Unit 4 Questions MEGATHREAD!

[Vincezor]

(From STAV 2009, question 4)

A block of nickel and a block of copper are joined together and then combined, the block is placed in a solution of 1.0M HCL (aq) @ 25 degrees C. It was obsereved that bubbles of gas are only evolved from the surface of the copper. However when the two blocks are separated and placed in the same acid @ 25 degrees C, the bubbles only appear on the Nickel surface.

Explain these observations and include half equations to support your explanation.

I don't really understand the solutions given, so if someone could explain it better I'd really appreciate it!

[nbhindi]

This is exactly right. If the reaction vessel cannot expand, pressure must increase. This is true for any gases. However, the equilibrium will not shift.

Cheers Mao for clarifying 

[HarveyD]

The neutralization of an acid and a base may be represented as
H3O + OH ---> 2H2O = - 57 kJ mol
At 25°C, the pH of water is 7.0.

How come at O°C, the pH of water will be greater?
If both the OH- and H3O+ concentration are decreasing as a result of the forward reaction, why would the pH change? :S

[b^3]

pH is given by log([H₃O⁺)
As temperature decreases, ionisation will be less, so [H₃O⁺ will decrease (OH^- will decrease aswell but it doesn't matter).
So as [H₃O⁺] decreases, pH increases. I think where you are getting confused with is that the solution will still be netural as the concentrations of H₃O⁺ and OH^- will be the same, (but the won't multiply to 10^-14 M² as it is not at 25 C).

I.e. at O C the solution is neutral but the pH is greater than 7. Remember this, it is a common trick on exams.

EDIT: fixed greater/less than

[HarveyD]

ah so that will apply to all situations where the temperature isnt 25 degrees?

[b^3]

ah so that will apply to all situations where the temperature isnt 25 degrees?

Well yeh kinda.
The way I remember is like this (this is whent he concentrations of both are the sam)
temp<25, less ionisation, concen. H3O+ is less, pH increases i.e. pH>7, solution is neutral
temp=25, Solution is neutral, pH=7
temp>25, more ionisation, concen. H3O+ is greater, pH decreases i.e. pH<7, solution is neutral

[HarveyD]

alright makes sense, thanks

also couple more questions ><
1) Do we need to include the electrolyte in a half equation?
Like for the lead-acid accumulator, if you wrote Pb --> Pb2+ + 2e-
instead of Pb(s) + (SO4)2- (aq) ---> PbS04(s) + 2e"
(the sulphuric acid is the electrolyte)
or would it be wrong?

2) Is the solution for the below question wrong? I thought the outer casing is the anode for primary cells

[vea]

alright makes sense, thanks

also couple more questions ><
1) Do we need to include the electrolyte in a half equation?
Like for the lead-acid accumulator, if you wrote Pb --> Pb2+ + 2e-
instead of Pb(s) + (SO4)2- (aq) ---> PbS04(s) + 2e"
(the sulphuric acid is the electrolyte)
or would it be wrong?

2) Is the solution for the below question wrong? I thought the outer casing is the anode for primary cells

1. I think you should include them, suggested solutions usually do.
2. If you look at the outer container, it is made of silver and in the equation given, silver is being reduced making it the cathode reaction.

QUESTION:
Is the concentration of a reaction vessel the concentration of all the reactants and products added together or the average of these concentrations? Can't seem to think it through but atm I'm thinking it's the average.

[b^3]

alright makes sense, thanks

also couple more questions ><
1) Do we need to include the electrolyte in a half equation?
Like for the lead-acid accumulator, if you wrote Pb --> Pb2+ + 2e-
instead of Pb(s) + (SO4)2- (aq) ---> PbS04(s) + 2e"
(the sulphuric acid is the electrolyte)
or would it be wrong?

2) Is the solution for the below question wrong? I thought the outer casing is the anode for primary cells

1. I think you should include them, suggested solutions usually do.
2. If you look at the outer container, it is made of silver and in the equation given, silver is being reduced making it the cathode reaction.

QUESTION:
Is the concentration of a reaction vessel the concentration of all the reactants and products added together or the average of these concentrations? Can't seem to think it through but atm I'm thinking it's the average.

I'd think it would be the total number of mole of particles (of say both products added together e.t.c) divided by the volume, although I'm not sure. Have you got a question.example related to this?

[vea]

It was on the 2011 Lisachem paper that I just did at school so I don't have the pdf files or anything. The question gave you the concentration of the 4 species in an equilibrium reaction and asked you to find the pressure of the system. The temperature (460 degrees) was given but no volume was given.

So I did PV=nRT
P=n/v RT
P=cRT which requires the concentration of the reaction vessel as a whole.

[b^3]

Yeh we did that for our practice exam too. Here are the solutions for that question and yey! I did it right on the exam, but made more work for us this time, just add the concentrations together.

[vea]

Thanks for that!
For bii) Isn't it impossible to tell whether the reaction is at equilibrium or not? The number of particles on the reactants side and products side are equal so the pressure of the reaction vessel should be constant regardless of whether or not it is in equilibrium?

[b^3]

Yeh what you said is what I think I wrote down so I think they are wrong, or it is just a bad question.

[golden]

alright makes sense, thanks

also couple more questions ><
1) Do we need to include the electrolyte in a half equation?
Like for the lead-acid accumulator, if you wrote Pb --> Pb2+ + 2e-
instead of Pb(s) + (SO4)2- (aq) ---> PbS04(s) + 2e"
(the sulphuric acid is the electrolyte)
or would it be wrong?

2) Is the solution for the below question wrong? I thought the outer casing is the anode for primary cells

What vea said is correct.

[david10d]

Got a few questions here:

1. Two half cells were connected by a salt bridge and wiring to produce an electric current. One half cell consisted of hydrogen gas bubbled over a Pt electrode in a slightly basic solution, and the other consisted of O2 gas bubbled over a Pt electronic in water. As time passed, the solution around the positive electrode.

A) Remained slightly basic
B) Remained neutral
C) Became slightly basic
D) Became neutral

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i can't seem to wrap my head around cells... in the galvanic cell, when an unreactive electrode is for the cathode do the ions in solution form onto the electrode (eg Platinum)? If an unreactive electrode is used for the anode what happens in this half cell? I don't get what happens with an unreactive electrode....

What does it mean when the battery is in equilibrium? The battery goes flat, but what happens inside the battery?

in the recharge process, why does the anode turn into the cathode, vice versa


SIGH IM SO FUCKING BEHIND LOL

what do we need to know about batteries? just the advantages/disadvantages and how they work?

how do you get the maximum voltage required to recharge the cell?


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how long hav eyou guys been doing exams?