HELP! HSC Chemistry 2010 Paper - Multiple Choice Question (1 Viewer)

[rosiearwen]

Hi guys,

I'm just doing the 2010 paper for exam practice and I'm completely stuck on multiple choice question 18:

Chromate and dichromate ions form an equilibrium according to the following equation.

2CrO₄²⁻(aq) + 2H⁺(aq) --> Cr₂O₇²⁻(aq) + H₂O(l)

Which solution would increase the concentration of the chromate ion (CrO₄²⁻) when added to the equilibrium mixture?

(A) Sodium nitrate
(B) Sodium chloride
(C) Sodium acetate
(D) Ammonium chloride

According to the sample answers, it's C, but I'm not sure why, and obviously with multiple choice questions, they don't give an explanation!

Thanks in advance for any help with this.

Rose

 

[hayabusaboston]

Hi guys,

I'm just doing the 2010 paper for exam practice and I'm completely stuck on multiple choice question 18:

Chromate and dichromate ions form an equilibrium according to the following equation.

2CrO₄²⁻(aq) + 2H⁺(aq) --> Cr₂O₇²⁻(aq) + H₂O(l)

Which solution would increase the concentration of the chromate ion (CrO₄²⁻) when added to the equilibrium mixture?

(A) Sodium nitrate
(B) Sodium chloride
(C) Sodium acetate
(D) Ammonium chloride

According to the sample answers, it's C, but I'm not sure why, and obviously with multiple choice questions, they don't give an explanation!

Thanks in advance for any help with this.

Rose

EDIT: I was wrong lolol, see below for AnimeX answer.

 

[AnimeX]

Hi guys,

I'm just doing the 2010 paper for exam practice and I'm completely stuck on multiple choice question 18:

Chromate and dichromate ions form an equilibrium according to the following equation.

2CrO₄²⁻(aq) + 2H⁺(aq) --> Cr₂O₇²⁻(aq) + H₂O(l)

Which solution would increase the concentration of the chromate ion (CrO₄²⁻) when added to the equilibrium mixture?

(A) Sodium nitrate
(B) Sodium chloride
(C) Sodium acetate
(D) Ammonium chloride

According to the sample answers, it's C, but I'm not sure why, and obviously with multiple choice questions, they don't give an explanation!

Thanks in advance for any help with this.

Rose

So to increase the concentration of the chromate ion (CrO4 2-) you want the equilibrium to shift to the left. This can be done a multitude of ways, let's go through the options.

adding:
(A) Sodium nitrate --> this will increase the concentration of nitrate ions --> no effect on this equilibrium (since no nitrates in the chemical equilibrium)
(B) Sodium chloride --> this will increase the concentration of chloride ions --> again, no effect
(C) Sodium acetate --> this will increase the concentration of acetate ions (CH3COO-), this reacts with the water to produce OH- ions as H2O + CH3COO- --> OH- + CH3COOH, as a result of the increasing OH- ions the H+ ions will react with the OH- ions to produce water, since water is being introduced into the chemical equation the equibirium will shift to minimise the disturbance (LCP) and thus shift it to the left.

(D) Ammonium chloride --> NH4+ and Cl- ions, they aren't in the solution at all so will not affect it in any way.

 

[HeroicPandas]

So to increase the concentration of the chromate ion (CrO4 2-) you want the equilibrium to shift to the left. This can be done a multitude of ways, let's go through the options.

adding:
(A) Sodium nitrate --> this will increase the concentration of nitrate ions --> no effect on this equilibrium (since no nitrates in the chemical equilibrium)
(B) Sodium chloride --> this will increase the concentration of chloride ions --> again, no effect
(C) Sodium acetate --> this will increase the concentration of acetate ions (CH3COO-), this reacts with the water to produce OH- ions as H2O + CH3COO- --> OH- + CH3COOH, as a result of the increasing OH- ions the H+ ions will react with the OH- ions to produce water, since water is being introduced into the chemical equation the equibirium will shift to minimise the disturbance (LCP) and thus shift it to the left.

(D) Ammonium chloride --> NH4+ and Cl- ions, they aren't in the solution at all so will not affect it in any way.

NH4+ is a special one and does affect the equilibrium

 

[hayabusaboston]

WAIT I WAS WRONG
lol good job animex, I didnt realise my error until just then.

 

[HeroicPandas]

WAIT I WAS WRONG
lol good job animex, I didnt realise my error until just then.
but m8, ammonium makes ammonia+ hydronium in water, so it does affect equilibrium, makes it go rightwards.

he said it all

 

[someth1ng]

Be careful when you say that equilibrium was shifted.

Equilibrium is shifted when change conditions like temperature or pressure. What you do when you add more/less material, you disturb the system so that it is no longer at equilibrium. Technically speaking, equilibrium is still at the same place (equilibrium constant, K is the same).

 

[rosiearwen]

Thanks everyone who replied, and thanks especially to AnimeX, that explanation was really helpful!

 

[AnimeX]

NH4+ is a special one and does affect the equilibrium

whoops my bad, yeah since it's amphoteric it will react with the water and produce NH3 + H3O+ ions, which is same as H+, favouring forward rxn.

Be careful when you say that equilibrium was shifted.

Equilibrium is shifted when change conditions like temperature or pressure. What you do when you add more/less material, you disturb the system so that it is no longer at equilibrium. Technically speaking, equilibrium is still at the same place (equilibrium constant, K is the same).

so when increasing the concentration would it be better to say favour the forward/backward rxn?

Thanks everyone who replied, and thanks especially to AnimeX, that explanation was really helpful!

no problem

 

[hayabusaboston]

Thanks everyone who replied, and thanks especially to AnimeX, that explanation was really helpful!

You're IN BRAH!!

 

[hayabusaboston]

love your signature

Is this a sarcastic comment implying since I oversimplified a multiple choice question that I will no longer have a 9 infront of my ATAR? lolol.
or u actually like it?
lol its impossible to tell.

 

[HeroicPandas]

whoops my bad, yeah since it's amphoteric it will react with the water and produce NH3 + H3O+ ions, which is same as H+, favouring forward rxn.

are u sure?

 

[someth1ng]

so when increasing the concentration would it be better to say favour the forward/backward rxn?

Yes, for a solution.