IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Sciences => Topic started by: kratos009 on November 19, 2009, 06:12:34 am

Title: AS Chemistry Question MCQ
Post by: kratos009 on November 19, 2009, 06:12:34 am

Hey guys
I got one Chemistry multi-choice question where i don't understand how they got the answer,

1.Aluminium chloride catalyses certain reactions by forming carbocations (carbonium ions) with
chloroalkanes as shown.
RCl + Al Cl3 ? R+ + AlCl4-


Which property makes this reaction possible?
A AlCl3 exists as the dimer Al2Cl6 in the vapour.
B AlCl3 is a covalent molecule.
C The aluminium atom in Al Cl3 has an incomplete octet of electrons.
D The chlorine atom in RCl has a vacant p orbital.


The answer was C. Could someone explain to me why the AlCl3 has an incomplete octet, because doesn't the aluminium ion give away its three electrons to the Cholride ions?

Title: Re: AS Chemistry Question MCQ
Post by: 123 on November 19, 2009, 07:49:01 am

Hey this is explained in that Chem MCQ helps book...i think its in pg. 18 (left bottom) : Q18
Heres that book if u don;t hav 1: http://www.megaupload.com/?d=7CBYEBYX (http://www.megaupload.com/?d=7CBYEBYX)

Title: Re: AS Chemistry Question MCQ
Post by: kratos009 on November 19, 2009, 07:57:10 am

Thanks a lot 123,  i understand it now.
If you have Chem exam tomorrow, GOOD LUCK.

Oh i just got one more question,

1.How does ammonia behave when its aqueous solution is used to dissolve silver chloride, AgCl ?

A as a base
B as a ligand with the Ag+ ion
C as a ligand with the Cl– ion
D as a reducing agent

the answer was B. I was wondering how does ammonia bind with the positive Ag+ ion, i thought it would bind with the negatively charged chloride ion as an ammonium chloride?thanks

Title: Re: AS Chemistry Question MCQ
Post by: ghl23 on November 19, 2009, 08:20:53 am

if i've not mistaken, when ammonia is added to silver chloride.. it reacts with silver to form a soluble complex Ag(NH3)2+ and also liberating Cl- ions

AgCl(s) + 2NH3(aq) ? Ag[(NH3)2]+(aq) + Cl?(aq)

Title: Re: AS Chemistry Question MCQ
Post by: top_dog77 on November 19, 2009, 08:21:50 am

thanks for the upload

Title: Re: AS Chemistry Question MCQ
Post by: ghl23 on November 19, 2009, 08:29:38 am

and to answer your question.. it forms the amine group(NH2-) which then reacts with the silver ions :-X

Title: Re: AS Chemistry Question MCQ
Post by: kratos009 on November 19, 2009, 08:51:32 am

Oh right i get it, thanks a lot.

Title: Re: AS Chemistry Question MCQ
Post by: 123 on November 19, 2009, 08:55:14 am

Quote from: kratos009 on November 19, 2009, 07:57:10 am

Thanks a lot 123,  i understand it now.
If you have Chem exam tomorrow, GOOD LUCK.

Oh i just got one more question,

1.How does ammonia behave when its aqueous solution is used to dissolve silver chloride, AgCl ?

A as a base
B as a ligand with the Ag+ ion
C as a ligand with the Cl– ion
D as a reducing agent

the answer was B. I was wondering how does ammonia bind with the positive Ag+ ion, i thought it would bind with the negatively charged chloride ion as an ammonium chloride?thanks

Even i thought it binds with -ve charged chloride ion as an ammonium chloride  :-\
But it turns out that when the ppt. of AgCl dissolves in NH3 (aq) the silver ion forms a complex ion (
This line is there in the CIE txtbook (pg. 240). Dunno if ur doing A2 levels but the detail involve complex ions & solubility product (A2 level stuff!!). This site explains it well: http://www.chemguide.co.uk/inorganic/group7/testing.html#top (http://www.chemguide.co.uk/inorganic/group7/testing.html#top)
Man the CIE txtbook is really confusing!! Some stuff is not explained and they ask that stuff  in a AS level paper1!!