Extracting metals & equilibria

Question 1

Reactivity of metals

Answer 1

The chemistry of the metals is seen by comparing their characteristic reactions.
Based on these reactions a reactivity series of metals can be produced.
The series can be used to place a group of metals in order of reactivity based on the observations of their reactions with water, acids and salts.

Question 2

Reactivity series

Answer 2

Potassium, sodium, lithium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold.

Question 3

Reaction with water

Answer 3

In general, when a metal reacts with water it produces a metal hydroxide and hydrogen gas.
Ca (s) + 2H2O (l) ⟶ Ca(OH)2 (aq) + H2 (g)
calcium + water ⟶ calcium hydroxide + hydrogen

Question 4

Reaction with dilute acids

Answer 4

Only metals above hydrogen in the reactivity series will react with dilute acids.
The more reactive the metal then the more vigorous the reaction will be.
Metals that are placed high on the reactivity series such as potassium and sodium are very dangerous and react explosively with acids.
metal + acid ⟶ salt + hydrogen

Question 5

Reactivity with metal salts

Answer 5

The reactivity between two metals can be compared using displacement reactions in salt solutions of one of the metals.
This is easily seen as the more reactive metal slowly disappears from the solution, displacing the less reactive metal.
Mg + CuSO4→ MgSO4 + Cu

Question 6

Oxidation

Answer 6

In all these reactions the more reactive metals lose electrons to become cations.
The more reactive the metal the more easily it becomes a cation.
The loss of electrons is oxidation.
The higher up the metal is in the reactivity series the more easily it will undergo oxidation.
Unreactive metals are therefore more resistant to oxidation.

Question 7

Redox

Answer 7

Redox reactions occur when electrons are transferred between substances.
* Oxidation is a loss of electrons. OIL
* Reduction is a gain of electrons RIG.
Both oxidation and reduction happen at the same time, hence the term redox. Oxidation and reduction can also be defined in terms of loss or gain of oxygen but on this page, they’re referring to the transfer of electrons.

Question 8

Sources of metal ores

Answer 8

The Earth’s crust contains metals and metal compounds such as gold, copper, iron oxide and aluminium oxide.
Useful metals are often chemically combined with other substances forming ores.
A metal ore is a rock that contains enough of the metal to make it worthwhile extracting.
They have to be extracted from their ores through processes such as electrolysis, using a blast furnace or by reacting with more reactive material.
In many cases the ore is an oxide of the metal, therefore the extraction of these metals is a reduction process since oxygen is being removed.
Common examples of oxide ores are iron and aluminium ores which are called haematite and bauxite respectively.

Question 9

Extracting metals

Answer 9

The most reactive metals are at the top of the series.
The tendency to become oxidised is thus linked to how reactive a metal is and therefore its position on the reactivity series.
Metals higher up are therefore less resistant to oxidation than the metals placed lower down which are more resistant to oxidation.
The position of the metal on the reactivity series determines the method of extraction.
Higher placed metals (above carbon) have to be extracted using electrolysis as they are too reactive and cannot be reduced by carbon.
Lower placed metals can be extracted by heating with carbon which reduces them.

Question 10

Sources of unreactive metals

Answer 10

Unreactive metals do not have to be extracted chemically as they are often found as the uncombined element.
They are known as native metals.
This occurs as they do not easily react with other substances due to their chemical stability.
Examples include gold and platinum which can both be mined directly from the Earth’s crust.

Question 11

Bioleaching & Phytomining

Answer 11

Extraction of metal ores from the ground is only economically viable when the ore contains sufficiently high proportions of the useful metal, such as iron ores and aluminium ores.
For low grade ores (ores with lower quantities of metals) other techniques are being developed to meet global demand.
This is happening in particular with nickel and copper as their ores are becoming more and more scarce.
Phytoextraction and bioleaching (bacterial) are two relatively new methods of extracting metals that rely on biological processes.
Both of these methods avoid the significant environmental damage caused by the more traditional methods of mining.
Traditional mining involves a great deal of digging, moving and disposing of large amounts of rock.
Biological methods are, however, very slow and also require either displacement or electrolysis to purify the extracted metal.
Both techniques are also used to extract metals from mining wastes, which may contain small quantities of metals or toxic metals that need to be removed from that environment.

Question 12

Phytomining

Answer 12

This process takes advantage of how some plants absorb metals through their roots.
The plants are grown in areas known to contain metals of interest in the soil.
As the plants grow the metals are taken up through the plants vascular system and become concentrated in specific parts such as their shoots and leaves.
These parts of the plant are harvested, dried and burned.
The resulting ash contains metal compounds from which the useful metals can be extracted by displacement reactions or electrolysis.

Question 13

Bioleaching

Answer 13

Bioleaching is a technique that makes use of bacteria to extract metals from metal ores.
Some strains of bacteria are capable of breaking down ores to form acidic solutions containing metals ions such as copper(II).
The solution is called a leachate which contains significant quantities of metal ions.
The ions can then be reduced to the solid metal form and extracted by displacement reactions or electrolysis.
This method is often used to extract metals from sulfides e.g. CuS or Fe2S
Although bioleaching does not require high temperatures, it does produce toxic substances which need to be treated so they don’t contaminate the environment.
Bioleaching is not only used for the primary extraction of metals, but it is also used in mining waste clean up operations.

Question 14

Recycling metals

Answer 14

Everyday materials such as metals are produced from natural but finite sources.
Some products made from these materials can be reused which saves energy and decreases the environmental impact.
Metals can be melted and recast into new shapes.
Sometimes the materials being recycled need to be kept separate, depending on what the use of the recycled material will be.

Question 15

Economical benefits to recycling metals

Answer 15

It is economically beneficial to recycle metals, especially those that are costly to extract such as aluminium.
Recycling is fast becoming a major industry and provides employment which feeds back into the economy.

Question 16

Environmental impact of recycling metals

Answer 16

Mining and extracting metal from ores has detrimental effects on the environment and ecosystems.
It is much more energy efficient to recycle metals than it is to extract them as melting and re-moulding requires less energy.
Recycling decreases the amount of waste produced, hence saving space at landfill sites and energy in transport.
There is a limited supply of every material on Earth.
As global populations increase there is greater need for effective recycling methods to attain sustainable development.
Mining and extraction use up valuable fossil fuels, which contributes to climate change.

Question 17

Disadvantages of recycling metals

Answer 17

Collection and transport of material to be recycled requires energy and fuel.
Workers, vehicles and worksites need to be organised and maintained.
Materials need to be sorted before they can be recycled which also requires energy and labour.
Products made from recycled materials may not always be of the same quality as the original.

Question 18

Life cycle assessment

Answer 18

A life cycle assessment (LCA) is an analysis of the overall environmental impact that a product may have throughout its lifetime.
The cycle is broken down into four main stages which are:
Raw Materials
Manufacture
Usage
Disposal
A life cycle assessment is carried out using the data of a given product and the criteria of the assessment.
Rarely is there a perfect product with zero environmental impact, so often a compromise is made between environmental impact and economical factors.

Question 19

Raw materials

Answer 19

Obtaining the necessary raw materials has an impact on the environment which may include:
Using up limited resources such as ores and crude oil.
Damaging habitats through deforestation or mining.

Question 20

Manufacturing

Answer 20

Manufacturing processes also have an impact on the environment which may include:
Using up land for factories.
The use of fossil fuelled machines for production and transport.

Question 21

Usage

Answer 21

Usage of a product may also affect the environment although it depends on the type of product.
For example, a wooden desk has very little impact whereas a car will have a significant impact (air pollution).

Question 22

Disposal

Answer 22

The disposal of outdated products has an impact on the environment which may include:
Using up space at landfill sites.
Whether the product or its parts can be recycled.

Question 23

Reversible reaction

Answer 23

Some reactions go to completion, where the reactants are used up to form the product molecules and the reaction stops when the reactants have been exhausted.
In reversible reactions, the product molecules can themselves react with each other or decompose and form the reactant molecules again.
A + B ⇌ C + D

Question 24

Dynamic equilibria

Answer 24

A reversible reaction is one that occurs in both directions.
When during the course of reaction, the rate of the forward reaction equals the rate of the reverse reaction, then the overall reaction is said to be in a state of equilibrium.
Equilibrium is dynamic i.e. the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate.
The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure).
It only occurs in a closed system so that none of the participating chemical species are able to leave the reaction vessel.

Question 25

Ammonia as a reversible reaction

Answer 25

An example of a dynamic equilibrium is the reaction between H2 and N2 in the Haber process.
When only nitrogen and hydrogen are present at the beginning of the reaction, the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest
As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease, so the rate of the forward reaction will decrease.
However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase (ammonia will decompose to reform hydrogen and nitrogen).
Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached.

Question 26

Haber process

Answer 26

Ammonia is manufactured using The Haber Process which occurs in five stages.
Stage 1: H2 and N2 are obtained from natural gas and the air respectively and are pumped into the compressor through pipe.
Stage 2: the gases are compressed to about 200 atmospheres inside the compressor.
Stage 3: the pressurised gases are pumped into a tank containing layers of catalytic iron beds at a temperature of 450°C. Some of the hydrogen and nitrogen react to form ammonia:
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

Stage 4: unreacted H2 and N2 and product ammonia pass into a cooling tank. The ammonia is liquefied and removed to pressurised storage vessels
Stage 5: the unreacted H2 and N2 gases are recycled back into the system and start over again.

Question 27

Temperature for haber process

Answer 27

A higher temperature would favour the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made.
If a lower temperature is used it favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made
However at a lower temperature the rate of reaction is very slow.
So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly.

Question 28

Pressure for haber process

Answer 28

A lower pressure would favour the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made.
A higher pressure would favour the forward reaction as it will try to decrease the pressure by creating less molecules (2 molecules of gaseous products) so a higher yield of products will be made.
However high pressures can be dangerous and very expensive equipment is needed.
So 200 atm is a compromise pressure between a lower yield of products being made safely and economically.

Question 29

Position of equilibrium

Answer 29

The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction.
This balance is framed in an important concept known as Le Chaterlier’s Principle, named after Henri Le Chatelier who was a French military engineer in the 19th century.
This principle states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change.
The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration.
Knowing the energy changes, states and concentrations involved allows us to use the principle to manipulate the outcome of reversible reactions.

Question 30

Temperature on equilibrium

Answer 30

All reversible reactions are exothermic in one direction and endothermic in the other.
If you raise the temperature of A+B ⇌ C+D where -> = exothermic, the equilibrium will shift to oppose the change. The system wats to absorb heat energy to cool it down. Therefore, the equilibrium will shift to the left (the endothermic direction).
If you reduce the temperature of A+B ⇌ C+D where -> = exothermic, the equilibrium will shift to oppose the change. The system wants to produce heat energy to heat it up. Therefore, the equilibrium will shift to the right (the exothermic direction).

Question 31

Pressure on equilibrium

Answer 31

Changing the pressure affects reactions where the reactants and products are gases. Many of these reactions have a greater volume on one side (either of products or reactants), Greater volume means there are more gas molecules on that side of the equation and less volume means there are fewer gas molecules.

Example - The reaction below is used to make hydrogen gas. It has two gas molecules on the left and four on the right.
CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g)

If you increase the pressure, the equilibrium shifts to oppose the change and moves to decrease the pressure. The position of equilibrium will shift to the left which generates less gas particles – CH4 and H₂O.
If you decrease the pressure, the equilibrium shifts to oppose the change and moves to increase the pressure. The position of equilibrium will shift to the right which generates more gas particles - CO and H₂.

Question 32

Concentration on equilibrium

Answer 32

If you change the concentration of either the reactants or the products, the system will no longer be at equilibrium. So, the system will respond to bring itself back to equilibrium again. A+B ⇌ C+D.
If you add/increase the concentration of A or B, the position of equilibrium will shift to oppose the change and so the equilibrium will shift to the right forming more C & D and decreasing the amount of A & B.
If you add/increase the concentration of C or D, the position of equilibrium will shift to oppose the change and so the equilibrium will shift to the left forming more A & B and decreasing the amount of C & D.

Question 33

Catalyst for haber process

Answer 33

The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached.
This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy).
As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst.
So a catalyst is used as it helps the reaction reach equilibrium quicker
It allows for an acceptable yield to be achieved at a lower temperature by lowering the activation energy required.
Without it the process would have to be carried out at an even higher temperature, increasing costs and decreasing yield as the higher temperature decomposes more of the NH3 molecules.