Reactivity Series and Metal Extraction: Chemistry 0620 (Cambridge O Level / IGCSE)

A materials engineer tests small offcuts of three metals (magnesium, zinc and iron) by dropping an identical-sized piece of each into separate beakers containing the same volume and concentration of dilute hydrochloric acid. Every beaker starts at $20\,°\text{C}$. The highest temperature reached in each beaker during the next two minutes is recorded: magnesium reaches $52\,°\text{C}$, zinc reaches $38\,°\text{C}$, and iron reaches $26\,°\text{C}$.

Based on this data, what is the correct order of these three metals, from most reactive to least reactive?

A chemistry teacher demonstrates reactions of Group I and Group II metals with water, using a large trough of cold water with a few drops of universal indicator added.

(a) A pea-sized piece of sodium is dropped onto the surface of the water. Describe two observations you would expect to make during this reaction. [2]

(b) Write a word equation for the reaction between sodium and water. [1]

(c) A similarly-sized piece of calcium is then dropped into a fresh trough of the same water. The reaction is noticeably less vigorous than with sodium. Explain this difference in terms of the reactivity series. [1]

(d) The teacher then heats a coil of magnesium ribbon and passes steam over it. State one observation you would expect to make, and write a word equation for this reaction. [2]

A photography-restoration workshop produces a waste solution of silver nitrate after developing archive film negatives. To recover the silver rather than discard it, a technician suspends a coil of iron wire in a beaker of the waste silver nitrate solution overnight.

(a) Describe two observations you would expect to make after the iron wire has been left in the solution overnight. [2]

(b) Explain, in terms of the tendency of iron and silver to form positive ions, why iron displaces silver from silver nitrate solution. [2]

(c) Write the ionic equation, including state symbols, for the reaction taking place. [2]

An aluminium smelting plant extracts aluminium from purified aluminium oxide (obtained from bauxite) by electrolysis. The aluminium oxide is dissolved in molten cryolite and electrolysed using carbon electrodes, at a temperature far below the melting point of pure aluminium oxide.

(a) State the role of cryolite in this process. [1]

(b) Write ionic half-equations for the reactions at: (i) the cathode [1] (ii) the anode [1]

(c) Explain why the carbon anodes in this cell need to be replaced at regular intervals, while the cathode does not need to be replaced in the same way. [2]

(d) Plant records show that, at a steady rate of operation, $300$ tonnes of anode carbon are consumed for every $900$ tonnes of aluminium produced. Assuming the plant maintains this same ratio, calculate the mass of anode carbon that would be consumed while producing $1500$ tonnes of aluminium. [2]

Four hypothetical metals, J, K, L and M, have positions in the reactivity series relative to carbon given by

$J > K > \text{carbon} > L > M$

(with J the most reactive and M the least reactive of the five).

Which statement about extracting these metals from their ores is correct?

Iron is extracted from iron(III) oxide ore in a blast furnace. The furnace is charged from the top with iron ore, coke and limestone, and a blast of hot air is blown in near the bottom.

(a) Near the bottom of the furnace, coke reacts with the hot air blast, releasing heat. Write a word equation for this reaction. [1]

(b) Further up the furnace, the hot gas formed in (a) reacts with more unburnt coke to form a different gas. Write a word equation for this reaction. [1]

(c) This second gas then reduces the iron(III) oxide to iron. Write a word equation for this reaction, and state the name given to a reaction in which oxygen is removed from a compound. [2]

(d) Explain the purpose of the limestone added to the furnace, including what happens to it on heating and what solid by-product it forms. [2]

Aluminium is above zinc and iron in the reactivity series. Yet when a clean strip of aluminium foil is placed in cold water, or in dilute hydrochloric acid, it appears to react barely at all, far more slowly than its position in the reactivity series would suggest. If the same strip is first rubbed with steel wool, or coated with a mercury salt, it then reacts rapidly with the acid.

Which statement correctly explains this apparent unreactivity of aluminium?

A student investigates the reactivity of four metals by adding an identical-sized rolled strip of each metal (magnesium, zinc, iron and copper) to separate test tubes, each containing the same volume and concentration of dilute hydrochloric acid. She counts the number of gas bubbles released from each metal strip in the first 30 seconds: magnesium 46 bubbles, zinc 19 bubbles, iron 7 bubbles, copper 0 bubbles.

(a) Name the gas produced in these reactions, and describe a test that could be used to confirm its identity. [2]

(b) Using the bubble-count data, write the order of these four metals from most to least reactive. [1]

(c) Write a word equation for the reaction between magnesium and dilute hydrochloric acid. [1]

(d) Explain, in terms of the reactivity series, why no bubbles were observed in the copper test tube. [1]

(e) Suggest why the student used metal strips of the same surface area in each test tube. [1]

A demonstration mixes powdered aluminium with powdered iron(III) oxide and ignites the mixture. The reaction proceeds without any external heat source, releasing an enormous amount of energy and producing molten iron together with solid aluminium oxide. This reaction (the "thermite" reaction) is sometimes used industrially to weld railway tracks together.

(a) Using the reactivity series, explain why aluminium is able to displace iron from iron(III) oxide in this reaction. [2]

(b) Write the balanced symbol equation, including state symbols, for this reaction. [2]

(c) Suggest why the fact that the iron produced is molten makes this reaction useful for welding two sections of railway track together. [2]

Heating iron(III) oxide with carbon reduces it to iron, but no amount of heating sodium oxide with carbon will produce sodium metal.

Which statement correctly explains this difference, in terms of the tendency of each species to form positive ions?