JEE Main · 2024 · 04 Apr 2024 · Shift 1 · Chemical Thermodynamics

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The enthalpy of formation of ethane (C₂H₆) from ethylene by addition of hydrogen where the bond energies of C-H, C-C, H-H are 414 kJ, 347 kJ, 615 kJ and 435 kJ respectively is - ____ kJ.

Answer: 125

Solution

C_{2} H_{4} (g) + H_{2} (g) \to C_{2} H_{6} (g)

Δ H = [B E_{C = C} + B E_{H - H}] - [B E_{C - C} + 2 B E_{C - H}]

(one C=C and one H–H break; one C–C and two extra C–H form).

= (615 + 435) - (347 + 2 \times 414) = 1050 - 1175 = - 125 kJ .

The enthalpy of formation is

- 125

kJ, so the reported magnitude is 125.

JEE Main · 2024 · 04 Apr 2024 · Shift 1 · Integer answer

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