Question

For which reaction is ∆H approximately (or exactly) equal to ∆U?

A. The reaction of A + B -> C + D
B. The reaction of X + Y -> Z
C. The reaction of P + Q -> R + S
D. The reaction of M + N -> O

Answer 1

For the reaction ΔH is approximately equal to ΔU when no gases are involved or when the number of moles of gases does not change, as in the case of the combustion of carbon to form CO₂ under conditions where no volume work is performed.

Step-by-step explanation:

The question asked for which reaction is ΔH approximately (or exactly) equal to ΔU. This will occur if there are no gases involved or if the number of moles of gaseous reactants and products is consistent because ΔH and ΔU differ only by the work done by/against the atmosphere when the system expands or contracts, which is significant in reactions involving gases. To apply Hess's law, and given the sample reactions presented, we will assume standard conditions and stoichiometry for reactions such as the combustion of carbon, where ΔH for the reaction can be calculated using the standard enthalpy changes of formation (ΔH°f) for reactants and products.

It is not directly clear from the provided reaction labels which reaction ΔH would be equal to ΔU. Still, we can state that reaction involving the combustion of 2 mol of carbon to give CO₂, which can be assumed to occur under conditions where no volume work is done (for instance, in a bomb calorimeter), would have ΔH approximately equal to ΔU.