Hess’s Law of Constant Heat Summation

Hess’s Law states that the total enthalpy change for a chemical reaction is the same, no matter how many steps the reaction is carried out in.

Explanation

Hess’s Law is based on the principle that enthalpy is a state function. The enthalpy change (ΔH) depends only on the initial and final states of the system — not on the intermediate path or steps taken.

Therefore, whether a reaction occurs in a single step or multiple steps, the overall enthalpy change remains the same.

Mathematical Expression

If a reaction can be expressed as a sum of steps:

ΔH_total = ΔH₁ + ΔH₂ + ΔH₃ + … + ΔH_n

Each ΔH corresponds to the enthalpy change of an individual step.

Example (CO₂ formation)

Formation of CO₂ from carbon and oxygen:

• C (graphite) + ½ O₂ → CO  ΔH₁ = -110.5 kJ
• CO + ½ O₂ → CO₂  ΔH₂ = -283.0 kJ

Overall reaction: C (graphite) + O₂ → CO₂

ΔH_total = ΔH₁ + ΔH₂ = (-110.5) + (-283.0) = -393.5 kJ

Importance and Applications

• Helps calculate enthalpy for reactions difficult to measure directly.
• Used in determining heats of formation and combustion.
• Forms the basis of Hess Cycles in thermochemistry.
• Useful in enthalpy and energy-efficiency studies in engineering and materials science.

Quiz

1. Hess’s Law is based on the fact that enthalpy is a:

• a) Path function
• b) State function ✅
• c) Variable function
• d) None

Explanation: Enthalpy depends only on initial and final states.

2. Total enthalpy change in a multi-step reaction is:

• a) Average of steps
• b) Sum of enthalpy changes ✅
• c) Zero
• d) Twice first step

Explanation: ΔH is additive for all steps.

Frequently Asked Questions

Q1: Why is Hess Law true?
A: Because enthalpy is a state function and independent of the reaction path.

Q2: What does Hess’s Law help calculate?
A: It helps determine enthalpy changes for reactions that are hard to measure experimentally.

Q3: Who discovered Hess Law?
A: Germain Henri Hess in 1840.

Up Next: Enthalpy — Understanding Heat Changes