Hi!
I need some help understanding electrolysis! I am quite rusty since learning it last year.
I'm confused with this question (the answers in this paper are sometimes wrong so if you disagree with the answer it could be wrong but maybe not - I'm not sure). The answer is in the textbox.
I know galvanic cells and electrolytic cells are the same in terms of reduction happens at the cathode and oxidation at the anode and I know the anode of positive and cathode negative but this question has two different electrodes and I'm confused why the half questions are Zinc and Nickel - what happens at the aluminium electrode? Any explanations for how electrolysis works, in general, will be much appreciated!
Thank you!
For this question,
you're going to want to look at page 10 of your data booklet. In an electrochemical cell - regardless of if it's galvanic or electrolytic - the chemicals that react are ALWAYS going to be the ones that give the BIGGEST difference (this will end up being the BIGGEST Ecell). So, first thing you need to do is figure out what chemicals you're working with. In this case, you have:
Al(s)
Ni(s)
Zn
2+ (aq)
NO
3- (aq)
So, look for ANY equations in the electrochemical series that contain these chemicals and ions. There's also a very sneaky one to be aware of - all of this is happening in an aqueous solution, so water could be involved, too! So you need to consider that reaction, too. In an exam, you could just highlight these - but I'm going to have to write them out, d'oh. They are:
Al (s) ---> Al
3+ (aq) + 3e
-, E
o=+1.68
2H
2O (l) + 2e
- ---> H
2 (g) + 2OH
- (aq), E
o=+0.83
Zn
2+ (aq) + 2e
- ---> Zn (s), E
o=-0.76
Ni (s) ---> Ni
2+ (aq) + 2e
-, E
o=-0.24
And looking further down, there are no nitrate equations - so we don't need to worry about it. Now, to make this easier to read, I'm going to display the data from most negative E to most positive E - the same way it's displayed in the data booklet:
Zn
2+ (aq) + 2e
- ---> Zn (s), E
o=-0.76
Ni (s) ---> Ni
2+ (aq) + 2e
-, E
o=-0.24
2H
2O (l) + 2e
- ---> H
2 (g) + 2OH
- (aq), E
o=+0.83
Al (s) ---> Al
3+ (aq) + 3e
-, E
o=+1.68
So, looking at this, what will make the biggest difference? Ionic zinc and solid aluminium! So, we calculate the Ecell and we get +0.92. Well, that's not going to work for an electrolytic cell - the Ecell needs to be negative! Okay, what if we try with water - then we get an Ecell of 0.07. Still problematic... That means, the only electrolytic cell that will work is the one with solid nickel and ionic zinc - which has an Ecell of -1.00 (huh, neat).
Basically, you can think of it as this - if the zinc reacted with the water or aluminium, then we'd be CREATING electricity. However, electrolytic cells don't create electricity - they consume it. So, the moment you start putting electricity into the system, the aluminium isn't going to want to react anymore (kinda like in equilibrium - you've changed the system by feeding it electricity, so the system won't want to produce electricity anymore).
Now, I know what you're thinking. "That's very long and tedious, I don't have the time to do this in an exam!" - well, I've got good news for you - we can cheat a little bit. Essentially, this method boils down to there's a difference between general reactivity, and electrochemical reactivity. In fact, electrochemical reactivity correlates exactly with the electrochemical series. When it's written the way it is in the QCE data booklet, the left side goes up by decreasing oxidant strength, and the right side goes up by increasing reductant strength. In any electrolytic cell, the weakest oxidant will react with the weakest reductant. In this case, the weakest oxidant (at the top of the left) is ionic zinc, and the weakest reductant (at the bottom on the right) is solid nickel.
EDIT: After doing all of this, I've realised that I read the equation for water wrong - so in fact, the answer should have nickel reacting with water. I imagine the textbook writers simply forgot they were doing everything in water, hence why they didn't include that equation - it's an easy mistake to make!