Chemistry 0620 | Paper 5 Mock 01 | Practical Test

This question ranks five metals by reactivity from how vigorously they react with cold water and with dilute acid. The principle is that a more reactive metal reacts faster and more vigorously; the fair test keeps mass, surface preparation, volume and time the same so only the metal changes.

(a) Add the calcium safely by picking it up with tongs, not fingers, and dropping it gently into the water while wearing eye protection [1]. The observations in that tube are steady bubbling, with the piece sinking then rising as gas bubbles cling to it [1], and the mixture turning cloudy or milky white while the tube warms up [1].

(b) In cold water the other four show little or nothing [4]: magnesium, no change or a very few small bubbles on the ribbon; zinc, no change; iron, no change, or a trace of rust after a long time; copper, no change.

(c) In dilute acid the differences appear [4]: magnesium, rapid bubbling, the tube warms and the metal disappears; zinc, steady but slower bubbling; iron, slow bubbling and the solution slowly turns pale green; copper, no bubbles and no change.

(d) Combining both sets of results, the order of reactivity most reactive first is calcium, magnesium, zinc, iron, copper [2] (one mark for four of the five in the correct order, marked consequentially on your own observations).

(e) The gas given off in the acid tubes is hydrogen; identify it by collecting it in an inverted test tube held over the mouth of the boiling tube [1], then holding a lighted splint at the mouth, which makes the gas burn with a squeaky pop [1].

(f) The two hardest to tell apart are zinc and iron, which both bubble slowly in acid and show nothing in cold water [1]. To separate them, pass steam over each metal in turn in the same apparatus, using the same mass of metal and the same heating [1]; time how long each takes before the gas at the delivery tube will light, or compare how fast the bubbles come [1]; the metal reacting sooner or faster is the more reactive, which is the zinc [1]. (Timing how long each takes to give a fixed volume of gas in the acid is also accepted.)

(g) The comparison was kept fair in any three ways [3]: the same volume of water and the same volume of acid in every tube; the same concentration of acid and the same mass of each metal; and every metal cleaned the same way and cut to a similar surface area, with all tubes starting at the same temperature and watched for the same time.

This measures the fraction of air used up when iron rusts. Rusting is the slow reaction of iron with oxygen and water, \( 4\,\mathrm{Fe} + 3\,\mathrm{O_2} + 2x\,\mathrm{H_2O} \rightarrow 2\,\mathrm{Fe_2O_3}\!\cdot\! x\mathrm{H_2O} \). As the damp iron wool takes oxygen from the trapped air, the pressure falls and water rises up the tube; the rise measures the oxygen removed.

(a) Record the starting length of the trapped air column, read from the ruler to the nearest millimetre, for example 120 mm [1], with the unit written [1].

(b) Enter a water height for every one of the six days [1], with the values increasing from day to day [1], the last two or three values equal [1], every value to the nearest millimetre [1], read at eye level against the ruler [1]. A worked set:

(c) When the level stops rising, the final air column \( = \) starting length \( - \) final rise, for example \( 120 - 24 = 96\,\mathrm{mm} \) [1].

(d) Percentage of air used \( = \dfrac{\text{rise in water level}}{\text{starting air column}} \times 100 \) [1]; substituting the figures \( = \dfrac{24}{120} \times 100 \) [1] \( = \) about 20 % [1]. Accept 15 % to 25 %, consequential on your own readings; the true value is about one-fifth because oxygen is roughly 21 % of air.

(e) The gas used up is oxygen [1].

(f) The iron wool has rusted, turning orange-brown and crumbly [1].

(g) A control tube is set up the same way with the same volume of trapped air [1], but with dry iron wool, or left empty, so that no rusting can take place [1]; clamp it in the same beaker at the same temperature and measure it each day, expecting no rise [1]. This shows the rise in the first tube is caused by the rusting and nothing else.

(h) The temperature must be kept constant because a rise in temperature makes the trapped air expand and a fall makes it contract [1]; either change would move the water level and give a change not caused by the rusting [1].

(i) The water may rise less than expected for any one reason such as: air leaked in past the mouth of the tube, the iron wool dried out, the wool was greasy so part did not rust, or it was not left long enough [1].

This question tests recognition of standard gas-collection apparatus and how to assemble it so no gas leaks before it is measured. The reaction of a carbonate with hydrochloric acid, \( \text{carbonate} + \text{HCl} \rightarrow \text{salt} + \text{H}_2\text{O} + \text{CO}_2 \), gives off carbon dioxide, and the rate is found from how the gas volume grows with time.

(a) Naming the apparatus [2]. Piece A is the wide-based vessel that holds the reactants, a conical flask [1]. Piece B, the graduated barrel with a moving plunger, is a gas syringe, which measures the volume of gas collected [1].

(b) Completing the assembly gas-tight [2]. The join must be sealed at both ends:

1. Push a bung carrying the delivery tube firmly into the neck of the conical flask so no gas escapes at the top [1].
2. Connect the delivery tube to the nozzle of the gas syringe, clamp the syringe horizontally, and check the plunger is pushed fully in and reads zero before the solid is added [1].

Starting from zero with an airtight seal matters because any gas lost before or during collection makes the measured volume too low.

(c) What to record and how often [2]. Record two quantities: the volume of gas in the syringe and the time on the stop-clock [1]. Take the volume at regular intervals, for example every 30 s (any stated regular interval from 10 s to 60 s is acceptable), continuing until no more gas is collected [1]. Fixed intervals are needed so the readings can be plotted as a smooth volume-time graph from which the rate is read.

Exam takeaway: for any gas-collection rate experiment, name the two variables as volume and time, and always state a regular time interval rather than just saying you would time it.