O-Level Chemistry

[Dr.033430Daniel]

Mojostar:

Hi all,


I'm weak in electrolysis...any way to improve in this area? Any book to recommend? Thanks.

Hi Mojostar – The first thing I would recommend is to read a textbook. These three by local authors are all good:

Chemistry Insights
Chemistry Matters
Discover Chemistry

For question practice, try Conceptual Learning Chemistry by Ong Yean Ling (Blue in Color). I selected this book as a supplement to our Chemistry class worksheets because it looked to be the best one out there.

To get a handle on electrolysis, practice drawing the setups. It is a common exam question to ask you to first draw the setup and then name the products. So once you have studied the topic draw the setup several times for practice, so that you can improve your speed at doing the drawing. Then make your own notes listing all the versions of electrolysis you can find, what goes to the anode, what goes to the cathode etc. There is a logic to it that you should understand in terms of the reactivity of the various ions. But it also helps to write the reactions out a few times so that both the logic and the more common examples stick with you.

Make your notes, then test yourself a week later and see if you can remember.

[spicy]

AdonciaTang:

Mojostar:

Hi all,


I'm weak in electrolysis...any way to improve in this area? Any book to recommend? Thanks.

Hi Mojostar. Get a tutor to help u. My niece was also failing in her Pure Chem for a year and she bought all sorts of books to read thinking it would help. It does but its so minimal. After she engaged my DS's tutor, her results improved, because now there is someone who can help to guide her along in her work and teach her the concept which she might get wrong.

Hi
I have PMed you. Would it be possible to share your tutor's contact?
Thanks !

[achiever2803]

spicy:

AdonciaTang:

[quote=\"Mojostar\"]Hi all,


I'm weak in electrolysis...any way to improve in this area? Any book to recommend? Thanks.

Hi Mojostar. Get a tutor to help u. My niece was also failing in her Pure Chem for a year and she bought all sorts of books to read thinking it would help. It does but its so minimal. After she engaged my DS's tutor, her results improved, because now there is someone who can help to guide her along in her work and teach her the concept which she might get wrong.

Hi
I have PMed you. Would it be possible to share your tutor's contact?
Thanks ![/quote]Hi,

wondering if you are still looking for a tutor for chemistry. if you, do drop me a pm and i can call you to tell you the details.!

Cheers

[hopefulparent]

My son didn’t do well in Chemistry in his lower sec. Maybe it’s because he didn’t put in the effort? But it’s a compulsory subject for his school, so he has to take it this year. Thankfully, he did well for his sec 3 mid year. Proper guidance from school and tuition is very important.

[hterbin]

Mojostar:

Hi all,


I'm weak in electrolysis...any way to improve in this area? Any book to recommend? Thanks.

Electrolysis is a complicated subject with really many different scenarios. However, once you master the basic key concepts which really isn't many, electrolysis would be a really simple subject for you! Do PM me if you need any help regarding electrolysis.

[KiausuLormee]

Need help to explain this question.

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[KiausuLormee]

http://i62.tinypic.com/33duy6x.jpg\">

[KiausuLormee]

http://i60.tinypic.com/2hyh00y.jpg\">

[hterbin]

KiausuLormee:

Need help to explain this question.

http://i57.tinypic.com/11sjpcp.jpg\">

Hi, is it possible that you take a better picture? Part of the question got cut off.

[hterbin]

KiausuLormee:

Need help to explain this question.

http://i57.tinypic.com/11sjpcp.jpg\">

In the question as shown, there are 4 metals, W, X, Y and Z. Now, W, X, Y reacts with cold water while Z does not. W and Y floats on water, so it must be either: Lithium, Sodium or Potassium as only these 3 metals are less dense than water.

Metal X is very unique. Metal X is denser than water and has less vigorous reaction than W and Y. From this we can infer the following - Metal X cannot be a Group I metal and is less reactive than Li, Na or K. Metal X is less reactive than Li, Na or K as it has less vigorous reactions than Metal W and Y, which has to be either Li, Na or K. That being said, the metal cannot be a group I metal as, it cannot be Li, Na or K, and any metal denser than water in Group I, Rb, Cs and Fr are all more reactive than W and Y. Hence, X cannot be a Group I metal. X must be therefore be Calcium since it can react with water and is not a Group I metal.

To question (a) the answer would be:
Metal X is Calcium
Ca + 2H2O -> Ca(OH)2 + H2

Metal Z does not react with cold water, and produce a reddish brown substance upon leaving in water. Whenever there is a presence of a reddish-brown compound, it is most probably the presence of Iron(III) ions. Z reacts with water to form rust, which is a reddish brown substance, also known as hydrated iron(III) oxide. Therefore metal Z is Iron. Now iron has a very unique characteristic, it has a reversible reaction: 3Fe(s) + 4H2O(g) <> Fe3O4(s) + 4H2(g). It reacts with steam to form Iron(II, III) oxide and hydrogen gas, and its oxide reacts with hydrogen gas to form elemental iron and steam. Since iron is ranked above hydrogen ion in the metal reactivity series, it reacts with hydrochloric acid.

To question (bi) the answer would be:
Metal Z is Iron
Reaction with steam: 3Fe + 4H2O -> Fe3O4 + 4H2
Reaction with hydrochloric acid: Fe + 2HCl -> FeCl2 + H2

Now, the reddish brown substance formed can either be iron(III) oxide or iron(III) hydroxide, however for simplicity sake, lets take it as iron(III) oxide. Oxidation involves the losing of electron, i.e. the charge of the species becomes more positive. E.g. Mn loses 2 electrons to form Mn2+, the oxidation state of Manganese increases from 0 in elemental manganese to +2 in manganese(ii) oxide. Note that oxidation state is written as +n and charge is written as n+.

To question (bii) the answer would be:
Iron gains 3 electrons from oxygen to form Iron(III) oxide. Since iron gained electrons to form iron(III) ions, the reaction is an oxidation reaction.

All the explanations are based off the best of what I can guess and see from the photo which cuts off half the questions.