KCET 2025 Chemistry Paper

To determine which reaction has ∆H=∆U, we use the equation:
∆H=∆U+∆ng⋅RT
Where:
∆H is the change in enthalpy.
∆U is the change in internal energy.
∆ng 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:
2HI(g)⇌H2(g)+I2(g)
The change in the number of moles of gas ( ∆ng ) can be calculated as follows:
On the reactant side, there are 2 moles of gas.
On the product side, there are 1 mole of H2 plus 1 mole of I2, totaling 2 moles.
Thus, ∆ng=2−2=0.
Since ∆ng=0, it follows that:
∆H=∆U
Therefore, for this reaction, the change in enthalpy is equal to the change in internal energy.