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.