H2 chemistry qns

Z een

22 Nov 09, 00:04

1) The numerical values of the solubility product at 25 degrees celsius for AgCl and AgI are 1.6X10^-10 and 8.0X10^-17 respectively.

What is the equilibrium constant for the reaction below?

AgCl (s) + I- (aq) -> AgI(s)+ Cl-(aq)

I sq rt the solubility prroducts to get conc of Cl- ions and I- ions and take e conc of I- divide by conc of Cl- but i couldnt get the ans.

(ans: 2.0X10^6)

2) Nickel (II) forms a red complex with ligand L at rm temp. 4X10^-3 mol dm^-3 of L reacted with 3X10^-3 solution of nickel (II) chloride.

Then a colorimeter graph shows max reading at 4cm3 of nickel (II) solution and 6cm3 solution.

Why is the co-ordination number of nickel in the complex 4?

3) (CH3)2C=CH2 -> intermediate -> (CH3)3CBr

What is the intermediate for this reaction?

Thank you!

tut4nkh4m3n

22 Nov 09, 00:23

for 1)

Ksp[AgCl] = [Ag+] [Cl-]
Ksp[AgI] = [Ag+] [I-]

divide Ksp[AgCl] by Ksp[AgI] and the [Ag+] will cancel out and you will have [Cl-] / [I-] which is what you want. its value will be 2 x 10^6

Z een

22 Nov 09, 00:28

oh, ty! But why is it that the conc. of Ag ions are same for both compounds?

tut4nkh4m3n

22 Nov 09, 00:29

for 2) i don't really understand the question

for 3) adding HBr(g) to the starting compound will form what you want. the intermediate is the cation formed before the Br- 'attacks' it.

tut4nkh4m3n

22 Nov 09, 00:34

Originally posted by Z een:

oh, ty! But why is it that the conc. of Ag ions are same for both compounds?

not too sure how to ans that. it should go back to the basic definition of Ksp. anyw this question is more of a math question and how you manipulate the relevant equations to get what you want.

UltimaOnline

22 Nov 09, 01:07

~ But why is it that the conc. of Ag ions are same for both compounds? ~ Because we're talking about the same solution within the same container. Hence although the [Ag+], [Cl-] and [I-] values are not fixed (they change in order to maintain the fixed Ksp values at any given temperature), but [Ag+] is [Ag+] within that container whether you are using it to multiply against [Cl-] or against [I-]. Solubility equilibria is always one of the trickiest aspects for students to understand. tut4nkh4m3n's is right, both in his solution (pun pun), as well as his suggestion that you don't really need to understand it fully conceptually, to work out the correct answer mathematically. ~ Why is the co-ordination number of nickel in the complex 4? ~ The question lacks relevant information. Is the ligand monodentate, bidentate, etc? If the question presented to you is exactly as you typed it, then it's flawed, and you can skip this question. 3) (CH3)2C=CH2 -> intermediate -> (CH3)3CBr ~ What is the intermediate for this reaction? ~ The pi bond accepts the proton, forming a tertairy cabocation intermediate, which is subsequently attacked by the bromide ion nucleophile.

youphoria

22 Nov 09, 06:04

2nd qn isnt wrong, its an mcq qn from rjc prelims 09...right?

FrozernFire

22 Nov 09, 09:19

Originally posted by youphoria:

2nd qn isnt wrong, its an mcq qn from rjc prelims 09…right?

my school... it's a bidentate ligand, a diagram of the compound L was provided

UltimaOnline

22 Nov 09, 11:10

If the ligand is bidentate, then the based on the molar ratio of metal cations to ligands, the co-ordination number of nickel in the complex is indeed 4.

Lin.thitlwin

22 Nov 09, 20:24

hey..where should I go to discuss about H1 Economics?

dkcx

22 Nov 09, 20:55

Originally posted by Lin.thitlwin:

hey..where should I go to discuss about H1 Economics?

Start a new thread lo...

Barani

25 Nov 09, 18:27

H2 Chemistry A lvl 2009 question...

Zn(OH)2 plus excess NH3...dissolves to form a complex...write an equation

can the eq be like this?---Zn(OH)2 + 6NH3 --->> [Zn(NH3)6]2+ + 2OH minus

I dun think it's in our syllabus that we need to know how many of the NH3 ligands will be binded to the central transition metal cation...Do you think well be penalised for writing the products as [Zn(NH3)6]2+ instead of the real answer (researched from the net) [Zn(NH3)4] ???

Barani

25 Nov 09, 18:32

Also, can I define co-ordination no. in an ionic lattice structure this way?

Co-ordination no. is the no. of ionic bonds each ion in the lattice structure forms with its surrounding ions?

dkcx

25 Nov 09, 18:37

Originally posted by Barani:

Also, can I define co-ordination no. in an ionic lattice structure this way?

Co-ordination no. is the no. of ionic bonds each ion in the lattice structure forms with its surrounding ions?

Are u sure a coordination bond is an ionic bond? They are not as far as i remember.

Barani

25 Nov 09, 18:42

No..cuz in paper 2..this qns is about a giant ionic lattice structure..the co-ordination no. here isn't the dative bond we learnt in transition metal complxes but the ionic lattice structure...

dkcx

25 Nov 09, 18:46

Originally posted by Barani:

No..cuz in paper 2..this qns is about a giant ionic lattice structure..the co-ordination no. here isn't the dative bond we learnt in transition metal complxes but the ionic lattice structure...

In a giant lattice, u still call it coordination no. meh? No idea what A's are teaching since uni don't bother about this...

I'll let UltimaOnline answer this then... I'm going back to my damn quantum chemisty and hope i survive that exam tml...

Lin.thitlwin

26 Nov 09, 14:11

ha2..I bet UltimaOnline is busy..

UltimaOnline

26 Nov 09, 16:09

The meaning of coordination number depends on context, and is somewhat calculated differently in crystal lattice structures vs complex ions. For a crystal lattice (eg. ionic lattice), coordination numer refers to number of closest neighbouring atoms touching a given (central) atom, and not "no. of ionic bonds" (to what extent you'll be penalized for it depends on how strict the mark scheme is). The usage of the term "bonds" mostly refer to covalent (dative or non) bonds; the so-called "ionic bonds" are really electrostatic forces of attraction between oppositely charged ions. This wiki page explains clearly everything relevant you need to know about coordination number. http://en.wikipedia.org/wiki/Coordination_number

Barani

26 Nov 09, 19:30

sry, hw abt this?

H2 Chemistry A lvl 2009 question...

Zn(OH)2 plus excess NH3...dissolves to form a complex...write an equation

can the eq be like this?---Zn(OH)2 + 6NH3 --->> [Zn(NH3)6]2+ + 2OH minus

I dun think it's in our syllabus that we need to know how many of the NH3 ligands will be binded to the central transition metal cation...Do you think well be penalised for writing the products as [Zn(NH3)6]2+ instead of the real answer (researched from the net) [Zn(NH3)4] ???

UltimaOnline

26 Nov 09, 19:46

Simply speaking, (I haven't personally seen the question, but assuming that no stoichiometric info is given, then) this is an unfair question. But it's not beyond Cambridge to give unfair questions at 'A' levels (eg. see 2008 paper, and now 2009 as well). The chemistry of complex ions is actually far more complex than can be handled at 'A' levels. Which means at 'A' levels, other than a list of common complex ions which you can use to memorize their coordination numbers and geometries (eg. tetraaminediaquocopper(II) ion [Cu(NH3)4(H2O)2]2+), for relatively unknown ions, the question must provide the relevant info for you to deduce the coordination number and geometry. As to whether they will accept or penalize your erroneous answer, it depends on how strict Cambridge sets their mark scheme for this paper. Sometimes (ie. in some papers for some years) they accept, sometimes they partially penalize, sometimes they totally penalize.

Lin.thitlwin

27 Nov 09, 13:46

What is the product formed when phenol reacts with bromine in a SUITABLE SOLVENT?

Is it tribromophenol or just 2-bromophenol/4-bromophenol? Water can also be a solvent right?

UltimaOnline

27 Nov 09, 14:03

Whenever the question doesn't is ambiguous or doesn't specify conditions, it is the duty of the exam-smart student to take the initiative to specify conditions and consequent products. Hence, the exam-smart student would give BOTH answers : In non-polar solvent such as CCl4, we obtain a mixture of 2-bromophenol and 4-bromophenol. In aqueous solvent, we obtain 2,4,6-tribromophenol. That said, less exam-smart students who only give either answer (but solvent MUST be named/identified together with product) should score full marks, but this does depend on the particular mark scheme. If only one answer is to be given (eg. if this is indicated in the question), the more appropriate answer would be the one in which the reaction proceeds further, ie. aqueous solvent and 2,4,6-tribromophenol.

Barani

27 Nov 09, 17:35

one last qns...

what is the product formed when 1,2-dihydroxybenzene is reacted with bromine in both CCl4 and in water?

Barani

27 Nov 09, 20:22

Will there be ECF marks for observations? For example, if part a is about deducing the product of a reaction and part b is regarding the observation of the reaction...and that if the observation in part b is correct, with reference to part a???

UltimaOnline

27 Nov 09, 20:26

ECF is usually only for calculations. For observations, it depends on the mark scheme (ie. up to the setters) for a particular paper. But it's rare. 4-bromo-1,2-dihydroxybenzene and hydrogen bromide (steamy fumes) are the only products formed when 1,2-dihydroxybenzene is reacted with bromine in both CCl4 and in water, due to steric hinderance of the large Br atom.