VCE Chemistry Question Thread

[blacksanta62]

Hey, can anyone remind my brain of what the value of K indicates? I remember that It allows us to see if a reaction has gone backwards of forward but I've forgotten if a large K means net forward or a small K means net forward etc.
Thank you

[zsteve]

Hey, can anyone remind my brain of what the value of K indicate? It allows us to see if a reaction has gone backwards of forward.
Thank you

The K for a specific chemical reaction gives us a quantitative indication of the extent to which the reaction occurs (i.e. the position of equilibrium, to the left or to the right). So a LARGE K value means the reaction occurs to a large extent, a small K value means the reaction occurs to a lesser extent.

[blacksanta62]

Cheers zsteve Did you see my edit after posting, I added some more info so that my question wasn't so broad

[knightrider]

Just want some clarification on something.

If we are talking about equilibrium shifts.

If we say there will be a net foward reaction, will we lose marks.

Do we have to say shift to right ?

[sweetiepi]

Just want some clarification on something.

If we are talking about equilibrium shifts.

If we say there will be a net foward reaction, will we lose marks.

Do we have to say shift to right ?

Teachers usually prefer you to say net forward/net backward reaction, however, afaik, both net forward/backward and shift left/right are acceptable.

[blacksanta62]

Yo, just have some questions that I would love some help with:
1) With the attachment, part e, is i reasonable to have this as an answer: The volume of the sealed vessel has been reduced, increasing the pressure. The system partially opposes this by trying to reduce the pressure ==> forward reaction favoured as less particles are made

I have the solutions but they didn't answer to this depth
2) I originally thought that more reactants and products were added, is this ever the case/even possible? I looked at the solutions and realised I was wrong, then thought about why pressure was the answer. It makes sense but is adding both products and reactants possible?
Thank you

[jyce]

Yo, just have some questions that I would love some help with:
1) With the attachment, part e, is i reasonable to have this as an answer: The volume of the sealed vessel has been reduced, increasing the pressure. The system partially opposes this by trying to reduce the pressure ==> forward reaction favoured as less particles are made

I have the solutions but they didn't answer to this depth
2) I originally thought that more reactants and products were added, is this ever the case/even possible? I looked at the solutions and realised I was wrong, then thought about why pressure was the answer. It makes sense but is adding both products and reactants possible?
Thank you

Part e asked for the change only; it did not ask you to explain why. So, while your reasoning is sound, you weren't expected to go into that much detail. Be careful of this in your end-of-year exam - giving more detail than is required is a) a waste of time and b) could see you losing marks if anything extra you state is wrong.

In regards to your second question, keep in mind that adding more reactants and products would increase the pressure. But it is more likely that the pressure was increased by reducing the volume of the container. The reason being because ALL of the concentrations of all the species involved increased, and they did so instantaneously.

Just want some clarification on something.

If we are talking about equilibrium shifts.

If we say there will be a net foward reaction, will we lose marks.

Do we have to say shift to right ?

Not sure why you would lose marks for using "net forward reaction"? 

[blacksanta62]

Thanks jyce, I'll make sure to not do more than required in the exam.
Another question, is this an okay generalisation: If the concentration of all the products and reactants increase instantly, the volume of the sealed vessel has been decreased leading to an increased pressure
Just need something to remember if I ever run into a question like this again

[jyce]

Thanks jyce, I'll make sure to not do more than required in the exam.
Another question, is this an okay generalisation: If the concentration of all the products and reactants increase instantly, the volume of the sealed vessel has been decreased leading to an increased pressure
Just need something to remember if I ever run into a question like this again

Yeah, that's fine.

[blacksanta62]

Yeah, that's fine.

Thanks

[knightrider]

Teachers usually prefer you to say net forward/net backward reaction, however, afaik, both net forward/backward and shift left/right are acceptable.

Not sure why you would lose marks for using "net forward reaction"? 

Thankyou so much insanipi and jyce 

yea thats what i thought as well jyce,   but apparently if i use "net forward reaction" examiners wont understand what i am saying.
i lost marks in the sac for not saying left or right(instead i used net foward/net backwards)

The funny thing is examiners use net fowards / net backwards in assessors reports lol  ( i will defs be showing that to my teacher, i was really annoyed after i lost a mark for that reason.)

[NimaCr7]

hey !

can someone plz explain this question to me especially where it says " decrease the yield of the reaction"?

thanks!

[zsteve]

hey !

can someone plz explain this question to me especially where it says " decrease the yield of the reaction"?

thanks!

Hi NimaCr7! Happy to help, but for future questions please explain exactly what it is you don't get about the question. It's important in that it makes it easier for me to help you, and also aids your own learning.

Clearly, decreasing the volume of the reaction vessel will increase the concentration of all chemical species in the reaction vessel, both reactants and products. This occurs regardless of equilibrium effects. Exactly what do I mean by this? Well, decreasing the volume will cause the system to shift LEFT, as there are fewer reactant particles. This causes more NH3 to be produced, and less N2/H2 in the system. HOWEVER, the new equilibrium concentrations of all species (NH3, N2, H2) will still be higher than the old equilibrium concentrations.

It's important to remember that Le Chatelier's Principle predicts only a partial opposition of change.

So we agree that this will increase product concentration. However, because equilibrium shifts to the left, we get a lower mole amount of product, hence a lower yield of reaction.

There's an important distinction between concentration and mole amounts here, too - here, we get fewer moles. But as we're decreasing the volume, the concentration can increase.

Hence the answer is B.

Hope that helps!

[jyce]

Thankyou so much insanipi and jyce 

yea thats what i thought as well jyce,   but apparently if i use "net forward reaction" examiners wont understand what i am saying.
i lost marks in the sac for not saying left or right(instead i used net foward/net backwards)

The funny thing is examiners use net fowards / net backwards in assessors reports lol  ( i will defs be showing that to my teacher, i was really annoyed after i lost a mark for that reason.)

The only way I could see you having lost a mark for that is if you just wrote "forward reaction", instead of "net forward reaction".

But yeah, if you wrote "net forward reaction" I'd be bringing that up with your teacher 

[knightrider]

The only way I could see you having lost a mark for that is if you just wrote "forward reaction", instead of "net forward reaction".

But yeah, if you wrote "net forward reaction" I'd be bringing that up with your teacher 

Thanks Jyce 

Just wondering whats wrong with using "forward reaction" ?