iGCSE 2012 Chemistry Discussion

Original post by Graceyfave

Can someone clarify electrolysis of aqueous solutions? The difference between concentrated and dilute? :smile:

This was posted from The Student Room's iPhone/iPad App

Actually, I can't see that this will come up in our exam, as the level of depth is a tiny bit higher than usual, but let's give it a shot anyway.

Basically, some molecules in solution are easier to oxidize than others: take NaCl aqueous solution. Water molecules or Cl- ions can both be produced at the anode, but Cl- ions are easier to oxidize than water molecules, so you'll generally produce more Cl2 gas at the anode: 2Cl- -> Cl2 + 2e-

However, the more dilute the solution (i.e. the higher the concentration of water molecules), the more O2 gas will be coming out as well: 4OH- -> 2H2O + 4e- + O2. So you'll get both, depending on the concentration of the solution.

Hope this helps :smile:

Reply 62

OP

Original post by Graceyfave

Can someone clarify electrolysis of aqueous solutions? The difference between concentrated and dilute? :smile:

This was posted from The Student Room's iPhone/iPad App

Not on GCSE syllabus.

Original post by Big-Daddy

Actually, I can't see that this will come up in our exam, as the level of depth is a tiny bit higher than usual, but let's give it a shot anyway.

Basically, some molecules in solution are easier to oxidize than others: take NaCl aqueous solution. Water molecules or Cl- ions can both be produced at the anode, but Cl- ions are easier to oxidize than water molecules, so you'll generally produce more Cl2 gas at the anode: 2Cl- -> Cl2 + 2e-

However, the more dilute the solution (i.e. the higher the concentration of water molecules), the more O2 gas will be coming out as well: 4OH- -> 2H2O + 4e- + O2. So you'll get both, depending on the concentration of the solution.

Hope this helps :smile:

Correct but not on our syllabus :smile:

Reply 63

Graceyfave

Original post by sahajkaur

Not on GCSE syllabus.

Correct but not on our syllabus :smile:

It is.. :/

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Reply 64

willbarnes

Original post by _ashT

ah okay, thank you so much! Also, there's another point that I'm not too sure of: "explain how the methods of extraction of the metals in this section are related to their positions in the reactivity series."?

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I think this is about why Aluminium is extracted by electrolysis whereas Iron is reduced with CO. Aluminium is more reactive than Iron and Carbon is in between them. Hence, Carbon Monoxide can reduced Iron Oxide to Iron. I think that this doesn't work for aluminium though because aluminium is more reactive than carbon, so you need to use electrolysis.

Reply 65

Graceyfave

describe simple experiments for the electrolysis, using inert electrodes, of aqueous
solutions of sodium chloride, copper(II) sulfate and dilute sulfuric acid and predict
the products
1.54 write ionic half-equations representing the reactions at the electrodes during electrolysis
1.55 recall that one faraday represents one mole of electrons
1.56 calculate the amounts of the products of the electrolysis of molten salts and aqueous
solutions.
describe the manufacture of sodium hydroxide and chlorine by the electrolysis of
concentrated sodium chloride solution (brine) in a diaphragm cell
5.29 write ionic half-equations for the reactions at the electrodes in the diaphragm cell
5.30 recall important uses of sodium hydroxide, including the manufacture of bleach,
paper and soap; and of chlorine, including sterilising water supplies and in the
manufacture of bleach and hydrochloric acid

Just so no one is caught out

Reply 66

In addition we also need to know aluminium extraction by electrolysis.

Reply 67

Original post by sahajkaur

Correct but not on our syllabus :smile:

It is in the textbook, so I think it may come up. Unlikely, but possible.

Reply 68

HerroKitty

6

Does anyone happen to know which calcualtions only appear on paper 2?

Reply 69

Original post by HerroKitty

Does anyone happen to know which calcualtions only appear on paper 2?

Electrolysis calculations. I never got my head around them - in the end I had to develop an equation to solve them.

Reply 70

octopauly

14

Original post by Big-Daddy

Electrolysis calculations. I never got my head around them - in the end I had to develop an equation to solve them.

Could you share it please?

Reply 71

OP

Original post by Big-Daddy

Actually, I can't see that this will come up in our exam, as the level of depth is a tiny bit higher than usual, but let's give it a shot anyway.

Basically, some molecules in solution are easier to oxidize than others: take NaCl aqueous solution. Water molecules or Cl- ions can both be produced at the anode, but Cl- ions are easier to oxidize than water molecules, so you'll generally produce more Cl2 gas at the anode: 2Cl- -> Cl2 + 2e-

However, the more dilute the solution (i.e. the higher the concentration of water molecules), the more O2 gas will be coming out as well: 4OH- -> 2H2O + 4e- + O2. So you'll get both, depending on the concentration of the solution.

Hope this helps :smile:

Original post by Graceyfave

Can someone clarify electrolysis of aqueous solutions? The difference between concentrated and dilute? :smile:

This was posted from The Student Room's iPhone/iPad App

You misunderstood the specification. It is not asking you about the DIFFERENCE between electrolysis of dilute and concentrated products (explained above) but rather it is asking you aout the electrolysis of CONCENTRATED sodium chloride solution which is one of the industrial process we have to know. So study that and please don't mislead others.

Reply 72

OP

Original post by Big-Daddy

It is in the textbook, so I think it may come up. Unlikely, but possible.

' The difference between concentrated and dilute?' isnt and it has never come up, but its easy to work out :smile:

Reply 73

Original post by sahajkaur

You misunderstood the specification. It is not asking you about the DIFFERENCE between electrolysis of dilute and concentrated products (explained above) but rather it is asking you aout the electrolysis of CONCENTRATED sodium chloride solution which is one of the industrial process we have to know. So study that and please don't mislead others.

In which hydrogen gas and chlorine gas will be produced. :smile:

Reply 74

OP

Original post by Big-Daddy

In which hydrogen gas and chlorine gas will be produced. :smile:

As well as sodium hydroxide :wink:

But yeah

Reply 75

the11thJD

Sorry to be a pain but could someone please point me in the way of like a revision list or something? :smile:

Our teachers gave us one for bio, and it had like: 3.2 know the how to label a diagram of the chest cavity, including thorax, diaphragm etc. I'm presuming it's an official one but not sure :erm:

would be really helpfull if someone could :colondollar:

Reply 76