The chemical industry
There are some chemicals that industries need thousands os tonnes of each year e.g. ammonia, sulphuric acid, sodium hydroxide and phosphoric acid.
Bulk chemicals - Chemicals produced on a large scale.
Fine chemicals - Chemicals produced on a small scale, e.g. drugs, food additives and fragrances.
Before new chemicals are produced a huge amount of research and development goes on. It can take years and be really expencive, but it is worth it is the company makes alot of money from it.
For example, to make a new process run efficiently a new catalyst might have to be found, this can involve:
- Testing potential catalysts using a process of trial and error.
- Making computer models of the reaction to try and work out which substance might work as a catalyst.
- Designing or refining the manufacture of the catalyst to make sure that the new product can be mass-produced safely, efficiently and cost effectively.
- Investigating the risks to the environment of using the new catalyst and trying to minimise them.
- Monitoring the quality of the product to make sure that is not affected by the catalyst.
These jobs and lots of other types of work are done by the chemical industry.
The government has strict controls on everything to do with chemical processes. They are there to protect workers, the general public and the environment.
There are regulations about:
- Using chemicals- e.g. sulphuric acid is sprayed on potato fields to destroy the leaves and stalks of the potato plants to make harvesting easier. Government regulations restrict how much of the chemical can be used and require signs to be used to warn the public.
- Storage- many dangerous chemicals have to be stored in locked storerooms. Poisonous chemicals must be stored in either sealed containers or well-ventilated store cupboards.
- Transport- e.g. lorries transporting chemicals must display hazard symbols and identification numbers to help the emergncy services deal safely with any accidents and spills.
The process of producing a useful chemical can be split into 5 stages:
- Preparation of feedstock
- Separation of products
- Monitoring the purity of product
- Handling of by-products and and wastes
1. Raw chemicals are converted into feedstocks
Raw chemicals- The naturally occuring substances which are needed e.g. crude oil and natural gas.
Feedstocks- The actual reactants needed for the process e.g. hydrogen and ethanol. Raw chemicals usually have to be purified or changed in some way to make the feedstock.
The feedstock are converted into products. The conditions have to be carefully controlled to make sure the reaction happens, and at a sensible rate.
3. The products are separated
Chemical reactions usually produce the chemicals you want and some other chemicals called by-products. The by-products might be useful or they might be waste. You might also have some left over reactants. Everything has to be separated out so it can be dealt with in different ways.
4. The purity of the product is monitored
The purity of the chemical has to be monitored to make sure it's betwen certain levels. Different industries need different levels of purity depending on what the product is used for. If a slightly impure product will do the job it is meant for, there's no point wasting money on purification.
5. By-products and waste are dealt with
Where possible by-products are sold or used in another reaction.
If the reaction is exothermic, there may be waste heat. Heat exchangers can use excess heat to produce steam or hot water for other reactions, saving energy and money.
Waste products have to be carefully disposed of so they don't harm people or the environment, there are legal requirements about this.
Sustainable processes are ones that meet peoples needs today without affecting the ability of the future generations to meet their own needs. There are 8 key questions about sustainability.
- Will the raw materials run out?
- How good is the atom economy? The atom economy of a reaction tells you how much of the reactants ends up as useful products. Reactions with low atom economy make alot of waste and use up resources very quickly.
- What do I do with the waste products? Waste products can be expencive to remove and dispose of. They are likely to take up space and cause pollution. One way of dealing with this is to find uses of waste products rather than just throwing them away. Or you can use a reaction that gives useful by-products.
- What are the energy costs? A reaction needing alot of energy will be expencive. Providing energy usually involves burning fossil fuels.
- Will it damage the environment?
- What are the health and safety risks?
- Are there any benefits or risks to society?
- Is it profitable?
- During a chemical reaction old bonds are broken and new bonds are formed.
- Energy must be supplied to break existing bonds- so bond breaking is an endothermic process.
- Energy is released when new bonds are formed- so bond formation is an exothermic reaction.
In exothermic reactions the energy released by forming bonds is greater than the energy used to break them.
In endothermic reactions the energy used to break bonds is greater than the energy released by forming them.
- Every chemical bond has a particular bond energy associated with it. This bond energy varies slightly depending slightly on the compound the bond occurs in.
- You can use these known bond energies to calculate the overall energy change for a reaction.
Catalysts and reversible reactions
Activation energy- The minimum amount of energy needed for bonds to break and a reaction to start. If the energy input is less than the activation energy there won't be enough energy to start the reaction so nothing will happen.
catalyst- A substance which changes the speed of a reaction without being used up in the reaction. Catalysts lower the amount of activation energy needed for reactions to happen by providing alternative routes.
A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants.
- If a reversible reaction takes place in a closed system then a state of equilibrium will allways be reached. (a closed system means none of the reactants or products can escape).
- Equilibrium means that the relative (%) of quantities of reactants and products will reach a certain balance and stay there.
- It is in fact a dynamic equilibrium, which means that the reactions are still taking place in both directions, but the overall effect is nil because the foreward and reverse reactions cancel each other out. The reactions are taking place at exactly the same rate in both directions.
The haber process
- Feedstocks for the haber process are nitrogen and hydrogen.
- The nitrogen is obtained easily from the air, which is 78% nitrogen.
- The hydrogen comes from the cracking of chemicals in natural gas using steam.
- The reaction is reversible so not all the nitrogen and hydrogen will convert to ammonia. The gases don't stay in the reactant vessel long enough to reach equilibrium though.
- The N2 and H2 which dont react are recycled and passed through again so none is wasted.
- Recycling N2 and H2 means that more ammonia will be made using the same amount of reactant - the yield will increase.
Pressure: 200 atmospheres
Catalyst: Iron- Makes the rate of reaction alot faster.
The haber process
- Higher pressures favour the foreward reaction (since there are four molecules of gas on te left-hand side, but only two molecules on the right).
- So the pressure is set as high as possible to give the best % yield, without making the plant too expencive to build (it'd be too expencive to build a plant that would stand pressures of over 1000 atmospheres) hence the 200 atmosphere pressure.
- The foreward reaction is exothermic, which means that increasing the temperature will actually move the equilibrium the wrong way- awayfrom the amonia and towards the N2 and H2. So the yield of ammonia would be greater at lower temperatures.
- The trouble is lower temperatures mean a slower rate of reaction. So what they do is increase the temperature anyway, to get a much faster rate of reaction.
- The 450°C is a compromise between maximum yield and speed of reaction. It's better to wait just 20 seconds for a 10% yield than 60 seconds for a 20% yield.
- The unused hydrogen and nitrogen are recycled, so nothing is wasted.
- Nitrogen fixation is the process of turning N2 from the air into useful nitrogen compounds like ammonia.
- The haber process is a non-biological way of fixing nitrogen.
- Most of the ammonia produced by the haber process is used to make fertilisers.
- Fertilisers play a vital part in world food production as they increase crop yield so help to feed more people.
- When used in large amounts though fertilisers can pollute water supplies and cause eutrophication.
Eutrophication- When fertilisers leach into lakes and rivers, stimulating rapid algal growth. The algae blocks out the light to other plants, which then die. Microorganisms then feed on dead plants, using up all the oxygen that aquatic animals need to survive. Eventually all of the plant and animal life in the water dies.
Ammonia is also important in industry where it is used to manufacture plastics, explosives and pharmaceuticals.
- In the haber process very high temperatures and pressures have to be used to turn nitrogen and hydrogen into ammonia.
- Using an iron catalyst makes the rate of reaction much faster, so the ammonia is produced faster.
- Without the catalyst the temperature would have to be raised even further to get a quick enough reaction, and that would reduce the % yield even further.
- Some living organisms such as nitrogen-fixing bacteria can fix nitrogen at room temperature and pressure. They do this using enzymes.
- Chemists would like to be able to make catalysts that mimic these enzymes, so that processes like the haber process can be carried out at room temperature and pressure.
- As it's expencive and time consuming to work at high temperatures and pressures, this would mean that processes involving nitrogen fixation would become much cheaper and more efficient.
Nitrogen fixation sustainability
- Hydrogen comes from fossil fuels, they're non-renewable and will run out. Nitrogen comes from the air so it's unlikely it will run out.
- All the H2 and N2 make ammonia, so the atom economy is xcellent.
- There are no waste products as the chemicals are all recycled.
- Lots of energy is needed to keep the reaction at 450°C and 200atm.
- Fertilisers made from NH3 can cause eutrophication and water pollution.
- Working at high temperatures and pressures can be very dangerous.
- Making ammonia can help world food production.
- Making ammonia is very profitable.
- Alkanes are a family of hydrocarbons. e.g. methane, ethane, propane and butane.
- Alkanes are made up of chains of carbon atoms surrounded by hydrogen atoms.
- Alkanes only contain single covalent bonds between carbon atoms, we say they are saturated compounds.
- The alkane family contains molecules that look similar but have different length chains of carbon atoms.
- All alkanes have the formula: C(n)H(2n+2)
Alkanes burn to produce cabon dioxide and water, provided there is plenty of oxygen around.
Alkane + Oxygen = Carbon dioxide + Water
- Alkanes are pretty unreactive towards most chemicals.
- They don't react with aqueous reagents (substances dissolved in water).
- Alkanes don't react because the C-C bonds and C-H bonds in them are difficult to break.
- The general formula for an alcohol is C(n)H(2n+1)OH.
- The first two alcohols are methanol (CH3OH) and ethanol (C2H5OH).
- The '-OH' bit is called the functionl group.
- All alcohols have similar properties because they all have the -OH functional group.
Physical properties of alkanes, alcohols and water compared:
- Ethanol is soluable in water. Alkanes are insoluable in water.
- Ethanol and water are both good solvents, lots of things dissolve in them.
- The boiling point of ethanol is 78°C. This is lower than water (100°C) but much higher than ethane (-103°C).
- Ethanol is liquid at room temperature. It evaporates easily and gives off fumes (is volatile) . Methane and ethane are also volatile, but are gases at room temperature. Water is liquid at room temperature, but not volatile.
- Alcohols can dissolve lots of compounds that water can't e.g. hydrocarbons and oils. This makes them useful sovents in industry.
- Methanol is also used in industry as a starting point for manufacturing other organic chemicals.
- Ethanol is used in perfumes and aftershave lotions as it can mix with both the oils (that give the smell) and the water (which gives the bulk).
- 'Methylated spirit' ('or meths') is ethanol with chemicals added to it (e.g.methanol). It's used to clean paint brushes and as a fuel
- Alcohols burn in air to produce carbon dioxide and water because they contain hydrocarbon chains. Pure ethanol is clean burning so it is sometimes mixed with petrol and used as fuel for cars to coserve crude oil.
Alcohols react with sodium
Alcohols react with sodium
- Sodium metal reacts gently with ethanol, to produce sodium ethoxide and hydrogen.
Sodium + Ethanol = Sodium ethoxide + Hydrogen
- Sodium metal reacts much more vigorously with water, even melting because of the heat of the reaction.
Sodium + Water = Sodium hydroxide + Water
- Alkanes do not react with water at all.
The ethanol in alcoholic drinks is usually made by fermentation:
- Fermentation uses yeast to convert sugars into ethanol. Carbon dioxide is also produced.
- The yeast cells contain zymase, an enzyme that acts as a catalyst in fermentation.
- Fermentation happens fastest at 30°C . That's because zymase works best at this temperature. At lower temperatures the reaction slows down. At higher temperatures the enzymes are denatured.
- Zymase also works best at a PH of about 4, a strong acid or alkali solution will stop it working.
- It's important to prevent oxygen getting to the fermentation process. Oxygen converts the ethanol into ethanoic acid, which lowers the PH and can stop the enzyme working.
- When the concentration of ethanol reaches about 10 to 20%, the fermentation reaction stops, because the yeast gets killed off by the ethanol.
The fermented mixture can then be distilled to produce more concentrated ethanol, which can then be used to make products like brandy or whisky.
- The ethanol solution is put in a flask below a fractioning column.
- The solution is heated, so that the ethanol boils. The ethanol vapour travels up the column, cooling down as it goes.
- The temperature is such that anything with a higher boiling point than ethanol (like water) cools to a liquid and flows back into the solution at the bottom.
- This means that only pure ethanol vapour reaches the top of the column.
- The ethanol vapour flows through a condenser, where it's cooled to a liquid, which is then collected.
Sustainability of fermentation
- Sugar beet and yeast grow quikly so won't run out.
- The waste CO2 produced shows that it has a low atom economy. And because the enzyme is killed off when the ethanol is produced the reaction is even less efficient.
- The waste CO2 can be released without any processing.
- Energy is needed to keep the reaction at the optimum temperature.
- Carbon dioxide is a greenhouse gas so adds to global warming.
- The chemicals and processes do not have any specific dangers.