Question

Explain hess law and internal energy with exaample

Answer 1

Hess Law

According to Hess’s law, if A reacts to form the product B, it doesn’t matter how many steps involved to get the product, the total enthalpy change will be same. So the reaction can happen in one step where the enthalpy change is ΔH1.

Example:-

Formation of carbon dioxide from carbon and oxygen can occur in one step or two steps. In two steps it can be formed via the production of carbon monoxide.

In one step,

C(s) + O2(g) → CO2 ΔH1 = -393 kJ/mol ……………..(i)

In two steps,

step 1: C(s) + 1/2O2(g) → CO ΔH2 = -111 kJ/mol

Step 2: CO(g) + 1/2O2(g) → CO2 ΔH3 = -282 kJ/moL

Internal Energy

The internal energy is defined in terms of the interactions of pairs of atoms in different shells, so that the theory is not confined to nearest neighbour interactions.

Example:-

A room temperature glass of water sitting on a table has no apparent energy