IGCSE CHEMISTRY DOUBTS

[Ivo]

May june 08 q 8 b 3 the first one my answer was N-H & C=O bonds is it correct & if not what is ?
the second one i dnt quite get what the ms says ??

Just say:

Similarity: They both have an amide linkage.

Difference: The protein has many different monomers (of amino acids) whereas the synthetic polyamide has only two monomers (dicarboxylic acid and diamine).

I did this paper as a mock and my teacher marked it, full marks for this section.

[Ivo]

Could someone rexplain please may june 04 q4 b 2 ( i didnt get Ivo's explanation) and b 3 ??

What specifically did you not get, I can try and elaborate.

And for iii) The graph would rise to maximum height, then decrease to zero again.  Why?  Well because aluminium hydroxide (the product formed) dissolves in excess.  Think of the test for aluminium cation:  add aqueous sodium hydroxide, soluble in excess giving a colourless solution.  Same applies here, when a small volume was added, there was precipitate, as you add more and more, it goes colourless, therefore no precipitate.  The common mistake is to think of reactivity.

I hope that clears the issue somewhat. 

[8T]

Oh for b 3 that was perfect mate
But b 2 the only thing i get  is that iron 2 is more reactive period. xD
Could u try and elaborate as u said?

[8T]

Just say:

Similarity: They both have an amide linkage.

Difference: The protein has many different monomers (of amino acids) whereas the synthetic polyamide has only two monomers (dicarboxylic acid and diamine).

I did this paper as a mock and my teacher marked it, full marks for this section.

U seem to make it very easy to understand i already  have + rep u

[Ivo]

Oh for b 3 that was perfect mate
But b 2 the only thing i get  is that iron 2 is more reactive period. xD
Could u try and elaborate as u said?

What, so you don't get why it's Fe(OH)₂?

[8T]

What, so you don't get why it's Fe(OH)₂?

no i know that its a formula NOT B1 B2 IVO PLEASE WHY DOES IT HAVE TO REACH 8 I MEAN I REAched 7 but i dnt get whats the reason behind that???

[Ivo]

no i know that its a formula NOT B1 B2 IVO PLEASE WHY DOES IT HAVE TO REACH 8 I MEAN I REAched 7 but i dnt get whats the reason behind that???

Just making sure, you're talking about sketching the graph and why the graph reaches 8cm³ maximum height?

[8T]

Just making sure, you're talking about sketching the graph and why the graph reaches 8cm³ maximum height?

YES, WHY?

[Vin]

Elaboration::

1) First reaction, FeCl3 + 3 NaOH ---> 3NaCl  +  Fe(OH)3

so the molar ratio is 1:3 <deduce this ratio by 1)

Second reaction, FeCl2 + 2 NaOH ---> 2NaCl  +  Fe(OH)2

molar ratio 1:2

so 4 cm3 of FeCl2 reacted fully with 8 cm3 of NaOH

Ivo pls. check!

[Ivo]

YES, WHY?

Well, as I explained before.  The sodium hydroxide is in excess, so the maximum needed to fully react with the iron(III) chloride is 12cm³.  So, simplify the ratio down to 12:4 = 3:1/  So 3 moles of sodium hydroxide reacted with 1 mole of iron(III) chloride.  So the balanced equation is:

3NaOH + FeCl₃ -> 3NaCl + Fe(OH)₃

OK, now if iron (II) chloride:

2NaOH + FeCl₂ -> 2NaCl + Fe(OH)₂

So 2 moles instead of 3 was required.  So if 3 moles was 12cm³, then 1 mole is 4cm³, then 2 moles must be 8cm³.  Done!  

[Ivo]

Elaboration::

1) First reaction, FeCl3 + 3 NaOH ---> 3NaCl  +  Fe(OH)3

so the molar ratio is 1:3 <deduce this ratio by 1)

Second reaction, FeCl2 + 2 NaOH ---> 2NaCl  +  Fe(OH)2

molar ratio 1:2

so 4 cm3 of FeCl2 reacted fully with 8 cm3 of NaOH

Ivo pls. check!

Yep, exactly the same!

[8T]

THANKS A LOT BOTH !

[jellybeans]

D, obviously.  Because during the day, the plants photosynthesise, so causing oxygen levels to increase.  At night, plants and animals respire so release carbon dioxide, thus reducing the oxygen levels.

OH. i thought the q meant O2 was already high in the morning. lol, :s ;S hhaahaaah thaaankyo

Is this chemistry

i was about to answer

By the way which paper? I mean which session?

LOLLLLL THANKS and sry   lol /facepalm. totally posted it in the wrong thread  
O/N 06 Bio 

[gaurav95]

some more doubts coming up...............

q4aii),iii)thensame q bii

q6bi its saying ammoniuum ion and ms is giving test abt nitrate ion............

q7b whole

thx in advance........

4) a) ii) Pb(NO₃)₂ + 2NaCl -> PbCl₂ + 2NaNO₃
  
        iii) Ag⁺ + Cl^- -> AgCl

    b) ii) Please click here.

6) b) i) Test: Add aqueous sodium hydroxide.  Result: turns damp red litmus paper blue.

7) b) i) Moles of Mg = 3/24 = 0.125
           Moles of CH₃COOH = 12/60 = 0.200
           Using ratios given in the equation, to compare, divide 0.2/2 = 0.100 moles of CH₃COOH.          
           Therefore, magnesium is in excess.

        ii) The one not in excess: ie. 0.1

        iii) 0.1*24 =2.4dm³

Got it?

Ya got it but q biii u didnt explain

6bi where is this kinda of test mentioned...?

thx a lot..

[Ivo]

Ya got it but q biii u didnt explain

6bi where is this kinda of test mentioned...?

thx a lot..

You mean 6) b) ii) or 7) b) ii)?

And the test for ammonium ions you just need to know.  Memorise them!