The Haber Process
The Haber Process is a reversible reaction:
N2 + 3H2 <--> 2NH3 (+heat)
The feedstocks for theHaber Process are nitrogen and hydrogen
The nitrogen is obtained easily from the air, which is 78% nitrogen (and 21% oxygen).
The hydrogen comes from the cracking ofchemicals in natural gas and steam.
The reaction is reversible so not all the nitrogen and hydrogen will be converted into ammonia. The gases don't stay in the reaction vessel long enough to reach equilibrium though.
The N2 and H2 which don't react are recycled and passed through again so nothing is wasted.
Recycling N2 and H2 means that more ammonia will be produced using the same ammount of reactant- the yield of ammonia will increase.
Industrial conditions: Pressure: 200 atmospheres Temperature: 450 degrees celsius Catalyst: Iron
The Haber Process continued
The reversible reaction means there has to be a compromise:
The high pressure in the reaction vessel favours the forward reaction.
The pressure is therefore set as high as possible to give the best % yield, without making the plant too expensive to build. Hence the 200 atmospheres operating pressure.
The forward reaction is exothermic, which means that increasing the temperature will actually move the equilibrium the wrong way- away from ammonia and towards N2 and H2. So the yield of ammonia would be greater at lower temperatures.
However, at lower temperatures, the rate of reaction is slower. The temperature is therefore increased anyway, to get a much faster reaction.
The 450C is a compromise between maximum yield and speed of reaction. It's better to wait just 20 seconds for a 10% yield than wait 60 seconds for a 20% yield.
The hydrogen and nitrogen are recycled anyway so nothing is wasted.
Nitrogen fixation is about 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 happens when fertilisers leach into lakes and rivers stimulating rapid algal growth. The algae blocks out the light to other plants, which die. Microorganisms then feed on the 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 very important in industry where it is used to manufacture plastics, explosives and pharmaceuticals.
Nitrogen Fixation continued
In the Haber Process, very high temperatures and pressures have to be used to turn nitrogen into ammonia.
Using an iron catalyst makes the rate of reaction must faster, so the ammonia is produced faster.
Without a catalyst the temperature would have to be raised further to get a quick enough reaction, and that would reduce the % yield even further. So the catalyst is very important.
Some living organisms such as nitrogen-fixing-bacteria can fix nitrgoen at room temperature and pressure. They do this using biological catalysts called enzymes.
Chemists would like to make catalysts that mimic these enzymes, so that processes like the Harber Process can be carried out at room temperature and pressure as it's expensive and time consuming towork at high temperatures and pressures. Working at room temperature and pressure using nitrogen-fixing-bacteria wouldbe uch cheaper and more efficient.
Alkanes are made up of chains of carbon atoms surrounded by hydrogen atoms.
Alkanes only contain single covalent bonds between carbon atoms (C-C). We call them saturated compounds; unsaturated compounds conatin double bonds between carbon atms (C=C).
The alkane family contains molecules that look similar, but have different length chains of carbon atoms.
All alkanes have the formula CnH2n+2 where n is the number of carbon atoms
The first 4 alkanes are: Methane, CH4 Ethane, C2H6 Propane C3H8 Butane C4H10
With a plentiful supply of oxygen, alkanes burn to give carbon dioxde and water
alkane + oxygen --> carbon dioxide and water
Alkanes are unreactive towards most chemicals. They don't react with aqueous reagents (substanes dissolved in water). Alkanes don't react because the C-C bonds and C-H bonds in them are dificult to break.
The general formula foran alcohol is CnH2n+1OH
They have the finctional group -OH and all alcohols havesimilar properties because of their functional group.
The first two alcohols are: Methanol, CH3OH and Ethanol, C2H5OH
Alchols, Alkanes and Water-the similarities and differences:
- Ethanol is soluble in water. Alkanes are soluble in water.
- Ethanol and water are both good solvents.
- The boiling point of ethanol is 78C. This is lower than the boiling point of water (100C), but much higher than the boiling point of a similar sized alkane (eg ethane has a boiling point of -103C).
- Ethanol is a liquid at room temperature. It evaporates easily and gives off fumes (it's volatile). Methane and ethane are also volatile, but are gases at room temperature. Water is a liquid at room temperature but not volatile.
- Sodium metal reacts gently with ethanol to produce sodium ethoxide and hydrogen. Sodium metal reacts much more vigorously with water to produce sodium hydroxide and hydrogen. Alkanes do not react with sodium at all.
The uses of alcohols:
Alcohols such as methanol and ethanol can dissolve many compounds that water can't e.g. hydrocarbons and oils. This makes methanol and ethanol very useful very useful solvents in industry.
Methanol is also used in industry as a starting point for manufacturing other organic chemicals.
Ethanol is used in perfumes and aftershaves as it can mix with both the oils, which give the smell, and the water, that makes up the bulk.
Methylated spirit is ethanol with chemicals, (e.g. methanol) added to it. 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 a fuel for cars to conserve crude oil.
Making Ethanol using Fermentation and Biotechnolog
Ethanol can be made by Fermentation:
- The ethanol in alcoholic drinks is made using 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 a temperature of about 30C because zymase works best at this temperature. At lower temperatures, the reaction slows down and at higher temperatures, the zymase is denatured. Zymase also works best at a pH of about 4.
- It's important to prevent oxygen getting into the fermentation process. Oxygen converts the ethanol to ethanoic acid, which lowers the pH and can stop the enzyme working.
- When the concentration of ethanol reaches about 15%, the fermentation reaction stops, because the yeast gets killed off by the ethanol.
Ethanol can be made from biomass:
- Waste biomass is the parts of a plant that would normally be thrown away e.g. corn stalks
- Waste biomass can't be fermented in the normal way because it contains a lot of cellulose.
- E.coli bacteria can be genetically modified so they can convert cellulose in waste biomass into ethanol.
- The optimum conditions for this process are a temperature of 35C and a pH of 6, a slightly acidic solution.
Distillation and Sustainability
The ethanol produced using the fermentation and biotechnology methods is of a poor quality and low concentration which may need to be distilled to produce more oncentrated ethanol, which can then be used to make products like whisky or brandy.
- The ethanol sollution is put in a flask below a frationing 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 collected.
Sustainablity of the processes:
- Both processes use renewable feedstocks
- Low temperatures and pressures are used, meaning little energy and low cost
- Carbon dioxide is released which is a green house gas so adds to global warming
- Atom economy is low
- You have to distill the ethanol which uses energy
Making Ethanol using the Synthetic method
Fermentation and biotecnology are too slow for making ethanol on a large scale. Instead, ethanol is made on an industrial scale using ethane. The synthetic method allows high quality ethanol to be produced continuously and quickly.
Ethane is one of the hydrocarbons found in crude oil and natural gas. It is 'cracked' (split) to form ethene (C2H4) and hydrogen gas.
Ethene will react with steam (H2O) to make ethanol.
The reaction needs a temperature of 300C and a pressure of 70 atmospheres.
Phosphorica acid is used as a catalyst.
- The raw materials; crude oil and natural gas are non-renewable
- High energy costs due to the high temperature and pressure needed
- High atom economy, hydrogen is the only waste from cracking the ethane, which is then re-used in the Harber Process
- It's very profitable
Carboxylic acids have the functional group -COOH
The functional group gives them all similar properties. Their names all end in '-anoic acid'
Carboxylic acids react with alkalis, carbonates and reactive metals just like any other acid. The salts formed in these reactions end in '-anoate' e.g. methanoic acid forms a methanoate.
Carboxylic acids react with metals to give a salt and hydrogen
Carboxylic acids react with carbonates to give a salt, water and carbon dioxide
Carboxylic acids react with alkalis to form salt and water
Carboxylic acids are weak acids. They are less reactive than the strong acids; hyrochloric, sulphuric and nitric acids.
Dilute solutions of these weak acids will have higher pH values than dilute solutions of strong acids. For example dilute ethanoic acid will have a higher pH than dilute sulphuric acid.
Carboxylic Acids continued
Carboxylic acids often have strong smells and tastes, they're the reason why socks smell after sport and why gone off(rancid) butter tates horrible.
If wine or beer is left in the open air, the ethanol is oxidised to ethanoic acid. This is why drinking wine after it's been open for a few days is like drinking vinegar-it is vinegar.
ethanol + oxygen --> ethanoic acid + water
Vinegar is a weak solution of ethanoic acid.
Esters have the functional group -COO-.
They're formed from an alcohol and a carboxylic acid. It's called an esterfication reaction.
To start the reaction you need to add a strong acid catalyst to the mix, concentrated sulphuric acid is often used.
Many esters have pleasant smells which are often sweet and fruity. The nice fragrances and flavours of lots of fruits come from esters.
They're also volatile. This makes them ideal for perfumes (the molecules evaporate easily, so can drift to the smell receptors in your nose).
Esters are also used to make flavourings and aromas e.g. there are esters that smellor taste of rum , apple, banana, grape, etc.
Some esters are used as solvents for paint, ink, glue and in nail varnish remover.
Esters are also used as plasticisers.
Fats and Oils
Fatty acids are carboxylic acids with long chains. They often have between 16 and 20 carbon atoms.
Glycerol is an alcohol. Fatty acids and glycerol combine to make fats and oils. Most of a fat or oil molecule consists of fatty acid chains which give them many of their properties.
Fatty acids can be saturated (only C-C single bonds) or unsaturated (with C=C double bonds).
Fats have lots of energy packed into them, so they're good at storing energy.
When an organism has more energy than it needs it stores the extra as fat. The fat can then be used later on when the organism needs more energy.
The fats that plants and animals make have different properties:
Animal fats have mainly saturated hyrocarbon chains. They contain very few C=C bonds. They're normally solid at room temperature.
Vegetable oils have mainly unsaturated hyrocarbon chains. They contain lots of C=C bonds. They're normally liquid at room temperature.
How to make an Ester
The reaction between carboxylic acid and alcohol to make an ester is reversible so some of the ester will react with the other product (water), and re-form the carboxylic acid and alcohol. To get a pure ester you need a multi-stepreaction and purification procedure.
1. Refluxing-the reaction
To make ethyl ethanoate you need to react ethanol with ethanoic acid, using a catalyst such as concentrated sulpuric acid to speed the reaction up. Heating the mixture also speeds the reaction up, but you can't use a bunsen burner as the ethanol will evaporate or catch fire before it can react. Instead, the mixture's gently heated in a flask fitted with a condenser which catches the vapours and recycles them back into the flask giving them time to react. This method is called refluxing.
Distillation separates the ester from the unreacted alcohol and carboxylic acid, the sulphuric acid and water.The mixture is heated below a fractioning column. As it starts to boil, the vapour goes up the fractioning colum. When the temperature at the top of the column reaches the boiling point of ethyl ethanoate, the liquid that flows out of the condenser is collected. This liquid is impure ethyl ethanoate.
How to make an Ester continued
The liquid collected (the distillate) is poured into a tap funnel and then treated to rempve its imprities as follows:
The mixture is shaken with sodium carbonate solution to remove acidic impuritie. Ethyl ethanoate doesn't mix with water in the sodium carbonate solution, so the mixture separates into two layers, and the lower layer can be tapped off(rmeoved).
The remaining upper layer is then shake with concentrated calcium chloride solution to remove any ethanol. Again, the lower layer can be tapped off.
Any remaining water in the ethyl ethanoatecan be removed by shaking it with lumps of anhydrous calcium chloride, which absorb the water, this is called drying. Finally, the pure ethyl ethanoate can be separated from the solid calcium chloride by filtration.
Qualitative analysis tells you which substances are present in a sample.
Quantitative analysis tells you how much of a substance is present in a sample.
Quantitative analysis can be used to work out the molecular formula of the sample.
Usually, just a sample of the material under test is anylsed because it may be difficult to test all of the material if there's a lot of it, or it may be desired to test only a small amount so that the rest can be used for something else. Taking a smaple also means that if something goes wrong with the test, you can go back for another sample and try again. A sample must represent the bulk of the material being tested.
Samples are usually tested in solution. A solution is made by dissolving the sample in a solvent. There are two types of solution: aqueous and non-aqueous. The type of solution used depends on the type of substace being tested.
An aqueous solution means the solvent is water. They're shown by the state symbol (aq).
A non-aqueous solution means the solvent is anything other than water, e.g. ethanol.
Standard procedures are agreed methods of working that are chosen because they are the safest, most effective and most accurate methods to use. They are clear instuctions describing exactly how to carry out a specific practical task.
Standard procedures can be agreed within a company, nationally or internationally.
They're useful because wherever and whenever a test is done, the result should always be the same; it should give reliable results every time.
There are standard procedures for the collection and storage of a sample, as well as how it should be analysed.
Chromotography is an analytical method used to separate out substances in a mixture. They can then be used to identify the substances. There are many types of chromotography which all have two 'phases':
A mobile phase- where the molecules can move. This is always a liquid or a gas.
A stationary phase- where the molecules can't move. This can be a solid or a very thick liquid.
The components in the mixture separate out as the mobile phase moves across the stationary phase.
How quickly a chemical moves depends on it 'distributes' itself between the two phases, this is why different chemicals separate out and end up at different points.
The molecules of each chemical constantly move between the mobile and stationary phases.
They are said to reach dynamic equilibrium. At equilibrium the amount leaving the stationary phase for the mobile phases is the same as the amount leaving the mobile phase for the stationary phase. However, this doesn't necessarily mean there is the same amount of chemical in each phase.
Paper and Thin-Layer Chromatography
In paper chromatography, a spot of the substance being tested is put onto a baseline on the paper. The bottom of the paper is placed in a beaker containing a solvent, such as ethanol or water. The solvent is the mobile phase. The stationary phase is the chromatography paper.
- 1. The solvent moves up the paper.
- 2. The chemicals in the sample dissolve in the solvent and move between it and the paper. This sets up an equilibrium between the solvent and the paper.
- 3. When they're in the mobile phase the chemicals move up the paper with the solvent.
- 4. Before the solvent reaches the top of the paper, the paper is removed from the beaker.
- 5. The diferent chemicals in the sample form separate spots on the paper. The chemicals that spend more time in the mobile phase than the stationary phase form spots further up the paper.
Thin-layer chromotography is very similar to paper chromatography, but the stationary phase is a thin layer of solid e.g. siica gel spread onto a glass plate. The mobile phase is a solvent such as ethanol.
In paper and thin-layer chromatography, the amount of time the molecules spend in each phase depends on two things:
- How soluble they are in the solvent
- How attracted they are to the paper
Molecules with higher solubility in the solvent, and which are less attracted to the paper, will spend more time in the mobile phase and they'll be carried further up the paper.
Gas chromotography is used to analyse unknown substances too. If they're not already gases, then they have to be vaporised. The mobile phase is an unreactive gas such as nitrogen. The stationary phase is a viscous (thick) liquid, such as an oil.
The process is quite different from the other two types of chromatography:
- 1. The unknown mixture is injected into a long tube coated on the inside with the stationary phase.
- 2. The mixture moves along the tube with the mobile phase until it comes out the other end. Like in the other methods, the substances are distributed between the phases.
- 3. The time it takes a chemical to travel through the tube is called the retention time.
- 4. The retention time is different for each chemical and it's what is used to identify it.
Analysis - Chromatography
The result of chromatography analysis is called a chromatogram.
For paper and thin-layer chromatography-Some of the spots on the chromatogram might be colourless . If they are, a locating agent must be used to show where they are, e.g. it may be necessary to spray the chromatogram with a reagent.
Working out the Rf values for spots (solutes) on a chromatogram is essential. An Rf value is the ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent. It is found using this formula:
Rf= distance travelled by the solute/distance travelled by the solvent
Chromatography is often carried out to see if a certain substane is present in a mixture. A pure, known sample is run alongside the unknown mixture. If the Rf values match, the substances may be the same (although it doesn't prove that they are the same). Chemists use a substance called standard reference materials (SRMs) to check the identities of the substances. These have carefully controlled concentrations and purities.
The chromatogram from gas chromatography is a graph. Each peak on the graph represents a different chemical. The distance along the x-axis is the retention time which can be looked up to find out what the chemical is. The peak height shows how much of the chemical was in the sample.