Chemistry • Year 11 • Module 4 • Lesson 9
Hess’s Law Applied: Photosynthesis & Respiration
Build core vocabulary, recall the reverse-reaction rule, and map out the photosynthesis–respiration energy cycle.
1. Term–definition match
Match each term in the left column to its correct definition by writing the matching letter in the right column. Use each letter once only. 10 marks
| # | Definition | Term (A–J) | Your answer |
|---|---|---|---|
| 1.1 | The overall equation for building glucose from CO₂ and H₂O using light energy; ΔH° = +2803 kJ mol⁻¹. | ||
| 1.2 | The overall equation for breaking down glucose with O₂ to release energy; ΔH° = −2803 kJ mol⁻¹. | ||
| 1.3 | The thermodynamic law stating that the total enthalpy change of a reaction is independent of pathway. | ||
| 1.4 | A reaction that absorbs heat from the surroundings; positive ΔH. | ||
| 1.5 | A reaction that releases heat to the surroundings; negative ΔH. | ||
| 1.6 | The energy currency of the cell; hydrolysis releases approximately −30.5 kJ mol⁻¹. | ||
| 1.7 | Linking an exothermic ATP hydrolysis to an endothermic biosynthesis so the combined ΔH is negative. | ||
| 1.8 | The enthalpy change when one mole of a compound is formed from its elements in their standard states. | ||
| 1.9 | The continuous cycling of carbon between living organisms and the atmosphere via photosynthesis and respiration. | ||
| 1.10 | A diagram showing two enthalpy levels connected by reaction arrows, used to verify Hess’s Law. |
Term list: A Cellular respiration • B Photosynthesis • C Hess’s Law • D Endothermic reaction • E Exothermic reaction • F ATP • G ATP coupling • H Standard enthalpy of formation (ΔH⁰f) • I Carbon cycle • J Hess’s Law energy cycle diagram
Stuck? Revisit the Key Terms panel and Cards 2–3 of the lesson.2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line provided. 10 marks (1 mark T/F + 1 mark correction where needed)
2.1 Photosynthesis is exothermic because it releases energy to the surroundings. T / F
2.2 According to Hess’s Law, reversing a thermochemical equation reverses the sign of ΔH but keeps the magnitude the same. T / F
2.3 In a Hess’s Law energy cycle for photosynthesis and respiration, the molecules CO₂ + H₂O sit at the higher enthalpy level. T / F
2.4 ATP coupling allows cells to run endothermic reactions by adding an exothermic ATP hydrolysis equation to give a combined negative ΔH. T / F
2.5 If ΔH(respiration) = −2803 kJ mol⁻¹, then ΔH(photosynthesis) must also be −2803 kJ mol⁻¹ because the same molecules are involved. T / F
3. Function recall
Answer each in one or two precise sentences. 10 marks (2 marks each)
3.1 What is the function of chlorophyll in photosynthesis from a Hess’s Law perspective?
3.2 What does it mean for the Hess’s Law cycle to “close” (sum to zero)?
3.3 What is the function of ATP in a cell’s biochemistry, and why is it described as the “energy currency” of the cell?
3.4 Why is glucose described as a “solar energy storage molecule”?
3.5 What role does the carbon cycle play in balancing the energy budgets of photosynthesis and respiration on a global scale?
4. Cloze — fill in the blanks
Complete the paragraph using the word bank below. Each word is used once. 8 marks
Word bank: endothermic • exothermic • reversed • state function • chlorophyll • ATP • 2803 • zero
Photosynthesis is ________________ (ΔH = +________________ kJ mol⁻¹), meaning it absorbs energy from sunlight captured by ________________. Cellular respiration is the chemical ________________ of photosynthesis and is therefore ________________ (ΔH = −2803 kJ mol⁻¹). Hess’s Law works here because enthalpy is a ________________ — it depends only on the initial and final states, not the pathway. When we add the two thermochemical equations together, all species cancel and the total ΔH equals ________________, confirming the cycle closes. Organisms apply this same logic through ________________ coupling, adding exothermic hydrolysis reactions to endothermic biosynthesis steps.
5. Concept map
Draw labelled arrows between the six terms below to show how they are connected. Each arrow must carry a brief linking phrase (e.g. “is the reverse of”, “stores energy from”, “applies”). Aim for at least 6 labelled arrows. 6 marks
Terms: Photosynthesis • Cellular respiration • Hess’s Law • Glucose • ATP • Carbon cycle
Q1 — Term–definition matches
1.1 B (Photosynthesis) • 1.2 A (Cellular respiration) • 1.3 C (Hess’s Law) • 1.4 D (Endothermic reaction) • 1.5 E (Exothermic reaction) • 1.6 F (ATP) • 1.7 G (ATP coupling) • 1.8 H (ΔH⁰f) • 1.9 I (Carbon cycle) • 1.10 J (Hess’s Law energy cycle diagram). 1 mark each.
Q2 — True/False with correction
2.1 False. Photosynthesis is endothermic (ΔH = +2803 kJ mol⁻¹); it absorbs energy (from sunlight), it does not release it.
2.2 True. Reversing a thermochemical equation flips the sign of ΔH and keeps the magnitude identical (a direct consequence of enthalpy being a state function).
2.3 False. CO₂ + H₂O sit at the lower enthalpy level. Glucose + O₂ sit at the higher level because photosynthesis (endothermic) builds glucose from CO₂ + H₂O, placing the products 2803 kJ mol⁻¹ higher.
2.4 True.
2.5 False. ΔH(photosynthesis) = +2803 kJ mol⁻¹ (positive/endothermic). Hess’s Law requires it to be equal in magnitude but opposite in sign to ΔH(respiration), not equal in both.
Q3 — Function recall
3.1 Chlorophyll absorbs solar (light) energy and converts it into chemical energy stored in glucose bonds. From a Hess’s Law perspective, it provides the +2803 kJ mol⁻¹ required to drive the endothermic photosynthesis reaction.
3.2 The cycle “closes” means that when you add the two thermochemical equations (photosynthesis + respiration), all chemical species cancel and the total ΔH = 0 kJ mol⁻¹. This confirms that enthalpy is a state function and Hess’s Law is satisfied.
3.3 ATP stores chemical energy in its phosphate bonds and releases it as needed when hydrolysed to ADP + Pᵢ (ΔH ≈ −30.5 kJ mol⁻¹). It is the “energy currency” because cells use it as a portable, controlled-release store of energy that can be spent on specific biological work without releasing a dangerous flood of energy all at once.
3.4 Glucose stores the solar energy captured during photosynthesis in its chemical bonds. The 2803 kJ mol⁻¹ of light energy absorbed by chlorophyll is converted to chemical bond energy in glucose, making it a molecular battery for solar energy.
3.5 Photosynthesis fixes atmospheric CO₂ into glucose (removing carbon from the atmosphere), while respiration releases CO₂ back (returning carbon to the atmosphere). In a balanced system the rates of carbon uptake and release are equal, maintaining a stable atmospheric CO₂ level and balancing the global energy budget. Combustion of fossil fuels disrupts this balance by releasing carbon that was sequestered millions of years ago.
Q4 — Cloze answers (in order)
endothermic • 2803 • chlorophyll • reversed • exothermic • state function • zero • ATP
Q5 — Sample concept map
A correct map should include arrows such as:
- Photosynthesis —builds→ Glucose
- Photosynthesis —is the chemical reverse of→ Cellular respiration
- Cellular respiration —breaks down→ Glucose
- Cellular respiration —produces→ ATP
- Hess’s Law —explains the equal and opposite ΔH of→ Photosynthesis and Cellular respiration
- Photosynthesis and Cellular respiration —together drive→ Carbon cycle
Any biologically and chemically valid linking phrases are accepted. Award 1 mark per correctly labelled, directionally correct arrow (max 6).