This practical is being planned for a company that recycles aluminium drinks cans. The aim is to prepare aluminium sulfate solution. Fig. 1 shows two possib...
Question 1 Report
This practical is being planned for a company that recycles aluminium drinks cans. The aim is to prepare aluminium sulfate solution. Fig. 1 shows two possible starting materials: cleaned aluminium strips and aluminium oxide powder. In a preliminary test, both are placed in separate beakers containing equal volumes of warm dilute sulfuric acid. The metal produces only a few bubbles at first, whereas the oxide powder reacts steadily and leaves a colourless solution.
| Starting material | Time until no solid remains / min | Observation |
|---|---|---|
| Aluminium strips | more than 20 | few bubbles |
| Aluminium oxide powder | 6 | no gas seen |
(a) Describe the difference in reaction rate shown in Table 1. [2]
(b) Explain why aluminium strips may react slowly when first placed in acid. [3]
(c) Use formulae to write a balanced equation for aluminium oxide reacting with sulfuric acid. [3]
(d) Give two reasons why aluminium oxide is a more suitable material than aluminium metal for this preparation. [3]
(e) Describe the method used to make dry aluminium sulfate crystals from the colourless solution. [4]
(f) Give one use of aluminium sulfate outside the laboratory. [1]
Answer Details
(a) Aluminium oxide reacts faster: all of its solid has disappeared after 6 minutes, whereas aluminium strips still remain after more than 20 minutes. [2]
(b) Aluminium metal has a protective layer of aluminium oxide. The acid must first react with or remove this layer, so initially little acid can contact the aluminium metal itself. This makes the initial reaction slow. [3]
(c) Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O [3]
(d) Aluminium oxide is more suitable because it reacts more quickly, its powder has a larger surface area, and it produces no hydrogen gas. Avoiding hydrogen makes the preparation safer because hydrogen is flammable. The oxide also gives the salt solution directly. Any three of these points gain credit. [3]
(e) Gently heat the colourless solution to evaporate some water and concentrate it. Stop heating before it becomes dry. Leave the concentrated solution to cool so crystals form, then filter the crystals and dry them using filter paper. [4]
(f) One use is treating drinking water to remove suspended particles. It is also used in paper manufacture or dyeing fabrics. [1]
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