Free Hess's Law MCQs with Answers
2 Hess's Law MCQs from Chemistry, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.
2 questions
1. 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.
Correct answer: is the same whether the reaction occurs in one step or several2. Given that C + O2 gives CO2 has an enthalpy change of minus 393 kJ per mole, and CO + half O2 gives CO2 has an enthalpy change of minus 283 kJ per mole, the enthalpy of formation of carbon monoxide is
- A. minus 676 kJ per mole
- B. plus 110 kJ per mole
- C. minus 110 kJ per mole
- D. minus 283 kJ per mole
Explanation: Forming carbon monoxide and then burning it must total the same as burning carbon directly, so the unknown equals minus 393 minus the value of minus 283, giving minus 110 kJ per mole. Adding the two values instead of subtracting gives minus 676, which is the trap. This calculation is the standard textbook illustration of Hess's law.
Correct answer: minus 110 kJ per mole