Study TipsMay 19, 20266 min read
MDCAT Thermochemistry 2026 — Enthalpy, Hess's Law, Bond Energies
Premeth Team
Meth Experts
Thermochemistry yields 2–3 of the 45 MDCAT Chemistry MCQs. Almost all are application of either Hess's Law or bond-energy calculations.
- Estimated MCQs — 2–3 of 45 Chemistry
- Enthalpy sign convention — ΔH negative = exothermic, positive = endothermic
- Hess's Law — total ΔH is path-independent
Source: PMDC MDCAT 2026 syllabus.
Types of Enthalpy
- Enthalpy of formation (ΔHf) — from elements in standard states. Standard elements have ΔHf = 0.
- Enthalpy of combustion (ΔHc) — always negative (exothermic).
- Enthalpy of neutralisation — strong acid + strong base ≈ −57 kJ/mol.
- Enthalpy of solution — energy for dissolution; can be + or −.
- Bond energy — energy required to break 1 mole of bonds in gaseous state.
Hess's Law
The total enthalpy change of a reaction is the same whether achieved in one step or several. ΔHreaction = ΣΔHproducts − ΣΔHreactants.
Bond Energy Method
ΔHreaction = Σ bond energies of reactants − Σ bond energies of products.
FAQ
Q: Bond breaking endothermic or exothermic?
Bond breaking = endothermic (requires energy). Bond formation = exothermic (releases energy).
Q: Standard state conditions?
25°C (298 K), 1 atm pressure, element in its most stable form.
Drill Chemical Energetics chapter MCQs alongside Chemical Equilibrium.
Related Tags
#MDCAT#Chemistry#Thermochemistry#Enthalpy
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