Haber Process

Section 5: part d) The industrial manufacture of chemicals

-for double award you only need to know the Haber Process, Single Award people, you also need to know the Contact Process for the manufacture of Sulphuric Acid!

5.21 recall that nitrogen from air, and hydrogen from natural gas or the cracking of hydrocarbons, are used in the manufacture of ammonia

So if they ask what are the raw materials used in the Haber process, you know it. It's nitrogen from the air and hydrogen either from natural gas, which is methane (CH₄); or from the cracking of hydrocarbons. (Cracking is in the Crude Oil post.) 

5.22 describe the manufacture of ammonia by the Haber process, including the essential conditions:

i. a temperature of about 450°C

ii. a pressure of about 200 atmospheres

iii. an iron catalyst

Remember these conditions!! And remember that the reaction is reversible. Also, the forwards reactions is exothermic. 

N₂ + 3H₂ ⇌ 2NH₃

So decreasing the temperature would actually increase the yield, however, it is still done at a fairly high temperature to speed up the reaction. It makes the rate of reaction faster so the manufacturers get their ammonia quicker, as they say, time is money. The reaction would be too slow otherwise at low temperatures. It would be useless to have a low temperature and achieve a high yield of ammonia if it's going to take ages. You need the gases to reach equilibrium within the very short time that they will be in contact with the catalyst in the reactor.So 450°C is a compromise, and still produces a reasonably high proportion of ammonia. 

The catalyst does NOT affect the amount of products made. The yield of ammonia stays the same, you just get it faster because it speeds up the reaction by lowering the activation energy needed for the reaction. :) 

Increasing the pressure would favour the forwards reaction which is what is wanted, to get more ammonia. This is because if you look at the balanced equation, there are 4 moles of gas on the reactants side (left) but 2 moles of gas (ammonia) on the right hand side (products). So according to Le Chatelier's principle where you try to remove the change, if you increase pressure, the equlibrium would move to the right hand side to decrease pressure. And the products have less pressure because there are only 2 moles there.

Also, just as an extra, just thought that this will be useful to know and is very logical: :)

Credit for the information goes to 

http://www.chemguide.co.uk/physical/equilibria/haber.html

And no plagiarism was intended! 

Rate considerations

Increasing the pressure brings the molecules closer together. In this particular instance, it will increase their chances of hitting and sticking to the surface of the catalyst where they can react. The higher the pressure the better in terms of the rate of a gas reaction.

Economic considerations
Very high pressures are very expensive to produce on two counts.
You have to build extremely strong pipes and containment vessels to withstand the very high pressure. That increases your capital costs when the plant is built.
High pressures cost a lot to produce and maintain. That means that the running costs of your plant are very high.

The compromise
200 atmospheres is a compromise pressure chosen on economic grounds. If the pressure used is too high, the cost of generating it exceeds the price you can get for the extra ammonia produced.

5.23 understand how the cooling of the reaction mixture liquefies the ammonia produced and allows the unused hydrogen and nitrogen to be recirculated

Separating the ammonia

When the gases leave the reactor they are hot and at a very high pressure. Ammonia is easily liquefied under pressure as long as it isn't too hot, and so the temperature of the mixture is lowered enough for the ammonia to turn to a liquid. The nitrogen and hydrogen remain as gases even under these high pressures, and can be recycled.

Recycling

At each pass of the gases through the reactor, only about 15% of the nitrogen and hydrogen converts to ammonia. (This figure also varies from plant to plant.) By continual recycling of the unreacted nitrogen and hydrogen, the overall conversion is about 98%.

5.24 recall the use of ammonia in the manufacture of nitric acid and fertilisers

So ammonia is used to make nitric acid and fertilisers, as you know from bio, plants need nitrates to grow.

Just in case you want to know, here are some properties of ammonia:

• alkaline gas (turns damp red litmus paper blue, which is the test for ammonia!)
• extremely soluble in water--it forms a weak alkali-->ammonia solution
• less dense than air
• colourless gas with pungent odour

Synthetic polymers

From Section 5, part c) Synthetic polymers


5.14 recall that an addition polymer is formed by joining up many small molecules called monomers

5.15 draw the repeat unit of addition polymers, including poly(ethene), poly(propene) and poly(chloroethene)

5.16 deduce the structure of a monomer from the repeat unit of an addition polymer

Monomers are small molecules that can join up to make a very large molecule called a polymer.

e.g. Starch is a polymer made up of small glucose monomers

The monomer in addition polymerisation is unsaturated, i.e. the monomer must have a double bond. This opens up to allow other monomers to join up to form the polymer. 

Watch- Polymers and Monomers made easy, it's a bit long, about 7 minutes, but it's definitely clear and precise.
If the video doesn't come up here, go to youtube, type

Polymers and Monomers BBLC, and watch! 

Repeat unit of ethene, excuse my poor 'paint' skills, I wish I had a tablet! 

Repeat unit of propene

According to my teacher, you shouldn't write the 'n' for the repeat unit, only for the polymer to show that there are many more monomers attached to the section you have drawn like the following picture, a lot of the repeat unit for chloroethene makes poly(chloroethene)-obviously you're not going to draw a thousand chloroethene monomers for polychloroethene..  

'n' represents a large but variable number. It simply means that the structure in the bracket repeats itself many times in the molecule. 

To deduce the structure of a monomer from the repeating unit or the structure of the monomer, first find the repeat unit, and then put back the original carbon-carbon double bond. (because this opened up to form the repeat unit, whereby other monomers would join end to end as their bonds open up as well). To find the name, count how many carbons there are in the repeat unit.

The reason why the repeat units only show 2 carbons is because we're showing what happened to the double bonds. So even if the monomer is pentene with 5 carbons, your repeating unit shows 2 carbons with 3 hydrogen atoms attached and a C₃H₇ on the remaining branch, as pentene is C₅H₁₀. And please don't forget that there is no double bond in the repeating unit, it has opened up so don't forget these bonds on the ends with a bracket around it too. :)

http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel/fuels/hydrocarbonsrev4.shtml

http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/substancesfromcrudeoil/polymersandethanolrev1.shtml

An update for people still confused about addition polymerisation: 

Ethene is one of the alkenes produced by cracking. It is the smallest hydrocarbon containing a carbon-carbon double bond. (Carbon-carbon double bonds obviously need 2 carbons, and ethene has 2. In alkanes it may start with methane, but there is no such thing as methene, as with one carbon atom only, a carbon-carbon double bond cannot exist.)

Under the right conditions, molecules contain carbon-carbon double bonds can join together to produce very long chains. Part of the double bond is broken, it 'opens up', and the electrons in it are used to join to neighbouring molecules. This is called addition polymerisation. (Don't worry too much about the electrons bit, it's just to help you understand how.)

Polymerisation is the joining up of lots of little molecules (the monomers) to make one big molecule (the polymer). In the case of ethene, lots of ethene molecules join together to make poly(ethene). 

Three ethene monomers

Double bonds 'opening up'

Joining of monomers to form long carbon chain-poly(ethene)

The chain length can vary from about 4000 to 40,000 carbon atoms. For normal purposes, this is written using displayed formulae. 

Please ask your teacher if you are to use the above as a displayed formula for poly(ethene) or  the following, both are technically right but I'm not sure which one would be preferred in exams, so please check, I try to not make mistakes, but you guys still have to beware that it's possible. 

propene (monomer) → poly(propene) (polymer)

The monomers used to make addition polymers are based on ethene, for e.g. propene. 

Although this is the usual way to draw the structural formula for propene, for the purposes of showing how the molecule acts as a monomer and can form a polymer it should be drawn in a different way:

Chloroethene monomer. Now you try to work out how it polymerises. Draw three of these monomers, open up the double bonds, and join them together!

If the structure of the polymer is given then the structure of the monomer can be worked out.

Look for the repeating unit i.e. the part of the molecule that is repeated. 

The repeating unit should be made up of two carbon atoms from the main. 

Now put a double bond between the two carbon atoms to get the monomer. 

Typed from an edexcel book: 

Uses for poly(ethene):

Poly(ethene) comes in two types: low-density poly(ethene) (LDPE) and high-density poly(ethene) (HDPE). Low-density poly(ethene) is mainly used as a thin film to make polythene bags. It is very flexible and not very strong. E.g. those supermarket plastic bags

High-density poly(ethene) is used where greater strength and rigidity is needed-for example to make plastic bottles such as milk bottles. If you can find a recycling symbol with the letters HDPE next to it, then the bottle is made of high-density poly(ethene). If it has some other letters there, then it is a different polymer. 

Uses of poly(propene):

Poly(propene) is somewhat stronger than poly(ethene). It is used to make ropes and crates (among many other things). If an item has a recycling mark with PP inside it or near it, it is made of poly(propene). 

Uses of poly(chloroethene):

Poly(chloroethene)-PVC-has a lot of uses. It is quite strong and rigid, and so can be used for drainpipes or replacement windows. It can also be made flexible by adding 'plasticisers'. That makes it useful for sheet floor coverings, and even clothing. These polymers don't conduct electricity, and PVC is used for electrical insulation. It is now replacing rubber to insulate wires as it doesn't crack so easily, thus it's safer. (My physics blog talks about electricity and its dangers.)