About this experiment
Construct an indirect reaction route by reversing and multiplying measured reaction steps. Cancel intermediates explicitly, then test why consistent thermochemical data give a path-independent enthalpy.
Learning goals
- Add chemical equations as signed stoichiometric vectors.
- Reverse and scale enthalpies together with reactions.
- Distinguish a balanced target equation from an accidental agreement of energy numbers.
How to use it
- 1Select the carbon-to-carbon-dioxide target and construct a route through carbon monoxide.
- 2Reverse a step to derive carbon-monoxide formation indirectly.
- 3Add a closed reaction cycle and verify unchanged net species and enthalpy.
- 4Introduce a measurement offset and repeat the cycle to expose inconsistent input data.
The model
Assumptions
- Reference enthalpies are rounded values at 298.15 K for graphite, gaseous oxygen, carbon monoxide and carbon dioxide.
- The deliberate measurement offset affects one reported step only. It demonstrates data inconsistency rather than a failure of energy conservation.
- The target match is an exact stoichiometric-vector test, including fractional oxygen coefficients. No chemical kinetics are predicted.
Try this in class
- Q1Why does reversing a chemical equation reverse the sign of its enthalpy?
- Q2Why is matching only the numerical enthalpy insufficient to prove a valid Hess route?
Frequently asked questions
Why can adding a cycle change the answer when an offset is present?
The modified data no longer represent one consistent set of state enthalpies. The cycle residual reveals that inconsistency.
Must the intermediate route be a practical laboratory synthesis?
No. Hess law combines thermochemical state changes algebraically; it does not assert that each chosen route is a convenient reaction mechanism.