Safety boundary
Use equation cards, simulations, and supplied energy data only. No combustion or reactive demonstrations are required.
Explicit chemistry instruction
Enthalpy is a state function: ΔH depends on initial and final states, not the route.
When equations are reversed, the sign of ΔH reverses; when coefficients are multiplied, ΔH scales by the same factor.
Adding valid reaction steps cancels intermediates and adds their ΔH values.
Vocabulary: Hess's law; state function; pathway; intermediate; thermochemical equation.
Worked example
If A→B has +20 kJ and B→C has −50 kJ, then A→C has −30 kJ.
Guided question
What happens to ΔH when a thermochemical equation is doubled?
Independent work
- Combine three two-step Hess problems.
- Reverse/scale equations correctly.
- Audit cancelled species.
Misconception check
Hess's law is not averaging pathway energies; it is algebraic addition of thermochemical steps.
Formative check
Why can different pathways give the same net ΔH?
Reveal response
Enthalpy depends only on initial and final states.
Mentor evidence conference
Ask Archaemenes to audit signs, pathway logic, units, and model limits.
Teacher guidance
Foundation: Use color-coded equation cards.
Core: Solve multi-step Hess cycles.
Extended: Connect state functions to energy diagrams.