PPT 5.2 Hess’s Law

PPT 5.2 Hess’s Law : According to Hess’s Law, the total enthalpy change for a reaction remains the same whether the reaction occurs in one step or in multiple steps.

The enthalpy change of a reaction is the heat absorbed or released during the reaction. It is measured in Joule or Kilo Joule.

Hess’s Law can be used to calculate the enthalpy change of a reaction than can not be measured directly.

If you know the enthalpy changes for reactions, you can calculate the missing enthalpy change or of any enthalpy change in a multi-step reaction.

The enthalpy changes for the steps can be added or subtracted to calculate the enthalpy change for the overall reaction.

According to Hess’s Law, the enthalpy change for a reaction is independent of the path taken to make the products.

We can use Hess’s Law to predict the heat of a reaction for a chemical process by adding the heat of multiple reactions of a reaction.

Enthalpy is a state function which means it depends only on the initial and final state of the system and not on the path followed to achieve that state.

It is a very useful tool in thermochemistry to calculate and interpret thermodynamic properties.

Calculate the enthalpy change for the reaction: 2H2 + O2 -> 2H2O using Hess’s Law, given the following data:

H2 + (1/2)O2 -> H2O, ΔH = -285.8 kJ/mol

2H2 -> 2H2, ΔH = 0 kJ/mol

2H2 -> 2H2, ΔH = 0 kJ/mol

(1/2)O2 -> (1/2)O2, ΔH = 0 kJ/mol

H2 + (1/2)O2 -> H2O, ΔH = -285.8 kJ/mol

ΔH = (2 x -285.8) kJ/mol = -571.6 kJ/mol

Using Hess’s Law, calculate the enthalpy change for the reaction:

2C2H5OH + 9O2 -> 4CO2 + 6H2O + heat.

C2H5OH + 3O2 -> 2CO2 + 3H2O , ΔH = -1366 KJ/mol

I hope you have understood the concept by PPT 5.2 Hess’s Law.