Hess's Law (DP IB Chemistry): Revision Note

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Philippa Platt

Updated on

Hess's Law

  • In 1840, the Russian chemist Germain Hess formulated a law which went on to be known as Hess’s Law

  • This went on to form the basis of one of the laws of thermodynamics. The first law of thermodynamics relates to the Law of Conservation of Energy

  • It is sometimes expressed in the following form:

Energy cannot be created or destroyed, it can only change form

  • This means that in a closed system, the total amount of energy present is always constant

  • Hess’s law can be used to calculate the standard enthalpy change of a reaction from known standard enthalpy changes

  • Hess’s Law states that:

"The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same."

  • This means that whether the reaction takes place in one or two steps, the total enthalpy change of the reaction will still be the same

Diagram to show Hess's Law

Hess's Law cycle diagram showing two routes from reactants A + B to products C. The direct route (Route 1) goes from A + B to C with enthalpy change ΔH₁. The indirect route (Route 2) goes via intermediate products X and Y + Z with enthalpy changes ΔH₂ and ΔH₃ respectively. The cycle illustrates that the total enthalpy change is the same regardless of the route taken.
The diagram above illustrates Hess’ Law: the enthalpy change of the direct route, going from reactants (A+B) to product (C) is equal to the enthalpy change of the indirect routes
  • Hess’ Law is used to calculate enthalpy changes which can’t be found experimentally using calorimetry, eg:

3C (s) + 4H2 (g) → C3H8(g)

  • ΔHf (propane) can’t be found experimentally as hydrogen and carbon don’t react under standard conditions

Calculating ΔHr from ΔHf using Hess’s Law energy cycles

Diagram to show Hess's Law 

Flowchart of Hess's Law with reactants to products via direct and indirect routes, showing enthalpy changes ΔH(1), ΔH(2), and ΔH_r.
The enthalpy change from elements to products (direct route) is equal to the enthalpy change of elements forming reactants and then products (indirect route)
  • According to Hess’s Law, the total enthalpy change is the same regardless of the pathway taken, because energy is conserved

  • There are two possible routes to form the products from the elements:

1. Direct formation (one-step route):

  • Elements → Products

  • Enthalpy change: ΔH2

2. Indirect formation (two-step route via reactants):

  • Elements → Reactants → Products

  • Enthalpy change: ΔH1 + ΔHr

By Hess’s Law:

ΔH2 = ΔH1 + ΔHr

  • Rearranged to solve for the enthalpy change of reaction (ΔHᵣ):

ΔHr = ΔH2−ΔH1

Examiner Tips and Tricks

You do not need to learn Hess's Law word for word as it is not a syllabus requirement, but you do need to understand the principle as it provides the foundation for all the problem solving in Chemical Energetics

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry Content Creator

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

Philippa Platt

Reviewer: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener