Read Work and Heat Sign Conventions Before Any P-V Problem
First-law problems reward fixing sign convention before anything else — work done ON the gas is positive for the system in most Indian-syllabus convention, and reversible vs irreversible changes the magnitude even for the same start and end states.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Reversible vs irreversible work for the same compression
these give different magnitudes for the same start and end states, so identify which process type the question describes before picking the formula.
Calorimetry is a heat-balance equation
Mixing ice and water (or any calorimetry problem) balances heat lost by the warmer component against heat gained by the cooler one, including any phase-change enthalpy (like fusion) that must be paid before temperature can rise — don't forget the latent-heat term when ice is involved.
Enthalpy of combustion: per mole vs per unit mass
Heat of combustion is usually reported per mole, but a question asking for energy released per gram needs that value divided by molar mass — check units carefully before comparing two fuels' 'energy release'.
Hess's law lets you build any enthalpy from measured steps
When a target reaction's ΔH isn't measured directly, construct it from a combination of given reactions (including phase-change steps like vaporisation) added or reversed as needed — the enthalpy of the target equals the same combination of the given ΔH values.
OlympIQ exam lens
- ✓Reversible and irreversible work formulas give different magnitudes for the same start/end states — identify the process type first.
- ✓Calorimetry balances heat lost against heat gained, including any latent heat for a phase change like ice melting.
- ✓Heat of combustion is per mole by default — convert to per gram only when explicitly asked.
- ✓Hess's law: combine given ΔH values (adding, reversing, or scaling reactions) to build the target reaction's enthalpy.
Don't apply the reversible work formula to a single-step (irreversible) expansion or compression, or vice versa — they give genuinely different numerical answers for the same P, V and T endpoints.
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