help as soon as possible!!! (1 Viewer)

[Vaishali]

i know i am asking a lot of questions but......... i am quite stressed abt the exams.....
1. i cant understand the le chatelier's principle figuring out... most importantly for the exothermic and endothermic reactions....... i know that when delta H is negative, its an exothermic reaction and when its positive its endothermic...
but how the hell to figure out in which direction the reaction wud proceed if we are not given the value of delta H??????? coz i saw in a HSC paper (2004) and it didnt say anything abt the value of delta H.

2. what the hell are we supposed to write for the catalytic cracking thing??/ coz i thought that its only related to the production of ethene and we need to remember it as one of the process for the production of ethene..... but then some ppl r trying to confuse me and make me stress more coz they think that mentioning of zeolites is also important...
what the hell do i do???
plz help NOW!!!

soonest as possible help needed!!!!

 

[Nodice]

1. LeChateliers principle is really easy if you just use a bit of logic. I see it similar, in a way, to Lenz's Law (if you do physics). If you could post up the question, I would answer - but you would probably have to know a little about the reaction anyways eg neutralisation reaction is always exothermic.

2. That catalytic cracking dotpoint is related to the industrial production of ethylene. I just wrote what "cracking" is, where it occurs and what is involved - ie involves a catalyst, known as a zeolite (type of aluminosilicate) which adsorbs reactants thereby weakening their bonds, and occurs in a catalytic crack at 500 degrees. Example reaction: C18H38 --> 4C2H4 + C10H22 (see, it produces ethylene). Knowing zeolites I suppose is partially important (I have seen a multiple choice question that asked what catalyst is used in catalytic crack), but its not the end of the world.
Remember - Relax (stress is one of the worst possible things that can happen and it is often unnecessary - I mean it), the probability of that dotpoint may not be tremendously high, you will do ok if you just have some confidence.

 

[Vaishali]

the question that i saw cannot be found by me right now..
but this is another question which i wanted to ask:
phosgene is prepared from the reaction of carbon monoxide and chlorine in the presence of a catalyst:
CO + CL2 --> COCl2 DELTA H = - 9.93 kJ / mol
which of the following sets of conditions would produce the highest yield of phosgene?
a. high temp, high pressure
b. low temp, low pressure
c. low temp , high pressure
d. high temp, low temp

acc to me, it shud be the pressure should be low coz its a gas and if the pressure is more then the gas will turn into liquid... so the options left are b and c.
then the value of delta H says that the reaction is exothermic... right?
therefore the reaction releasses heat and if we give more heat the reaction may not actually speed up....
so is the answer, b?
plz reply soon
god i think i need hydrogen cynide now... the strongest poison.
to die!!!!!!!!!

 

[Nodice]

Ok first off, when the change in heat is negative, that means the reaction is exothermic and is getting warmer. If we increase the temperature, LeChateliers Principle states that the equilibrium must oppose this and try to get colder, so here the equilibrium will be to the left - this is not what we want, since we are increasing the reactants not the products, so therefore we must have the temperature low (so that it will try to warm up and choose the exothermic path).
The next step is the pressure. "if the pressure is more then the gas will turn into liquid..." This is incorrect thinking. If the pressure is increased, then we have a greater concentration of particles than what was present before. LeChateliers principle says we should do the opposite of this, and so the opposite of increasing the number of particles is decreasing the number of particles. In your equation, there are fewer particles on the products side of the equilibrium, so the we will increase the pressure to increase the yield.
Thus the answer to your question is to reduce temperature and to increase pressure (c).

 

[Riviet]

Just adding on to Nodice's nicely explained answers, decreasing the temperature in a system means that by Le Chatelier's principle, the system will counteract the change by increasing the temperature of the system. To do this it must add heat to the system. It adds heat to the system by "releasing" heat, therefore the equilibrium shifts in the direction of the exothermic reaction. In other words, the system produces more heat and phosgene, which is what we want.
Increasing the pressure in a system, means that by Le Chatelier's principle, the system will try to counteract the change by lowering the pressure, therefore the equilibrium shifts to the side with less particles. In this case we have 1 CO and 1 chlorine, which makes 2, but on the right side of the equilibrium we have 1 phosgene. Since there are less particles on the right side, the equilibrium shifts to the right side, because the equilibrium shifts to the side with less moles of gas.