KCET 2025 Chemistry Paper

To determine which reaction has $∆ H=∆ U$, we use the equation:$∆ H=∆ U+∆ n_g ⋅ R T$ Where:$∆ H$ is the change in enthalpy.$∆ U$ is the change in internal energy.$∆ n_g$ is the change in the number of moles of gas.$R$ is the ideal gas constant.$T$ is the temperature in Kelvin. For the reaction:$2 {HI}_{( {g})} ⇌ {H}_{2( {g})}+ {I}_{2( {g})}$ The change in the number of moles of gas ( $∆ n_g$ ) can be calculated as follows:On the reactant side, there are 2 moles of gas.On the product side, there are 1 mole of ${H}_2$ plus 1 mole of ${I}_2$, totaling 2 moles. Thus, $∆ n_g=2-2=0$.Since $∆ n_g=0$, it follows that:$∆ H=∆ U$Therefore, for this reaction, the change in enthalpy is equal to the change in internal energy.