Equilibrium Double Equation Question

[tcg93]

Why is the answer to this one A and not C?

[Shark 774]

My teacher explained this. I thought I understood, but now I've lost it!! Either way I think this is way beyond the scope of the course. Anyway I will recheck and if nobody has replied by then I will get back to you

[jayna]

i would of said C as well =/...
what exam is this from?

[Mao]

I am not too sure about this one. My calculations show the amount of AgCl(s) actually decreases with dilution... I'll spend more time on this another day, but you guys don't have to worry about running across this.

[Shark 774]

I've got it! Teacher explained it to me

If you add water the bottom reaction will go left (according to LCP) and the top reaction will go right (according to LCP). But because AgCl is not soluble in water (As you see the value of K is very very small) the effect is not evident for the first equation, but it is much more evident for the second one. Therefore more Ag+ is produced by the second equation, which pushes the first equation to the left, making more undissolved AgCl (s). Hope this makes sense.

Also is you add HCl, the Cl- will push the first equation to the left, but the H+ will react with NH3, pushing the second equation to the left also. So the first equation is trying to use up Ag+ and the second equation is trying to produce it, so they will just cancel out.

By the way this is way beyond the scope of the course!

Hope this is all good.

[kenhung123]

I get how to eliminate the the wrong ones however, I still do not understand your explanation as to why A is correct. Even if the K is small it would still shift to the right in the correct proportion to partially eliminate the effects of the addition of water? Your saying that it does not do anything at all therefore accumulation of products?

[fady_22]

I've got it! Teacher explained it to me

If you add water the bottom reaction will go left (according to LCP) and the top reaction will go right (according to LCP). But because AgCl is not soluble in water (As you see the value of K is very very small) the effect is not evident for the first equation, but it is much more evident for the second one. Therefore more Ag+ is produced by the second equation, which pushes the first equation to the left, making more undissolved AgCl (s). Hope this makes sense.

Also is you add HCl, the Cl- will push the first equation to the left, but the H+ will react with NH3, pushing the second equation to the left also. So the first equation is trying to use up Ag+ and the second equation is trying to produce it, so they will just cancel out.

By the way this is way beyond the scope of the course!

Hope this is all good.

Yes, but you are making the assumption that there is Ag(NH3)2+ in the reaction mixture. Which, unless NH3 is added, is an unreasonable assumption.

[toshibaj]

Yes, but you are making the assumption that there is Ag(NH3)2+ in the reaction mixture. Which, unless NH3 is added, is an unreasonable assumption.

It says right there what happens when NH3 is added. It's not that unreasonable an assumption.

Anyway, poorly worded, and overly difficult question IMO.

[meefz]

Soooooo whats the answer?

[Martoman]

This is TSFX. Take it on face value.

More water, second equation goes left.

More Ag+ then the first equation goes left as well. A

For reasons very well explained by Shark 774, adding water to the first one is negligeable due to K.

B is silly, drives the 2nd equation right.

C is partially silly as Shark 774 explained, ammonia is a base so it will mop up some of the HCL's.

D is again silly because it would drive the second equation to the right.

[Mao]

These analysis are logically correct, if the second equilibrium is much stronger than the first equilibrium. That is, dilution affects the second equation much more than the first equation.

When water is added, the immediate changes are all aqueous species have a lower concentration. This implies the first equation will move to the right to increase [Ag+] and [Cl-], and second equation will move to the left to increase [Ag+] and [NH3]. If the second equilibrium is much stronger than the first, [Ag+] will be increased by a significant amount, and thus pushing the first equilibrium to the left. But this occurs only if the maths agrees with 'second equilibrium is much stronger than the first'.

My initial calculations are showing that this is not the case. This system is not easily solved analytically, I am numerically approximating equilibrium concentrations and then altering the system after equilibrium is achieved. What I have found is that diluting this system already in equilibrium (with some NH3/Ag(NH3)+), dilution by a factor of 10 actually decreases the amount of AgCl(s), not increase. I am in the process of debugging the simulation to see if I have made any mistakes, but this could be an epic fail on TSFX's account.

[Linkage1992]

This may be a really dumb question, but why wouldn't increasing the pressure work also? Yeah, it would drive the second equation to the right, but it would also move the first to the left. Or would the  decrease on the bottom mean there is not enough on the top to go to the left?  :idiot2:

[Mao]

This may be a really dumb question, but why wouldn't increasing the pressure work also? Yeah, it would drive the second equation to the right, but it would also move the first to the left. Or would the  decrease on the bottom mean there is not enough on the top to go to the left?  :idiot2:

There's no gas, increasing pressure doesn't change anything.

[Shark 774]

I've got it! Teacher explained it to me

If you add water the bottom reaction will go left (according to LCP) and the top reaction will go right (according to LCP). But because AgCl is not soluble in water (As you see the value of K is very very small) the effect is not evident for the first equation, but it is much more evident for the second one. Therefore more Ag+ is produced by the second equation, which pushes the first equation to the left, making more undissolved AgCl (s). Hope this makes sense.

Also is you add HCl, the Cl- will push the first equation to the left, but the H+ will react with NH3, pushing the second equation to the left also. So the first equation is trying to use up Ag+ and the second equation is trying to produce it, so they will just cancel out.

By the way this is way beyond the scope of the course!

Hope this is all good.

Yes, but you are making the assumption that there is Ag(NH3)2+ in the reaction mixture. Which, unless NH3 is added, is an unreasonable assumption.

As toshibaj said, it clearly says in the question that NH3 IS added.