Energetics Problem Solving

This final lesson brings together all the topics from the course into exam-style problem solving. The key to success in energetics questions is a systematic approach: identify what data you have, decide which method to use, set up the calculation clearly, and check your answer makes sense.

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Strategy for Multi-Step Hess's Law Problems

Some exam questions give you several thermochemical equations and ask you to calculate ΔH for a target reaction. The approach is:

1. Write the target equation clearly.
2. Examine each given equation — decide if it needs to be reversed, multiplied, or used as-is to build the target.
3. Reverse equations as needed (and change the sign of ΔH).
4. Multiply equations as needed (and multiply ΔH by the same factor).
5. Add the manipulated equations — intermediate species should cancel.
6. Add the ΔH values to get the answer.

Worked Example

Calculate ΔH for: C₂H₂(g) + 2H₂(g) → C₂H₆(g)

Given:

• (1) C₂H₂(g) + 5/2 O₂(g) → 2CO₂(g) + H₂O(l), ΔH₁ = −1300 kJ mol⁻¹
• (2) C₂H₆(g) + 7/2 O₂(g) → 2CO₂(g) + 3H₂O(l), ΔH₂ = −1560 kJ mol⁻¹
• (3) H₂(g) + ½O₂(g) → H₂O(l), ΔH₃ = −286 kJ mol⁻¹

Solution: