Daily Exam Challenge: Combined Science (Higher)
Mark Scheme — 17 marks total
Higher TierMarking Instructions
- • Accept equivalent answers unless the mark scheme states otherwise.
- • M marks are for method — award even if the final answer is wrong.
- • A marks are for accuracy — only award if the method mark has been earned.
- • B marks are independent and can be awarded without other marks.
- • Where a range of answers is acceptable, this is indicated in the mark scheme.
A student measures the rate of photosynthesis at different temperatures. The rate increases from 10°C to 35°C, then rapidly decreases. (a) Explain why the rate increases between 10°C and 35°C. (b) Explain why the rate rapidly decreases above 35°C. (c) Sketch a graph to show the expected results.
Acceptable Answer
(a) As temperature increases, molecules have more kinetic energy. Enzyme-substrate collisions are more frequent and more energetic. More enzyme-substrate complexes form per unit time, increasing the rate of the reactions in photosynthesis. (b) Above the optimum temperature (35°C), the enzymes involved in photosynthesis begin to denature. The bonds holding the enzyme's 3D shape break, causing the active site to change shape. The substrate no longer fits, so the rate of reaction drops rapidly. (c) Graph: x-axis = temperature, y-axis = rate. Curve rises steadily from 10°C, peaks at ~35°C, then drops sharply. The curve is not symmetrical — the decrease is steeper than the increase.
Explain, using ideas about electron transfer, why the reaction between sodium and chlorine is an example of both oxidation and reduction.
Acceptable Answer
Sodium atoms each lose one electron: Na → Na⁺ + e⁻. This is oxidation (loss of electrons — OIL). Chlorine atoms each gain one electron: Cl + e⁻ → Cl⁻. This is reduction (gain of electrons — RIG). Because both oxidation and reduction occur simultaneously, this is a redox reaction. The sodium is the reducing agent (it causes reduction of chlorine by donating electrons). The chlorine is the oxidising agent (it causes oxidation of sodium by accepting electrons).
A crane lifts a 500 kg steel beam 12 m in 8 seconds. The motor is rated at 10 kW. (g = 10 N/kg) (a) Calculate the useful energy transferred. (b) Calculate the total energy input from the motor. (c) Calculate the efficiency of the crane.
Acceptable Answer
(a) Useful energy = GPE = mgh = 500 × 10 × 12 = 60,000 J (b) Total energy = Pt = 10,000 × 8 = 80,000 J (c) Efficiency = 60,000/80,000 × 100 = 75%