Reactivity and Metal Extraction
The Reactivity Series
Metals can be placed in order of how readily they form positive ions. The more easily a metal loses its outer electrons, the more reactive it is. Chemists have arranged metals in a reactivity series, with potassium, sodium, lithium and calcium at the top, then magnesium, aluminium, zinc and iron in the middle, and copper, silver and gold at the bottom. Carbon and hydrogen are included as non-metal reference points — their positions determine how metals can be extracted and which metals react with dilute acids.
The order from most to least reactive is: potassium, sodium, lithium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold. The reactivity of a metal is related to how easily it forms its positive ion — metals higher in the series hold their outer electrons less strongly, so they lose them more readily.
A metal’s position in the reactivity series determines how it reacts with water and dilute acids, whether it can displace other metals, and which extraction method is used. The higher the metal, the more easily it loses electrons to form positive ions.
Reactions with Water and Dilute Acid
Very reactive metals (potassium, sodium, calcium) react vigorously with cold water to produce a metal hydroxide and hydrogen gas. For example, sodium reacts rapidly: \(2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)\). Calcium reacts more gently, producing a cloudy solution as calcium hydroxide has limited solubility.
Moderately reactive metals (magnesium, zinc, iron) react slowly or not at all with cold water but react well with dilute acids to produce a salt and hydrogen gas. For example: \(Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)\). Copper, silver and gold are below hydrogen in the reactivity series and do not react with dilute acids at all, because they do not lose their electrons readily enough.
Displacement Reactions
A displacement reaction occurs when a more reactive metal replaces a less reactive metal from a solution of its salt. The more reactive metal loses electrons more readily and ‘pushes out’ the less reactive metal, which is deposited as a solid.
A classic example is iron placed in copper sulfate solution. Iron is above copper in the reactivity series, so it displaces copper:
$$Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$$
The blue solution turns pale green (iron(II) sulfate) and a brown copper deposit forms on the iron. No reaction occurs if a less reactive metal is placed in a solution of a more reactive metal’s salt — for example, copper will not displace iron from iron sulfate solution.
You can determine a reactivity order by systematic displacement experiments. Place different metals into solutions of other metals’ salts and observe whether a displacement occurs (colour change, a solid deposited, or a temperature rise). If displacement occurs, the added metal is more reactive. By testing several combinations, you can rank the metals in order. You can also compare how vigorously metals fizz with dilute acid — faster bubbling and greater heat released indicates higher reactivity. This kind of evidence-gathering is the practical foundation of the reactivity series.
Extracting Metals from Their Ores
The method used to extract a metal depends on its position in the reactivity series. Metals below carbon in the series can be extracted by reduction with carbon. The carbon is more reactive, so it displaces the metal from its oxide. In this redox reaction, the metal oxide is reduced (loses oxygen) and the carbon is oxidised (gains oxygen). For example, extracting zinc from zinc oxide:
$$2ZnO(s) + C(s) \rightarrow 2Zn(l) + CO_2(g)$$
Metals above carbon in the reactivity series (such as aluminium, sodium and potassium) cannot be extracted by carbon because they are too reactive — carbon cannot pull the oxygen away from them. Instead, these metals must be extracted by electrolysis, which uses electrical energy and is therefore more expensive. The need to use a finite non-renewable ore and large amounts of energy makes metal extraction an important sustainability issue.
Scientists are developing biological alternatives for extracting metals from low-grade ores. Phytoextraction uses plants to absorb metal compounds through their roots; the plants are harvested and burned, and the metal is recovered from the ash. Bioleaching uses bacteria to produce acidic leachate that dissolves metal compounds from the ore; the metal is then recovered by displacement or electrolysis. These methods are slower than conventional extraction but can work on ores that would otherwise be waste, reduce energy costs, and lower the amount of toxic metals left in landfill. Deciding whether to use them involves weighing these benefits against the slow production rate (IaS4 — risk–benefit reasoning under practical constraints).
Metals below carbon in the reactivity series are extracted by reduction with carbon (a cheaper process). Metals above carbon are extracted by electrolysis (more expensive — uses large amounts of electricity). The position of carbon in the series is the key to explaining the extraction choice.