Metals and Reactivity
The Reactivity Series
Different metals react with different levels of vigour with water, oxygen and acids. Chemists arrange metals in order of this reactivity to produce the reactivity series. From most to least reactive, the key metals to know are:
Potassium (K) → Sodium (Na) → Calcium (Ca) → Magnesium (Mg) → Aluminium (Al) → Carbon (C) → Zinc (Zn) → Iron (Fe) → Hydrogen (H) → Copper (Cu) → Silver (Ag) → Gold (Au)
Carbon and hydrogen are not metals, but they are included as reference points because they are important in extraction reactions. Metals above hydrogen in the series react with dilute acids to produce hydrogen gas; metals below hydrogen do not. The most reactive metals (potassium, sodium) react explosively with cold water, while moderately reactive metals like magnesium react slowly with cold water but vigorously with steam.
Reactivity series (most to least): K, Na, Ca, Mg, Al, C, Zn, Fe, H, Cu, Ag, Au. Metals above carbon need electrolysis to extract; metals below carbon can be extracted by reduction with carbon. Metals above hydrogen react with acids to give H₂ gas.
Displacement Reactions
A more reactive metal will displace a less reactive metal from a solution of one of its compounds. This is a practical way to confirm relative reactivity. For example, placing iron into copper sulfate solution displaces copper, because iron is higher in the reactivity series:
$$Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$$
Observations: the iron becomes coated with a reddish-brown layer of copper, and the blue colour of the solution fades as copper(II) ions are removed and replaced with paler iron(II) ions. Displacement reactions are also redox reactions — the more reactive metal is oxidised (loses electrons) and the metal ion in solution is reduced (gains electrons).
In the iron/copper displacement reaction, iron atoms each lose two electrons: \(Fe \rightarrow Fe^{2+} + 2e^-\) — this is oxidation. The copper ions each gain two electrons: \(Cu^{2+} + 2e^- \rightarrow Cu\) — this is reduction. Remember: OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain.
You can write ionic half-equations for each electrode process separately, then combine them to give the overall ionic equation.
Extracting Metals from Their Ores
Metals are found in the Earth's crust, often combined with other elements in ores. The method of extraction depends on the metal's position in the reactivity series. Metals that are less reactive than carbon can be extracted by heating their oxide with carbon — the carbon removes the oxygen (reduction):
$$2Fe_2O_3(s) + 3C(s) \rightarrow 4Fe(l) + 3CO_2(g)$$
This reaction occurs in a blast furnace. Iron ore (haematite, \(Fe_2O_3\)), coke (carbon) and limestone (\(CaCO_3\)) are added at the top. Hot air is blasted in at the bottom. The coke burns to form carbon monoxide, which reduces the iron oxide to molten iron that trickles to the bottom and is tapped off. Limestone removes acidic impurities (silica) as molten slag.
Metals below carbon in the reactivity series (e.g. iron, copper, zinc) are extracted by reduction with carbon — cheaper but limited. Metals above carbon (e.g. aluminium, sodium) must be extracted by electrolysis — more expensive because it uses large amounts of electricity.
Biological Methods of Metal Extraction
Traditional mining is expensive and environmentally damaging. Low-grade ores — where metal concentrations are too low to justify conventional extraction — can instead be processed using biological methods. Two important techniques are phytomining and bioleaching.
In phytomining, specific plant species (called hyperaccumulators) are grown on soil containing low-grade metal ore. The plants draw metal ions up through their roots and store them in leaves and stems. When the crop is harvested and incinerated, the ash contains a concentrated form of the metal compound, which is then processed to obtain the metal. The technique is slow but has a much lower environmental footprint than open-cast mining.
In bioleaching, certain bacteria (such as Acidithiobacillus ferrooxidans) are used to break down metal sulfide ores. The bacteria oxidise the sulfide minerals, releasing metal ions into an acidic solution called a leachate. The metal is then recovered from the leachate by displacement — scrap iron is added to displace copper, for example, because iron is higher in the reactivity series: \(Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)\).
Bioleaching produces acidic waste water that can pollute nearby rivers and streams — a significant environmental concern that requires careful management. However, compared with conventional smelting, it avoids the production of sulfur dioxide air pollution.
Phytomining: grow hyperaccumulator plants on low-grade ore → harvest and burn → recover metal from ash. Bioleaching: bacteria break down ore → metal ions in leachate → recover metal by displacement with scrap iron. Both are lower-impact alternatives to traditional mining.
Recycling Metals
Recycling metals saves energy — often a large fraction of the energy required compared with extracting from ore — and conserves finite ore reserves. It also reduces waste going to landfill and cuts the environmental damage from mining operations. However, sorting and collection of scrap metal has its own costs, and contamination can reduce the quality of recycled metal. Despite this, recycling aluminium, for example, uses a small fraction of the energy that electrolysis of aluminium ore demands.