Normally, we calculate the enthalpy change that occurs during a chemical reaction. An enthalpy change is the amount of heat absorbed or released as a chemical reaction takes place. Based on the information available to you, you carry out the calculation differently. Here, let us discuss the different ways we use to calculate the enthalpy change in a reaction:
When we know the enthalpy of the products and reactants, we can use the following formula to calculate the overall enthalpy change:
∆H = Hproducts − Hreactants
Let us consider an example of sodium chloride which is formed by the addition of sodium ion and chloride ion.
Ionic sodium has an enthalpy of −239.7 kJ/mol, chloride ion has enthalpy −167.4 kJ/mol and Sodium chloride (table salt) has an enthalpy of −411 kJ/mol.
Now inserting these values in the above formula, we get
−411 kJ/mol – (−239.7 kJ/mol −167.4 kJ/mol)
−411 kJ/mol – (−407.1 kJ/mol)
−411 kJ/mol + 407.1 kJ/mol = −3.9 kJ/mol
We understand that the formation of salt releases almost 4 kJ of energy per mole from the calculation.
Calculating Enthalpy Change during Phase Transition
When substances change from one state to another, there are specific enthalpies involved in these changes. For water, the enthalpy of melting is ∆Hmelting = 6.007 kJ/mol. Suppose you heat the ice from 250 K until it melts, and then heat the water to 300 K. The enthalpy change for the heating parts is just the heat required, so you can find it using:
∆H = nC∆T
where (n) is the number of moles, (∆T) is the change in temperature and (C) is the specific heat.