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# Half equations for electrolysis.

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1. Please can anyone teach me how to do them. Im really stuckkkk
Thanks xx
2. I'm not a great teacher but I'll try explain.

Basically, Electrolysis breaks down the compound into its elements.

So if we Potassium Chloride broken down it would make Potassium and Chlorine. The Potassium ions are positively charged so they go to the cathode to gain electrons and become stable and so the equation would be:
2K+ + 2e- (arrow) 2K , this means that two positively charged potassium ions gain two electrions (reason for the plus) to become stable potassium atoms.

The chlorine ion is negatively charge so it is attracted to the anode. The lose electrons to become stable chlorine atoms so the equation is:
2Cl- - 2e- (arrow) Cl2 , this means that two negatively charged chlorine ions lose two electrions (reason for the minus sign) to become stable chlorine atoms.

If you need more help with it just ask, hope I was of any use.
3. (Original post by kai4321)
I'm not a great teacher but I'll try explain.

Basically, Electrolysis breaks down the compound into its elements.

So if we Potassium Chloride broken down it would make Potassium and Chlorine. The Potassium ions are positively charged so they go to the cathode to gain electrons and become stable and so the equation would be:
2K+ + 2e- (arrow) 2K , this means that two positively charged potassium ions gain two electrions (reason for the plus) to become stable potassium atoms.

The chlorine ion is negatively charge so it is attracted to the anode. The lose electrons to become stable chlorine atoms so the equation is:
2Cl- - 2e- (arrow) Cl2 , this means that two negatively charged chlorine ions lose two electrions (reason for the minus sign) to become stable chlorine atoms.

If you need more help with it just ask, hope I was of any use.
I don't think you need 2K, just K is fine. With Cl you do need the 2 as it is diatomic.
4. (Original post by DaveJ)
I don't think you need 2K, just K is fine. With Cl you do need the 2 as it is diatomic.
Well generally speaking you don't, but in the exams they nearly always (from experience at least) ask you to balance a half equation and so far ALL of them have been with 2 atoms of the elements and 2 electrons so I just remembered it like that. I realise that it could be more or less.
5. Thankss, thats made it much clearer x
6. How do you know whether it will go to the anode or cathode? And also, why does is become Cl2, I thought it would just be Cl?
7. (Original post by Ooo'Yeah)
How do you know whether it will go to the anode or cathode? And also, why does is become Cl2, I thought it would just be Cl?

Positive cation to negative cathode.
Negative anion to positive anode.

It becomes Cl2 because Chlorine is diatomic and so cannot exist as Cl.
8. Thank you!
But how do you know if it's postive or negative? Because I thought it could only be that if it lost or gained electrons, how do you know this?

I bet it's a really obvious answer, but I really don't get it atm
9. (Original post by Ooo'Yeah)
Thank you!
But how do you know if it's postive or negative? Because I thought it could only be that if it lost or gained electrons, how do you know this?

I bet it's a really obvious answer, but I really don't get it atm

Well from what I remember, you'll be splitting up an ionic compound. The metal ion will be the cation (loses electrons to bond ionically), and the non-metal ion will be the anion (gains electrons to bond ionically).
10. Ohh okay, so is one always a metal ion?

Sorry for all the questions!
11. (Original post by Ooo'Yeah)
Ohh okay, so is one always a metal ion?

Sorry for all the questions!

Tends to be in ionic compounds.

But it doesn't really matter - the atom which has lost electrons is the positive cation, the atom which has gained electrons is the negative anion.
12. Thank you very, very much! You have cleared up what I have been trying to figure out all week
13. Cations are pussytive.

Therefore they will go to the cathode.

Knowing that cations are postive, you know that the anions are negative, and therefore the anode is postive.

14. Right... im stuck also.. i understand all you've said.. but what i dont get is how to like make up your own half equations... for example... "Silver Nitrate was electrolyzed for 40 minutes using a current of 0.2 amps."
a. Write the half equation for the reaction at the negative electrode (cathode)
b. Write the half equation for the reaction at the positive electrode (anode)

ALSO... can someone explain to me about coulombs and faradays please.. i dont understand that AT ALL ..thank you all xx
15. (Original post by OllieM)
Right... im stuck also.. i understand all you've said.. but what i dont get is how to like make up your own half equations... for example... "Silver Nitrate was electrolyzed for 40 minutes using a current of 0.2 amps."
a. Write the half equation for the reaction at the negative electrode (cathode)
b. Write the half equation for the reaction at the positive electrode (anode)

ALSO... can someone explain to me about coulombs and faradays please.. i dont understand that AT ALL ..thank you all xx
I suppose you mean silver nitrate solution. This is important as you need to consider the ions present in the solution.

Silver nitrate solution = silver ions, nitrate ions, hydrogen ions and hydroxide ions (these last two from the water)

Ag+, H+, NO3-, OH-

There is competition between the two positive ions at the cathode:
Ag+, H+

The most reactive ION gets to gain the electrons. In this case it is the silver ion:

Ag+ + 1e --> Ag(s)

At the anode
-------------

Competition between NO3- and OH- to release electrons. The OH- wins.

4OH- - 4e --> 2H2O + O2

----------------------------------------------------------

You are probably asking at this point how I know which ion gets to react at each electrode. This is something that you must learn.

At cathode: The more unreactive element gets released (it has the more reactive ion). Ions of elements higher up the reactivity series than hydrogen are not released, hydrogen is instead.

At anode: The order of ion reactivity is halide > hydroxide > all others

Therefore in ordinary solutions containing halide ions the halogen is released (unless it's fluorine, which can't get released as it oxidises water). In very dilute halide ion concentrations the hydroxide ions are oxidised and oxygen is released (see the equation above and learn it).

In solutions with all other negative ions hydroxide ions are released (that's why you are better off learning the equation)

---------------------------------------------------------------

The actual amount of a substance released depends on the number of moles of electrons:

Ag+ + 1e --> Ag

This equation tells me that 1 mole of electrons (1e) releases 1 mole of silver.

Moles of electrons = 96348 Coulombs of charge and 1 Coulomb = 1 amp of current per second.

Hence Coulombs = Current x time

Moles of electrons (Faradays) = Coulombs /96485
16. Actually... can you help me with this also...
" Find the mass of zinc liberated if 6 amps flows for 20 minutes during the electrolysis of zinc chloride, ZnCl2." (the 2 is small tho haha)
Thank you very muuuch... x
17. (Original post by OllieM)
Actually... can you help me with this also...
" Find the mass of zinc liberated if 6 amps flows for 20 minutes during the electrolysis of zinc chloride, ZnCl2." (the 2 is small tho haha)
Thank you very muuuch... x
1. The total charge in Coulombs = amps x time(seconds)

2. The number of moles of electrons = charge /96485

The equation is Zn2+ + 2e --> Zn(s)

3. So 2 moles of electrons makes 1 mole of zinc...

see if you can figure it out from this.
18. OK... so it would be...
Charge = amps x time (seconds)
Charge = 6 x 1200 = 7200
Number of Electron moles = (7200/96485)
= 0.075 (2s.f)
What do i do now?? thanks btw
19. (Original post by OllieM)
OK... so it would be...
Charge = amps x time (seconds)
Charge = 6 x 1200 = 7200
Number of Electron moles = (7200/96485)
= 0.075 (2s.f)
What do i do now?? thanks btw
Now you look at the equation and you will see that 2 moles of electrons are needed to make 1 mole of zinc..

Zn2+ + 2e --> Zn(s)

Therefore as you have 0.0746 molesof electrons you can only form 0.0746/2 moles of zinc.

Mass of zinc = moles x relative atomic mass
20. At this point im still a bit confuse. So half equation is basically writing an equation for 1 side of the electrodes. But why is it like this 2Cl---> Cl2+2e- instead of Cl2+2e- ----> 2cl. At this loint im really confused by that

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