective surroundings. When different parts of an isolated system are at different temperatures, several degrees of heat transfer occurs. The heat lost by the part at a higher temperature is equal to the heat gained by the part at a lower temperature.

Calorimetry means the measurement of heat. When a body at a higher temperature is brought in contact with another body at a lower temperature, there is a transfer of heat from the body with the higher temperature that is gained by the body with a lower temperature. No heat escapes or is lost in the surrounding. The calorimeter is a device used to measure the activity of this heat across the bodies.

Heat Lost = Heat Gained

Within a system, heat transfer is calculated using the formula,

Q = mcΔt

Here,

Q –  the measure of heat transfer

m – the mass of the body

c –  the specific heat capacity of the body

Δt is the change in the temperature

What is a calorimeter?

A calorimeter consists of a metallic vessel and stirrer of the same material as Copper or aluminum. The vessel is placed inside a wooden jacket that contains heat-insulating materials such as glass, wool etc. The outer part of the jacket acts as a heat shield. It, therefore, reduces the heat loss from the inner vessel. A mercury thermometer is inserted through an opening in the outer jacket.  Generally, in a calorimeter, a fixed amount of fuel is burned. Since the vessel is filled with water, the fuel burning results in the water heating up. So, according to the calorimetric principle, heat loss by the fuel is equal to the heat gained by the water. Therefore, it is important to insulate the calorimeter from the environment so that there is no heat loss to the environment that helps in improving 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 Calorimeters

There are generally four types of calorimeters that are commonly used. They are:

Adiabatic calorimeters: An adiabatic calorimeter is a calorimeter used to examine a runaway reaction. Since the calorimeter runs in an adiabatic environment, any heat generated by the material sample under test causes the sample to increase in temperature, thus fuelling the reaction. Normally, some amount of heat is usually lost by the sample to the sample holder, and hence the environment is not completely adiabatic. Therefore, a mathematical correction factor, referred to as the phi-factor, is often applied to adjust the calorimetric result to account for these heat losses.

Reaction calorimeters: A reaction calorimeter is a calorimeter in which a reaction is initiated within a closed insulated container. The heat produced due to reaction is measured, and therefore, the heat content is obtained by integrating heat flow versus time. There are 4 primary types

1. Heat Flow Calorimeter – The cooling/heating jacket plays an important role in controlling either the temperature of the method or the jacket’s temperature. Heat is measured by monitoring the temperature difference between heat transfer fluid and the process fluid. This heat transfer process between the jacket and the reaction vessel is monitored through thermally conductive thermopiles. These contain a large number of thermocouple junctions.
2. Heat Balance Calorimeter – Heat balance calorimetry involves heating/cooling the liquid inside the surrounding jacket to create a variable temperature. It allows the reaction vessel to maintain a constant temperature (or within a required range). These calorimeters require precise knowledge of the heat capacity of the reaction systems. Here, both the reacting sample and the parts of the system in contact with the sample need to be kept under consideration.
3. Power Compensation Calorimeter – Power compensation uses a heater placed within the vessel to maintain a continuing temperature. The energy supplied to the present heater is often varied as reactions require, and therefore the calorimetry signal is only derived from this electric power.
4. Constant Flux Calorimeter – A constant flux calorimeter utilizes a constant cooling/heating flux jacket. This manages to keep the internal liquid temperature at a constant. These calorimeters use jackets with varying geometries and can often work at low temperatures. So, this type of calorimeter can thermally regulate reactions that require cold reagents or a cold reaction temperature.

Bomb Calorimeters: A bomb calorimeter is a constant-volume calorimeter utilised to measure the warmth of combustion of a specific reaction. Electrical energy is employed to ignite the fuel; it’ll heat the encompassing air. This air thereby expands and escapes through a tube that leads the air out of the calorimeter because the fuel is burning. The air thereby escaping through the copper tube will also heat the water outside the tube. The temperature difference of the water allows for calculating the calorie content of the fuel.

Differential Scanning Calorimeter: In the differential scanning calorimeter or DSC, heat flows into a sample usually contained within a small aluminum capsule or ‘pan’. This heat flow is measured differentially, i.e., by comparing it to the flow into an empty reference pan.

Conclusion

Calorimetry is the means of measuring the heat of chemical reactions or physical changes, which is made with the help of measurements of a calorimeter. When these two bodies with dissimilar temperatures are in contact with one another, heat transfer occurs. The body at a higher temperature transfers heat to the body at a lower temperature until a thermal equilibrium is obtained.