Unit 4 Questions MEGATHREAD!

[HarveyD]

This is the answer :/

At the anode (+) bubbles of a gas are formed. This is a pale yellow-green gas that has a
distinctive strong smell. It is chlorine gas formed in the oxidation reaction:
2Cl-(aq) → Cl2(g) + 2e-. Note that the concentration of the chloride ion in the solution is
4.0 M (not the standard 1.0 M) and so the chloride ion will be oxidised in preference to
water.

Is it a mistake in the solutions or...?

[Milkshake]

/facepalm.
Nah it's right, I took the concentration of Cl2 as 1.0M

[HarveyD]

how do we know what the concentration of the chlorine ions is?

Edit: Oh is it because theres 2 lots of Cl2 that we multiply the normal concentration by 4?

[Milkshake]

Since Conc of MgCl2 is 1.0M

then 1.0M=n(MgCl2)/(lets say 1L for simplicity)

Therefore n(MgCl2) = 1.0

n(Cl-) = n(MgCl2) x 2 = 2.0

Do the same thing for the CuCl2, and you get a total of 4 moles of Cl- in the entire solution of 1L, so [Cl-]=4.0M

Edit: Oh is it because theres 2 lots of Cl2 that we multiply the normal concentration by 4?

For this question yeh you can do that since the concentration of the MgCl2 and CuCl2 are the same, but if the concentrations were different, then you would have to work it out separately.

[HarveyD]

ahh okay, thanks for that

[Christiano]

There's a question in STAV 2008 where solutions say the answer to a pH question is 14.58 .. I thought the ph range was from 1-14? Is the solutions wrong then?

[pi]

There's a question in STAV 2008 where solutions say the answer to a pH question is 14.58 .. I thought the ph range was from 1-14? Is the solutions wrong then?

The pH scale is usually between 1-14, but you can definitely get pHs out of that range - ie. negative pHs or over 14 The pH scale, in fact, has no limits

eg. Try the ph of 10M HCl

[golden]

I have a graphs question:

Lisachem 2009, question 3 (SA): the graph shows the concentrations to 1 significant figure. The answer for the subsequent working (K value) also had their answers to 1 significant figure. Are you allowed to use 2 significant figures in your working eg. 1.0 M instead of 1 M? Because if the C were say 1.5 M that wouldn't that mean our answers would be way off if we had to round to 1 significant figure?

EDIT:
Another question; with question 1 a, vi, is it wrong not to have the equilibrium arrows?

[Cappuccinos]

I have a graphs question:

Lisachem 2009, question 3 (SA): the graph shows the concentrations to 1 significant figure. The answer for the subsequent working (K value) also had their answers to 1 significant figure. Are you allowed to use 2 significant figures in your working eg. 1.0 M instead of 1 M? Because if the C were say 1.5 M that wouldn't that mean our answers would be way off if we had to round to 1 significant figure?

EDIT:
Another question; with question 1 a, vi, is it wrong not to have the equilibrium arrows?

I was always told to use calculator values when working out and then using significant figures for the final answer?
So I think it's okay to use two significant figures when working out... can someones please correct me if I'm wrong?

I'm not sure about the equilibrium arrows... I'd say so,  they penalize you for not having states don't they? Yeah not so sure...
 ____________________________________________________________
I have a question of my own

An electrolytic cell consists of two carbon electrodes and an electrolyte of aqueous sodium chloride to which phenolphthalein has been added.

When an electric current is passed through the cell:
A bubbles of chlorine appear at the cathode
B bubbles of hydrogen appear at the anode
C a pink colour surrounds the cathode
D sodium metal collects at the cathode.

The answers C.   
What does the pink mean? Does that mean theres a base (chlorine???) around the cathode?  But doesn't Chlorine get oxidised at the anode?

And why isn't it D? Don't Sodium ions get reduced to form Sodium Metal at the cathode? Or does carbon play a part? Carbon's not even on the electrochemical series so how do I know its a stronger reductant or whatever??
Yeah I'm so confused :/

[mickeymouse]

Lisachem 2009 MC Q7

The K value is less than at equilibrium so I thought the backwards reaction would be occurring faster than the reverse at that point in time?
it doesn't say in order to get to equilibrium....

[b^3]

I have a graphs question:

Lisachem 2009, question 3 (SA): the graph shows the concentrations to 1 significant figure. The answer for the subsequent working (K value) also had their answers to 1 significant figure. Are you allowed to use 2 significant figures in your working eg. 1.0 M instead of 1 M? Because if the C were say 1.5 M that wouldn't that mean our answers would be way off if we had to round to 1 significant figure?

EDIT:
Another question; with question 1 a, vi, is it wrong not to have the equilibrium arrows?

I was always told to use calculator values when working out and then using significant figures for the final answer?
So I think it's okay to use two significant figures when working out... can someones please correct me if I'm wrong?

I'm not sure about the equilibrium arrows... I'd say so,  they penalize you for not having states don't they? Yeah not so sure...
 ____________________________________________________________
I have a question of my own

An electrolytic cell consists of two carbon electrodes and an electrolyte of aqueous sodium chloride to which phenolphthalein has been added.

When an electric current is passed through the cell:
A bubbles of chlorine appear at the cathode
B bubbles of hydrogen appear at the anode
C a pink colour surrounds the cathode
D sodium metal collects at the cathode.

The answers C.   
What does the pink mean? Does that mean theres a base (chlorine???) around the cathode?  But doesn't Chlorine get oxidised at the anode?

And why isn't it D? Don't Sodium ions get reduced to form Sodium Metal at the cathode? Or does carbon play a part? Carbon's not even on the electrochemical series so how do I know its a stronger reductant or whatever??
Yeah I'm so confused :/

The key word here is aqueous solution. That means the two water reactions are invovled. (i.e H₂O is present)
O₂+4H⁺+4e^-->2H₂O +1.23
2H₂O+2e^-->H₂+2OH^- -0.83

Since water is a stongest oxidant than Na⁺ and a stronger reductant than Cl^-, it is reduced and oxidise preferibly. (So that is why D is not correct)
So the cathode reaction is 2H₂O+2e^-->H₂+2OH^- (include states)
And the anode reaction is 2H₂O->O₂+4H⁺+4e^- (include states)
Now according to the data booklet 11. Acid-base indicators, phenolphthalein is colorless in acid form and red in basic form.
So that means as the OH^- is produced around the cathode, it is becoming more basic and so the solution turns red (or pink) around the cathode.

[Cappuccinos]

I see! Thanks for your help, I get it now

EDIT: Wait, what about acidic solutions? (e.g of sodium sulfate and iron (II) sulfate)
Does that mean theres a H+ reaction and a water reaction are occuring?

O₂ + 4H⁺ + 4e^-  --> 2H₂O    +1.23
2H⁺ + 2e^-   --> H₂    0.00

H+ ions are stronger oxidant  then Na⁺ and Fe²⁺ and H₂ is the only reductant?

Therefore the products are hyrdogen at the cathode, and oxygen at the anode ?

Is this correct?

[meli001]

A 0.1 M solution of HCL has been diluted by a factor of 10.
The Corresponding concentrations of:
[OH^-]  increased or decreased
[H^+]   increased or decreased

[Mao]

A 0.1 M solution of HCL has been diluted by a factor of 10.
The Corresponding concentrations of:
[OH^-]  increased or decreased
[H^+]   increased or decreased

pH 1 --> 2

H+: 0.1 M --> 0.01 M
OH-: 10^-13 M --> 10^-12 M

[HarveyD]

Would you expect to detect a flow of electrons through a wire connecting Two half cells: AgNO3 with 0.5M and AgNO3 with 5.0M. Explain your choice.