Chemistry 3/4 2013 Thread

[SocialRhubarb]

Electrolysis is occurring during recharge.

For the question:

Firstly, we'll need to identify what's being reduced and what's being oxidised in the discharge reaction given.

In discharge, Cd ---> Cd²⁺, and hence is undergoing oxidation. Ni³⁺ ---> Ni²⁺ and is hence being reduced. Therefore, when the cell is being recharged, the reverse reactions will be occurring and the cadmium ions will be reduced to form cadmium metal and the nickel (II) ions will be oxidised to form nickel(III) ions.

The question asks for the products formed at the positive electrode, and since it's being recharged, it's undergoing electrolysis, which means that the positive electrode will be the anode, which is where oxidation occurs. So we're looking to form NiO(OH), and since it occurs in an alkaline electrolyte, we can't form H+ ions. Therefore our answer is B.

[barydos]

Electrolysis is occurring during recharge.

For the question:

Firstly, we'll need to identify what's being reduced and what's being oxidised in the discharge reaction given.

In discharge, Cd ---> Cd²⁺, and hence is undergoing oxidation. Ni³⁺ ---> Ni²⁺ and is hence being reduced. Therefore, when the cell is being recharged, the reverse reactions will be occurring and the cadmium ions will be reduced to form cadmium metal and the nickel (II) ions will be oxidised to form nickel(III) ions.

The question asks for the products formed at the positive electrode, and since it's being recharged, it's undergoing electrolysis, which means that the positive electrode will be the anode, which is where oxidation occurs. So we're looking to form NiO(OH), and since it occurs in an alkaline electrolyte, we can't form H+ ions. Therefore our answer is B.

Thanks, you're a legend
Umm sorry haha I've got another follow up question.
If we were told Ni(OH)₂ was oxidised to become NiO(OH), how would we fully write that half reaction?

[psyxwar]

Thanks, you're a legend
Umm sorry haha I've got another follow up question.
If we were told Ni(OH)₂ was oxidised to become NiO(OH), how would we fully write that half reaction?

Ni(OH)₂, Ni has an oxidation state of +II.
NiO(OH), NiO has a formal charge of 1+, therefore Ni's oxidation number is +III

Hence, Ni(OH)₂ --> NiO(OH) + H⁺ + e-

[SocialRhubarb]

Be careful though, you can't write it with H+ if it occurs in an alkaline electrolyte.

[lzxnl]

That's what I hate about writing half equations. The question NEVER states what sort of electrolyte we have. I mean, if we have a frigging sodium nitrate electrolyte...what H+?

[psyxwar]

I hear we only need to worry about balancing stuff in acidic solutions for VCE?

[lzxnl]

In a fuel cell, when we have to write half equations, we have to know what the electrolyte is, but we're NEVER told what it is. Sigh. Stupid VCE.
They should troll everyone and give a pure sulfuric acid electrolyte or something and give fuel cell questions based on that.

[sugarbie]

So I'm doing a prac report now and I got a question on fuel cell, This is the electrochemical series:

O2(g)+4H+(aq)+4e-⇌ 2H2O(l)    -Acid soultion
O2(g)+ 2H2O(l) +4e-⇌ 4OH-(aq)    -Alkaline solution

2H+(aq)+2e-⇌H2(g)    -Acid solution
2H2O(l)+2e-⇌H2(g)+2OH-(aq)   -Alkaline solution

So the O2 gas is the strongest oxidant and the H2 gas is the strongest reductant, so in theory they should react with each other. However, given that the equation is Alkaline and acid solution, I  my knowledge that Alkaline react with alkaline and acid react with acid, can someone please explain this?
I know that the overall is 2H2O(l) →O2(g)+2H2(g), i just don't know how to explain this. Thanks

[Stevensmay]

If it is an alkaline solution then we will have lots of OH-, but no H+. This means that the two half-equations that require H+ cannot occur, as there is none present.

[thushan]

If it is an alkaline solution then we will have lots of OH-, but no H+. This means that the two half-equations that require H+ cannot occur, as there is none present.

Technicality. There will be SOME H+, but extremely small amounts. So small that you could not fathom a reaction involving H+.

[thushan]

So I'm doing a prac report now and I got a question on fuel cell, This is the electrochemical series:

O2(g)+4H+(aq)+4e-⇌ 2H2O(l)    -Acid soultion
O2(g)+ 2H2O(l) +4e-⇌ 4OH-(aq)    -Alkaline solution

2H+(aq)+2e-⇌H2(g)    -Acid solution
2H2O(l)+2e-⇌H2(g)+2OH-(aq)   -Alkaline solution

So the O2 gas is the strongest oxidant and the H2 gas is the strongest reductant, so in theory they should react with each other. However, given that the equation is Alkaline and acid solution, I  my knowledge that Alkaline react with alkaline and acid react with acid, can someone please explain this?
I know that the overall is 2H2O(l) →O2(g)+2H2(g), i just don't know how to explain this. Thanks

So basically - if the solution were acidic, there would be a hell of a lot of H+ swimming around compared to OH-. This would mean that there would be negligible amounts of OH- to act as a reactant. Also, if theoretically OH- were to be a product in an acid solution reaction, since there is a lot of H+ swimming around, the H+ would gobble up the OH- to form water and the net reaction would be an acidic reaction.

The opposite holds for an alkaline solution.

[clıppy]

I've got a question here with oxidisers listed in increasing order of strength under standard condition and in salt water solutions.
In the salt water solutions at the bottom is:
Al³⁺ + 3e -> Al
Zn²⁺ + 2e -> Zn

The question is, could a piece of aluminum electrically connected to a zinc structure protect the zinc from corrosion when it is placed in sea water? Explain your answer
Any help?

[lzxnl]

I've got a question here with oxidisers listed in increasing order of strength under standard condition and in salt water solutions.
In the salt water solutions at the bottom is:
Al³⁺ + 3e -> Al
Zn²⁺ + 2e -> Zn

The question is, could a piece of aluminum electrically connected to a zinc structure protect the zinc from corrosion when it is placed in sea water? Explain your answer
Any help?

Last time I checked, aluminium was a stronger reductant than zinc.
So aluminium would be oxidised preferentially to zinc and would stop the zinc from corroding as long as there was aluminium metal.

[aestheticatar]

Aye yo,

Study design highlights key knowledge including:
'the construction and operation of fuel cells: advantages and disadvantages of fuel cells compared to conventional energy sources;'

Does this mean we have to know the equations and applications of the different fuel cells similar to table 27.1 of the Heinemann textbook (page 438)?

Sorry if it sounds stupid. I just want clarification.
Thanks again

[psyxwar]

Hey guys,

why do we titrate a solution of known concentration (ie. in burette) against one of unknown concentration (and not the other way around)? Apparently it has something to do with safety?