TYS 'O' LVL chem questions

last SG · 2008-10-15T00:35:24+00:00

relax relax.just do ur paper, mind ur own business.don look left right centre, just focus.

MyPillowTalks SG

ssccareredd

i duno anything

hhheeellp

UltimaOnline SG

No, the qn didn't say "valence", the qn asked
"how many electrons in 1 molecule". So nobody got the correct ans.

Solution

3 bonds = 6 electrons involved in bonding

2 valence electrons not involved in bonding + 2 inner shell electrons not involved in bonding = 4 electrons not involved in bonding.

Ah sian SG

ya what wishboy get it right

i think he forgot to add the inner electron

Ah sian SG

how you know what is a redox reaction

how u determine the oxidation state

and what is a precipation

Ah sian SG

uppppppppppp

UltimaOnline SG

how you know what is a redox reaction

how u determine the oxidation state

and what is a precipation

O.S. = Oxidation State
Eg. KMnO4

K is always K+, so O.S. = +1

O is always O2- (unless peroxide ion, not in 'O' levels), so O.S. = -2

So (+1)+(x)+4(-2) = 0

Hence O.S. of Mn = x = +7

Precipitation is when a cation and an anion bond together and come out of solution as a solid.

Eg. Sulfuric acid + Calcium nitrate = calcium sulfate precipitate + nitric acid.

SO4 2-(aq) + Cat2+(aq) --> CaSO4(s)

Ah sian SG

why no one reply

Ah sian SG

finallyyyyyyyyyyyyy

thank you

Ah sian SG

any more Q

eagle SG

Flourine forms useful compounds called hexaflourides.

(a) Uranium forms 2 types of gaseous uranium hexaflouride, ²³⁵UF₆ and ²³⁸UF₆, based on the 2 isotopes of uranium.
(i) What are isoptopes?
(ii) Which isotope of uranium is radioactive?
(iii) Which gas, ²³⁵UF₆ or ²³⁸UF₆, diffuses faster? Explain your answer.

(b) Sodium aluminium hexaflouride or cryolite is used in the extraction of aluminium by electrolysis.
(i) Explain the role of cryolite in the extraction of aluminium.
(ii) Why is it not possible to extract aluminium by electrolysing a solution of an aluminium salt, like aluminium chloride, using carbon electrodes?

Ah sian SG

wa all i nv see b4 one

expect isopotopes

atom that share the same proton but differ in nucleon

bi i guess ar

catalyst

cii isit too expensive or i think is produce poisionous gas

Ah sian SG

cii i know liao

aluminium is not reacivity cause of oxide layer

Ah sian SG

bo answer

davidche SG

Atoms of the same element having different number of neutrons.

(ii) Which isotope of uranium is radioactive?

Who knoes.
(iii) Which gas, ²³⁵UF₆ or ²³⁸UF₆, diffuses faster? Explain your answer.

235UF. Both molecules have the same kinetic energy. However, since 235UF has a lower Mr of 235 compared to 238, 235 UF diffuses faster.

(b) Sodium aluminium hexaflouride or cryolite is used in the extraction of aluminium by electrolysis.
(i) Explain the role of cryolite in the extraction of aluminium.

Catalyst
(ii) Why is it not possible to extract aluminium by electrolysing a solution of an aluminium salt, like aluminium chloride, using carbon electrodes?

It sublimes.

btw, cyollite is out of syllabus.

gunner77 SG

another 2 hours!

UltimaOnline SG

Since Eagle gave the questions, I feel it's appropriate for him (and not me) to give the answers (it's appropriate only for me to comment on genuine qns from students, rather than Qns given by other Mods).

But I agree with David, that a(ii) and b(i) is not in 'O' level syllabus. My only comments are :

The best definition of isotopes are :

Isotopes of elements have atoms that have the same number of protons but different number of neutrons.

As to "Why is it not possible to extract aluminium by electrolysing a solution of an aluminium salt, like aluminium chloride, using carbon electrodes?"

Aluminium is too reactive, hence Al3+ is too stable to be reduced in aqueous solution; H+ or H2O would instead be reduced at the cathode, producing hydrogen gas.

UltimaOnline SG

FYI, "the role of cryolite in the extraction of aluminium" is NOT "Catalyst".

It's role is actually rather complex, and in any case, is not in the 'O' level syllabus.

http://en.wikipedia.org/wiki/Hall-H%C3%A9roult_process

Ah sian SG

thanks better not to read

i am already very scare

Aluminium is too reactive, hence Al3+ is too stable to be reduced in aqueous solution; H+ or H2O would instead be reduced at the cathode, producing hydrogen gas.

if al is very reactivity why al3+ is too stable

UltimaOnline SG

thanks better not to read

i am already very scare

Aluminium is too reactive, hence Al3+ is too stable to be reduced in aqueous solution; H+ or H2O would instead be reduced at the cathode, producing hydrogen gas.

if al is very reactivity why al3+ is too stable

It's *because* Al is so reactive, that's why Al3+ is so stable. (Same with Na, K, all reactive metals; and fluorine too : F- is stable because F is very reactive).

Analogy :

If you're very passionate about your girlfriend, once you got her liao, you die die won't let anyone take her away right?

Similarly, once Na (very desperate to get rid of it's 1 valence electron) has managed to throw away it's electron to become Na+, it's very stable and die die doewan to kena reduce back to Na.

Ah sian SG

got more Qs

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