Chemistry: Post your doubts here!

[Student12]

How do you do the questions mentioned below of the paper attached?
Question 1 : The ans. is A. what i did is 25dm x 1.0x10^-2 you get the moles of KOH but we need to find the concentration of calcium sulphate. It doesnt make sense what i did !! EXPLAIN mee?
Q4 : why is it D?
Q8: why is it D ?
Q11: how do you do this question ?
Q14 ??
Q17: why D and not A
Q20: how do you know alcohols have the ratio of hydrogen to carbon the highest.
Q23: why B and not A ?
Q32 ??

I know they are aloot of questions but Help would be appreciated !

ANYONE ?!?!

 

[Utsav.]

any tips for P5 how to prepare cuz i saw the pastpares and i couldnt solve any of em!!! so confusing!
each and ever question ny suggestions plz!!

 

[elbeyon]

any tips for P5 how to prepare cuz i saw the pastpares and i couldnt solve any of em!!! so confusing!
each and ever question ny suggestions plz!!

Check this out:-
http://www.xtremepapers.com/community/threads/9701-chemistry-paper-5-any-tips.9519/

 

[smartangel]

hey guys...please explain Q7, 8, 13 and 40!
http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_12.pdf

 

[Xtremite]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w11_qp_11.pdf

in MCQ 8, how is A correct? isn't base a specie that accepts a H+?

Also MCQ 9. I dont get that eithr

 

[Jaf]

http://www.xtremepapers.com/CIE/index.php?dir=International A And AS Level/9701 - Chemistry/&file=9701_w04_qp_1.pdf

Question 12, question 23, question 24, question 36, question 37 and question 38 please?

Thank You

12) You need to know the Boltzmann curve for this question. There's no other way of doing this question. When you increase the temperature the peak moves to the right indicating the average energy of the molecules has increased. Answer is B.

23) Most of us will require some sort of 'structure-drawing' in this question. Here are the all the possible substitutions:
1-bromoethane; 1,1-dibromoethane; 1,2-dibromoethane; 1,1,1-tribromoethane; 1,1,2-tribromoethane; 1,2,2-tribromoethane; 1,1,1,2-tetrabromoethane; 1,1,1,2,2-pentabromoethane and finally 1,1,1,2,2,2-hexabromoethane. (It's not as complicated as it sounds. Try drawing all of them)
Ans: D

24) We haven't learnt electrophilic substitution yet, so C and D can be ruled out. (It usually only happens in aromatic compounds). Carbonyls react with CN- ions by nucleophilic substitution so B is the answer.

25) Saturated compounds = alkanes
A,B and C can be ruled out since these reactions require a double bond. D is the correct answer.
(oops sorry, I accidentally did this question; I see it's not on your list)

36) Thermal stability decreases down the group of Hydrogen Halides. (HI decomposes with just a lit match) So 1 is ruled out. This renders C as the answer. However, let's look at the other two statements anyway. 2 - Bond length increases down the group of HXs. This is because the radii of halogens increase down the group so their nuclei move further and further away from the bonding electrons with hydrogen. 3 - Ease of oxidation increases down the group of HXs. Recall that H2SO4 can not oxidize HCl but oxidizes HI to I2.

37) This question is pretty confusing. I need to confirm it from my teacher, first. Thank you so much for bringing this to our attention. :-]
The marking scheme says the answer is A.

38) 1 - this has more Cl atoms so obviously the amount of NaCl formed will be greater. 2 - the bond length of C-Br is more that that of C-Cl and hence it's broken more easily, so obviously NaBr should also be greater. 3 - the bond length of C-I is still longer. So answer is A.

 

[Jaf]

How do you do the questions mentioned below of the paper attached?
Question 1 : The ans. is A. what i did is 25dm x 1.0x10^-2 you get the moles of KOH but we need to find the concentration of calcium sulphate. It doesnt make sense what i did !! EXPLAIN mee?
Q4 : why is it D?
Q8: why is it D ?
Q11: how do you do this question ?
Q14 ??
Q17: why D and not A
Q20: how do you know alcohols have the ratio of hydrogen to carbon the highest.
Q23: why B and not A ?
Q32 ??

I know they are aloot of questions but Help would be appreciated !

4) Write the equations for all. (Remember water is NOT a gas at rtp)
CH4 + 2O2 = CO2 + 2H2O (10+ 50 = 60 dm^3 left)
C2H6 + 3.5O2 = 2CO2 + 3H2O (20 + 35 = 55 dm^3 left)
C3H8 + 5O2 = 3CO2 + 4H2O (30 + 20 = 50 dm^3 left)
C4H10 + 6.5O2 = 4CO2 + 5H2O (40 + 5 = 45 dm^3 left)
Hence D.

8) It's D because only in D is 1 mole of a substance formed from its elements in their standard physical states. (in italics is the definition of enthalpy of formation)

11) This is a mathematical question.
Let the moles of the acid and alcohol that reacted be x moles. So the left over acid and alcohol are (1-x) moles each.
So obviously the moles of each of ethyl ethanoate and water will be x moles.
So putting these in the Kc equation gives:
[(x^2)/(1-x)^2] = 4
I've given you the equation, you do the math. The quadratic equation that results after simplifying gives you answers 2 and 2/3. Since there were originally only 1 mole of the acid/alcohol that reacted, 2 moles can not react. So answer is 2/3 or B.

14) *shrugs* I tried this for 20 mins but I'm not getting the correct answer. I'm sure my method is correct, though. I'm probably doing some mathematical error. (yes, you may have noticed I don't like math very much )

17) It's not A because HBr can further be oxidized by H2SO4 into Br2 + SO2 + H2O. This is a group VII reaction we have to learn.

20) Notice the word 'general formula' in the equation.
GF of alcohols - CnH(2n+1) OH (2:1)
GF of aldehydes - CnH(2n+1) CHO (1:2)
GF of carboxylic acids - CnH(2n+1) COOH (1:2)
GF of halogenoalkanes - CnH(2n+1) X (1:1)
So A.

23) It's not A because the carbon on the far right has 2 hydrogens attached to it. The condition for cis-trans isomerism to be possible is that the carbon atoms between which is the double bond should each be attached to 2 different atoms or groups of atoms. This is only possible in B.

32) 1 - Conc. of H+ has to be high because both dissociations produce H+ ions. 2 - Conc. of SO4(2-) ions can not be high since the dissociation of HSO4(-) is only partial as it is a weak acid and not complete like the strong acid H2SO4. This renders D the answer (there's no other choice which has 1 but not 2 in it).

 

[panda222]

Ive got a really simply question to ask.

Sometimes in the exam, they ask you for the equation for Contact Process. What equation do we give? cuz there are like 3 or 4 of them.

1) S + O2--------SO2
2) 2SO2 + O2-------2SO3
3) SO3 + H2So4---------H2S2O7
4) H2S2O7 + H2O----------2H2SO4

Out of those four which one do we give?

 

[zain786]

ccan anybody please give me all the conditions of the chemical reactions needed in As level organic

including reagents pressure temperature and catalysts.

thank you :

 

[smzimran]

ccan anybody please give me all the conditions of the chemical reactions needed in As level organic

including reagents pressure temperature and catalysts.

thank you :

http://www.xtremepapers.com/community/threads/organic-chemistry-as-level.12534/
Maybe this will help!

 

[trgirl]

do you think solving aqa question will help me or not? i dunno how to improve my chemistry?

 

[hm12]

anyone please reply to my post above????????

 

[Zishi]

The decreasing order of ease of hydrolysis of acyl chloride , alkyl chloride and aryl chloride is as follows : CH3COCl>CH3CH2Cl>C6H5Cl

can anyone explain why is that so?

The carbon in Ethanoyl chloride has a large partial positive charge because of two most electronegative atoms being attached it. So it's very attractive to nucleophiles(hydroxide ions in case of hydrolysis). The chlorine atom in chlorobenzene is directly attached to the benzene ring, so a lone pair on it gets delocalised with the delocalised ring electrons by overlapping. This makes the carbon-chlorine bond much stronger than in alkyl and acid chlorides. I hope that you get the idea now.

do you think solving aqa question will help me or not? i dunno how to improve my chemistry?

If you've solved CIE's past papers already, then they'll definitely help.

 

[trgirl]

i have a question?? i dunno how to answer this??
use your knowledge of structure and bonding to deduce why sodium metal can be beaten into shape with a hammer but solid sodium chloride breaks into pieces?
good question right??

 

[trgirl]

The carbon in Ethanoyl chloride has a large partial positive charge because of two most electronegative atoms being attached it. So it's very attractive to nucleophiles(hydroxide ions in case of hydrolysis). The chlorine atom in chlorobenzene is directly attached to the benzene ring, so a lone pair on it gets delocalised with the delocalised ring electrons by overlapping. This makes the carbon-chlorine bond much stronger than in alkyl and acid chlorides. I hope that you get the idea now.


If you've solved CIE's past papers already, then they'll definitely help.

thanx

 

[hassam]

i have a question?? i dunno how to answer this??
use your knowledge of structure and bonding to deduce why sodium metal can be beaten into shape with a hammer but solid sodium chloride breaks into pieces?
good question right??

well here u ll have to tell why a metal is malleable but an ionic compound is not.....
metals malleability is due to the fact that atoms are arranged in layers surrunded by sea of elecctrons acting as glue....so since these layerrs can slide past easily we cn give metal different shapes....bt i guessin ionic compound ions are held in place by strong forces of electrostatic attraction and there is no layered structure so its not malleable

 

[Student12]

4) Write the equations for all. (Remember water is NOT a gas at rtp)
CH4 + 2O2 = CO2 + 2H2O (10+ 50 = 60 dm^3 left)
C2H6 + 3.5O2 = 2CO2 + 3H2O (20 + 35 = 55 dm^3 left)
C3H8 + 5O2 = 3CO2 + 4H2O (30 + 20 = 50 dm^3 left)
C4H10 + 6.5O2 = 4CO2 + 5H2O (40 + 5 = 45 dm^3 left)
Hence D.

8) It's D because only in D is 1 mole of a substance formed from its elements in their standard physical states. (in italics is the definition of enthalpy of formation)

11) This is a mathematical question.
Let the moles of the acid and alcohol that reacted be x moles. So the left over acid and alcohol are (1-x) moles each.
So obviously the moles of each of ethyl ethanoate and water will be x moles.
So putting these in the Kc equation gives:
[(x^2)/(1-x)^2] = 4
I've given you the equation, you do the math. The quadratic equation that results after simplifying gives you answers 2 and 2/3. Since there were originally only 1 mole of the acid/alcohol that reacted, 2 moles can not react. So answer is 2/3 or B.

14) *shrugs* I tried this for 20 mins but I'm not getting the correct answer. I'm sure my method is correct, though. I'm probably doing some mathematical error. (yes, you may have noticed I don't like math very much )

17) It's not A because HBr can further be oxidized by H2SO4 into Br2 + SO2 + H2O. This is a group VII reaction we have to learn.

20) Notice the word 'general formula' in the equation.
GF of alcohols - CnH(2n+1) OH (2:1)
GF of aldehydes - CnH(2n+1) CHO (1:2)
GF of carboxylic acids - CnH(2n+1) COOH (1:2)
GF of halogenoalkanes - CnH(2n+1) X (1:1)
So A.

23) It's not A because the carbon on the far right has 2 hydrogens attached to it. The condition for cis-trans isomerism to be possible is that the carbon atoms between which is the double bond should each be attached to 2 different atoms or groups of atoms. This is only possible in B.

32) 1 - Conc. of H+ has to be high because both dissociations produce H+ ions. 2 - Conc. of SO4(2-) ions can not be high since the dissociation of HSO4(-) is only partial as it is a weak acid and not complete like the strong acid H2SO4. This renders D the answer (there's no other choice which has 1 but not 2 in it).

For question 4 why do you do 10+50 and all ? & Q20 the ratios ?? Can you explain in more detail :$ Anyone else who can do question no. 14 and these same questions in more detailed explaination ?? Though.. thanks for taking the trouble

 

[hm12]

hm12 said: ↑

The decreasing order of ease of hydrolysis of acyl chloride , alkyl chloride and aryl chloride is as follows : CH3COCl>CH3CH2Cl>C6H5Cl

can anyone explain why is that so?​

The carbon in Ethanoyl chloride has a large partial positive charge because of two most electronegative atoms being attached it. So it's very attractive to nucleophiles(hydroxide ions in case of hydrolysis). The chlorine atom in chlorobenzene is directly attached to the benzene ring, so a lone pair on it gets delocalised with the delocalised ring electrons by overlapping. This makes the carbon-chlorine bond much stronger than in alkyl and acid chlorides. I hope that you get the idea now.


yes i get it now thanks a lot for explaining

 

[hmlahori]

Hey fellows need explanations for some paper 1 mcqs​

Here are the links and questions....​

Thanks.......​

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s08_qp_1.pdf​

Q 8 Why is the answer D not A?​

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s07_qp_1.pdf​

Q 12 Shouldnt the answer be C not B?​

And please also explain Q 16,18,22,34,40 of this same paper.​

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w05_qp_1.pdf​

Q 14 Why is the answer not C?​

Q 33?​

 

[Zishi]

Hey fellows need explanations for some paper 1 mcqs​

Here are the links and questions....​

Thanks.......​

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s08_qp_1.pdf​

Q 8 Why is the answer D not A?​

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s07_qp_1.pdf​

Q 12 Shouldnt the answer be C not B?​

And please also explain Q 16,18,22,34,40 of this same paper.​

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w05_qp_1.pdf​

Q 14 Why is the answer not C?​

Q 33?​

Q8: You've to think about pV=nRT. At constant T, p is inversely proportional to V, and A doesn't show a graph of inversely proportional. At constant p, V and T are directly proportional, which D shows correctly.
Q12: The electro negativity difference between Al and Cl isn't that great as between Mg and Cl, so AlCl3 is mainly covalent.
As for rest of the questions, could you tell why are you not able to do them and where are you facing the problem?