H2 Chemistry Hess’ Law: Reverse and Scale the Equations Before Adding

Let the target equation decide the arithmetic

In a public 2025 H2 Chemistry examination discussion, a student asked how to combine two supplied equations rather than arrive at a number by trial and error. That question is a good starting point for practising Hess' Law. The method below uses original teaching data and is not a reconstruction or validation of that examination answer.

The official 2026 H2 Chemistry 9476 syllabus includes Hess' Law, energy cycles and calculations involving relevant energy terms. Its cover identifies 2026 as the first examination year for that syllabus. The older discussion is used for its learning question; match practice to the syllabus code and year assigned to you.

Write the target before looking for a shortcut

Suppose an exercise asks for the enthalpy change for C(s, graphite) + ½O₂(g) → CO(g). You are supplied with these illustrative data under the same stated conditions: C(s, graphite) + O₂(g) → CO₂(g), ΔH₁ = −394 kJ mol⁻¹; and CO(g) + ½O₂(g) → CO₂(g), ΔH₂ = −283 kJ mol⁻¹. Treat these as rounded inputs for this exercise, rather than precision reference data.

Underline CO in the target. It must be a product. In the second supplied equation it is a reactant, so reverse that equation. You now have CO₂(g) → CO(g) + ½O₂(g), with enthalpy change +283 kJ mol⁻¹. Changing the reaction direction changes the sign; it does not change the size of the numerical value.

Add the species before adding the numbers

Add the first equation to the reversed second equation. Before cancellation, the left side contains C + O₂ + CO₂ and the right side contains CO₂ + CO + ½O₂. The CO₂ cancels completely. Subtracting ½O₂ from each side leaves C + ½O₂ → CO, which is exactly the target.

Only now add the corresponding enthalpy changes: −394 + 283 = −111 kJ mol⁻¹. The negative sign describes an exothermic change for the target reaction as written. Writing the cancellation is useful because it demonstrates that the number belongs to the requested reaction, rather than to another combination of the same data.

Scale the enthalpy when you scale an equation

If the target is instead 2C(s, graphite) + O₂(g) → 2CO(g), double the entire derived equation and its enthalpy change. The answer for that reaction as written is −222 kJ mol⁻¹. If the question asks for the standard enthalpy change of formation of CO, however, the formation equation produces one mole of CO, so use the one-mole equation and −111 kJ mol⁻¹ for these illustrative inputs.

Keep the distinction between an enthalpy change attached to a specified reaction equation and the amount of heat associated with a particular sample. Do not double a coefficient while leaving the reaction's enthalpy unchanged. Equally, do not divide a final result just because the question contains a coefficient 2 somewhere; first establish which quantity and reaction the question requests.

Check states and conditions before cancelling

A substance must represent the same chemical species in the same state to cancel directly. Water vapour and liquid water are different entries in an energy calculation. If a route produces one but the target requires the other, an additional enthalpy term may be needed. Similarly, do not silently replace graphite with another form of carbon.

When drawing a cycle, label each arrow with both its direction and its enthalpy term. Follow a complete route from the same starting materials to the same products. An arrow followed backwards contributes the negative of its labelled change. This is more reliable than memorising that the answer is always “top minus bottom”, because diagrams can be arranged differently.

Use a short error log for practice

Choose appropriate energetics questions from the TestPapersFree H2 Chemistry collection. Record the target, each reversal, each multiplier, the uncancelled sum and the final result with units. Classify mistakes as direction, coefficient, state or arithmetic. Start with two-equation combinations before returning to a longer cycle. A correct number with the wrong net reaction needs another attempt; a clear cancellation provides a method you can transfer to less familiar data.