Level: AS Levels, A Level, GCSE – Exam Boards: Edexcel, AQA, OCR, WJEC, IB, Eduqas – Chemistry Revision Notes

Using resources

 

Using the Earth’s resources

Humans have always used the earth’s resources to provide themselves with food, water, transport and shelter.  Many of the resources that humans have used in the past are finite and are now running out.

 

Humans now need to look for sustainable resources that can be used for future generations.

• Finite resources are those which there is a limited supply of, for example coal, oil and gas.
• Renewable resources will not run out in the near future because the reserves of these resources are high. Examples include solar power, wind power, hydropower and geothermal energy.
• Sustainable development is development that meets the needs of current generations without compromising the needs of future generations.

 

Water

• Potable water is water that is safe to drink.Potable water:
• Is not pure-it contains impurities and minerals.
• Is odourless, colourless and tasteless.
• Contains low levels of microbes and salts (safe for human use and consumption).

 

Making water safe

Fresh water

Rainwater collects in the ground, lakes and rivers and has low levels of dissolved substances. It is treated to make it potable by:

• Sedimentation: large solid particles drop to the bottom.
• Filtration: fine particles are removed (by passing through filter beds).
• Sterilising: microbes are killed using UV light, ozone or chlorine.

In the UK, this process is used to provide potable water.

 

Sea water

Sea water is undrinkable due to the high number of substances dissolved in it. Desalination is needed to remove salt  and the dissolved substances.

Desalination can be carried out by:

Distillation

• Water is evaporated and then condensed.
• The salt is left behind.
• The disadvantage to this process is that often, not all of the water evaporates which leaves behind a salty wastewater that can be difficult to sustainably dispose of without harming aquatic organisms.

 

Reverse osmosis

Reverse osmosis involves forcing water through a membrane at high pressure.

• The membrane has tiny holes in it so that it will only allow water to pass through.
• Other substances are prevented from passing through the membrane as they are too large to fit through the holes.
• This method produces large amounts of wastewater and requires the use of expensive membranes.
• The efficiency of this method is very small.

 

These processes are expensive due to the large amounts of energy required.

The methods of making water safe vary depending on where you live. Many countries with little fresh water have to rely on desalination of sea water.

 

Wastewater treatment

Water cycles

Water must be continually recycled. Sewage and wastewater must be treated to provide clean water.

• Human waste contains high levels of harmful bacteria and nitrogen compounds which can be a danger to aquatic organisms.
• Industrial/agricultural waste contains high levels of toxic metal compounds, fertilisers and pesticides which also damage the ecosystem.

Cleaning sewage requires several steps:

1. Screening: branches, twigs and grit are removed.
2. Sedimentation: wastewater is placed in a settlement tank. The heavier solids sink to the bottom and form a sludge, whilst the lighter effluent floats on the surface.
3. Aerobic digestion: the effluent is transferred to another tank, where the organic matter undergoes aerobic digestion. This water can be safely released back into the environment.
4. Anaerobic digestion: the sludge is placed in another tank where the organic matter undergoes anaerobic digestion. This can produce fertiliser and methane gas which can be used as fuel.

In extremely rural areas a septic tank can be used. Each household has their own individual sewage system which allows anaerobic bacteria to develop to treat the sewage by decomposing it.

 

Required practical: Analysis and purification of water samples from different sources

 

pH of water samples

• Test the pH of each water sample using a pH meter or universal indicator.
• Identify the pH of the sample.
• Identify whether the substance is an acid/alkali

Mass of dissolved solids

• Measure out a defined amount of the sample using a measuring cylinder.
• Measure the mass of an evaporating basin and record the mass in a table.
• Place the measured amount of water into the evaporating basin and heat using a Bunsen burner until all the liquid has evaporated.
• Once the evaporating basin has cooled, place it on the balance again and record its mass.
• Calculate the mass of the solid left behind.

 

Distillation of the water sample

• Set up the equipment for distillation as shown in Figure 1.

Figure 1: Source AQA practical sheets

• Heat the water gently, using a Bunsen burner until it boils.
• The distilled water will collect in the cooled test tube. Collect about 1 cm depth of water, then stop heating.
• Analyse the collected water by determining its boiling point.

 

Life cycle assessment and recycling (LCA)

• An LCA is carried out to assess the environmental impact in each stage of a products life.

 

There are four main stages that need to be considered:

Stage 1 – Extracting the raw materials needed to make the products and then processing them: Energy and environmental costs.

Stage 2 – Manufacturing and packaging of the product: How much energy and resources are needed to manufacture the product?

Stage 3 – The environmental impact of the use of the product during its lifetime: This will depend on the type of product.

Stage 4 – Disposal at the end of a product’s life: Methods include landfill, incineration and recycling.

Comparative LCAs can be used to evaluate products in order to find which one will have a lower environmental impact. E.g. comparing the lifecycle of a plastic bag to that of a paper bag.

The disadvantage of this process is that opinions may differ on the impact of a stage in the life cycle and therefore judgements must be made.

 

Recycling

Some products such as glass bottles can be re-used but some need to be recycled for a different use.

 

Advantages

• If more items are recycled, less would end up in landfill sites.
• Fewer resources and finite materials are removed from the ground.
• Crude oil used to produce plastics, is not needed. Saving on high energy costs.
• Less greenhouse gases are released into the environment.

 

Disadvantages

• Collection and transport of materials involves vehicles and the use of fuel.
• Materials can be difficult to sort; and is dependent on the purity of the materials and the level of purity required for the final product.

e.g. copper used in electrical wires must have a very high purity. In order to recycle copper, it needs to be processed and then melted down again to make copper wiring.

Alternative methods of metal extraction

Biological methods of extraction are useful as traditional methods of mining involves digging out huge parts of the ground and metal ores on earth are in short supply.

Biological methods have a lower impact on the environment and make use of ores that contain small amounts of copper. These methods are slow, but they do reduce the need to obtain new ore through mining.

 

Phyto-mining

This process is used to extract copper and nickel.

Plants called hyperaccumulators are used: they concentrate toxic metals in their tissue.

• Plants absorb the metal compounds found soil.
• The metal ions e.g. copper, builds up in the leaves.
• The plants are then cut, dried and then placed in a furnace.
• The ash produced from burning contains soluble metal compounds that can be extracted.
• The ash is dissolved in an acid and the metal is then extracted by electrolysis or through a displacement reaction with iron.

 

Bioleaching

Bioleaching uses bacteria to produce leachate– an acidic solution which contains metal ions.

Unfortunately bioleaching produces toxic substances that are harmful to the environment.

To extract copper, the copper undergoes a displacement reaction with iron or electrolysis can be used. Iron is cheaper and a more cost-effective way of producing copper from the leachate.

 

Corrosion of metals and its prevention

Metals react with oxygen to form metal oxides. This, over time weakens the metal and causes corrosion.

Rusting

Rusting is a form of corrosion, when iron or steel reacts with oxygen in the air and/or water.

iron + oxygen + water    —–>   iron(III) oxide

 4Fe + 3O₂ + 6H₂O    —–>      4Fe(OH)₃

Prevention

Corrosion and rusting can be prevented by creating a barrier on the surface of the metal. Barrier methods include:

• Greasing
• Painting
• Coating with plastic
• Electroplating: a thin layer of a metal can be applied to an object using electrolysis.
• Sacrificial protection: a more reactive metal is layered on top, so it reacts more readily with oxygen and the metal underneath does not corrode.

E.g. galvanising is coating iron with zinc. Zinc reacts with oxygen and water instead of iron.

Other methods include:

• Storing the metal in an unreactive atmosphere such as argon prevents it from reacting with oxygen.
• Calcium chloride can be used to absorb water vapour and keep the metal dry.

 

Alloys

An alloy is when a metal is mixed with other elements or metals to change its properties.

• Bronze is an alloy of copper and tin.
• Brass is an alloy of copper and zinc.
• Gold is often mixed with other metals such as copper, zinc or silver to change its appearance.

 

Steel

Steel is an alloy made up of iron mixed with different amounts of carbon.

• High-carbon steel contains a high percentage of carbon. It is strong and brittle and is used in construction.
• Low-carbon steel contains a low percentage of carbon. It is softer and more easily shaped.
• Stainless steel is iron mixed with the elements chromium and nickel. It is often used for making cutlery as it does not rust.

 

Ceramics, polymers and composites

Glass

Soda-lime glass is a mixture of melted sand, limestone and sodium carbonate. The molten liquid is cooled and solidified. It is used for windows jars and bottles.

Glassware that is used in baking and in laboratory equipment contains boron trioxide, which increases the melting point of the glass. This is called Borosilicate glass.

 

Ceramics

Ceramics include pottery and bricks. They are made by shaping wet clay and heating the result in a furnace. Crystals form in the clay and join it together. The pottery can be coated in a glaze which hardens over time and forms a waterproof layer.

 

Polymers

The properties of a polymer can be dependent on the conditions used to create them.

HDPE and LDPE

High-density poly(ethene) and Low-density poly(ethene) are both made from the monomer ethene but using different catalysts and reaction conditions.

• LDPE is flexible and is used in carrier bags and bubble wrap.
• HDPE is much stronger, flexible and resists chemical attack. It is used in plastic bottles, drainpipes and buckets.
• Both of these materials are thermosoftening polymers

 

Thermosoftening plastics

• Contain individual tangled polymer chains and melt when heated.
• They are flexible.
• Polymer chains can move over each other freely when heated.

Thermosetting plastics

• Consist of polymer chains with cross-links (covalent bonds) between the chains.
• They are hard and brittle.
• They cannot be reshaped and do not melt when heated.

Figure 2: thermosetting polymers have crosslinks between the chains

 

Composite materials

Composites are made up of a reinforcement material and a matrix material which binds the reinforcement together.

Carbon fibre reinforced polymer (CFRP) contains fibres that are strong under tension. CRFP contains carbon fibres used as reinforcement. The fibres are flexible but do not easily stretch. They are held together by polymer resin (matrix) which helps to bind the fibres together, making them stiff.

 

The Haber Process

In the Haber process, ammonia is made. The reaction requires a high temperature, high pressure and an iron catalyst. The forwards reaction is exothermic.

• Nitrogen is obtained by extraction from the air.
• Hydrogen is obtained from natural gas.
• A temperature of 450^oC is used.
• A pressure of 200 atmospheres is used.

N₂(g)  +   3H₂(g)  2NH₃(g)

• If the pressure of the system is increased, the equilibrium position will shift to the right as there are fewer molecules, so more NH₃ is made.
• If the pressure of the system is decreased, the equilibrium position will shift to the left as there are a larger number of molecules, so the yield of NH₃ is reduced.
• Increasing the temperature of the reaction will result in the equilibrium shifting in favour of the reverse direction (endothermic) to reduce the temperature. The yield of NH₃ decreases.

 

To obtain the most economic yield for the Haber process, a compromise must be used. The reaction is carried out at a high pressure (200 atm) to force the equilibrium to the right and increase the yield.

If too high a temperature is used,  then the yield of ammonia is reduced. But if the temperature is too low then the rate of reaction is slow. So a temperature of 450^o C is used is a compromise.

• The reaction mixture is cooled, the ammonia liquifies and is then removed.
• The hydrogen and nitrogen that has not reacted is recycled to make the process more efficient.

 

NPK fertilisers

These are fertilisers that contain the elements nitrogen, phosphorus and potassium.

NPK fertilisers are formulations of different salts containing specific percentages of the elements. Different ratios are used for different plants and stages of plant growth.

Examples:

• Ammonium nitrate – NH₄NO₃
• Ammonium sulfate – (NH₄)2SO₄
• Ammonium phosphate – (NH₄)₃PO₄
• Potassium nitrate – KNO₃

Raw materials:

• Ammonia (made in the Haber process) is used to manufacture ammonium salts and nitric acid.
• Potassium Chloride, potassium sulphate and phosphate rock (calcium phosphate) are mined.

 

Phosphate rock cannot be used directly as a fertiliser as it is insoluble so must be treated with acid.

• When reacted with nitric acid it produces calcium nitrate and phosphoric acid.
• When reacted with sulfuric acid produces a mixture of calcium sulfate and calcium phosphate which is called single superphosphate.
• When reacted with phosphoric acid produces calcium dihydrogen phosphate which is called triple superphosphate.

 

 

 

Questions:

 

1. Describe the steps required to make water from a river potable.
2. Give two ways that corrosion of iron can be prevented.

 

 

 Answers:

 

1. Sedimentation, filtration and sterilisation.

 

Leave the water in a holding tank so sediments can sink to the bottom. Filter the water through sand to remove small pieces of waste and any finer particles. Kill microbes using chlorine.

 

 

2. Painting/greasing/ sacrificial protection such as galvanising with zinc.