Hess's law states that the enthalpy change of a reaction
- A. depends on the number of steps taken
- B. is the same whether the reaction occurs in one step or several
- C. is always negative
- D. depends on the catalyst used
Explanation
Because enthalpy is a state function, the total change depends only on the initial and final states, so a reaction that cannot be measured directly can be calculated through a series of steps that can. This is how the enthalpy of formation of carbon monoxide is obtained, since burning carbon always gives some carbon dioxide as well. Hess's law is essentially the first law applied to chemical change.