Chemistry Year 11 - Module 4 - Lesson 9
Hess's Law Applied — Photosynthesis & Respiration
1. Key Ideas
Every cell in your body right now is breaking down glucose and releasing 2803 kJ mol⁻¹ of energy. Plants do the exact reverse — absorbing 2803 kJ mol⁻¹ of sunlight to build glucose. Hess's Law predicts this perfectly: reverse a reaction, reverse the sign of ΔH. But here is the puzzle — photosynthesis is endothermic, so how do plants actually run it continuously without violating the laws of thermodynamics?
- ΔH(photosynthesis) = +2803 kJ mol⁻¹; ΔH(respiration) = −2803 kJ mol⁻¹
- Why Hess's Law requires the ΔH values to be equal and opposite (reverse reaction rule)
2. Success Criteria
By the end, you should be able to:
- ΔH(photosynthesis) = +2803 kJ mol⁻¹; ΔH(respiration) = −2803 kJ mol⁻¹
- The two values are equal and opposite because the equations are exact reverses
- ATP hydrolysis releases ≈ −30.5 kJ mol⁻¹ per mole — used to power endothermic reactions
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: "ΔH(photosynthesis) = +2803 kJ mol⁻¹; ΔH(respiration) = −2803 kJ mol⁻¹". Use one specific example from the lesson.
2. Apply this idea to a new example: "The two values are equal and opposite because the equations are exact reverses". Show your reasoning clearly.
3. Analyse why this idea matters for understanding Hess's Law Applied — Photosynthesis & Respiration: "ATP hydrolysis releases ≈ −30.5 kJ mol⁻¹ per mole — used to power endothermic reactions".
6. Extend: Apply the Idea
A student gives a memorised answer about Hess's Law Applied — Photosynthesis & Respiration 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 Applied — Photosynthesis & Respiration?
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 Applied — Photosynthesis & Respiration?
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.