# Enthalpy and Hess law

Enthalpy change and Hess’s law.

Introduction: Enthalpy is the total energy of a system, some of which is restored as chemical potential energy in the chemical bonds. During reactions, bonds are broken and formed. As a result, all reactions are accompanied by a change in the potential energy of the bonds, and hence an enthalpy change. This enthalpy change of reaction can be measured and is given the symbol H. The temperature of a system is a measure of the average kinetic energy of the particles present. Therefore, during a reaction, as bonds are broken and formed, there is a change in this average kinetic energy and this will result in a change in the temperature of the system. On the other hand, heat is the measure of the total energy of the substance. When the temperature of a substance increases or decreases, heat energy is absorbed or released from the environment. In order to measure this change in temperature, the following formula can be used:-

Heat energy = m.C.T    …...where, m= mass of the substance

C= specific heat capacity

T= change in temperature

Aim of experiment: - This particular experiment focuses on Hess’s law. Hess’s law states that the total enthalpy change on converting a given set of reactants to a particular set of products is constant, irrespective of the way in which the change is carried out. For this experiment, the conversion of NaOH to NaCl by two different routes was used and the change in enthalpy for each path was measured to test Hess’s law.

Route one: -   NaOH + 2M HCl   2M NaCl

2M NaCl + H2O    1M NaCl

Route two: -      NaOH + H2O  2M NaCl

2M NaOH + 2M HCl  1M NaCl

Materials: - beakers, NaOH, 2M HCl, stopwatch, thermometer, pipettes, measuring cylinder

Methods: -   for the first route, 4g of NaOH was weighed. Then using a measuring cylinder 50 ml ...