Predicting Chemical Reactions — Reactivity
The Reactivity Series
Metals can be arranged in order of their reactivity series. The most reactive metals (such as potassium and sodium) are at the top, and the least reactive (such as gold and platinum) are at the bottom. The reactivity of a metal depends on how easily it forms a positive ion by losing its outer-shell electrons. Metals higher in the series lose electrons more readily because their outer electrons are further from the nucleus and more shielded by inner electron shells.
The order from most to least reactive is: potassium, sodium, lithium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold, platinum. Carbon and hydrogen are included as reference points even though they are not metals — carbon’s position determines how metals can be extracted, and hydrogen’s position determines which metals react with dilute acids.
A metal’s reactivity depends on how easily it loses its outer electrons to form a positive ion. More reactive metals react more vigorously with water and dilute acids. The tendency to form positive ions increases up the reactivity series.
Reactions of Metals
You can deduce the reactivity of a metal from its reactions with water and dilute acids. Very reactive metals (K, Na, Li, Ca) react vigorously with cold water to produce a metal hydroxide and hydrogen gas. For example, sodium reacts with water: \(2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)\). Calcium reacts more gently, producing a cloudy solution as calcium hydroxide is only slightly soluble.
Moderately reactive metals (Mg, Zn, Fe) react very slowly or not at all with cold water but react well with dilute acids to produce a salt and hydrogen gas. For example, magnesium reacts with hydrochloric acid: \(Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)\). Unreactive metals (Cu, Ag, Au) do not react with water or dilute acids because they do not lose their electrons easily enough.
Displacement Reactions
A displacement reaction occurs when a more reactive metal replaces a less reactive metal from a compound. For example, if you place iron in copper sulfate solution, the iron displaces the copper:
$$Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$$
The iron is more reactive than copper, so it displaces it. The blue copper sulfate solution fades to the pale green of iron(II) sulfate, and a brown coating of copper appears on the iron. A less reactive metal cannot displace a more reactive one — copper in iron sulfate solution would show no reaction. This principle can be applied to any pair of metals: if the added metal is higher in the reactivity series than the metal in the salt, displacement occurs.
You can work out a reactivity order by doing a series of displacement experiments. Place different metals into solutions of other metal salts. If a displacement occurs (colour change, solid deposited, temperature rise), the added metal is more reactive. If nothing happens, the added metal is less reactive. By testing several combinations systematically, you can rank the metals in order of reactivity. You can also compare how vigorously different metals react with dilute acid — faster fizzing and more heat indicates higher reactivity.
Extracting Metals
How a metal is extracted from its ore depends on its position in the reactivity series. Metals below carbon in the series (such as iron, zinc and copper) can be extracted by reduction with carbon. The carbon is more reactive, so it displaces the metal from its oxide and takes the oxygen. For example, extracting iron from iron oxide:
$$2Fe_2O_3 + 3C \rightarrow 4Fe + 3CO_2$$
In this reaction, the iron oxide is reduced (loses oxygen) and the carbon is oxidised (gains oxygen). This is why carbon reduction is also a redox reaction. Metals above carbon in the series (such as aluminium, sodium and potassium) are too reactive to be extracted by carbon. They must be extracted by electrolysis, which is more expensive because it uses large amounts of electricity. For example, aluminium is extracted from aluminium oxide (bauxite) by electrolysis at around 960°C.
Biological methods offer alternatives to traditional extraction, particularly for low-grade ores where conventional mining is not economically viable. Phytoextraction uses plants to absorb metal compounds from the soil through their roots; the plants are harvested and burned, and the metal is extracted from the ash. Bioleaching uses bacteria to produce acidic solutions that dissolve metal compounds from the ore; the metal can then be extracted from the solution by displacement or electrolysis. These methods are slower but have lower environmental impact and can extract metals from ores that would otherwise be waste.
Metals below carbon in the reactivity series are extracted by reduction with carbon (heating the oxide with carbon). Metals above carbon must be extracted by electrolysis. The choice of method depends entirely on the metal’s reactivity relative to carbon.