Measurement of change in H and U Calorimetry

Calorimetry is a part of physics that measures the energy changes of a body following chemical or physical changes. All matter possesses some amount of heat energy. This energy is always undergoing transfer and exchange. Whether this change is due to a change in chemical state or physical state, calorimetry calculates the rate and the amount of change of energy of a body or system. All the measurements in calorimetry are usually carried out at constant volume and under constant pressure.

Measurement of delta U

Calorimetry refers to the methods and procedures for calculating the changes in the energy of any system. To that end, there are certain types of changes identified by studies. One of them is ΔU which is the change in internal energy. A type of calorimeter known as a bomb calorimeter is used for measuring this type of energy change. A bomb calorimeter consists of a steel vessel contained in another steel vessel that contains water and is insulated. This inner steel vessel can withstand high pressure. A stirrer and a thermometer are suspended in the water.

Procedure

•     A known mass of a compound is taken in a platinum cup.
•     The inner chamber or the bomb vessel is filled with oxygen at high pressure.
•     Current is passed through an element submersed in the known compound.
•     The compound combusts.
•     The rise in the temperature of the water is noted.
•     This rise in temperature helps calculate the heat capacity of the apparatus.

 The heat produced in this process is the internal heat of combustion because the experiment takes place at a constant volume in a closed vessel. 

The following formula is used for calculating ΔU:

 ΔU = (Q x ΔT x m) / m

Where

Q = the heat capacity

ΔT= rise in temperature

m= molecular mass of the substance

 Measurement of ΔH

Delta H is the heat of the formation of the products without the heat of the reactants. The letter H is representative of enthalpy or the total heat of the system. Changes in delta H can be measured by a simple calorimeter consisting of a water bottle fitted with a cork through which a thermometer and a stirrer are inserted. 

The chemical reaction of heat neutralisation between an acid (HCl) and a base (NaOH) can be used to demonstrate the calculation of ΔH.

Procedure

•     Take a known volume and a known concentration of HCl in a beaker.
•     Take the same volume and a concentration of NaOH in another beaker.
•     Keep both the beakers in a water bath to bring them to the same temperature.
•     Note the initial temperature of both the acid and the base.
•     Pour both the acid and the base into the bottle of the calorimeter.
•     Stir them together.
•     Note the highest temperature attained by the solution.
•     Note the total mass of the solution.

 Heat produced = mass x specific heat x rise in temperature

 Measurement of ΔH for any reaction

For calculating the ΔH for reactions involving reaction in solutions calorimetry, follow  the steps described above. In an exothermic reaction, heat is released, so there is a rise in temperature, and in an endothermic reaction, there is a fall in temperature. So for an exothermic reaction, ΔH is positive and for an endothermic reaction, ΔH is negative. The reaction is carried out in a vessel made of a conducting material and is placed in an insulated vessel containing water and fitted with a thermometer and a stirrer. The change in temperature is recorded and ΔH is calculated.

 Types of calorimeters

 Adiabatic calorimeter: The term adiabatic refers to any system in which no heat can enter or leave. So the heat produced by the reaction raises the temperature of the whole system. So in this type of calorimeter, the heat produced speeds up the rate of the reaction as well. But some heat is lost to the container in which the reaction takes place. This heat loss is called the phi factor. However, it is calculated mathematically, and the end results are adjusted accordingly.

1. Differential scanning calorimeter: A small aluminium capsule called a pan contains the materials for the reaction. The temperature of the pan is measured and compared to that of an empty pan, which is kept only for reference.
2. Calvet type calorimeter: This type of calorimeter uses a fluxmeter sensor. It contains several thermocouples arranged in a circular manner. These types of calorimeters can be used for measuring heat changes during sublimation reactions. The sensor measures the latent heat of transitions or the heat capacity of a system. These calorimeters do not require standards or calibrations and they can withstand temperatures up to 1600 degrees Celsius. Drop calorimetry experiments which measure the calorific capacities and enthalpies of formation can be performed using Calvet type calorimeters. 
3. Constant pressure calorimeters: These calorimeters maintain constant pressure and are used for measuring enthalpy changes in solutions. These are the simplest type of calorimeters. One example would be calorimeters made of polystyrene cups used by students in schools. 
4. Isothermal titration calorimeter: The reactions and the interactions between biomolecules are measured using these calorimeters. Enthalpy, stoichiometry, entropy, binding affinity of solution binding reactions can be calculated using this type of calorimeter. This method gives quantitative analysis when a titrated solution reacts with a substrate. The heat which is either released or absorbed when a molecule binding occurs is measured by the calorimeter. The procedure takes place in a constant volume and under constant pressure. 
5. Bomb calorimeters: These types of calorimeters measure the delta H of combustion type reactions. These calorimeters are generally made of steel, are sturdy, and work in an atmosphere of oxygen because it is necessary for combustion. These calorimeters can bear high pressures and the exothermic release of heat that happens because of the combustion type reactions.

 Conclusion

Calorimetry is essential for several studies conducted in physics and chemistry. It provides essential knowledge about the specific heat capacities which can then be used for making several calculations and predictions about the different substances. Like most scientific methods, calorimetry too requires careful measurement and meticulous recording of the factors involved and the results obtained. A lot can be said about how a substance will interact with changes in the conditions of temperature and pressure if its interaction with heat is known.