A-LevelChemistryReaction kinetics9701/51 · Oct/Nov 2021 · Q216 Marks

The rate of reaction between calcium carbonate, CaCO3, and hydrochloric acid, HCl, can be followed by collecting and measuring the volume of carbon dioxide produced at 30-second intervals. The equation for the reaction is: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(I)

(a)A student plans to collect the carbon dioxide by displacement of water. Draw a labelled diagram of the apparatus that could be used to carry out this experiment. The apparatus should allow the accurate recording of the volume of carbon dioxide produced.[3]
(b)(i)The student carried out the investigation using an excess of calcium carbonate with dilute hydrochloric acid. The student stopped timing after 330 seconds had passed. The volume of carbon dioxide produced was 93 cm³. Vfinal is the final volume of carbon dioxide collected at 330 seconds. V is the volume of carbon dioxide collected at each interval of time, t. Vfinal−V is proportional to the concentration of hydrochloric acid at a given time. | time, t/s | volume of carbon dioxide collected/cm³ | Vfinal−V/cm³ | |---|---|---| | 0 | 0 | | | 30 | 22 | | | 60 | 37 | | | 90 | 50 | | | 120 | 61 | | | 150 | 68 | | | 180 | 75 | | | 210 | 78 | | | 240 | 79 | | | 270 | 87 | | | 300 | 90 | | | 330 | 93 | | Complete the table.[1]
(b)(ii)Plot a graph of Vfinal−V₁ (y-axis) against time, t (x-axis). Use a cross (x) to plot each data point. Draw a curved line of best fit. [Grid for graph][2]
(b)(iii)Circle the point which you consider to be most anomalous.[1]
(b)(iv)Suggest one reason for this anomalous point.[1]
(b)(v)Draw construction lines on the graph to calculate two consecutive half-lives for this reaction. Use these half-lives to determine the mean half-life, t₁. first half-life = ..................................... s second half-life = ..................................... s mean half-life, t₁ = ..................................... s[2]
(b)(vi)The rate constant, k, for this reaction can be calculated using the following expression. t₁ = 0.693 / k Calculate k. If you did not obtain a value for t₁ in (v) you may use 95 seconds. This is not the correct answer. k = ..................................... s⁻¹[1]
(c)State how an increase in temperature would affect the value of k for this reaction. Explain your answer.[1]
(d)(i)Calcium carbonate is a component of antacid tablets. An alternative method of studying the rate of reaction between calcium carbonate and hydrochloric acid is: • Place one antacid tablet into a beaker. • Add 50 cm³, an excess, of 2.0 mol dm⁻³ hydrochloric acid and start the stop-clock immediately. • Record the time taken for the fizzing to stop. An antacid tablet typically contains 1.0 g of CaCO3. Complete columns A, B and C in the table to show four more concentrations of excess HCl(aq) which would allow this method to be carried out. Each sample of HCl(aq) must be made by dilution of 2.0 mol dm⁻³ HCl. [Ar: Ca, 40.1; C, 12.0; O, 16.0] | A volume of 2.0 mol dm⁻³ HCl /cm³ | B volume of distilled water /cm³ | C concentration of HCl /mol dm⁻³ | D time taken for fizzing to stop /s | |---|---|---|---| | 50.0 | 0.0 | 2.0 | | | | | | | | | | | | | | | | | | | | | |[2]
(d)(ii)Identify the dependent variable in this investigation.[1]
(d)(iii)Suggest how the reliability of the results could be improved.[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

[3] M1 Addition of solid and acid must not lead to initial escape of gas from sealed system or produce excess volume measurement leading to a false volume reading e.g. Adding acid to solid already in the flask or vice versa. M2 Allow any leakproof apparatus capable of delivering a sample of gas into one collection vessel over water. M3 A vertical inverted measuring cylinder or burette filled with water positioned to collect the gas (via delivery tube).
[1] Vfinal - Vt / cm³ 93, 71, 56, 43, 32, 25, 18, 15, 14, 6, 3, 0
[2] M1 Selects and labels appropriate linear scales for axes AND All points plotted correctly AND covering at least half of the grid for each axis. M2 Smooth curve of best fit line drawn going through most points.
[1] The point at 240 s.
[1] The volume of gas may have been read before 240.
[2] M1 Suitable construction lines shown on the graph. M2: Two half-lives read correctly from graph AND mean half-life calculated using values read correctly from graph. (Likely to be approx. 78 s)
[1] 0.693 / average t1/2 calculated in 2(b)(v)
[1] The value of k would increase AND as t½ would decrease.
[2] M1 Table complete with four different sets of values that would produce different concentrations, total volume = 50 cm³ AND All volumes of 2.0 mol dm-³ acid must be 10 cm³ or greater (to ensure an excess). M2 All concentrations correctly calculated.
[1] time (taken for fizzing to stop)
[1] The experiment should be repeated to identify / eliminate anomalies.