OCR AS - Chemistry Unit F322 - Chains, energy and resource - REVISION!

cant remeber module 4 properly ;/

heres a question - what is le chatiliers principle and how does it effect the production of ammonia

Can anyone explain ozone and the chlorine radicals equations?

le Chateliers principle states that is the conditions of an equilibrium are changed (e.g. a temperature change), then the equilibrium will shift to minimise this change.

For ammonia production the reaction has less moles of gas on the right hand side of the equation (which contains ammonia only), and it is exothermic when producing ammonia. Therefore, if you want the best yield of ammonia, then a low temperature (so more ammonia is produced to create more heat) and low pressure (so less moles are produced when creating ammonia, reducing volume and pressure) are ideal conditions.

However, at low temperatures such as room temp, the reaction isn't noticeable because its too slow. Therefore a temperature of 400-500 degrees celsius is used to maximise rate. A pressure of 200 atmospheres is used, as going much higher would create a safety risk for workers in the factory. Although the iron catalust has little effect, as it speeds up both forward and reverse reactions, it allows less energy to be used to heat the mixture.

Hope this helps...



Chlorine radicals are formed when UV light shines on something like a CFC (chlorofluorocarbon), as it contains chlorine. The UV light comes from sunlight, and uses breaks the bond by homolytic fission-this produces radicals, not ions. Radicals are a species with an unpaired electron, and are very reactive. The radicals are shown by dots near the symbol, but they often dont care in exam questions.

So, the radicals react with an ozone molecule as Cl. + O3 -> O2 + ClO . None of these are radicals. However, the second reaction in ClO + O -> Cl. + O2. The single O atoms are formed during the ozone production cycle. This essentially breaks O3 -> O2 + O. As the chlorine isn't used up, the chlorine radicals act as a catalyst-they ask that all the time in the exam questions.

Hope this helps...

can someone explain how to calculate enthalpy of combustion if u r given the values for enthalpy of formation. do u just use enthalpy of formation so make a digram that is based on enthalpy of formation so A=C-B

ooooh had my biology today, now just my chemistry -_- quite confident though, only a few things to brush up on, anyone need anything?

Help...I dont understand the equilibrium stuff on le chateliers principal and the last topic on green chemistry!

right. the equilibrium is ok once you understand it:

when pressure increases the position of equilibrium moves to the side with the lowest amount of gas.
when temperature increases the position usually moves to the right (if exothermic reaction) never seen an endothermic one though.


Green chemistry is how a process can be improved to make it more economic.
such as using all products, using sustainable raw materials, using catalysts to reduce costs.

any help?

Help...I dont understand the equilibrium stuff on le chateliers principal and the last topic on green chemistry!

kinda, but where does CFC's come into it?

write it as:

A + B =============> C + D
....../\........................................./\
.......|..........................................|
.......enthalpy products of formation

write in the values and follow the route round the bottom way, if you go against arrows the its - the answer, if its with the arrows its plus

with combustion values you do the arrows the other way round

here is a question guys, I'd like it in Your words.


Describe ways that research chemists are trying to minimise climate change resulting from global warming caused by the release of greenhouse gases.

i had the same problem in january as i'm having now, chemistry always comes last and i'm always shattered by then when i need to reviseeee!

this isnt how we learnt it we have to draw a diagram with reactants and products and eithe rcombustion products or elements depending on which reaction we are doing. its on page 199 of the textbook