Electrolysis help?

[username]

Following my other post on galvanic cell things, I've made some other generalisations on electrolysis. Could someone clarify it please?

Molten electrolysis
eg: Electrolysis of NaCl

- Na will be the reductant, reducing Cl and it itself will be oxidised. Occurs at anode (+).
- Cl will be the oxidant, oxidising Na and it itself will be reduced. Occurs at the cathode  (-)

Can it be safe to say that in all molten electrolysis of stuff, the first element (eg: the Na in NaCl) will always be oxidised at the anode?

Electrolysis in aqueous solutions

So here's how I assume to approach it...

eg: ZnCl2

The possible equations are

Cl2 + 2e --> 2Cl-
O2 + 4H + 4e --> 2H2O
Zn2+ + 2e- --> Zn
2H2O + 2e --> H2 + 2OH-

The Zn will be the oxidant because it is higher in the electrochemical series than 2H2O + 2e --> H2 + 2OH-?
The water will be the reductant because it's lower than Cl2 on the electrochemical series?

I think overall I'm being confused in how to decide what is oxidised/reduced compared to water...

Please help?  I'll bake you cookies.

[Collin Li]

Molten electrolysis

Can it be safe to say that in all molten electrolysis of stuff, the first element (eg: the Na in NaCl) will always be oxidised at the anode?

Maybe... Just do what you normally do. Circle what reactants you have present in your electrochemical series, and predict what will happen. But you are right, most likely, that will be the case.

Electrolysis in aqueous solutions

So here's how I assume to approach it...

eg: ZnCl2

The possible equations are

Cl2 + 2e --> 2Cl-
O2 + 4H + 4e --> 2H2O
Zn2+ + 2e- --> Zn
2H2O + 2e --> H2 + 2OH-

The Zn will be the oxidant because it is higher in the electrochemical series than 2H2O + 2e --> H2 + 2OH-?
The water will be the reductant because it's lower than Cl2 on the electrochemical series?

I think overall I'm being confused in how to decide what is oxidised/reduced compared to water...

It helps if you circle the reactants. You have , and .

On the left-hand side, is the strongest oxidant (highest on the list).

On the right-hand side, is the strongest reductant (lowest on the list).

I coloured "Zn" in maroon, because you should say . We don't have any Zn.

[username]

Thaaank you Coblin. You're amaazing. ^_^

By the way, did you mean "Zn2+ is the strongest oxidant (highest on the list)" in relation to itself and the water?

[Collin Li]

By the way, did you mean "Zn2+ is the strongest oxidant (highest on the list)" in relation to itself and the water?

I meant out of all the circled reactants. So yeah, you'd have zinc ions and water circled on the left-hand side (from the bottom two equations), and the zinc ions are stronger than water. We don't have chlorine gas, or oxygen and acidic protons, so we don't need to worry about those top two equations undergoing reduction (reactants acting as oxidants).

[username]

Ooh okay. So in another example, perhaps:

Aqueous electrolysis of... NaI

So there's

O2 + 4H + 4e --> 2H2O
I2 + 2e- --> 2I-
2H2O + 2e --> H2 + 2OH-
Na+ + e- --> Na

At the cathode the reaction would be 2H2O + 2e --> H2 + 2OH-
And at the anode: 2I- --> I2 + 2e-

??
 

[Collin Li]

Correct.

[dusty_girl1144]

so does this mean water is the strongest reductant too? :s and oxygen gas and hydrogen ions are produced in the first question?

[Collin Li]

so does this mean water is the strongest reductant too? :s and oxygen gas and hydrogen ions are produced in the first question?

Which question are you referring to? For the electrolysis of zinc chloride, yes. For the electrolysis of sodium iodide, no.

[Mao]

so does this mean water is the strongest reductant too? :s and oxygen gas and hydrogen ions are produced in the first question?

if you tried to electrolyse deionised water or dilute sodium chloride solutions [or all the other combinations of stuff], yes.

in the first question [electrolysis of molten sodium chloride], the reactants were molten rather than dissolved in water, i.e. water is not present. Hence sodium will be reduced and chloride will be oxidised.