Haber Process - balance (1 Viewer)

[nerdd]

hey guys.. looking at the 2nd syllabus dot point in Chemical Monitoring & Management..

I'm looking at the textbook, the 'chem in a month' and my class notes but i can't really find anything that answers the question "Why the Haber process is based on a delicate balancing act involving reaction energy, reaction rate and equilibrium".

can anybody help?

 

[chunder]

because otherwise the process would become economically unsound.

Too much reaction energy means the reaction rate would be fast but the yield would be low (i.e. an increased tendency for the reverse reaction) as the Haber process is an exothermic reaciton. Therefore a balance is required in order for an acceptable yield to be achieved over an economically sound period of time.

 

[Giant Lobster]

Yeah, the N2 + 3H2 --> 2NH3 reaction is an equilibrium and exothermic

High temp = equilibrium lies to the left (Le Chatelier's principle)
Low temp = low reaction rate
High pressure = high reaction rate + more ammonia produced (but i dunno why they dont increase the pressure to 3000 atms or higher.. something i may have missed)

Also more balancing that are probably not required by that particular syllabus point is the fact that hydrogen is obtained from methane and nitrogen from air. Balancing is required to eliminate all H2O, CO and CO2 byproducts before the reactants are sent to the haber process.

 

[kini mini]

Originally posted by Giant Lobster
High pressure = high reaction rate + more ammonia produced (but i dunno why they dont increase the pressure to 3000 atms or higher.. something i may have missed)

Increasing the pressure forces the equilibrium to the right by Le C's principle as the reactants have four times the volume of the products. It's expensive to do so I think, so you have to do a cost-benefit analysis...

edit: clearly I can't read

My notes, from Pathways IIRC.

Explain why the Haber process is based on a delicate balancing act involving reaction energy, reaction rate and equilibrium

 As the temperature is increased more reactant molecules collide with sufficient kinetic energy to exceed the reaction activation energy
 the reaction rate between nitrogen and hydrogen to form ammonia increases.
 However increasing temperature favours decomposition of the ammonia product (Le Cs principle). A compromise temperature providing a satisfactory reaction rate and satisfactory yield of ammonia is selected.
 To achieve an economic yield of about 30%, the temperature is raised to 525C and pressure of 35 000 kPa (35 MPa or 345 atm) is used.
 The ammonia product is also extracted from the reaction chamber as it is produced
 by Le Cs principle this increases the forward rate as the system tries to re-establish equilibrium

 

[chunder]

one should not forget the use of an iron-iron oxide catalyst. without the catalyst a temperature much greater than 500 degress would be needed.

 

[Giant Lobster]

Magnetite is the catalyst

and increasing the pressure fill force the equilibrium to the right, doesn't it?

3H2 + N2 --> 2NH3

4:2 part ratio, increase pressure = more NH3 right?

 

[inasero]

why is it a delicate balancing act? because you could just decrease the temperature and increasre the pressure indefinitely in order to 'theoretically' drive the yield to a maximum...but in reality this isn't the case because what you'll find is that although the yield does in fact increase, the rate of reaction at the end is significantly lower ...this is because some energy (ie that provided by kinetic energy, which in turn is provided by heat energy) is required to overcome the activation energy barrier otherwise you'd be sitting there all day waiting for the conversion into ammonia products....as for pressure its a delicate balancing act because you'd ideally want to have the highest possible pressure without damaging the catalysys or blowing up the reaction apparatus, thus posing a safety hazard

 

[iambored]

Low temperature = high yield = long time, even with catalyst
High temperature = low yield = equilibrium reached quickly

so, if its high temp, u get equilibrium quickly but LOW yeild, so its no good

if its low temp, u get a good yield, but it takes AGES

so if u balance, u can get a moderate yield, moderately fast

 

[Giant Lobster]

Originally posted by inasero
as for pressure its a delicate balancing act because you'd ideally want to have the highest possible pressure without damaging the catalysys or blowing up the reaction apparatus, thus posing a safety hazard

Thanks, that cleared up my query on the effect of excessive pressures.

 

[Constip8edSkunk]

cost is also a factor y u dun want 2 high temperature(along wif rate, which is more important) and superhigh pressure... mainly effects pressure tho.