Unit 4 questions thread

[charmanderp]

Because if you actually look at the electrochemical series, you'll see it can't act as anything other than an oxidant.

[VCEstudentguy]

What are the 2 MAIN sources of Hydrogen for the use as a fuel? I know one is Natural Gas, but what would be the 2nd most common source?

[aznxD]

Electrolysis of water: 2H2O(l) ---> 2H2(g) + O2(g)

[atom]

Why is it that galvanic cells require two half cells, but electrolytic cells allows the two half cells to be combined in one cell? Is it due to the need for salt bridge/contamination?

[charmanderp]

In a galvanic cell you have two species that would react spontaneously to produce a current (transfer electrons), hence they must be kept separate in order to conduct that current. With electrolysis the reactants are oriented in the electrochemical series such that they won't react spontaneously, hence they can be held in a singular cell.

[soccerboi]

A student wishes to electrolyse some water by setting up an electrolytic cell.

b. i. Circle the best solution below for the student to choose for the electrolyte in the cell.
deionised water
sodium sulfate
copper sulfate
sodium iodide

Solution
sodium sulfate

ii. Give an explanation for your choice.
Solution
Sodium sulfate will provide ions that can carry charge in the solution but which will not react
in preference to the H2O(l).

I dont understand it, can someone elaborate please
Cheers

[Nobby]

A student wishes to electrolyse some water by setting up an electrolytic cell.

b. i. Circle the best solution below for the student to choose for the electrolyte in the cell.
deionised water
sodium sulfate
copper sulfate
sodium iodide

Solution
sodium sulfate

ii. Give an explanation for your choice.
Solution
Sodium sulfate will provide ions that can carry charge in the solution but which will not react
in preference to the H2O(l).

I dont understand it, can someone elaborate please
Cheers

So there are two main things you're looking for in your electrolyte - it must conduct, and it can't react. So you can't use deionised water, because it won't conduct electricity. You can't use copper sulfate, because once if you run a current through the solution, copper is going to deposit at the cathode instead of H₂ gas, just look at your E^o table. You can't use sodium iodide, because as you can glean from the E^o table I₂ gas will bubble off the anode instead of O₂. And finally, you can use sodium sulfate, because sodium is so far down the electrochemical series it's not going to reduce in preference to water.

[charmanderp]

Another thing; sulfate, despite being a stronger oxidant than water will not be reduced as it is a negatively charged ion and in electrolysis the cathode is also negative, hence sulfate will be repelled and not take part in the reaction.

And finally, you can use sodium sulfate, because sodium is so far down the electrochemical series it's not going to reduce in preference to water.

Also Nobby, it's referring to Na+ here which is an oxidant, not a reductant. So it wouldn't be reduced in preference to water rather than it not reducing in preference to water. Sorry for being pedantic but I just thought I'd point that out (:

[Nobby]

Also Nobby, it's referring to Na+ here which is an oxidant, not a reductant. So it wouldn't be reduced in preference to water rather than it not reducing in preference to water. Sorry for being pedantic but I just thought I'd point that out (:

Ah yeah probably should have said 'sodium ions' and 'be reduced'.

[charmanderp]

Also Nobby, it's referring to Na+ here which is an oxidant, not a reductant. So it wouldn't be reduced in preference to water rather than it not reducing in preference to water. Sorry for being pedantic but I just thought I'd point that out (:

Ah yeah probably should have said 'sodium ions' and 'be reduced'.

Yeah I figured you probably knew that that was the case but others might have read it and taken it at its word.

[soccerboi]

A student wishes to electrolyse some water by setting up an electrolytic cell.

b. i. Circle the best solution below for the student to choose for the electrolyte in the cell.
deionised water
sodium sulfate
copper sulfate
sodium iodide

Solution
sodium sulfate

ii. Give an explanation for your choice.
Solution
Sodium sulfate will provide ions that can carry charge in the solution but which will not react
in preference to the H2O(l).

I dont understand it, can someone elaborate please
Cheers

So there are two main things you're looking for in your electrolyte - it must conduct, and it can't react. So you can't use deionised water, because it won't conduct electricity. You can't use copper sulfate, because once if you run a current through the solution, copper is going to deposit at the cathode instead of H₂ gas, just look at your E^o table. You can't use sodium iodide, because as you can glean from the E^o table I₂ gas will bubble off the anode instead of O₂. And finally, you can use sodium sulfate, because sodium is so far down the electrochemical series it's not going to reduce in preference to water.

Thanks a lot for the explanation but could you reexplain why sodium iodide is not a suitable electrolyte? I always have trouble with knowing which half eqns to look at on the E.S
Also, you refered to I2 gas in your explanation, but on the E.S it solid, so do we till refer to that half eqn?

[thushan]

Al can't be deposited because if you look at your electrochemical series, water (at -0.83V) is a stronger oxidant than Al3+ and will reduce in preference to Al3+. the word 'solution' refers to involving water.

Oh ok thank you, but how do you know which water eqn to look at? I can see 4 on the electrochemical series that contain water.

You see the one that uses water as a REACTANT.

[charmanderp]

A student wishes to electrolyse some water by setting up an electrolytic cell.

b. i. Circle the best solution below for the student to choose for the electrolyte in the cell.
deionised water
sodium sulfate
copper sulfate
sodium iodide

Solution
sodium sulfate

ii. Give an explanation for your choice.
Solution
Sodium sulfate will provide ions that can carry charge in the solution but which will not react
in preference to the H2O(l).

I dont understand it, can someone elaborate please
Cheers

So there are two main things you're looking for in your electrolyte - it must conduct, and it can't react. So you can't use deionised water, because it won't conduct electricity. You can't use copper sulfate, because once if you run a current through the solution, copper is going to deposit at the cathode instead of H₂ gas, just look at your E^o table. You can't use sodium iodide, because as you can glean from the E^o table I₂ gas will bubble off the anode instead of O₂. And finally, you can use sodium sulfate, because sodium is so far down the electrochemical series it's not going to reduce in preference to water.

Thanks a lot for the explanation but could you reexplain why sodium iodide is not a suitable electrolyte? I always have trouble with knowing which half eqns to look at on the E.S

I- is a stronger reductant (left hand side of the serieS) than H2O, so you'd have to apply a current for a certain amount of time before this I- supply was exhausted (oxidised to I2) and you started producing O2(g) and electrolysing H2O at the anode.

[soccerboi]

So in this quesrtion, because we are electrolyising water, which half eqn/s do we look at and why? sorry if ive asked this before, i just still am not confident with doing these questions

[charmanderp]

Look at the half-equations which involve the species you have present. So look for equations which have H2O on its own on one side and I-.