Electrolysis

zed963

How do we know which ion goes to the electrode in electrolysis?

Eg. the electrolysis of sodium chloride solution.

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Reply 1

shengoc

Original post by zed963

How do we know which ion goes to the electrode in electrolysis?

Eg. the electrolysis of sodium chloride solution.

electrodes are of the cathode or anode. find out what they are in your books/notes

also look up what electrolysis is and what it does - in terms of breaking a compound by chemical means.

Reply 2

zed963

Original post by shengoc

electrodes are of the cathode or anode. find out what they are in your books/notes

also look up what electrolysis is and what it does - in terms of breaking a compound by chemical means.

AFAIK there are two electrodes which are in the electrolyte. The cathode is negative and attracts cations. The anode is positive and attracts anions.
Electrolysis is the decomposition of a compound.

What I'm asking is how do I know which ion discharges at the electrode.

Eg. In sodium chloride solution we have Na+ H+ and OH- and Cl-

So the cations go to the cathode and the anions go to the anode but which ion on the cathode or anode discharges first?

Reply 3

charco

Study Forum Helper

Original post by zed963

There is an simple and a more complete answer to your question.

The easy answer is that only metals less reactive than lead will be released at the cathode, otherwise it is hydrogen. Only halogens are released at the anode otherwise it's oxygen.

If you require a more complete discussion ...

Reply 4

Enigma Machine

I have no idea what you're asking; what do you mean by 'discharges' first?
Hydrogen is formed at the cathode, and chlorine is formed at the anode.

Reply 5

zed963

Original post by Bude8

I have no idea what you're asking; what do you mean by 'discharges' first?
Hydrogen is formed at the cathode, and chlorine is formed at the anode.

Okay, so why isn't Na discharged or OH?

Reply 6

zed963

Original post by charco

How will I know if the metal is less reactive than lead?

Reply 7

Enigma Machine

Original post by zed963

Okay, so why isn't Na discharged or OH?

Aqueous solutions contain four ions:
A cation and an anion from the solute
H+ and OH- from water

At each electrode, only ONE ion can be discharged.
The metal forms at the cathode ONLY IF it is below hydrogen in the reactivity series. If it's above hydrogen, then hydrogen itself is formed.

At the anode the negative ion in the dissolved solid will be either mono atomic (eg Cl-, Br-, I-) or polyatomic (eg SO4 2-, NO3 -)
If mono atomic, that is the ion that will be discharged.
If polyatomic then hydroxide ions will be discharged, producing oxygen.

Reply 8

chapmouse

Original post by zed963

How do we know which ion goes to the electrode in electrolysis?

Eg. the electrolysis of sodium chloride solution.

Are you having problems with the names of the electrodes, and which accepts which?
I know it sounds silly, but this is the tactic I use to remember the electrodes:
Cations are always positive. This is because, when you think of a cat ironing, it's probably happy. The Cation just moves towards the negative Cathode.

Spoiler

And once you know that cations are positive, you can infer that anions are negative.

My high school Chemistry teacher used to use a misspelling of anode to explain that.
the Annode is Not Negative, and therefore it is positive, so the negative anion moves toward it.

I don't like that example, because you might put two n's in anion by accident. So yeah, i'm gonna advise the happy cat example.

The above posts seem to explain pretty well which ions form where and why, so i'll leave that.
Hope this helps!

Reply 9

zed963

Original post by Bude8

What's the difference between mono atomic and polyatomic?

Reply 10

charco

Study Forum Helper

Original post by zed963

What's the difference between mono atomic and polyatomic?

mono = one
poly = more than one (actually, many)

Reply 11

zed963

Original post by charco

mono = one
poly = more than one (actually, many)

Will I have to memorise the reactivity series?

Reply 12

Elm Tree

Original post by zed963

What's the difference between mono atomic and polyatomic?

Monoatomic are ions containing only one element. e.g Cl-

Polyatomic are ions containing more than one element. e.g OH-

Reply 13

Enigma Machine

Original post by zed963

Will I have to memorise the reactivity series?

Depends on your syllabus, we do.
Easy way to remember it:

Please - Potassium
Stop - Sodium
Calling - Calcium
My - Magnesium
Annoying - Aluminium
CARBON
Zebra - Zinc
In - Iron
The - Tin
Language - Lead
HYDROGEN
Class - Copper
Silver
Gold
Platinum

Reply 14

Elm Tree

Original post by zed963

Will I have to memorise the reactivity series?

If you're on AQA then no, It will be on the data sheet that is provided during the exam.

Reply 15

zed963

Is this site wrong http://alexteoh.com/emap-electrochem/FactorsaffectingPreferentialDischarge.html

It says that OH- is easier to discharge than Cl-

Reply 16

Efemena15

Trust me on this, have a look on GCSE bitesize at the video and revision notes. They're amazing

Reply 17

zed963

Original post by Efemena15

Trust me on this, have a look on GCSE bitesize at the video and revision notes. They're amazing

I find them to be basic.

Reply 18

zed963

Original post by Bude8

Oh, Why's carbon is red for?

Also, what's the half equation for OH-?

Reply 19

Elm Tree

Original post by zed963

Oh, Why's carbon is red for?

Also, what's the half equation for OH-?

There isn't a half equation for it because OH- doesn't react with the electrode, Cl- does.

Half equation for Cl- would be 2Cl- - 2e-(electron) -> Cl2