O' level Chemistry Electrolysis Concept

last SG · 2008-10-14T12:14:09+00:00

Hes the man!No questions for now XD

iamapebble SG

Hi all, I'm afraid I need help again. D:

Given, a simple cell for the production of electrical energy. One electrode is copper and the other is one of aluminium, copper, iron, magnesium or zinc. Suggest a suitable solution to use as electrolyte in this cell.

Answer: Copper (II) sulphate.

My concern here is, why can't I use an electrolyte like aluminium sulphate, iron (II) sulphate, magenisium sulphate or zinc sulphate? Isn't it that for voltage to be produced, as long as one metal is more reactive than the other and electrons are allowed to flow then there'll be a electrical energy, no? I know it sounds like a basic concept question but apparantly my textbook only has a small section on 'simple cells' and their concepts are completely wrong (as verified by our teachers as well).

Anyone care to enlighten? (: Thank you!

Garrick_3658 SG

Hi again :D

Firstly, this isn't electrolysis. This is a simple cell question, although it is under the topic: Electrolysis.

Hey, you're right. It CAN be any salt solution. Hell, it can even be sodium chloride solution.

Or am I missing something?

EDIT: Ok wait, it depends on what products you want. I believe the product is copper metal.

And are you sure that you are using TWO copper electrodes in one simple cell experiment?

iamapebble SG

Hello :D Haha.

It's actually the first part of a full question and this is the only part I'm unsure of, that's why there's the copper there too, ignore that actually.

But then, it is a simple cell right, what does it has to do with the products? My concept on the electrical energy produced is right, right?

For the products, it is only a matter of concern if I'm purifying copper or electroplating something with copper, so I have to use copper (II) sulphate so that the Cu2+ can be discharged right? So in the case of a simple cell, it shouldn't matter, will it? Can anyone confirm? ^^ Thanks!

Garrick_3658 SG

Your concept that a current flows between two metals with different positions on the reactivity series is correct. Electrons flow from the more reactive metal to the less reactive metal.

For example, the more reactive metal is zinc. Thus zinc ions are discharged into the electrolyte. Since zinc has been oxidised, there are electrons, right? Electrons go onto the zinc rod to make it negative. The electrons then go along the connecting wire to the copper electrode. Here, copper and hydrogen cations are attracted to the copper electrode. By preferential discharge, therefore, copper cations are reduced to copper metal, hence explaining why the need for a copper salt electrolyte in the first place.

In this case, I would assume that this is a ELECTROPLATING reaction using a simple cell (weird, never heard/seen such question before)

davidche SG

Yes you can use any electrolyte.

But you normally use an electrolyte of the same metal ion so that there wouldnt be displacement i guess. It may happen when your cathode is zinc and the electrolyte is any thing less than zinc.

Even for electroplating of copper, you can still use sodium sulphate electrolyte and nothing wrong will happen except that you will have very little copper ions to be discharged and thus very little copper to be electroplated on the object.

Please correct me if i am wrong XD

Garrick_3658 SG

First, do realise that this is a simple cell question. It is not electrolysis.

Correct. I'm not sure whether sodium will be discharged at the initial start of reaction (since it is the only cation, unless you mean solution, then it's hydrogen), but it most likely will. Once copper cations are enter the solution from the ionized anode, then copper will be plated.

In short, using an electrolyte different to the metal plated will screw up the process.

davidche SG

hmm i realised, which leads me to ask, does it matter what electrolyte you use in a simple cell? If the metal at the cathode is zinc and the electrolyte is copper sulphate.

tinuviel07 SG

hmm i realised, which leads me to ask, does it matter what electrolyte you use in a simple cell? If the metal at the cathode is zinc and the electrolyte is copper sulphate.

If the cathode is zinc, wouldn't the metal displace the copper from the salt solution, so instead of the electrons flowing to the anode, the metals would be taken in by the copper ions directly at the cathode?

davidche SG

True~

Garrick_3658 SG

This is why I emphasised on it being a SIMPLE CELL question.

If the cathode is zinc, then the anode cannot be copper. I believe (based on what I have learnt) that in a SIMPLE CELL, cathode = +ve while anode = -ve

So electrons flow from anode to cathode.

Please, don't confuse this with electrolysis.

A good analysis of a displacement reaction taking place (I didn't realise), now this makes the question debatable. Using copper (II) sulphate causes a displacement reaction taking place, hence copper metal forms on the zinc electrode. Which really spoils the whole idea of using a simple cell.

davidche SG

Or maybe there aint a ''cathode'' in a simple cell?

tinuviel07 SG

it depends actually..

Garrick_3658 SG

Alright, let's not use cathode and anode.

More reactive metal (MR) = -ve

Less reactive metal (LR) = +ve

Electrons flow from MR to LR, hence cations are discharged there to form elements.

But since this is a copper (II) sulphate solution electrolyte, by right copper metal should be formed, but then, displacement takes place between zinc and copper, hence after a while, reaction stops (you don't throw the whole zinc metal into the electrolyte). Or rather, copper doesn't form anyway. Hydrogen gas instead, is evolved.

UltimaOnline SG

Productive discussion going on here. Not much to add, other than :

1) Always visualize a "Red Cat riding An Ox". Reduction occurs at the Cathode; at the Anode, Oxidation occurs.

This will always hold true regardless of whether you're talking about an electrolytic cell (ie. in which the setup includes a battery or electrical source, and the direction you choose to place the battery, which side of the battery negative and which side positive, will determine which electrode is the cathode and which electrode is the anode), or a galvanic/voltaic/simple cell which generates electricity.

2) As to the electrolyte, for the purpose of a simple cell, as you guys have already pointed out correctly, you can use any solution that conducts electricity, as long as the ions in the electrolyte solution does not react (IN A REDOX REACTION) with, or precipitate out with, the new ions generated by at the anode (if this happens, it would RESULT IN NO CURRENT FLOWING IN THE EXTERNAL WIRE, OR reduce the availability of mobile ions, and the current would decrease to zero in a short amount of time, defeating the purpose of the electric cell setup).

(EDITED : FOR THE ELECTRIC CELL TO WORK, COMPARMENTALIZE ELECTRODES & USE 2 ELECTROLYTES & USE SALT BRIDGE TO AVOID ANY DIRECT REDOX BETWEEN ELECTROLYTE AND ELECTRODE)

If you're not sure whether the electrolyte you think of can be used, simply write the expected reduction half equation (at the cathode) and the expected oxidation half equation (at the anode), and check that the redox reaction of the cell is feasible. At 'O' levels, you need not calculate using reduction and oxidation potentials (ie. numerical values), you only need to apply your recall of the reactivity series to conclude if the setup is feasible or not.

Garrick_3658 SG

Back to the original post's question then. How feasible is that?

UltimaOnline SG

Back to the original post's question then. How feasible is that?

The original post gave too many possible setups (permutations and combinations). Link a specific suggested electrolyte to a specific setup (ie. specify both electrodes), and write out the reduction and oxidation half equations, and then I'll tell you if your equations are correct and feasible or not.

iamapebble SG

Hey ultimaonline, that was a very clear explanation, thank you! =) Let's say I let one of the electrode to be copper and the other one to be aluminium, and I choose aluminium sulphate as the electrolyte.

Aluminium being more reactive will be the anode and copper will be the cathode.

At the anode(+):
Al(s) -> Al3+(aq) + 3e

At the cathode(-):
Cu(s) + 2e -> Cu2+(aq)

Overall reaction:
2Al(s) + 3Cu(s) -> 2Al3+(aq) + 3Cu2+(aq)

Correct? >< Then how do I know if it's feasible or not.

UltimaOnline SG

(Hmmm... my post kena truncated and data lost... wait... )

UltimaOnline SG

>>> Let's say I let one of the electrode to be copper and the other one to be aluminium, and I choose aluminium sulphate as the electrolyte.

Aluminium being more reactive will be the anode and copper will be the cathode.

At the anode(+):
Al(s) -> Al3+(aq) + 3e

At the cathode(-):
Cu(s) + 2e -> Cu2+(aq)

Overall reaction:
2Al(s) + 3Cu(s) -> 2Al3+(aq) + 3Cu2+(aq)

Correct? >< Then how do I know if it's feasible or not. <<<

Your anode equation is correct, but your cathode equation is wrong. You can only reduce metal cations to metal atoms, you cannot reduce metal atoms any further.

The problem with using aluminium sulfate electrolyte, is that you'e not providing a feasible reduction process at the cathode. Why would the universe bother to painstakingly extract electrons from aluminium metal at the anode, simply so that these electrons can be used to reduce aluminium cations at the cathode back to aluminium metal? Where's the gain (in stability) in such a silly act?

If human beings behaved senselessly like this, then the world financial markets would collapse... oh... ummm... ok.

So basically, iamapebble, you'd want to use copper(II) sulfate as the electrolyte because being an unreactive metal, Cu2+ ions would be eager to be reduced to Cu metal at the cathode, meaning they happily accept the electrons that came from the oxidation process (Al ---> Al3+ + 3e-) at the anode, and so if you've a happy (electron) donor and a happy (electron) recipient, the current can flow and we're all good and happy.

Note : if you had used aqueous aluminium sulfate electrolyte, protons from water, and/or water molecules themselves, would be reduced at the cathode and a current would still flow, but the current would be weaker. Besides, at 'O' levels, your teachers/examiners want you to be able to think sharply to produce an optimal current and therefore to give the most appropriate answer of "copper(II) sulfate electrolyte" in which Cu2+ ions are reduced to copper metal and joins/coats the copper cathode; rather than rely on brute force (just because your aluminium anode very reactive, so you think you can bully unhappy-to-be-reduced water into being reduced at the cathode! Buay sui lah!).

(EDITED : FOR THE ELECTRIC CELL TO WORK, COMPARMENTALIZE ELECTRODES & USE 2 ELECTROLYTES & USE SALT BRIDGE TO AVOID ANY DIRECT REDOX BETWEEN ELECTROLYTE AND ELECTRODE)

>>> Hey ultimaonline, that was a very clear explanation, thank you! =) <<<

You're welcome, iamapebbel! If you've any further questions on this, feel free to write out more scenarios (ie. specify the electrolyte, the cathode, the anode, and the reduction half equation, the oxidation half equation, the overall equation), and I will comment and advise you on them.

iamapebble SG

Hey thanks :D Your explanation made me laugh, which helped me absorb it better, which is a good thing! Chemistry paper 2 in exactly a week's time >< I'll practice more electrolysis questions and clear my doubts here :D