$word_{count}$: 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.