Interesting questions (chem)

[Martoman]

I've done this all over the forum, so I might as well do it here. Check back periodically for updates.

, phase either liquid or gaseous, its inconsequential.

Addition of C by Le Chatileir's principle, means that the forward reaction is favoured, yet the rate of the forward reaction is very low. With specific referece to collision theory, and assuming there is sufficient energy to overcome the activation complex, propose why the rate of the forward reaction is negligible.

[stonecold]

Is it to do with the fact that the concentrations of reactants A and B are very low, so while the forward reaction will be favoured, the the frequency of collisions and therefore successful collisions between reactants A, B and C will be so infrequent, that there will be no noticeable shift in the overall position of the reaction?

[fady_22]

In order for the three reactants A, B and C, to react, they must all collide with enough energy to overcome than the activation energy, and with the correct orientation. Since there are three reactants, the probability that all these conditions are met is low. Hence, the frequency of successful collisions between reactants is low, causing the rate of the forward reaction to be negligible.

[Martoman]

In order for the three reactants A, B and C, to react, they must all collide with enough energy to overcome than the activation energy, and with the correct orientation. Since there are three reactants, the probability that all these conditions are met is low. Hence, the frequency of successful collisions between reactants is low, causing the rate of the forward reaction to be negligible.

Orientation is the key here. Good work fady, this question is meant to probe understanding on that idea of collision theory, which is usually fogotten.

[fady_22]

In order for the three reactants A, B and C, to react, they must all collide with enough energy to overcome than the activation energy, and with the correct orientation. Since there are three reactants, the probability that all these conditions are met is low. Hence, the frequency of successful collisions between reactants is low, causing the rate of the forward reaction to be negligible.

Orientation is the key here. Good work fady, this question is meant to probe understanding on that idea of collision theory, which is usually fogotten.

I just realised that you wrote "assuming there is sufficient energy to overcome the activation complex".

[stonecold]

I clearly fail...

[Martoman]

Yeah there is need to mention that otherwise people will just use the rote learned -> isn't sufficient energy to overcome blah blah.

[Russ]

I clearly fail...

Not really, your answer was a more practical one and fady's was a more theoretical one which was what the question was based on (a la carbon production in stars requiring an intermediate stage)

[Mao]

I'm not sure I understand what you are asking. In real chemical systems, trimolecular elementary steps are very rare, but the products usually undergo a much faster reaction immediately after it is produced, and thus backward reaction is negligible. In this sense, the trimolecular step has a rate that is significant, and the kinetics of the entire system usually rest on this step.

Whilst orientation is important, this depends on a lot of things, including the type of solvents, the shape of the reactants themselves, etc. You will often find that orientation doesn't affect things that much, as room temperature is a fair amount of thermal energy and each molecule rotates/vibrates wildly. There are a lot of question marks still, such as is there a catalyst (an enzyme can speed up such a reaction to a ridiculous rate), or are the molecules small, or are they large macro-biomolecules that can only react at certain sites?

Also, if the original system is at equilibrium, then the forward rate is not negligible. Forward rate = backward rate at equilibrium always, this means if the forward rate is tiny, the backward rate is equally as tiny.

[Martoman]

this was taken from an exam, tsfx if i believe, i thought it was interesting if only because orientation isn't talked about much at all.

[Mao]

this was taken from an exam, tsfx if i believe, i thought it was interesting if only because orientation isn't talked about much at all.

haha, wouldn't surprise me. It's vague as hell, in line with their style.

[kenhung123]

Another exam had it as well but they used "geometry" to describe orientation..

[Blakhitman]

I'm not sure I understand what you are asking. In real chemical systems, trimolecular elementary steps are very rare, but the products usually undergo a much faster reaction immediately after it is produced, and thus backward reaction is negligible. In this sense, the trimolecular step has a rate that is significant, and the kinetics of the entire system usually rest on this step.

Whilst orientation is important, this depends on a lot of things, including the type of solvents, the shape of the reactants themselves, etc. You will often find that orientation doesn't affect things that much, as room temperature is a fair amount of thermal energy and each molecule rotates/vibrates wildly. There are a lot of question marks still, such as is there a catalyst (an enzyme can speed up such a reaction to a ridiculous rate), or are the molecules small, or are they large macro-biomolecules that can only react at certain sites?

Also, if the original system is at equilibrium, then the forward rate is not negligible. Forward rate = backward rate at equilibrium always, this means if the forward rate is tiny, the backward rate is equally as tiny.

Yea nah that is EXACTLY what I was about to post, good work Mao, I see you've been studying your chemistry.

[Martoman]

Equal masses of the following substances are dissolved in seperate samples of water to give 500ml of solution. Explain which solution would have the lowest pH.

Options to be dissolved are:

A) NH3
B) HNO3
C) HCl
D) HClO4

[fady_22]

Equal masses of the following substances are dissolved in seperate samples of water to give 500ml of solution. Explain which solution would have the lowest pH.

Options to be dissolved are:

A) NH3
B) HNO3
C) HCl
D) HClO4

HCl.
A) is a base, so would have a higher pH than the rest of the options (which are acids).
The rest are all strong acids, but HCl would have the greatest number of mole in a given mass (as it has the lowest molar mass) and hence would produce the greatest concentration of H3O+, causing it to have the lowest pH.