Physical Chemistry
8 marks · NormalA student investigates the displacement reaction between zinc powder and copper(II) sulfate solution. She uses calorimetry and also measures how fast the blue colour disappears when she changes the reactant concentrations.
Describe how you would carry out a calorimetry experiment to determine the enthalpy change for the reaction between zinc and copper(II) sulfate solution. Include the apparatus needed and key steps in your answer.
In one experiment, the student adds 1.20 g of zinc to 50 cm³ of 0.200 mol/dm³ copper(II) sulfate solution in a polystyrene cup at 18.0°C. The maximum temperature reached is 27.5°C.
Calculate the final concentration of copper(II) sulfate after the reaction, assuming all of it is used up. Give your answer in mol/dm³.
State one way the rate of this reaction can be measured during the experiment.
The student repeats the experiment with 25 cm³ of 0.200 mol/dm³ copper(II) sulfate and 25 cm³ of distilled water (total volume still 50 cm³), keeping the amount of zinc the same.
Calculate the new concentration of copper(II) sulfate before mixing with zinc.
Predict and explain how the lower concentration affects the rate of reaction. Use collision theory in your answer.
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) — · must show polystyrene cup or calorimeter, measuring cylinder for solutions, temperature reading before and after, adding zinc last, stirring, recording maximum temperature
(b(i)) 0 mol/dm³
(b(ii)) Measure the change in temperature over time, or measure the time taken for the blue colour to disappear, or measure the change in mass if a gas is given off · must show rate can be measured from colour change or temperature change
(c(i)) 0.100 mol/dm³
(c(ii)) The rate will be slower because the concentration of copper(II) sulfate is lower, meaning fewer particles in a given volume, and so there will be fewer collisions per second between reactant particles. · must show fewer collisions per second, collision theory, rate slower
Common mistakes: Uses mass instead of concentration; Describes measuring temperature change as a rate measurement; Ignores dilution; Misses step in calorimetry setup
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Describes use of polystyrene cup or suitable calorimeter with solutions.
M2 (a) · 1 mark · Mentions recording initial and final temperature, adding zinc last, and stirring.
C1 (a) · 1 mark · Procedure outlined is logical and complete.
A1 (b(i)) · 2 marks · Correct calculation of final copper(II) sulfate concentration as 0 mol/dm³ (all used up).
B1 (b(ii)) · 1 mark · Any valid measurement of rate is stated.
A2 (c(i)) · 1 mark · Correct calculation of concentration after dilution (0.100 mol/dm³).
E1 (c(ii)) · 1 mark · Explanation uses collision theory: rate is lower because fewer collisions per second.
Worked solution
a) Fill a polystyrene cup with a measured volume of copper(II) sulfate solution. Measure and record the initial temperature. Weigh out the zinc and quickly add it to the solution in the cup. Stir the mixture constantly and record the highest temperature reached. The apparatus includes: a polystyrene cup (acts as an insulator), a thermometer, a measuring cylinder, a balance, and a stirring rod.
b(i) If all copper(II) sulfate reacts, there is none left. Final concentration = 0 mol/dm³.
b(ii) The rate can be measured by timing how long the blue colour takes to disappear (colour change), or by measuring temperature changes over time.
c(i) Diluting 25 cm³ of 0.200 mol/dm³ solution to 50 cm³: Using c₁V₁ = c₂V₂: (0.200 mol/dm³) x 25 cm³ = c₂ x 50 cm³; c₂ = (0.200 x 25) / 50 = 0.100 mol/dm³.
c(ii) The rate will be slower because a reduced concentration of copper(II) sulfate means fewer Cu²⁺ ions per volume, resulting in fewer collisions with zinc atoms per second (collision theory).