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AQA CHEMISTRY 2
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Niki
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Joined: 02/01/2009 - 14:22
Posts: 10
AQA CHEMISTRY 2
Niki
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Joined: 02/01/2009 - 14:22
Posts: 10
AQA CHEMISTRY 2

Need helpi dont undersnt habers process and stuff i have the worst teacher in the world :mad::(

chemistrysubjec...
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Joined: 10/06/2007 - 22:31
Posts: 347

Do you understand the ideas about reversible reactions and equilibria?

Haber process is one special case of a reversible reaction that forms an equilibrium.

You need to know the conditions for the industrial process and then understand the reasons for these choices.

For any equilibrium there are two things that you can change and three things you need to think about.

The two things you can change are pressure and temperature.

The three things to think about are percentage yield, rate and cost.

Think about yield:

The choice of high pressure or low pressure depends on how many gas moles there are on the left and right of the equation. If there are more moles of gas on the left hand side, a high presure favours the forward reaction. Thew choice of high or low temperature depends on whether the reaction is endothermic or exothermic in the forward direction. An exothermic reaction is favoured by low temperature.

For Haber process you would want a high pressure and a low temperature.

Think about rate:

Both high temperature and high pressure would lead to a fast rate of reaction.

Think about cost:

The cheapest conditions would be normal ("ambient") temperature and pressure. Slight variations can be achieved quite cheaply but a huge pressure is expensive to produce (energy required) and maintain (high grade plant (pumps, tanks etc)).

Compromise:

Although a high pressure would be great for the yield and rate, it would be very expensive. Although a low temperature would give a good yield, the reaction would be very slow. A higher temperature for a better rate would give a poorer yield.

A catalyst can help (iron-based catalyst is used)

Hope this helps, get back online and let me know if you need more help.

MKIA
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Joined: 31/05/2009 - 14:46
Posts: 4

Also...
it's one of those reactions where you can receive 100% yeild, being reversible.
The best thing about the harbor process is that it recyles the left over reactants, returning them to the reation vessle.
A negative point of it could be the source of raw material (hydrogen) which would most likely come from unsustainable sources or from the electolysis of brine, which used ample amount of energy thus harming the enviroment and using up fossil fuels!
Nitrogen on the other hand, is fairly easy to get hold off, there's plenty of it in the air!
... that's all i can think of so far..

chemistrysubjec...
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Joined: 10/06/2007 - 22:31
Posts: 347

Please don't confuse % yield with the idea of reversibility in this way.

When the exam asks about % yield it is referring to the percentage of the mixture that has been converted to ammonia under the conditions of pressure and temperature that have been chosen. This is nothing to do with the fact that the "waste" products are recycled to try to react again.

By increasing the pressure, the percentage of ammonia is increased. By increasing the temperature, the percentage of ammonia is decreased. These changes to the percentage yield have to be balanced against the changes in the rate of reaction and the cost of creating those conditions.

coco123
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Joined: 02/06/2009 - 19:03
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harber process

the harber process is an important industrail process.it produces amonia which is used to make fertilisers

we need nitrogen and hydrogen to make amonia
the reaction is reversible-it occurs in both directions-not all the nitrogen and hydrogen will convert to ammonia. the reaction reaches a dynamic equilibrium

hope dis will do u sum gud

Niki
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Joined: 02/01/2009 - 14:22
Posts: 10
thnx

thnx i did my exam and it helped thankyou:D

chemistrysubjec...
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Joined: 10/06/2007 - 22:31
Posts: 347

It is good to hear that it helped. Remember the site when you go on to other exams in the future.