Originally posted by Ah sian: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)
why no one reply
finallyyyyyyyyyyyyy
thank you
any more Q
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?
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
cii i know liao
aluminium is not reacivity cause of oxide layer
bo answer
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?
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.
another 2 hours!
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.
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.
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
Originally posted by Ah sian: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.
got more Qs
what is ar
atomic mass of 1/12 of 12 carbon atom (isit)
eh 1pm liao leh. Go makan a good lunch and relax urself (think of someone u love and feel happy) till the exam starts.
if u don't makan, empty stomach, blood glucose level low, brain cannot function properly, u cannot perform at ur best.
go makan some nice lunch, and relax and be happy. the more relax and happy u are, the better u will score in the exam.
Originally posted by Ah sian:what is ar
atomic mass of 1/12 of 12 carbon atom (isit)
Biangz eh. Fundamentals like definitions don't do last minute work lah.
Relative Atomic Mass is the mass of an atom of (a particular isotope of) an element, relative to one-twelfth the mass of an atom of carbon-12.
i suddenly very nervous ahhaha
Originally posted by Ah sian:i suddenly very nervous ahhaha
Ok Ok, stop asking qns liao. Go makan, go relax. Enjoy urself, enjoy the exams, enjoy ur holidays.
(And next year come for my 'A' level tution.)
Originally posted by UltimaOnline: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.
but for 'O' levels, they need to know that it helps lower the melting point of aluminium oxide, no?
Originally posted by tinuviel07:
but for 'O' levels, they need to know that it helps lower the melting point of aluminium oxide, no?
In old 'O' level syllabus yes. New 'O' level syllabus taken out liao.
i too nervous liao
i now
ar = sum of isotopic mass X percentage
Originally posted by Ah sian:i too nervous liao
i now
ar = sum of isotopic mass X percentage
In other words, a weighted average of the isotopes.
For instance, given that average molar mass of Cl is 35.5, and that there are 2 isotopes, Cl-35 and Cl-37, find the relative frequency of each isotope.
Solution :
(x/100)(35) + ((100-x)/100)(37) = 35.5
x = 75
Hence 75% of all chlorine atoms have a molar mass of 35g, and 25% of all chlorine atoms have a molar mass of 37g.
this one the answer given by my teacher last time
(a)
(i) They are elements with the same chemical properties and number of protons, but with different number of neutrons.
(ii) Uranium-235, 23592U
(iii) 235UF6 diffuses faster. This is because it has a smaller molecular mass than 238UF6.
(b)
(i) The mixture of cryolite and aluminium oxide has a lower melting point than pure aluminium oxide.
(ii)
Aluminium is very reactive; it is higher in the reactivity series than
hydrogen. Thus, hydrogen ions will be preferably discharged, not
aluminium.