Q.25 Δ H for formation of ethane gas is −84.4 kJ at 300 K. Calculate Δ U for the reaction.

Answer: ΔU = −79.41 kJ Formation reaction: 2C(s) + 3H₂(g) → C₂H₆(g) Δ n_g = 1 - 3 = -2 Δ H = Δ U + Δ n_g RT ⇒ Δ U = Δ H - Δ n_g RT Δ n_g RT = -2 × 8.314 × 10⁻³ × 300 = -4.988 kJ Δ U = -84.4 - (-4.988) = -84.4 + 4.988 = -79.41 kJ

\(\Delta H\) for formation of ethane gas is −84.4 kJ at 300 K. Calculate \(\Delta U\) for the reaction.

Q.25 \(\Delta H\) for formation of ethane gas is −84.4 kJ at 300 K. Calculate \(\Delta U\) for the reaction.

Doubtly Asked 28 Oct 2026 Board exam · March 2025 hsc 12th Save WhatsApp Report

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Answer: ΔU = −79.41 kJ

Formation reaction: \(\mathrm{2C(s) + 3H_2(g) \rightarrow C_2H_6(g)}\)

\(\Delta n_g = 1 - 3 = -2\)

\(\Delta H = \Delta U + \Delta n_g RT \Rightarrow \Delta U = \Delta H - \Delta n_g RT\)

\(\Delta n_g RT = -2 \times 8.314 \times 10^{-3} \times 300 = -4.988\) kJ

\(\Delta U = -84.4 - (-4.988) = -84.4 + 4.988 = -79.41\) kJ

Doubtly AI · 28 Oct 2026 AI-verified · Oct 2026 Report

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\(\Delta H\) for formation of ethane gas is −84.4 kJ at 300 K. Calculate \(\Delta U\) for the reaction.

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