🧪 Chemistry · Thermodynamics · NEET & JEE

For H<sub>2</sub>(g) + Cl<sub>2</sub>(g) → 2HCl(g), using bond enthalpies H–H = 435, Cl–Cl = 242 and H–Cl = 431 kJ/mol, ΔH is:

Answer: −185 kJ.

A +185 kJ
B −620 kJ
C −185 kJ
D +862 kJ

Correct answer: C. −185 kJ

Explanation: ΔH = bonds broken − bonds formed = (435 + 242) − 2(431) = 677 − 862 = −185 kJ.

Isothermal expansion follows a gentler curve (heat flows in to keep T constant) while adiabatic expansion drops in pressure more steeply (no heat exchange, so internal energy and temperature fall as the gas does work).

Concept context

Study energy changes in chemical reactions. Understand enthalpy, entropy, Gibbs free energy, and the laws of thermodynamics that decide whether a reaction will occur spontaneously or not.

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For H<sub>2</sub>(g) + Cl<sub>2</sub>(g) → 2HCl(g), using bond enthalpies H–H = 435, Cl–Cl = 242 and H–Cl = 431 kJ/mol, ΔH is:

Concept context

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