Energetics
5 marks · EasyA student adds 2.00 g of magnesium powder to 50.0 cm³ of dilute sulfuric acid in a polystyrene cup. The temperature of the mixture increases from 22.0°C to 27.4°C.
State whether the reaction between magnesium and sulfuric acid is exothermic or endothermic. Explain your answer using the temperature change.
Use your answer from (a). Calculate the heat energy change, Q, in joules for this reaction. [Assume the specific heat capacity of the solution is 4.18 J/g°C and its mass is 50.0 g. Use Q = mcΔT.]
Is the value of Q calculated in part (b) positive or negative for this reaction? Suggest a reason for the sign based on your answer to part (a).
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Exothermic; the temperature increases, showing heat is released to the surroundings.
(b) Q = 4.18 × 50.0 × (27.4 – 22.0) = 1,132 J
(c) Q should be negative for an exothermic reaction, because energy is released.
Common mistakes: Incorrect use of temperature change sign in calculation; Forgetting to use the correct units; Not linking positive/negative Q to exo-/endothermic nature
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly identifies the reaction as exothermic.
B2 (a) · 1 mark · Explanation links to temperature rise and heat being released to the surroundings.
M1 (b) · 1 mark · Uses Q=mcΔT with correct substitution of values and temperature change.
A1 (b) · 1 mark · Correct final answer for Q to 3 significant figures (1,130 J or 1,132 J) with unit.
B3 (c) · 1 mark · Links sign (+/–) of Q to exo-/endothermic nature and explains.
Worked solution
a) The reaction is exothermic because the temperature increases. An exothermic reaction releases heat to the surroundings, causing the temperature to rise.
b) Q = mcΔT
m = 50.0 g, c = 4.18 J/g°C, ΔT = (27.4 – 22.0) = 5.4°C
To 3 significant figures: Q = 1,130 J
c) The calculated Q is positive as heat is gained by the solution. But for an exothermic reaction, the enthalpy change ΔH is negative, indicating heat is released by the chemicals to the surroundings.