Hess's Law

Hess's Law – Definition, Explanation, and Example

Hess’s Law of Constant Heat Summation explains that:

The total enthalpy change in a chemical reaction is the same, no matter how many steps the reaction is carried out in.

This law is based on the First Law of Thermodynamics, which states that energy cannot be created or destroyed, only transformed from one form to another.

Importance of Hess’s Law

Hess’s Law is particularly useful when the enthalpy change of a reaction cannot be measured directly. It allows us to determine the overall enthalpy change by using known values from other reactions.

Example: Formation of CO₂ from Carbon

Direct Method:

C(graphite) + O₂(g) → CO₂(g)    ΔH = -393.5 kJ/mol

Indirect Method (Two Steps):

1. C(graphite) + ½O₂(g) → CO(g)    ΔH₁ = -110.5 kJ/mol
2. CO(g) + ½O₂(g) → CO₂(g)    ΔH₂ = -283.0 kJ/mol

Using Hess’s Law:

ΔH = ΔH₁ + ΔH₂ = -110.5 + (-283.0) = -393.5 kJ/mol

This confirms that the total enthalpy change is the same in both methods.

How to Apply Hess's Law in JEE Problems

• Align chemical equations by adding, subtracting, or multiplying them.
• Multiply ΔH when multiplying an equation.
• Reverse the sign of ΔH when reversing a reaction.
• Cancel out common substances on both sides.

JEE Main Example Question

Given:

1. H₂(g) + ½O₂(g) → H₂O(l); ΔH = -286 kJ
2. H₂(g) + Cl₂(g) → 2HCl(g); ΔH = -184 kJ

Find ΔH for: H₂O(l) + Cl₂(g) → 2HCl(g) + ½O₂(g)

Solution:

• Reverse equation 1: H₂O(l) → H₂(g) + ½O₂(g); ΔH = +286 kJ
• Add it to equation 2: ΔH = 286 + (-184) = +102 kJ

Answer: ΔH = +102 kJ

Hess’s Law helps in calculating enthalpy changes for reactions that are hard to measure directly. It's a vital concept in thermochemistry and is frequently tested in JEE Main, both in theory and numerical problems.