Chemistry teacher - Post your ''A'' level questions here

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Chemistry teacher - Post your ''A'' level questions here

Postby Mr David » Sat Jun 25, 2011 3:54 am

Hi all,

I am opening this thread to assist students who have difficulties in their ''O'' level and ''A'' level chemistry.
I will try my best to answer your questions.

Cheers,
Chemistry teacher

Mr David
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Re: Chemistry teacher - Post your ''A'' level questions here

Postby daisyt » Mon Jun 27, 2011 1:56 pm

Hi, I have a question here. Really appreciate your help. :smile:

W has 12 electrons
Y has 1 electron
What are the elements make up W & Y, it can be covalent or ionic using dot and cross diagrams. (cannot remember the exact words used in the question, but something like that).

Thank you so much!

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Re: Chemistry teacher - Post your ''A'' level questions here

Postby Mr David » Mon Jun 27, 2011 10:52 pm

Hi. Firstly, we have to be clear on one thing: Are you talking about valence electrons or the total number of electrons? For instance, if A has 3 electrons in total, then A has to be from Group 1 ( Remember your electronic configuration-2,1) If A has 3 valence electrons then, A has to be from Group 3.

In this question, there is no ambiguity because there is no way an element has 12 valence electrons (octet rule), although elements in Period 3 can expand their octet configuration. Therefore, it is safe to declare that W has 12 electrons in total. If that is the case, then W has to be from Group 2 i.e. magnesium (electronic configuration-2,8,2). Logically, Y has to be hydrogen.

The result is an ionic compound which is magnesium hydride. In this special scenario, the hydrogen takes on a -1 charge i.e. H-. H+ is more common, but in the presence of a highly electropositive element such as magnesium, H takes on a -1 charge. Therefore, you have to draw the dot and cross diagram for magnesium hydride (ionic compound) i.e. MgH2 or WY2

Chemistry Teacher
Last edited by Mr David on Sat Jul 30, 2011 2:28 am, edited 1 time in total.

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Re: Chemistry teacher - Post your ''A'' level questions here

Postby daisyt » Tue Jun 28, 2011 9:10 pm

Thanks for your explanation. The 12 & 1 are total electrons. :thankyou:

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Re: Chemistry teacher - Post your ''A'' level questions here

Postby Mr David » Tue Jun 28, 2011 10:48 pm

Sure, no problem.

Cheers,
Chemistry Teacher

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Re: Chemistry teacher - Post your ''A'' level questions here

Postby daisyt » Tue Jul 19, 2011 8:44 am

Hi Mr David, i have another question. We know there are 4 basic types of chemical reaction and we need to understand which one before makinh up the equation.

Eg. When sodium hydroxide solution is added to aqueous iron(II) sulfate, iron(II) hydroxide precipitate is formed.
FeSO4(aq) + 2NaOH(aq) --> Na2SO4(aq) + Fe(OH)2(s)

How to know which type of reaction the above belongs to, if we are only given the left sides equation and ask to form the right equation.

Thanks again.

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Re: Chemistry teacher - Post your ''A'' level questions here

Postby Mr David » Fri Jul 22, 2011 4:56 am

Hi.
As far as ''O'' level is concerned, there are 3 standard reactions for acids i.e. 1) acid + metal, 2) acid + base and 3) acid + metal carbonate. Meanwhile, there are 2 standard reactions for bases i.e. 1) base + acid (same as that for acid) and 2) base + ammonium salt. Therefore, we have the so-called 4 standard reactions since the acid-base neutralization repeats itself in both categories. Of course, there are cases when a given reaction doesn't come under any of these 4 categories. Then, we just have to adopt the displacement reaction i.e. exchanging partners, such as the example which you had given. Sodium is a more reactive metal than iron(II), therefore it will displace iron(II) from its sulfate. If you cannot categorize a given reaction under any of these 4 categories, it is ''safe'' to adopt the displacement reaction, as far as ''O'' level is concerned. Actually, there is one more reaction too i.e. metal + water to give salt and hydrogen.

Chemistry Teacher
Last edited by Mr David on Sat Jul 30, 2011 2:31 am, edited 1 time in total.

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Re: Chemistry teacher - Post your ''A'' level questions here

Postby daisyt » Fri Jul 22, 2011 6:10 am

Hi Mr David, thanks. Do you have good reference book for O and A lvl?

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Re: Chemistry teacher - Post your ''A'' level questions here

Postby FrekiWang » Fri Jul 22, 2011 6:21 pm

Mr David wrote:Hi.
As far as ''O'' level is concerned, there are 3 standard reactions for acids i.e. 1) acid + metal, 2) acid + base and 3) acid + metal carbonate. Meanwhile, there are 2 standard reactions for bases i.e. 1) base + acid (same as that for acid) and 2) base + ammonium salt. Therefore, we have the so-called 4 standard reactions since the acid-base neutralization repeats itself in both categories. Of course, there are cases when a given reaction doesn't come under any of these 4 categories. Then, we just have to adopt the displacement reaction i.e. exchanging partners, such as the example which you had given. Sodium is a more reactive metal than iron(II), therefore it will displace iron(II) from its sulfate. If you cannot categorize a given reaction under any of these 4 categories, it is ''safe'' to adopt the displacement reaction, as far as ''O'' level is concerned. Actually, there is one more reaction too i.e. metal + water to give salt and hydrogen.

Hope it makes chemistry simpler for you.
Chemistry Teacher


Sorry, but I strongly disagree with your answer.

This reaction is clearly a 'precipitation' reaction under preparation of salts.

True, a more reactive pure metal displaces a less reactive metal from its solution, but this applies only to e.g. Fe+CuSO4 ->FeSO4 + Cu, but its compound does not, e.g. NaCl + CuSO4 do not react with each other.

The reaction stated is a precipitation(when an aqueous acid/alkali/salt and another aqueous salt are mixed, if one of the compound is insoluble in water after exchanging the ions, a reaction will occur. Otherwise there will be no reaction), precipitation reaction has no link to reactivity series.

Examples of precipitation:

HCl + AgNO3 - > HNO3 + AgCl (s)
2KOH + ZnCl2 -> 2KCl + Zn(OH)2 (s)
CuSO4+BaCl2 -> CuCl2 + BaSO4 (s)

Some examples of no reaction:

HCl + CuSO4 (after ions are exchanged, no insoluble compound will form)
KOH + CaCl2 (after ions are exchanged, no insoluble compound will form)
CuSO4+FeCl2(after ions are exchanged, no insoluble compound will form)

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Re: Chemistry teacher - Post your ''A'' level questions here

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