What is the Hess’ law?

According to the Hess’s Law of constant heat summation, the total amount of heat evolved or absorbed in a reaction is same whether reaction takes place in one step or multiple steps.

All chemical reactions that take place around us might not be using heat energy always for there completion but there are some reactions which account to heat energy for there completion and use the same amount of heat energy if we complete the reaction process only in one step or in multiple number of steps. FOR EXAMPLE,

• FORMATION OF CARBON DIOXIDEThe formation of carbon dioxide can take place in two steps which are given below:-

• Firstly, we can directly react 1 mole of carbon with 1 molecule of oxygen we will give 1 mole of carbon dioxide. As, this reaction is an exothermic reaction there will be a liberation of -393.5 KJ/mol of heat energy.

• OR we can break this whole reaction process into two parts: 1. To the first part we can combine 1 mole of carbon with half mole of oxygen molecule which will lead to the formation of 1 mol of carbon monoxide (CO) with the liberation of -110.5 KJ/mole of heat energy. 2. To the second part we can combine 1 mole of the formed carbon monoxide with half mole of an oxygen molecule which will lead to the formation of exactly 1 mole of carbon dioxide with the liberation of -283.0 KJ/mol of heat energy.In total this two part reaction will also liberate – 393.5 KJ/mol of heat energy which is exactly the same amount of heat energy that was liberated when we performed the reaction process directly in one step.(The -ve sign used above indicates the liberation of heat energy).