Chemistry: Post your doubts here!

[smzimran]

Need help with the following 2 MCQs:
35. Since BaNO3 is the oxidising agent, shouldn't it be reduced? Why is BaO formed? Why can't Mg(NO3)2 be formed? (The correct option is B.)
30. The er mentions that in this exceptional circumstance, options A and B were both credited. Why do you think B was credited? (I can come up with just one case cis-trans isomerism.)

30.
I cant figure that out myself, what i think is that it has something to do with ethers and their reactions with conc. H2SO4

 

[Wonder girl]

how can i prepare for chemistry paper 5.....i need some guidelines and some tips for doing calculation

 

[abcde]

35.
Barium is reduced check it either by o.s or just by the fact that there is loss of oxygen from BaNO3 to form BaO. (Removal of oxygen is reduction)
As the fuel is 'lit up' it means heat is provided and when heat is given to nitrates, they decompose to metal oxide, NO2 and O2....
Mg(NO3)2 cannot be formed due to the same reason: The nitrate decomposes due to heat so there cannot be a displacement reaction of Mg to form magnesium nitrate...

Thank you. I get it better now. However, the oxidation state of Ba in both compounds is still +2. So there has been no change in the oxidation state in my opinion.

 

[smzimran]

Thank you. I get it better now. However, the oxidation state of Ba in both compounds is still +2. So there has been no change in the oxidation state in my opinion.

Its +3 in BaNO3.
Oxygen.... -2
Nitrogen...+3

 

[abcde]

Its +3 in BaNO3.
Oxygen.... -2
Nitrogen...+3

Barium nitrate has the formula Ba(NO3)2! NO3 ions are always -1. Oxidation state of Ba is still +2. Can you please tell me where I'm wrong?

 

[smzimran]

Barium nitrate has the formula Ba(NO3)2! NO3 ions are always -1. Oxidation state of Ba is still +2. Can you please tell me where I'm wrong?

Oh,.. iam sorry totally forgot the 2 in the (NO3)2
BTW, the removal of oxygen thing must be true then

 

[abcde]

Oh,.. iam sorry totally forgot the 2 in the (NO3)2
BTW, the removal of oxygen thing must be true then

Exactly. Yeah, I think so.

 

[tom ed]

heyy everyone
http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s11_qp_12.pdf
questions 17, 22 and 29.
for question 29 i get how 2 new chiral carbon atoms are added on creating the diol but how is one chiral atom removed with hot, acidified KMnO4?

1)in question no 29....the chiral carbon is removed from where "OH" is attached ..this is because this secondary alcohol part becomes a ketone in which there is a double bond between C and O so chiral carbon is no more there.......
2)in question no 22 the reaction is.......Propane-2-ol +SOCl2----->2-chloropropane +SO2(g)+HCl(g)........since SO2 and HCl(the byproducts) are gases...they escape...leaving behind pure chloroderivative...so this reagent is more preferable...

 

[DragonCub]

heyy everyone
http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s11_qp_12.pdf
questions 17, 22 and 29.
for question 29 i get how 2 new chiral carbon atoms are added on creating the diol but how is one chiral atom removed with hot, acidified KMnO4?

For Q17,
"a strong acid" means concentrated sulphuric acid (conc. H2SO4) can neutralise bases and/or form conjugate bases; "an oxidising agent" indicates that conc. H2SO4 can make another element's oxidation state rise during reaction.
In the choices:
A, ethanol reacts with conc. H2SO4 but the reaction is esterification. This shows neither the "acid" nor "oxidant" property of conc. H2SO4. A is incorrect.
B, magnesium carbonate is a base; it reacts with conc. H2SO4 to show the "acid" property of conc. H2SO4. However, this is a pure neutralisation reaction, which does not involve any shift of oxidation state. The "oxidant" propety cannot be represented. B is incorrect.
C, propanenitrile reacts with conc. H2SO4 to form propanoic acid. Conc. H2SO4 oxidises a nitrile to a carboxylic acid - this is its "oxidant" property, but the reaction does not show any trace of it being an acid. C is also incorret.
D, the reaction equation is 2NaBr + 3H2SO4 = 2NaHSO4 + Br2 + SO2 +2H2O The NaHSO4, or sodium hydrogensulphate, is a conjugate base of sulphuric acid, indicating that H2SO4 is an acid; the oxidation state of Br is raised, from -1 (bromide) to zero (bromine gas), showing that conc. H2SO4 acts as an oxidising agent. This reaction can show both propeties of conc. H2SO4. D is the correct choice.

Answer: D.

Hope this can be of help to you.

 

[abcde]

AoA!
Can anyone explain question 31 from the paper below? I know of no reaction in which sulfuric acid reacts with ethanol. :S
http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s11_qp_11.pdf
An explanation of 24 would also be helpful.

 

[chesterfield]

Are there compiled past papers of chemistry like physics. If someone can link me through if there are. Thanks.

 

[Muhammad Talha]

Plz solve this mcq:
A sample of chlorine containing isotopes of mass numbers 35 and 37 was analysed in a
mass-spectrometer.
How many peaks corresponding to
Cl2(charge on CL +1) were recorded?
A 2 B 3 C 4 D 5

 

[Muhammad Talha]

Use of the Data Booklet is relevant to this question.
What mass of solid residue can be obtained from the thermal decomposition of 4.10 g of
anhydrous calcium nitrate?
A 0.70g B 1.00g C 1.40g D 2.25g

 

[Ahmed Ali Akbar]

need help in oct/nov 09 p12 mcq 40,,,4, and 10..and in may/june 04 mcq 28....detail explanation is needed....

 

[abcde]

AoA!
Can anyone explain question 31 from the paper below? I know of no reaction in which sulfuric acid reacts with ethanol. :S
http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s11_qp_11.pdf
An explanation of 24 would also be helpful.

I understand these questions. No need for an explanation.

 

[abcde]

This (from w11_qp_12) remains unanswered:

30. The er mentions that in this exceptional circumstance, options A and B were both credited. Why do you think B was credited? (I can come up with just one case cis-trans isomerism.)

Attached files on pg.21.

 

[DANGERBP]

do u guyz know how can i get chem exam 2012 june ??

 

[smzimran]

do u guyz know how can i get chem exam 2012 june ??

What do u mean?

 

[abcde]

do u guyz know how can i get chem exam 2012 june ??

Yes. The quickest way is appearing for it this June session.

 

[DANGERBP]

What do u mean?

u can apply for the exam either on june or jan !
so i was wonderin if ide b able to get the questions of this year !?