Hess’s law is one of the most powerful tools in thermochemistry because it allows you to calculate enthalpy changes for reactions that cannot be measured directly. IB Chemistry students encounter Hess’s law throughout Topic 5 (Energetics/Thermochemistry), especially in Paper 2 and in IA calculations. This article breaks the idea down into simple steps so you can confidently approach any Hess’s law problem.
What Is Hess’s Law?
Hess’s law states that the total enthalpy change of a reaction is the same regardless of the pathway taken.
In simpler terms:
If you can’t measure a reaction directly, but you can combine known reactions to get the same overall change, the enthalpy of the overall reaction is the sum of the enthalpies of the individual steps.
This works because enthalpy is a state function—it depends only on the initial and final states, not on the route taken between them.
Why Hess’s Law Works (The Concept)
Hess’s law is based on the principle of energy conservation.
Energy cannot be created or destroyed; it can only be transferred.
So if a reaction can be broken down into multiple intermediate steps, each step has an enthalpy value. Adding these stepwise values gives the same result as if the reaction happened all at once. This is why Hess’s law is sometimes called the Law of Constant Heat Summation.
When Do You Use Hess’s Law in IB Chemistry?
You use Hess’s law when:
- The enthalpy of a reaction cannot be measured directly
- You are given multiple chemical equations with known enthalpies
- You need to calculate enthalpy of formation, combustion, or reaction pathways
- You need to manipulate equations by reversing or scaling them
- You work with energy cycles (Born–Haber cycles)
This makes Hess’s law a versatile calculation tool across many thermochemical topics.
How to Apply Hess’s Law
There are two main approaches IB students use:
1. Algebraic Method
You rearrange given equations—reversing, multiplying, dividing—until they match the target reaction.
Rules:
- Reversing an equation → change sign of ΔH
- Multiplying an equation → multiply ΔH by the same factor
- Adding equations → add their ΔH values
This is the method most often tested on IB Paper 2.
2. Enthalpy Cycles (Diagram Method)
You draw an energy cycle showing multiple pathways from reactants to products.
The key principle:
All pathways from the same starting point to the same end must have equal total enthalpy.
This method is especially useful for:
- Bond enthalpy cycles
- Combustion cycles
- Formation cycles
- Born–Haber lattice energy cycles
Worked Example (IB-Style)
Calculate ΔH for this reaction:
C(graphite) + ½O₂ → CO
You are given:
- C(graphite) + O₂ → CO₂ ΔH = –393 kJ/mol
- CO + ½O₂ → CO₂ ΔH = –283 kJ/mol
Goal: Combine these to isolate C + ½O₂ → CO.
Step 1: Reverse the second equation
CO₂ → CO + ½O₂ ΔH = +283
Step 2: Add the reversed equation to the first one
(C + O₂ → CO₂)
- (CO₂ → CO + ½O₂)
CO₂ cancels out.
Final result:
C + ½O₂ → CO
ΔH = –393 + 283 = –110 kJ/mol
Common Mistakes IB Students Make
- Forgetting to flip the sign when reversing an equation
- Not scaling ΔH when multiplying equations
- Trying to cancel species that do not appear on both sides
- Mixing up cycle directions
- Forgetting that enthalpy values are per mole, not per reaction mixture
Careful equation alignment avoids nearly all of these errors.
FAQs
Why is enthalpy a state function?
A state function depends only on the initial and final conditions, not on the process or path. Because enthalpy reflects stored chemical energy, the route taken does not change the total energy difference.
Is Hess’s law always accurate?
Hess’s law depends on consistent energy measurements under standard conditions. Experimental errors or non-standard conditions can produce slight deviations, but the law itself is exact.
Do I need to draw cycles in IB exams?
Not always, but diagrams help visualize enthalpy pathways. They are particularly helpful for Born–Haber cycles and formation/combustion cycles.
Conclusion
Hess’s law simplifies complex thermochemical calculations by letting you add or manipulate known enthalpy values to find unknown ones. Understanding how to reverse equations, scale them, and add them correctly gives you a powerful tool for solving energetics questions confidently. With consistent practice, Hess’s law becomes one of the most intuitive parts of IB Chemistry.
