Chemistry Year 11 - Module 4 - Lesson 8
Hess's Law
1. Key Ideas
Steel production requires carbon to reduce iron ore — and chemists needed to know the exact ΔH for C(s) + O₂(g) → CO₂(g) via the intermediate CO(g). The problem: you can't burn carbon to CO without also producing CO₂. The solution: Hess's Law. Add two measurable equations together, cancel the intermediate, and the answer falls out — no impossible experiment required.
- Hess's Law: ΔH is the same regardless of pathway — enthalpy is a state function
- Why Hess's Law follows from enthalpy being a state function (First Law of Thermodynamics)
2. Success Criteria
By the end, you should be able to:
- Hess's Law: ΔH is the same regardless of pathway — enthalpy is a state function
- Reverse equation → ΔH changes sign; scale equation by n → ΔH scales by n
- The NESA prototype: C → CO₂ via CO(g) as intermediate
3. Key Terms
4. Activity: Build the Lesson Map
Use the lesson to complete the table. Keep answers brief but specific.
| Prompt | Your answer |
|---|---|
| Main concept | |
| Important example | |
| Common mistake to avoid | |
| How this links to the next lesson |
5. Short Answer Questions
1. Explain this lesson goal in your own words: "Hess's Law: ΔH is the same regardless of pathway — enthalpy is a state function". Use one specific example from the lesson.
2. Apply this idea to a new example: "Reverse equation → ΔH changes sign; scale equation by n → ΔH scales by n". Show your reasoning clearly.
3. Analyse why this idea matters for understanding Hess's Law: "The NESA prototype: C → CO₂ via CO(g) as intermediate".
6. Extend: Apply the Idea
A student gives a memorised answer about Hess's Law but does not use evidence or reasoning.
Improve the answer by writing a stronger response that uses accurate terminology, a relevant example and a clear explanation.
7. Multiple Choice
1. What is the best first step when answering a question about Hess's Law?
A. Identify the key concept being tested
B. Write every fact from memory
C. Ignore the command word
D. Skip examples and evidence
2. Which answer would show stronger understanding of Hess's Law?
A. An answer with accurate terms and reasoning
B. A copied definition only
C. A single-word response
D. An answer with no example
3. What should you do if a question asks you to explain?
A. Link the idea to a reason or cause
B. List unrelated facts
C. Only draw a diagram
D. Write the shortest possible answer
8. Success Criteria Proof
Finish with evidence that you can do each success criterion.