word_count: 200

H = U + PV. At constant pressure: delta_H = qp. The key relationship connecting delta_H and delta_U for gas-phase reactions: delta_H = delta_U + delta_ngas * RT, where delta_ngas = moles of gaseous products - moles of gaseous reactants. Only count gaseous species — liquids and solids contribute negligibly to the PV change. If delta_ngas = 0 (e.g., H2 + Cl2 -> 2HCl): delta_H = delta_U. If delta_ngas > 0 (more gas moles produced): delta_H > delta_U. If delta_ngas < 0 (fewer gas moles): delta_H < delta_U. Common numerical: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l). delta_ngas = 1 - 3 = -2. Given delta_H = -890 kJ: delta_U = -890 + 2(8.314 x 10^-3)(298) = -885 kJ. Important: H2O(l) is NOT counted as a gas. This distinction between liquid and gaseous water in combustion products is a frequent trap in JEE questions. Always verify the physical state given in the problem.