Measuring enthalpy changes of reactions
5 marks · HardCopper(II) sulfate crystals dissolve in water in a polystyrene cup calorimeter. A student wants to determine the enthalpy change of the process.
State two measurements the student must take to calculate the enthalpy change. (2 marks)
The student dissolves 10.0 g of copper(II) sulfate ( ) in 50.0 g of water. The temperature rises by 7.4 °C. Calculate the heat absorbed or released by the water. (Specific heat capacity of water = 4.2 J/g°C) Give your answer to three significant figures. (2 marks)
Explain whether the dissolving of copper(II) sulfate in water is exothermic or endothermic, based on your calculation. (1 mark)
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(a) — · must show Initial temperature, Final temperature, Mass of water/solution
(b) 1550 J · must show q = mcΔT, Correct values substituted (m=50, c=4.2, ΔT=7.4), Correct final answer to 3 sig. fig.
(c) Exothermic · must show Correctly links temperature rise to exothermic process
Common mistakes: Using mass of CuSO4 instead of water for heat calculation; Not identifying reaction type from temperature change
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States one correct measurement (e.g. initial and final temperature, or temperature change)
B2 (a) · 1 mark · States a second correct measurement (e.g. mass of water or solution)
M1 (b) · 1 mark · Uses q = mcΔT or substitutes all values correctly
A1 (b) · 1 mark · Correct calculation to 3 significant figures (q = 50 × 4.2 × 7.4 = 1554 J ≈ 1550 J)
B3 (c) · 1 mark · States it is exothermic with correct reason (temperature of water increases)
Worked solution
a) The student must measure the initial temperature and the highest (final) temperature of the solution, and the mass of water (or total solution) used.
b) q = mcΔT = 50.0 g × 4.2 J/g°C × 7.4 °C = 1554 J = 1550 J (3 sig. fig.)
c) The reaction is exothermic because the temperature of the water increased.