**Chemistry...Before/After

reznov

The reaction is clearly going from left to right. What backward arrow are you talking about? It just means that the reaction is dynamic. We had Triioidie at equilibrium I.e. the product. It goes on top. Why would we invert anything or am I just missing something?

It couldn't be any other way as far as I can see. We werent taught how to find the Qc to determine to which side the equilibrium shifts. Every worked problem has simply gone from left to right.

FaoiSin

reznov wrote: »

The reaction is clearly going from left to right. What backward arrow are you talking about? It just means that the reaction is dynamic. We had Triioidie at equilibrium I.e. the product. It goes on top. Why would we invert anything or am I just missing something?

Yeah I'm pretty sure tri-iodide was the product. It's just unsettling that the arrows were different this year :P

reznov

YeaaahFaoiSin wrote: »

Yeah I'm pretty sure tri-iodide was the product. It's just unsettling that the arrows were different this year :P

How were they different? They were exactly the same. The arrows indicate a dynamic reaction. WHICH it should be at equilibrium. :P

FaoiSin

http://www.chemistry.mtu.edu/~kmsmith/PChem/Experiments/3511/Keq/Keq2.pdf

Boo ya!

FaoiSin

reznov wrote: »

How were they different? They were exactly the same. The arrows indicate a dynamic reaction. WHICH it should be at equilibrium. :P

Ah it's just they had the reverse arrow on top and forward on bottom which never happened in previous years. Doesn't mean anything though :P

reznov

YeaaahFaoiSin wrote: »

http://www.chemistry.mtu.edu/~kmsmith/PChem/Experiments/3511/Keq/Keq2.pdf

Boo ya!

Exactly. It'd be illogical any other way.

Liveforrugby

How is nobody seeing this!!!! :eek: God damn it people it was a trick question ffs. You'll see what I mean come the marking schemes

reznov

YeaaahFaoiSin wrote: »

Ah it's just they had the reverse arrow on top and forward on bottom which never happened in previous years. Doesn't mean anything though :P

Don't know what that should change. :P

FaoiSin

It doesn't! It just confused me :P

Liveforrugby I've posted proof so kindly just stop.

reznov

How is it a trick question? There is only one symbol for a dynamic reaction at equilibrium and it is two opposite facing arrows. Nothing on Wikipedia states anything about arrows being significant and also, in my chemistry live book, not a single problem exists with any other arrows but the normal ones. Did I miss something? Please give me where you're sourcing your hypothesis.

hollingr

Liveforrugby wrote: »

How is nobody seeing this!!!! :eek: God damn it people it was a trick question ffs. You'll see what I mean come the marking schemes

Question starts with Iodine crystals reacting with Iodide.

By scientific convention the PRODUCTS form the numerator.. The arrow direction is completely irrelevant.

The Kc value is an indication of which side of the reaction is favoured, if Kc>1 the forward reaction is favoured, if less than 1 the reverse reaction is favoured. By inverting it you are throwing the whole convention off. This whole arrow thing you are on about is just not logical, it's about products and reactants, and the Iodine is a reactant NOT a product in the question.

there is absolutely no way they can justify taking marks away on this, in this house we obey the laws of thermodynamics!!!!!!!!

http://www.youtube.com/watch?v=Xy0UBpagsu8

Heatproof

Is a mixed bed resin the answer to Q7 a) for the hard water softening question ?

Liveforrugby

reznov wrote: »

How is it a trick question? There is only one symbol for a dynamic reaction at equilibrium and it is two opposite facing arrows. Nothing on Wikipedia states anything about arrows being significant and also, in my chemistry live book, not a single problem exists with any other arrows but the normal ones. Did I miss something? Please give me where you're sourcing your hypothesis.

Well I suppose think of it this way. What is the forward reaction? Had the reaction been the other way around would you still have done the same thing or would you have noticed?

What is the inverse of Kc, why does it even exist? How can you tell what the forward reaction is? Does it matter?

RedTexan

Tankosaur wrote: »

The na2co3 is 0.05 M/ 500 cm^3 so the titration value you get for hcl most also be M/500cm^3 which came to 0.12 or summit like that.

But the question asked for M/L so you must multiply by 2 and it comes to 0.24 Moles/litre

It's just Va x Ma all over Na equal to the equivalent base, it's all the same, there's no need for multiplying by 2

reznov

Liveforrugby wrote: »

Well I suppose think of it this way. What is the forward reaction? Had the reaction been the other way around would you still have done the same thing or would you have noticed?

What is the inverse of Kc, why does it even exist? How can you tell what the forward reaction is? Does it matter?

I don't know what the inverse of Kc is. Care to enlighten me? I haven't had to invert anything in the past papers or Declan Kennedy's Chemistry Live workbook questions.

RedTexan

Oh and for all this equilibrium trouble, the directions of the arrows is irrelevant, it is clearly stated that the products of the reaction form the numerator of the Kc, which in this case were the triiodide ions. You can read it here if you want http://en.wikipedia.org/wiki/Chemical_equilibrium

hollingr

reznov wrote: »

I don't know what the inverse of Kc is. Care to enlighten me? I haven't had to invert anything in the past papers or Declan Kennedy's Chemistry Live workbook questions.

Inverse of Kc is the equilibrium constant if the reactants were forming the products, so in this case if

I3 -> I2 + I-

In the question the reaction starts with Iodine and Iodide though so he's wrong.

If the question said: "what is the Kc value for triiodide decomposing to form I2 and I-" he'd be right. They would have to specifically ASK for the opposite. The arrows are not an indication of which is the reactant and which is the product, which is the basis for equilibrium equations.

reznov

The form of equations is simple too Stalin:
Reactants to Products.

Doubt the SEC decided to screw everyone over.

hollingr

reznov wrote: »

The form of equations is simple too Stalin:
Reactants to Products.

Doubt the SEC decided to screw everyone over.

Even if he IS right, it'd be 3 marks for the inversion, not worth getting panties in a bunch over.

Even a phd chemist would have gotten that "wrong"

Dicksboro_man

and realistically, with the chemistry paper there is no such thing as trick questions. Especially not on the topic thats not the most popular among students (comparing to organic or atomic theory)..

You tried to be too smart for your own good buddy, unlucky, you'll only lose a few marks.

RedTexan

How many marks will I lose for having the heat of formation equation backwards and coming out with a heat change of 224 instead of-224?

hollingr

If they wanted to add a reaaaaaal sneaky trick, they would have added the crystals to 1 L of water each and then combined them so the "reaction vessel" was 2 litres, so we'd have to divide by 2.. but they were nice this time around!

@tex: 3 marks usually, going by previous past papers, as long as you wrote out the full equations and balancing.. I made the same mistake

reznov

hollingr wrote: »

If they wanted to add a reaaaaaal sneaky trick, they would have added the crystals to 1 L of water each and then combined them so the "reaction vessel" was 2 litres, so we'd have to divide by 2.. but they were nice this time around!

Even then, that's just simple and anyone who had solved a couple of equilibrium questions would be are of that "trick".

Damn SEC trying the arrow method. :rolleyes:

Liveforrugby

reznov wrote: »

I don't know what the inverse of Kc is. Care to enlighten me? I haven't had to invert anything in the past papers or Declan Kennedy's Chemistry Live workbook questions.

Pg 241 point 4. Inverse kc

Dicksboro_man wrote: »

and realistically, with the chemistry paper there is no such thing as trick questions. Especially not on the topic thats not the most popular among students (comparing to organic or atomic theory)..

You tried to be too smart for your own good buddy, unlucky, you'll only lose a few marks.

I didn't do it just to be sure. There was a fair amount choice

paddyhes

Wait now :L
What was the actual Kc value? :L

RedTexan

paddyhes wrote: »

Wait now :L
What was the actual Kc value? :L

I'm saying 704.95

hollingr

Liveforrugby wrote: »

Pg 241 point 4. Inverse kc



I didn't do it just to be sure. There was a fair amount choice

In that question they actually explain why you would have been wrong to get the inverse, since Kc' is when the reactants and products are swapped, absolutely nothing to do with the arrows..

paddyhes

RedTexan wrote: »

I'm saying 704.95

Would you mind explaining how you got that :L

RedTexan

paddyhes wrote: »

Would you mind explaining how you got that :L

.0793 divided by (.08-.0793)x(.24-.0793)

hollingr

paddyhes wrote: »

Would you mind explaining how you got that :L

Kc= [I3] / [I2][I-]

initially: [I2]= 0.08, [I-] = 0.24, [I3] = 0
change: [I2]= -0.0793, [I-] = -0.0793, [I3] = +0.0793
@equilibrium: [I2]= 0.0007, [I-] = 0.1607, [I3] = 0.0793

So, Kc at equilibrium = (0.0793) / (0.0007)(0.1607)

Kc = 704.95