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Hess's law of constant heat summation Concepts for JEE

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Concept explainer

Hess's law of constant heat summation is a core JEE Main Chemistry subtopic under Chemical Thermodynamics. Master the definitions, standard results, and typical MCQ patterns tested in JEE Main and Advanced.

Key points

  • Understand the definition and scope of Hess's law of constant heat summation in the JEE syllabus
  • Memorise key formulas and standard results linked to Hess's law of constant heat summation
  • Practise 20–40 syllabus-aligned MCQs with step-by-step solutions

JEE tips

  • Revise Hess's law of constant heat summation with a one-page formula sheet before attempting mixed tests
  • After each practice set, log mistakes specific to Hess's law of constant heat summation and reattempt after 48 hours

Common trap

Students often rush Hess's law of constant heat summation questions without checking units, sign conventions, or boundary conditions — always verify assumptions before calculating.

Syllabus context

Part of Chemical Thermodynamics in JEE Main Chemistry.

Free sample questions

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Q1ChemistryUnit 4: Chemical Thermodynamics
Heat of combustion of CH4,C2H6\boldsymbol{C H}_{4}, \boldsymbol{C}_{2} \boldsymbol{H}_{6} and C3H8C_{3} H_{8} are respectively -210,-368.4 and −526.8Kcal-526.8 \mathrm{Kcal} mol 1_{1} Hence, heat of combustion of C8H16C_{8} H_{16} is approximately:
Q2ChemistryUnit 4: Chemical Thermodynamics
The statement "The change of enthalpy of a chemical reaction is same whether the reaction takes place in one or several steps" is:
Q3ChemistryUnit 4: Chemical Thermodynamics
If the heats of formation of C2H2C_{2} H_{2} and C6H6C_{6} H_{6} are 230KJmol−1230 \mathrm{KJ} \mathrm{mol}^{-1} and 85KJmol−185 \mathrm{KJ} \mathrm{mol}^{-1} respectively, the ΔH\Delta \mathrm{H} value for the trimerisation of C2H2C_{2} \mathrm{H}_{2} is:
Q4ChemistryUnit 4: Chemical Thermodynamics
(i) CaO(s)+H2O(l)=Ca(OH)2(s)\boldsymbol{C a O}(\boldsymbol{s})+\boldsymbol{H}_{2} \boldsymbol{O}(l)=\boldsymbol{C a}(\boldsymbol{O H})_{2}(\boldsymbol{s}) ΔH180∘C=−15.26kcal\Delta H_{180^{\circ} C}=-15.26 k c a l (ii) H2O(l)=H2(g)+12O2(g)\boldsymbol{H}_{2} \boldsymbol{O}(\boldsymbol{l})=\boldsymbol{H}_{2}(\boldsymbol{g})+\frac{1}{2} \boldsymbol{O}_{2}(\boldsymbol{g}) ΔH180∘C=68.37kcal\boldsymbol{\Delta} \boldsymbol{H}_{180^{\circ} C}=\mathbf{6 8 . 3 7} \boldsymbol{k c a l} (iii) Ca(s)+12O2(g)=CaO(s)C a(s)+\frac{1}{2} O_{2}(g)=C a O(s) ΔH180∘C=−151.80kcal\boldsymbol{\Delta} \boldsymbol{H}_{180^{\circ} C}=-151.80 k c a l From the following data, the heat of formation of Ca(OH)2(s)\boldsymbol{C a}(\boldsymbol{O H})_{2}(\boldsymbol{s}) at 18∘C\mathbf{1 8}^{\circ} \boldsymbol{C} is:
Q5ChemistryUnit 4: Chemical Thermodynamics
Given the following: C(s)+O2(g)→CO2(g);ΔH=\boldsymbol{C}(s)+\boldsymbol{O}_{2}(\boldsymbol{g}) \rightarrow \boldsymbol{C} \boldsymbol{O}_{2}(\boldsymbol{g}) ; \boldsymbol{\Delta} \boldsymbol{H}= −394kJ/mol-394 k J / m o l 2H2(g)+O2(g)→2H2O(l);ΔH=2 H_{2}(g)+O_{2}(g) \rightarrow 2 H_{2} O(l) ; \Delta H= −568kJ/mol⁡C2H5OH(l)+-\mathbf{5 6 8} k J / \operatorname{mol} C_{2} H_{5} O H(l)+ 3O2(g)→2CO2(g)+\mathbf{3} O_{2}(\boldsymbol{g}) \rightarrow \mathbf{2} \boldsymbol{C} \boldsymbol{O}_{2}(\boldsymbol{g})+ 3H2O(l);ΔH=−1058kJ/mol\mathbf{3} \boldsymbol{H}_{2} \boldsymbol{O}(\boldsymbol{l}) ; \boldsymbol{\Delta} \boldsymbol{H}=-\mathbf{1 0 5 8} \boldsymbol{k} \boldsymbol{J} / \boldsymbol{m o l} Using the given data, the heat of formation of ethanol is:
Q6ChemistryUnit 4: Chemical Thermodynamics
S+32O2→SO3+2x\boldsymbol{S}+\frac{\mathbf{3}}{\mathbf{2}} \boldsymbol{O}_{2} \rightarrow \boldsymbol{S} \boldsymbol{O}_{3}+\mathbf{2} \boldsymbol{x} kcal SO2+12O2→SO3+y\boldsymbol{S O}_{2}+\frac{1}{2} \boldsymbol{O}_{2} \rightarrow \boldsymbol{S} \boldsymbol{O}_{3}+\boldsymbol{y} kcal The heat of formation of SO2S O_{2} is:

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