Metals and their Ores
Reactive metals react with other elements to form compounds, which are found in the Earth's crust. When a compound contains enough of the metal for it to be appropriate for extracting, it is called a 'metal ore'. The more reactive a metal is, the harder to extract it from the compound.
Many metals react with oxygen, forming oxides - a process referred to as oxidation.
When a metal is separated from its oxide, it is referred to as a reduction reaction. Often, carbon is used for this, but only with metals that are less reactive than itself. Elements that are more reactive have to be extracted using electrolysis.
Electrolysis: K, Na, Ca, Mg, Al.
Carbon: Zn, Fe, Sn, Pb.
Reduction of Metal Ores.
Carbon: iron(III) oxide + carbon ----> iron + carbon dioxide.
Electrolysis: Aluminium. The main ore of Al is bauxite, which contains aluminium oxide. Al. Ox. is melted and used as the electrolyte. It is broken down into aluminium and oxygen atoms. The aluminium sinks to the bottom.
Electrolysis is very expensive, and is only used as a last resort.
Properties of Metals
Metals are: - strong
- good at conducting heat
- good at conducting electricity
They are good for making into things such as bridges and cars, because of their strength, but they are also good for making saucepans and similar, because heat travels through them. Their electrical conductivity makes them good for things like electrical wires.
Metals have different properties - hence, they are used for different things.
For example, aluminium forms hard, strong alloys, and so is good for building structures with, and is of course used for drink cans.
Some metals corrode - this happens thanks to oxidisation. The more reactive metals are, the more likely they are to corrode, because they react better with oxygen.
iron + oxygen + water ----> hydrated iron(III) oxide
N.B. 'rust' is only applied to iron, not other metals.
You can make metals more useful - pure iron is fairly bendy, so carbon and small amounts of other metals are often added to the iron to create steel, an alloy which is a lot stronger, and more resistant to corrosion.
Alloys are harder than 'pure' metals, because different elements have different sized atoms - e.g. when carbon is added to pure iron, the smaller carbon atoms will upset the layers of pure iron atoms, making it harder for them to slide over each other.
Many common metals are alloys. For example, pure gold is too soft to make jewellery, so other metals such as copper, nickel and zinc are used to harden it. The amount of pure gold in a piece of jewellery is described by carats, or fineness.
Some smart alloys have a 'shape memory' property. This allows them to remember their original state. An example is nitinol, an alloy of nickel and titanium - if you bend this alloy, when heated it will go back to it's original shape. Nitinol is often used for things like glasses frames, because if they get bent, they can be easily put back into shape. The alloys are also good for use in the tubes in damaged blood vessels.
It's important to recycle metals, because:
- It uses less resources.
- It uses less energy.
- It uses less money.
- It makes less rubbish.
There are also economic and environmental benefits. Recycling isn't free, but if you didn't recycle metals, you'd have to harvest more, probably making a mess of the landscape, and, of course, there are then various costs.
For example, for every 1 kg of aluminium recycled, 95% of the energy needed needed to mine and extract the aluminium, 4 kg of aluminium ore, and lots of waste.
Crude oil is a fossil fuel, which is extracted by drilling and pumping. It is a mixture of different sized hydrocarbon molecules. Hydrocarbons are fuels such as petrol and diesel, but are made from just carbon and hydrogen. The hydrocarbon molecules aren't chemically bonded to one another, so they all keep their original properties - e.g. their condensing points.
The fractionating column works continuously, with heated crude oil piped in at the bottom. It rises up, and the fractions are tapped of at their level of condensation.
The shorter the molecules, the more flammable the hydrocarbon is.
Going up: Bitumen, Fuel Oil, Diesel, Kerosene, Naphtha, Petrol and Gases.
When there's lots of oxygen, complete combustion occurs. The hydrocarbons burn to produce only carbon dioxide and water.
e.g. hydrocarbon + oxygen ----> carbon dioxide + water (+ energy)
The hydrogen and carbon have been oxidised.
Complete combustion releases lots of energy, and only produces these two waste products. When there is plenty of oxygen and combustion is complete, the gas burns with a clear blue flame.
Complete combustion of methane:
CH4 + 2O2 ----> 2H2O + CO2
The incomplete combustion of hydrocarbons is NOT safe. This occurs when there is not enough oxygen. Although carbon dioxide and water are still produced, so are carbon monoxide and carbon. This will produce a smoky yellow flame. The carbon monoxide (CO) is a colourless, odourless, and poisonous gas. People can be killed in their sleep due to faulty boilers and gas fires giving off CO.
Burning Fuels.2 - Choosing the Best Fuel
When choosing the best fuel, you need to consider the following:
- Ease of ignition,
- Energy value/Amount of energy released,
- Ash and smoke.
- Storage and transport.
Power stations release huge amounts of fossil fuels to make electricity. Cars are also a major culprit. When fossil fuels are burnt, carbon dioxide and water vapour are always released into the air. Sulphur impurities are found in petrol and diesel - consequently sulphur dioxide is also put into the air. If there is a lack of oxygen, particles of soot (carbon) and CO are also released.
Sulphur dioxide causes acid rain. It mixes with the clouds, forming dilute sulphuric acid, which falls as acid rain.
Acid rain causes lakes to become acidic, killing plants and animals as a result. It also kills trees and damages limestone buildings and stone statues. There have also been links suggested, between acid rain and human health problems.
There are international agreements to reduce the emissions of these air pollutants. Today, a lot of petrol and diesel is being replaced by low-sulphur versions.
Sulphur can be removed from fuels before burning, but this costs a lot and uses lots of energy, releasing more carbon dioxide. Acid rain itself can be prevented by using Acid Gas Scrubbers in power stations, and catalytic converters in cars, although, obviously the simplest way is to reduce our usage of fossil fuels.
Eco - Problems: Carbon Dioxide
Carbon dioxide is a greenhouse gas. The temp. of the Earth is a balance between the heat it gets from the Sun and the heat it radiates back out into space. Gases in the atmosphere, such as carbon dioxide, methane and water vapour naturally act like an insulating layer. They are also greenhouse gases. They absorb most of the heat that would usually be radiated, and re-radiate it in all directions, including back, towards the Earth. There is a correlation between the temperature of the Earth and the presence of carbon dioxide in the atmosphere. Human activity greatly influences carbon dioxide levels - mainly because of deforestation - carbon dioxide is released when trees are burnt to clear land, micro-organisms feeding on dead wood release CO2 as a product of respiration, and plants 'breath in' or absorb CO2 for photosynthesis, so less of it is being removed from the atmosphere. Also, burning fossil fuels increases the levels.
Here are two ways that scientists are trying to restore the balance with:
Iron Seeding: - Iron is needed for photosynthesis, therefore injecting iron into the upper ocean promotes the growth of phytoplankton. They then absorb CO2. However, some plankton that will grow may be toxic, creating inhabitable 'dead zones' in the ocean.
Converting CO2 into Hydrocarbons: - It may be possible to convert waste CO2 into hydrocarbons, often using high pressure, high temperatures and a metal catalyst. Short chain hydrocarbons can be generated fairly easily, although longer ones can't.
Biofuels are made from chemicals obtained from living things. The two main biofuels are biogas and ethanol.
It is made by micro-organisms. When they decompose living organisms' waste, and dead plants, they create biogas, which can be used as fuel. It can be burnt to heat water, and used in Central Heating Systems. It can also power turbines; which is useful in rural areas. Biogas can also be used as a fuel for cars and buses. Also, it is renewable, unlike fossil fuels. The crops can quickly be replaced. It is usually about 70% methane and 30% CO2. Although CO2 is released, the crops used photosynthesised when they were alive, therefore balancing out the release of CO2. It is also a clean fuel, and the raw materials for biogas are cheap and readily available.
Ethanol burns to give out just CO2 and water. It is made by using yeast to ferment the sugars it is made from. 'Gasohol' is a mixture of 90% petrol and 10% ethanol, and can be used to power cars. Brazil makes extensive use of gasohol, as it is best used in areas with lots of fertile land, ready to grow the crops on. However, the more land used for this, the less land available to grow food. Using gasohol means that less crude oil is being used, and the crops absorb CO2. Unfortunately, distilling the ethanol after fermentation uses a lot of energy.
A fuel cell is an electrical cell that's supplied with a fuel and oxygen and uses energy from the reaction between them to generate electricity.
Fuel cells use H2 and O2 to make electricity. Hydrogen and oxygen react to create water, which isn't a pollutant. You can also get energy from doing this.
Fuel cells were developed in the 1960s as part of the space programme, to provide electrical power on board, as they were more practical than solar cells, and safer than nuclear power. They're still actually used on the Space Shuttle missions. Unlike batteries, they don't run down, or need re-charging from the mains. It produces electrical and heat energy, as long as fuel is supplied.
Hydrogen fuel cells = efficiency >80%. The electricity is generated directly from the reaction, hence, no turbines or generators need get used. Because the process is not made up of numerous stages, there are less chances for the energy to be lost as heat, or friction, as there are no moving parts. Also, no pollutants such as g.house gases and CO are produced. Eventually, they could replace petrol and diesel in transport, and batteries. However, fossil fuels and power stations will still need to be used, because of storage issues (H2 is a gas), hydrogen's explosive tendencies, and because it is often made of hydrocarbons, which are generated via electricity, which usually uses fossil fuels.
Energy of Fuels
You can calculate energy content by heating water.
Mass of fuel burned = Final mass of fuel and burner - Initial mass of fuel and burner.
The Calorimetric Method:
- Put fuel into spirit burner, or bottled gas burner.
- Measure out about 200 cm^3 of water into a copper calorimeter.
- Take water's initial temperature.
- Put burner under calorimeter and light wick.
- Stir water occasionally, to ensure even distribution of heat.
- When water temp. has risen by 20-30 *c, blow out spirit burner.
- Make note of the highest temperature reached by the water.
- Reweigh the burner and fuel.
- For fuel comparisons, repeat with second fuel.
Of course, it needs to be a fair comparison.
Alkanes and Alkenes
When you crack crude oil, you end up with alkanes and alkenes.
Alkanes have all C-C single bonds. They're made up of chains of carbon atoms with single bonds between them. They're called saturated hydrocarbons because they have no spare bonds. They turn bromine water colourless for this reason. The won't form polymers, again, because they have no spare bonds. The first three alkanes are methane, ethane and propane.
Alkenes have a C=C double bond. They're chains of carbon atoms with one (or more) double bonds. They;re called unsaturated hydrocarbons because they have some spare bonds left. They will decolourise bromine water, as they form bonds with bromine atoms. They form polymers by opening up their double bonds to 'hold hands' in a long chain. The first two alkenes are ethene and propene.
'meth-' means one carbon atom.
'eth-' means two.
'prop-' means three etc. .
After the crude oil is distilled, you still have long and short hydrocarbons.
Splitting up long-chain hydrocarbons is known as 'cracking'.
Long-chain hydrocarbons from tar-like liquids which aren't very useful. Cracking turns them into shorter, more useful molecules. Cracking is a form of thermal decomposition - that is, breaking molecules down with heat. A lot of them are cracked because there is more demand for products like petrol than for diesel and lube. Also, cracking produces alkenes, which are needed to make plastics.
How to Crack Paraffin:
- Heat the paraffin. Quickly move the Bunsen to heat the porcelain. Alternate between both until paraffin vapourises and porcelain glows red.
- Heated paraffin vapour cracks as it passes over the heated porcelain.
- Smaller alkanes and alkenes travel down the delivery tube.
- Smallest ones are gases, so they collect in the gas jar.
- If the gas in the jar contains alkenes, it will decolourise bromine water.
Alkenes ----> Polymers.
Alkenes - polymerisation. Joining together lots of monomers (small molecules, e.g. alkenes) to form v. large molecules - these are called polymers. Many ethene molecules join up to produce poly(ethene) or polythene. Molecules form polymers by opening up their double bonds.
Polythene is stretchy and light, and so can be used for everything from plastic carrier bags to hose-pipes to laminating paper. Poly(propene) is tough but flexible, and can be used for thermal underwear, carpets and plastic containers.
Joining lots of chloroethene molecules together gives you poly(chloroethene), or 'PVC'. This can be used for clothing, electric cables and pipes. Poly(tetrafluoroethene) or 'PTFE' is un-reactive, flame resistant and very resistant to wear, and so is used as a non-stick pan coating, hence the name 'Teflon'.
Most plastics don't rot. When burned, they give off toxic gases. It's best to recycle them.
Some polythene bags are now made with starch granules in them. If buried, the starch is broken down by micro-organisms in the soil, causing the bag to break down into little pieces of polythene.
You can also get plastics that break down in sunlight - they tend to get used in agriculture.