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Tuesday, 24 April 2012

Electrolysis

Electrolysis


1.47 understand an electric current as a flow of electrons or ions

Current is basically a flow of charged particles and in a metal wire, it is a flow of electrons. These have a negative 1 charge. Ions are charged particles too as they have lost or gained electrons. The movements of these ions are responsible for the conduction of electricity. 

1.48 understand why covalent compounds do not conduct electricity

Covalent compounds do not have spare free electrons that can move and carry the charge; and neither do they contain ions-it’s a covalent compound not an ionic compound.

1.49 understand why ionic compounds conduct electricity only when molten or in solution

When they are a solid the ions are not free to move and carry the charge. When they are molten-as in it is in melted form-the ions are free to move. Remember in liquids the particles are able to slide over each other and move whereas in solids the particles can only vibrate around a fixed position. When the ionic compound is dissolved in a solvent to form a solution the ions are also made free to move.

1.50 describe simple experiments to distinguish between electrolytes and non-electrolytes


1.51 recall that electrolysis involves the formation of new substances when ionic compounds conduct electricity
Passing an electric current through a compound which is either molten or in solution causes chemical changes, the chemical reactions produce new products—new substances.

1.52 describe simple experiments for the electrolysis, using inert electrodes, of molten salts such as lead (II) bromide


Right hand electrode: Cat
hode-attracts cations-is the negative electrode, as it is attracting positive ions.
Left hand electrode: Anode-attracts anions-is the positive electrode, as it is attracting negative ions.
[Opposite charges attract]




Nothing happens until the lead (II) bromide melts.
Lead (II) bromide is an ionic compound. The solid consists of a giant structure of lead (II) ions and bromide ions packed regularly in a crystal lattice. It doesn’t have any mobile electrons, and the ions are locked tightly in the lattice and aren’t free to move. The solid lead (II) bromide doesn’t conduct electricity.
As soon as the solid melts, the ions do become free to move around, and it is this movement that enables the electrons to flow in the external circuit.

Electrodes are made out of carbon-which is inert (unreactive).

As soon as you connect the power source, it pumps any mobile electrons away from the left-hand electrode towards the right-hand one. The excess of electrons in the right-hand electrode makes it negatively charged-called the cathode. The left-hand electrode is positively charged because it is short of electrons. There is a limit to how many electrons can squeeze into the negative electrode (Cathode) because of the repulsion by the electrons already there.

The positive lead (II) ions are attracted to the cathode, which is the negative electrode. When they get there, they gain 2 electrons each from the electrode and forms neutral lead atoms. These fall to the bottom of the container as molten lead.
Pb2+ (l) + 2e- à Pb (l)

This leaves spaces in the cathode that more electrons can move into. The power source pumps new electrons along the wire to fill those spaces.

Bromide ions are attracted to the positive anode. When they get there, the extra electron which makes the bromide ion negatively charged moves onto the anode because this electrode is short of electrons.

The loss of the extra electron turns each bromide ion into a bromine atom. These join in pairs (bond covalently) to form bromine molecules. Overall:
2Br-(l) à Br2 (g) + 2e-

The new electrons on the anode are pumped away by the power source to help fill the spaces being created at the cathode.

The ions are discharged at the electrodes. Discharging an ion simply means that it loses its charge-either giving up electron(s) to the electrode or receiving electron(s) from it.

Redox reaction
Look back at the ionic equations, see that the lead (II) ions gain electrons at the cathode. Gain of electrons is reduction. The lead (II) ions are reduced to lead atoms.

The bromide ions lose electrons at the anode. Loss of electrons is oxidation. The bromide ions are oxidized to bromine molecules.


1.54 write ionic half-equations representing the reactions at the electrodes during electrolysis
For the electrolysis of lead (II) bromide, PbBr2
Cathode:
Pb2+ (l) + 2e- à Pb (l)

Anode:
2Br-(l) à Br2 (g) + 2e-

19 comments:

  1. SO HELPFULLLLLL THANKS

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  2. i still get confused\:

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  3. I have an exam tomorrow and this really helped, thanks :)

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  4. thanks you really helped me understand electrolysis

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  5. This was a really helpful and interesting way to recap!
    Thanks soo much....you are doing a great job!

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  6. The questions are really helpful

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  7. i have exams coming up and this was a great recap since i never understood what my teachers explained :zzzzzzz
    =))

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  8. It was easy to comprehend. It had easy diagrams and useful equations, Thanks for posting this.

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  9. Thankyou so so so much! these pages have been such a help to me I have my mocks coming up in a couple of weeks and I really struggle with science. I have managed to make a lot of flash cards and notes to help me study all because of your wonderful information. Thanks again! Hope you are doing well in IB and eventually university I suppose, you are an inspiration and have really motivated me today. Thankyou!!

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  10. Thank you various anonymous people who have posted nice comments!! I'm really glad the blog helped and is not going to waste even though I'm not doing IGCSEs anymore. :) :)

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  11. I would just like to say thank you for doing this blog, it's very helpful! But, I'm a bit confused after reading so many sources on this topic...please would you help me? So, is this right:
    At the anode (positive electrode) the anion (negative ion) from a compound will form an atom and at the anode, the cation forms (or is this at the cathode, the cation forms?)
    Please reply as soon as possible but thankyouu!

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  12. usefulthankxx

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  13. Thank you so much! This was so helpful and got me a great grade! Hope you're IB's are going well!

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  14. great though my dick hurts at times

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  15. Probably still gonna fail my test..........But at least on a happy note, I can fail with confidence!

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  16. This comment has been removed by the author.

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Note: This blog will no longer be updated as I have finished IGCSEs, and am busy with IB. Sorry! :/