Qualification > Sciences
ALL CIE CHEMISTRY DOUBTS HERE !!
elemis:
--- Quote from: Deadly_king on February 17, 2011, 05:42:10 am ---That's true but before turning into a carboxylic acid......the compound will first be converted into an aldehyde. We need to oxidise much more for it to get converted to carboxylic acid.
--- End quote ---
The question says it is oxidised completely THEN it is tested with DNPH
Deadly_king:
--- Quote from: Ari Ben Canaan on February 17, 2011, 06:13:10 am ---The question says it is oxidised completely THEN it is tested with DNPH
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Yupz........but note that in test 5, the reaction says warm with acidified dichromate ions. This is a weak reagent which is not sufficiently strong to convert the aldehyde formed to carboxylic acid readily.
If manganate ions had been used, then no aldehyde would have been present since all of them would have readily been converted to carboxylic acid.
This is the difference, your answer would have been okay had manganate ions been used instead of dichromate. ;)
narnia:
Why is SiO2 insoluble in water buh dissolves in concentrated alkali ???
Also,why acid-base character change as we go down group4?
Amelia:
--- Quote from: mixed on February 19, 2011, 11:33:22 am ---Why is SiO2 insoluble in water buh dissolves in concentrated alkali ???
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Silicon dioxide is a giant molecule (macromolecular structure due to extensive network of covalent bonds in the crystalline lattice structure that extends to infinity). since the solubility of a substance is related to the similarity in bond strength in the solvent (water in this case) AND the solute (silicon dioxide in this case), and the extensive network of covalent bonds is much stronger than the water-water interactions (i.e. hydrogen bonds), therefore silicon dioxide CANNOT be hydrated by water molecules (i.e. water cannot break down the giant crystal lattice structure of the macromolecule SiO2) to form aqueous ions and therefore it remains insoluble in water. (silicon dioxide only dissolves by heating with strong alkali. in this case water is not alkaline enough to hydrolyse silion dioxide to give sillicate ions).
Also Silicon dioxide is an acidic compound.
In one line, Silicon dioxide doesn't react with water, because of the difficulty of breaking up the giant covalent structure.
--- Quote ---Also,why acid-base character change as we go down group4?
--- End quote ---
http://www.thestudentroom.co.uk/wiki/Revision:Acid-Base_Character_of_Group_4_Oxides
Nera_egypt:
I have a question in the application booklet related to Biochemistry
In section 1.3 – Genetic information. I couldn't solve SAQ 11 part c on page 41 :( .
I would be really glad if someone helps me work it out :)
P.S: I attached the question
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