Chemistry: Post your doubts here!

[xhizors]

column headings for winter 10 53 Q2
please any one
May Allah give you and me world wides distinctions Ameen

 

[ZainH]

Reagent ..Na(OH) condition dipped in ethanol/heat under reflux .. for haloalkanes
Reagent .. Conc. H2SO4 /Al2O3 catalyst and 170C temperature.. for alcohols

Thanks a ton, any specific reason why these reagents are chosen?

 

[syed1995]

Thanks a ton, any specific reason why these reagents are chosen?

Nope.. cuz it's written in the book and those are the only reactions which do elimination

 

[ZainH]

Nope.. cuz it's written in the book and those are the only reactions which do elimination

Ah I see, thanks again

 

[knowitall10]

Two questions:
Q1. We write 'white fumes' as an observation if HCl is formed. Do we write the same thing for HBr and HI?
Q2. In reactions b/w PCl3 and alcohols/carboxylic acids heat is required, but not for those b/w them and PI3. Is this correct?
Also, does any one have Quick revision notes for Inorganic Chemistry? i just don't get it.. :/

Ans 1) I think we write "white fumes" for all three (I suggest u c what the mark scheme says when such a question comes up)
Ans 2) yes, heat PCl3 with alcohols/ carboxylic acids but you warm PI3 with alcohol.
And there's a revision guide for chemistry by Mary Jones: type it on google "download free Chem Revision Guide A level MAry Jones" or something like that.

 

[PhyZac]

Read the book.

1.) NaCl(s) + H2O --> NaCl(aq) .. no pH Change...
2.) MgCl2 + H2O --> only slightly hydrolyzes .. so pH 6.5 ..
3.) AlCl3 + 3H20 --> [Al(H2O)6]3+ and 3Cl-
[Al(H2O)6]3+ --> [Al(H2O)5(OH)]2+ and H+

so pH around 3..

Never so such a thing before, honestly. Thanks alot. (which book?)

Jazaka Allah Khairan, thank you sooooo much...May Allah S.W.T grant you the highest results, and have mercy on you and your family. Aameen. In Sha Allah the exam will be easy for you Ameeen!!

 

[xhizors]

column headings for winter 10 53 Q2
please any one
May Allah give you and me world wides distinctions Ameen

 

[meobunny]

Which question is it?
Generally, to make any concentrations, we use serial dilution.. we add water to the given and make at least five concentrations. But in some mark schemes, they say "Reject/Ignore Serial dilution" so can you tell me which question you're talking about?

EXactly, well it is may june 2007 question 1. Please help thankyou so much for the response !!

 

[knowitall10]

EXactly, well it is may june 2007 question 1. Please help thankyou so much for the response !!

alright you prepare a minimum of five different concentrations of HCl by using serial dilution:
1) Plan what concentrations you're going to make:
1.20, 1.40, 1.60, 1.80, and 2.00(provided to u)
2) Total volume of the solution must remain the same throughout, e.g 50 cm3
3) So,
C1V1=C2V2
C1= 2.00 (the conc. provided/ initial conc)
V1= x (volume of water that you must add)
C2= 1.20 (the conc you're planning to make)
V2= 50 (the total volume of the solution)
so now you calculate how much of the water you must add into the 2.00 mol/dm3 solution of HCl to make 1.20 mol/dm3.
2x=1.20 x 50
x= 25cm3
similarly, you make the rest of the concentrations.
am i clear enough?

 

[knowitall10]

column headings for winter 10 53 Q2
please any one
May Allah give you and me world wides distinctions Ameen

look at the mark scheme...

 

[Soldier313]

the question says aq. solutions of AgF, FeSO4 and MgBr2:
According to electrode potential values, the answer must be:
AgF
anode: O2
cathode: Ag
FeSO4 But when electrolysing FeSO4, why is SO2 wrong? beacause if you read pg 291 of
anode: SO2 x the chem textbook, you'll see that the OH- ions are discharged off at the anode
cathode: H2 in preference to sulfate ions. so the right answer is O2.

MgBr2 Same with MgBr2: on the same page, it also shows that Br- ions are discharged in preference to
anode: O2 x OH- ions so the right answer is Br2.
cathode: H2

JazakAllah khair sis
erm i think you were referring to the coursebook, right? coz the textbook i have has organic chem on page 291



according to electrode potential values, all the cathode products are correct
but when it comes to the anode products, for AgF, F2 should be given off

This is the problem
(for the other two compounds MgBr2 and FeSO4, our answers agree with the ms........ )
Anybody who gets an answer to this qn please do let us know, (or perhaps there's a mistake in the ms??)

 

[knowitall10]

JazakAllah khair sis
erm i think you were referring to the coursebook, right? coz the textbook i have has organic chem on page 291

View attachment 24944

according to electrode potential values, all the cathode products are correct
but when it comes to the anode products, for AgF, F2 should be given off

This is the problem
(for the other two compounds MgBr2 and FeSO4, our answers agree with the ms........ )
Anybody who gets an answer to this qn please do let us know, (or perhaps there's a mistake in the ms??)

Wa iyyak brother.
F2 is not given off because:

F2+2e- --> 2F- E value= +2.87 V
O2+ 2H2O+ 4e- ----> 4OH- E value= +0.40 V

the aq solution contains OH- ions that must convert to O2 to be discharged, so the equation can be rewritten as:
4OH- ----> O2+2H2O+ 4e- E value= -o.40 V (the value became -ve cuz the the reaction should happen in the reverse direction in this case)
Similarly, F- ions must convert to F2:
2F- ---> F2+2e- E value= -2.87 V

The E value of the OH-/O2 reaction is higher than that of F-/F2, so the OH-/O2 reaction proceeds in the forward direction, forming O2 and the
F-/F2 reaction proceeds to the backward direction forming the F- ions in solution. So O2 is given off at the anode.
Am i clear enough?

And nope, I'm talking about our Chemistry text book... Are you using the orange one?

 

[knowitall10]

Assalamu alaykum.
Can someone please explain the mark scheme of M/J 11 Paper 53 Q1 c?
I don't understand how we use the water bath with the reagents provided and i do not know what the thermometer range must be.
Jazakum Allahu Khair.

 

[PANDA-]

For AS Chemistry... What flame tests do we need to know, as in which elements give coloured flames when burnt?

 

[PhyZac]

For AS Chemistry... What flame tests do we need to know, as in which elements give coloured flames when burnt?

I was solving a paper, and a question asked for this.

Na and Cl2, Yellow flame.
P and Cl2, white flame.

 

[PANDA-]

I was solving a paper, and a question asked for this.

Na and Cl2, Yellow flame.
P and Cl2, white flame.

Yes, these kind of questions seem pretty common...
Thanks for the information But these are not the only ones.

 

[knowitall10]

Can anyone please help me? In M/J 11 p5? i posted my question but no one's replying...

 

[meobunny]

alright you prepare a minimum of five different concentrations of HCl by using serial dilution:
1) Plan what concentrations you're going to make:
1.20, 1.40, 1.60, 1.80, and 2.00(provided to u)
2) Total volume of the solution must remain the same throughout, e.g 50 cm3
3) So,
C1V1=C2V2
C1= 2.00 (the conc. provided/ initial conc)
V1= x (volume of water that you must add)
C2= 1.20 (the conc you're planning to make)
V2= 50 (the total volume of the solution)
so now you calculate how much of the water you must add into the 2.00 mol/dm3 solution of HCl to make 1.20 mol/dm3.
2x=1.20 x 50
x= 25cm3
similarly, you make the rest of the concentrations.
am i clear enough?

Thankyou so much :') Allah apko jaza de, you made it so clear ! If you could check my experiment i did before you tellin me about all this ! Could you just check it and mark it ? Thankyou sommuch once agaian

 

[knowitall10]

Thankyou so much :') Allah apko jaza de, you made it so clear ! If you could check my experiment i did before you tellin me about all this ! Could you just check it and mark it ? Thankyou sommuch once agaian

Aameen Sister.
I saw your work, it's a bit vague...
always mention what volume, what temp, how many concentrations, what apparatus you're using:
So, here, mention that the total volume must be 50 cm3
we have to prepare a minimum of 5 diff conc from 2.00 moldm3 (e.g the ones i mentioned in my last reply)
what mass of Mg ribbon? E.g 5grams, weighed by using a digital scale with the accuracy of 0.01 g
volume of HCl measure using a burette
(mention the method alone will get you a mark, for eg: here, you mention that you are preparing your concentrations by serial dilution and then you state your method)
And always remember that when you're planning an experiment, you are explaining somebody who does not know anything. Examiner ko kuch bhi nahi pata to aap nai usai sara method sehi tareekay say explain kar na ha hai.
U get what i mean?
and you're most welcome.

May Allah Succeed Us All in this world and in the Hereafter

 

[goodluckayesha]

why is there no hydrogen bonding btw two CH3F molecules and aldehydes n plz any1 give a brief explanation y these 2 have permanent dipole interactions n not hydrogen bonding n in which cases wud hydrogen bonding occur??