Hess's law says that the total enthalpy change for a chemical reaction depends only on the initial reactants and final products, not on the path taken between them. This matters because many reaction enthalpies are hard to measure directly in a lab. By combining reactions with known enthalpy changes, chemists can calculate an unknown enthalpy change accurately.
It is a powerful bookkeeping method for energy in chemical reactions.
Key Facts
- Hess's law: ΔHtotal = ΔH1 + ΔH2 + ΔH3 + ...
- If a reaction is reversed, the sign of ΔH changes: A -> B, ΔH = x means B -> A, ΔH = -x.
- If a reaction is multiplied by n, its enthalpy change is also multiplied: ΔHnew = nΔH.
- Enthalpy change is path independent because enthalpy is a state function.
- For formation enthalpies: ΔHreaction = ΣnΔHf products - ΣnΔHf reactants.
- For bond enthalpies, an estimate is ΔHreaction = Σ bonds broken - Σ bonds formed.
Vocabulary
- Hess's law
- Hess's law states that the overall enthalpy change of a reaction is the same no matter how many steps the reaction takes.
- Enthalpy change
- Enthalpy change, ΔH, is the heat energy transferred by a reaction at constant pressure.
- State function
- A state function is a quantity that depends only on the current state of a system, not on how the system got there.
- Formation enthalpy
- Formation enthalpy, ΔHf, is the enthalpy change when one mole of a compound forms from its elements in their standard states.
- Energy-cycle diagram
- An energy-cycle diagram shows multiple reaction pathways between the same substances so their enthalpy changes can be added and compared.
Common Mistakes to Avoid
- Forgetting to change the sign when reversing a reaction. This is wrong because the energy flow reverses when products become reactants.
- Multiplying the chemical equation but not multiplying ΔH. This is wrong because enthalpy is proportional to the amount of substance reacting.
- Adding reactions before canceling identical substances on opposite sides. This can leave extra species in the final equation and gives the wrong target reaction.
- Using products minus reactants backward in formation enthalpy calculations. The correct formula is ΔHreaction = ΣnΔHf products - ΣnΔHf reactants.
Practice Questions
- 1 Given C + O2 -> CO2, ΔH = -394 kJ and CO + 1/2 O2 -> CO2, ΔH = -283 kJ, use Hess's law to find ΔH for C + 1/2 O2 -> CO.
- 2 Use ΔHf values CO2(g) = -394 kJ/mol, H2O(l) = -286 kJ/mol, CH4(g) = -75 kJ/mol, and O2(g) = 0 kJ/mol to calculate ΔH for CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l).
- 3 A student says a two-step pathway must have a different ΔH than a one-step pathway because it has more steps. Explain why Hess's law shows this reasoning is incorrect.