Which of the following statements best describes Hess’s Law?
A. If a chemical equation for a reaction is the sum of two or more reactions, then the for the final reaction must be the difference of the Ho of the other reactions.
B. None of the options are correct; Hess’s Law does not deal with entropy or internal energy.
C. If a chemical equation for a reaction is the sum of two or more reactions, then the for the final reaction must be the negative of the first reaction.
D. If a chemical equation for a reaction is the sum of two or more reactions, then the for the final reaction must be the sum of the of the other reactions.
The correct answer and explanation is :
Correct Answer: D. If a chemical equation for a reaction is the sum of two or more reactions, then the ΔH° for the final reaction must be the sum of the ΔH° of the other reactions.
Explanation:
Hess’s Law is a fundamental principle in thermochemistry that states:
The total enthalpy change for a chemical reaction is the same, regardless of the number of steps or pathway taken, as long as the initial and final conditions are the same.
In other words, enthalpy (ΔH) is a state function. This means that its value depends only on the initial and final states of the system, not the path taken to get there. Therefore, if a chemical reaction can be expressed as the sum of two or more intermediate steps, the overall enthalpy change (ΔH°) is simply the sum of the enthalpy changes of each step.
For example, suppose we have:
- Reaction 1: A → B ΔH₁
- Reaction 2: B → C ΔH₂
Then the overall reaction:
- A → C ΔH_total = ΔH₁ + ΔH₂
This is the essence of Hess’s Law and is used extensively to determine the enthalpy changes of reactions that are difficult or impossible to measure directly.
Why the Other Options Are Incorrect:
- Option A: This says the final ΔH is the difference of the others. This is incorrect. We add enthalpy changes, not subtract, unless a reaction is reversed.
- Option B: This wrongly claims Hess’s Law doesn’t deal with enthalpy—it actually deals specifically with enthalpy (ΔH), not entropy (ΔS) or internal energy (U).
- Option C: This says the final ΔH is the negative of the first. Again, this is only true if the reaction is reversed. Hess’s Law uses addition or subtraction depending on reaction direction.
Conclusion:
Hess’s Law is a powerful tool in thermodynamics for calculating enthalpy changes. It allows chemists to use known enthalpy values to deduce unknown ones, ensuring accuracy and simplifying complex reaction pathways.