11.Obtaining and Using Metals GCSE EDEXCEL CHEMISTRY

Specification

The reactivity series of metals

The reactivity series of metals is a chart showing metals in order of decreasing reactivity. In general, the more reactive a metal is:

• the more vigorous its reactions are
• the more easily it loses electrons in reactions to form positive ions (cations)

The table summarises some reactions of metals in the reactivity series. Hydrogen is shown for comparison.

Reactions of metals with water

When a metal reacts with water, a metal hydroxide and hydrogen are formed

Metal + water → metal hydroxide + hydrogen

For example, sodium reacts rapidly with cold water, melting into a ball, and 'fizzing' about the surface:

sodium + water → sodium hydroxide + hydrogen

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

In general, the more reactive the metal, the more rapid the reaction is.

Reactions with steam

Metals that react slowly with cold water can react quickly with steam. In these reactions a metal oxide and hydrogen are produced.

Metal + steam → metal oxide + hydrogen

For example, magnesium reacts slowly with cold water. However, if steam is passed over hot magnesium, a vigorous reaction occurs:

Magnesium + steam → magnesium oxide + hydrogen

Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Question

State the difference between the products formed when calcium reacts with cold water and when it reacts with steam.

Calcium hydroxide forms when it reacts with water, but calcium oxide forms when it reacts with steam.

Reactions of metals with dilute acids

When a metal reacts with a dilute acid, a salt and hydrogen are formed.

Metal + acid → salt + hydrogen

For example, magnesium reacts rapidly with dilute hydrochloric acid:

Magnesium + hydrochloric acid → magnesium chloride + hydrogen

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

The more reactive the metal, the more rapid the reaction is. A metal below hydrogen in the reactivity series will not react with dilute acids.

Question

Platinum is placed below gold in the reactivity series. Predict its reaction with dilute acids and explain your answer.

Platinum will not react with dilute acids. Metals below hydrogen in the reactivity series do not react with dilute acids, and both gold and platinum are placed below hydrogen.

Hydrogen is always given off when a metal reacts with water, steam or a dilute acid.

n the reactions of metals with water, steam and acids, the metals lose electrons and form cations. The metal is oxidised and the water is reduced.

A metal's relative tendency to form cations and its resistance to oxidation are both related to its position in the reactivity series. In general:

• the higher up a metal, the greater the tendency to form cations
• the lower down a metal, the greater its resistance to oxidation

Metals and displacement reactions

Displacement in solutions

A more reactive metal can displace a less reactive metal from its compounds. For example, magnesium is more reactive than copper. It displaces copper from copper sulfate solution:

magnesium + copper sulfate → magnesium sulfate + copper

Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)

In this displacement reaction:

• magnesium becomes coated with copper
• the blue colour of the solution fades as blue copper sulfate solution is replaced by colourless magnesium sulfate solution
  

  Determining a reactivity series

  A reactivity series can be deduced by carrying out several displacement reactions. A piece of metal is dipped into a salt solution. Different combinations of metal and salt solution are tested. The table shows the results of one of these investigations.

	Magnesium sulfate solution	Copper sulfate solution	Iron sulfate solution	Reactions
Magnesium	Not done	Brown coating	Black coating	2
Copper	No visible reaction	Not done	No visible reaction	0
Iron	No visible reaction	Brown coating	Not done	1

Question

Use the results in the table to deduce an order of reactivity, starting with the most reactive metal.

The order of reactivity is: magnesium > iron > lead. This is because magnesium could displace lead and iron, iron could only displace lead, but lead could not displace magnesium or iron.

Question

Explain why three combinations of metal and salt solution were not done in the investigation.

A metal cannot displace itself from a solution of one of its salts. There would be no reaction, so these combinations were not done.

Displacement reactions as redox reactions - Higher

A balanced equation for the reaction between magnesium and copper sulfate solution can be written in terms of the ions involved:

Mg(s) + Cu²⁺(aq) + SO₄^2-(aq) → Mg²⁺(aq) + SO₄^2-(aq) + Cu(s)

Sulfate ions, SO₄^2-, appear on both sides of the equation. They do not take part in the reaction and are called spectator ions. The equation can be rewritten without them:

Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)

This equation is an example of a balanced ionic equation. It can be split into two half equations:

Mg(s) → Mg²⁺(aq) + 2e^- (oxidation)

Cu²⁺(aq) + 2e^- → Cu(s) (reduction)

Notice that:

• magnesium atoms lose electrons - they are oxidised
• copper ions gain electrons - they are reduced

Reduction and oxidation happen at the same time, so the reactions are called redox reactions.

Displacement reactions are just one example of redox reactions. Electrolysis reactions are also redox reactions.

Note that the reaction of metals with acids can also be described as a displacement reaction or a redox reaction. Only metals above hydrogen in the reactivity series will react and displace hydrogen from acids.

For example:

Magnesium + sulfuric acid → zinc sulfate + hydrogen

Written with the ions involved:

Zn(s) + 2H⁺(aq) + SO₄^2-(aq) → Zn²⁺(aq) + SO₄^2-(aq) + H₂(g)

Removing the spectator ions this becomes an ionic equation:

Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)

The half equations are:

Zn(s) → Zn²⁺(aq) + 2e^-

2H⁺(aq) + 2e^- → H₂(g) 

GCSE EDEXCEL Biological methods of metal extraction - Higher

Biological methods of metal extraction - Higher

Some alternative methods to extract metals use living organisms. These have advantages and disadvantages compared to the usual extraction methods.

Phytoextraction

Plants absorb mineral ions through their roots. Phytoextraction makes use of this to extract metals:

1. plants are grown on a low-grade ore that contains lower amounts of metal
2. the plants absorb metal ions through their roots and concentrate these ions in their cells
3. the plants are harvested and burnt
4. the ash left behind contains a higher concentration of the metal than the original ore
5. the ash is processed to obtain the metal

Phytoextraction is slow, but it:

• reduces the need to obtain new ore by mining
• conserves limited supplies of more valuable ores with higher metal content

Question

Suggest reasons why phytoextraction decreases the damage done to the environment.

Mining for metal ores involves quarries, which are large holes in the ground. These create noise, dust and traffic. They also destroy natural habitats. Phytoextraction reduces the need for mining, so reduces this damage.

Question

Give one disadvantage of phytoextraction.

It is a slow process.

Bioleaching

Certain bacteria can break down low-grade ores to produce an acidic solution containing metal ions. The solution is called a leachate and the process is called bioleaching.

Bioleaching can be used to extract copper metal. It does not need high temperatures, but it produces toxic substances, including sulfuric acid, which damage the environment.

Using scrap iron

Iron is more reactive than copper. It can displace copper from the leachate. For example:

Iron + copper sulfate → iron(II) sulfate + copper

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Since iron is cheaper than copper, the use of scrap iron is a cost-effective way to produce copper from the leachate.

GCSE EDEXCEL Life-cycle assessments

Life-cycle assessments

A life-cycle assessment or LCA is a 'cradle to grave' analysis of the impact of a manufactured product on the environment. The main stages are:

1. obtaining the raw materials needed
2. manufacturing the product
3. using the product
4. disposing of the product at the end of its useful life

The four main stages of the life-cycle assessment

The four main stages

At all stages, an LCA is likely to include information about the use of energy, transport of materials, and the release of waste substances into the environment.

1. Raw materials

All the raw materials we need come from the Earth's crust, atmosphere or oceans, or are due to living organisms. Obtaining these materials has an impact on the environment, including:

• using up limited resources such as ores and crude oil
• damaging habitats through quarrying, mining, or felling trees

2. Manufacture

The manufacture of products has an impact on the environment, including:

• using up land for factories
• the use of machines and people

3. Use

The impact of a product on the environment during its use depends on the type of product. For example, a wooden chair has very little impact, unless it needs cleaning or repair. On the other hand, a car will have a significant impact.

4. Disposal

The disposal of old products has an impact on the environment, including:

• using up land for landfill sites
• whether any or all of the product can be recycled or reused

Question

The table shows some data about the energy needed in the life cycle of a pillow case.

Life cycle stage	Energy use
Raw materials	10%
Manufacture	15%
Use	70%
Disposal	5%

Discuss the use of energy during the life cycle of the pillowcase, and describe one way in which the energy use could be significantly reduced.

15% of the energy use is associated with making the pillowcase, and very little in disposing of it. The greatest use of energy happens when it is being used, probably because of washing, drying and ironing the pillowcase. Energy use could be reduced by drying it outside and without ironing it.