Physical Chemistry
7 marks · EasyA student carries out an experiment to investigate the effect of surface area on the rate of reaction between excess powdered calcium carbonate and 50 cm³ of hydrochloric acid at 2.00 mol/dm³. The loss in mass, due to carbon dioxide being produced, is measured every 30 seconds. The experiment uses a conical flask as shown.
Describe how you could carry out this experiment to measure the rate of carbon dioxide production.
In a second experiment, the student repeats the reaction using the same amount of powdered calcium carbonate and acid, but keeps the flask sealed with a stopper and a delivery tube connected to an inverted measuring cylinder filled with water, to collect the gas formed.
The volume of carbon dioxide produced in the first minute is 45 cm³.
Calculate the average rate of reaction (in cm³/s) for the first minute.
State one way in which the method used in this experiment allows the rate to be measured more accurately than using just a balance.
The student then investigates the following gas-phase reversible reaction in a sealed container:
Predict and explain how increasing the pressure will affect the position of equilibrium of this reaction.
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 accurate mass measurement using balance, cotton wool to prevent loss of solid, regular mass readings over time, powdered CaCO3 and excess HCl
(b(i)) 0.75 cm³/s · must show correct calculation (45/60)
(b(ii)) All gas produced is measured directly; loss by escape is prevented. · must show collects complete gas produced, avoids over/under-estimation from splashing/evaporation
(c) Increasing pressure shifts equilibrium to the right (towards SO₃), as this side has fewer gas molecules. The system reduces pressure by favouring the forward reaction. · must show states equilibrium shifts to SO3/right, explains fewer gas molecules on product side, links shift to counteracting higher pressure
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Describes placing conical flask with powder and acid on a balance with cotton wool, measuring mass loss at regular intervals.
M2 (a) · 1 mark · Mentions importance of regular timing or use of excess CaCO3 to ensure accurate rate measurement.
A1 (b(i)) · 1 mark · Correct division of volume by time: 45 cm³ ÷ 60 s = 0.75 cm³/s.
B1 (b(ii)) · 1 mark · States that all gas is collected/measured accurately or prevented from escaping.
M3 (c) · 1 mark · States equilibrium shifts to right (SO₃ side).
E1 (c) · 1 mark · Recognises right side has fewer gas molecules (3 to 2).
E2 (c) · 1 mark · Explains shift as system counteracts the pressure increase.
Worked solution
a The experiment is carried out by placing powdered calcium carbonate and excess hydrochloric acid in a conical flask. Place the flask (with a loose cotton wool plug) on a balance and set it to zero. Start the reaction by adding the acid, then immediately record the mass at regular intervals (e.g. every 30 seconds). The decrease in mass corresponds to carbon dioxide loss. Record until the reaction stops.
b (i) The average rate = volume of CO₂ / time = 45 cm³ / 60 s = 0.75 cm³/s.
b (ii) This method ensures that all the gas produced is collected and measured accurately, as none escapes, reducing errors compared with measuring mass loss on a balance (where splashing or evaporation might affect readings).
c In the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), increasing pressure shifts the equilibrium to the right (towards SO₃). This is because there are 3 moles of gas on the left and only 2 on the right, so increasing pressure favours the side with fewer gas molecules, reducing the overall pressure. This means more SO₃ will be formed at equilibrium.