Chemistry: Post your doubts here!

[Pals_1010]

The mixture is distilled...

Aaaah ok.... I guess I missed that part Thanks

 

[Amy Bloom]

Hi,
I need help as i am not able to understand fully the markscheme answer.
Q. the trend of solubility of group2 sulfates.
Q. the trend of stability of carbonates and nitrates of group2 metals.

thankyou...

Hello there!
The trend of solubility of group 2 sulfates decreases down the group. remember that barium sulfate is sparingly soluble salt.
The stability of carbonates and nitrates increase down the grp because carbonates and nitrates have to be heated more strongly b4 they are completely decomposed.
Try to think this way: The carbonate ion has a small +2 charge into a small volume hence it has a high charge density. The distorting effect on a negative ion will increase.

 

[Mobeen]

Hello there!
The trend of solubility of group 2 sulfates decreases down the group. remember that barium sulfate is sparingly soluble salt.
The stability of carbonates and nitrates increase down the grp because carbonates and nitrates have to be heated more strongly b4 they are completely decomposed.
Try to think this way: The carbonate ion has a small +2 charge into a small volume hence it has a high charge density. The distorting effect on a negative ion will increase.

Can you explain it in terms of L.E and enthaloy of Hydration...

 

[Pals_1010]

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_w07_qp_1.pdf

Question 10 please?

 

[smzimran]

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_w07_qp_1.pdf

Question 10 please?

D
2NO2(g) --> 2NO(g) + O2(g)
initial conc= 4 : 0 : 0
equilibrium conc= (4-2x) : (2x) : (x)
It is given that equilibrium conc of O2 is 0.8, which means x = 0.8
equilibrium conc= (4-2(0.8)) : (2(0.8)) : (0.8)
equilibrium conc= 2.4 : 1.6 : 0.8

Kc = [NO]^2[O2] / [NO2]^2
Kc = (1.6)^2 (0.8) / (2.4)^2

 

[Noor]

www.chemguide.co.uk
contains notes classified according to our syllabus.

I think this one's good as well, http://en.wikibooks.org/wiki/Category:A-level_Chemistry

 

[Pals_1010]

D
2NO2(g) --> 2NO(g) + O2(g)
initial conc= 4 : 0 : 0
equilibrium conc= (4-2x) : (2x) : (x)
It is given that equilibrium conc of O2 is 0.8, which means x = 0.8
equilibrium conc= (4-2(0.8)) : (2(0.8)) : (0.8)
equilibrium conc= 2.4 : 1.6 : 0.8

Kc = [NO]^2[O2] / [NO2]^2
Kc = (1.6)^2 (0.8) / (2.4)^2

Thanks but why ''4-2x'' for the concentration of NO2?

 

[smzimran]

Thanks but why ''4-2x'' for the concentration of NO2?

It was initially 4 moles,
and 2 moles are there at equilibrium so that means 2x
That is why it is (4 - 2x)

 

[aliya_zad]

Hi,
I need help as i am not able to understand fully the markscheme answer.
Q. the trend of solubility of group2 sulfates.
Q. the trend of stability of carbonates and nitrates of group2 metals.

thankyou...

Down Group 2 the solubility of sulfates decreases..This is because with increasing cation size both hydration enthalpy and lattice enthalpy decreases..
But the decrease in hydration enthalpy is more than the decrease in lattice enthalpy..the breaking of the lattice therefore becomes tougher down the group and hence they are more stable..
LE increases coz the intermolecular(vanderwaals)forces of attraction increases with increasing size and proton number!

The stability of carbonates and nitrates increases down the group..
ie..down the group cation size increases..
the charge density therefore decreases..
the polarizing ability of cation decreases..
less distortion of anion(carbonate/nitrate)
hence they are more stable down the group..

Don't confuse solubility and stability!!
Hope this helps!!)

 

[saudha]

Given the equation for the complete combustion of ethylbenzene ...​

2C8H10(g)+ 21O2(g)==> 16CO2(g)+ 10H2O(l)​

what volume, in cm3, of ethylbenzene, is burned completely by 225cm​

of air? (3 sig fig answer, all gas volumes are measured at the same temperature and pressure, assume air is 21% oxygen) ​

SMI HELP MEEEEEEEEEEEE!!!!!!!!!!!!!!!!!!!!!!!!!1​

 

[hmlahori]

Given the equation for the complete combustion of ethylbenzene ...​

2C8H10(g)+ 21O2(g)==> 16CO2(g)+ 10H2O(l)​

what volume, in cm3, of ethylbenzene, is burned completely by 225cm​

of air? (3 sig fig answer, all gas volumes are measured at the same temperature and pressure, assume air is 21% oxygen) ​

SMI HELP MEEEEEEEEEEEE!!!!!!!!!!!!!!!!!!!!!!!!!1​

percentage of oxygen in 225 cm^3 of air= 21/100 multiplied by 225= 47.25

according to equation 2 mol C8H10 reacts with : 21 mol of O2
convert moles to (2 into 24 into 1000)cm^3: (21 into 24 into 1000)cm^3
volume in cm^3 48ooo cm^3 : 504000 cm^3
? cm^3 : 47.25
cross multiply
volume in cm^3 of ethylbenzene = (48000 multiplied by 47.25) divided by 504000
= 4.5 cm^3. (answer)
Hope this helps.............................

 

[saudha]

thx hmlahori..... loads

 

[Mobeen]

Hello,
i need help in kinetics...as i knoow slowest step is the rate determining step but how to find this, thats my qs.
its Qs.2 part3 of b.
Thankyou.

 

[Mobeen]

Down Group 2 the solubility of sulfates decreases..This is because with increasing cation size both hydration enthalpy and lattice enthalpy decreases..
But the decrease in hydration enthalpy is more than the decrease in lattice enthalpy..the breaking of the lattice therefore becomes tougher down the group and hence they are more stable..
LE increases coz the intermolecular(vanderwaals)forces of attraction increases with increasing size and proton number!

The stability of carbonates and nitrates increases down the group..
ie..down the group cation size increases..
the charge density therefore decreases..
the polarizing ability of cation decreases..
less distortion of anion(carbonate/nitrate)
hence they are more stable down the group..

Don't confuse solubility and stability!!
Hope this helps!!)

Thankyou...

 

[hmlahori]

Hey fellows need help with some paper 1 questions
Here are the links and questions.....
Thanks.......

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s04_qp_1.pdf Q16,37,39

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_w04_qp_1.pdf Q3,23,37

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s05_qp_1.pdf Q13,18,38

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s07_qp_1.pdf Q12,16,18,22,34,40

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s08_qp_1.pdf Q8,14​

​

 

[smzimran]

​

Hey fellows need help with some paper 1 questions​

Here are the links and questions.....​

Thanks.......​

​

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s04_qp_1.pdf Q16,37,39​

16.
Answer is A
It is the property of Group II carbonates and nitrates that they decompose on heating, lithium is the first element so its carbonate decomposes easily to LiO and CO2
B is wrong bcoz Lithium nitrate decomposes on heating to LiO, NO2 and O2 not into nitrite !
C and D are wrong bcoz Li burns readily into oxygen and it does not react with cold water.

37.
Answer is A
All bond angles are almost 120, maybe you are confused about C=O bond angle, well just remember we dont take into consideration pie bonds when checking angles.
There can be an electrophilic attack on C=C bond
There can be a nucleophilic attack on C=O bond

39.
Answer is D
2 will be oxidised to carboxylic acid but will not be soluble in water due to aromatic structure (benzene ring)
3 will not be oxidised, it is a ketone !

 

[smzimran]

​

Hey fellows need help with some paper 1 questions​

Here are the links and questions.....​

Thanks.......​

​

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_w04_qp_1.pdf Q3,23,37​

3.
Answer is C
These are gases at RTP: O2, CO2, SO2
Since there is a 1:3 ratio with O2, O2 used is 30cm3
That means 60 - 30 = 30cm3 is left,
CO2 and SO2 are each produced 10 cm3
So,
Total volume of gases left = 30 cm3 O2 + 10 cm3 CO2 + 10 cm3 SO2
=50 cm3

23.
Answer is D
These are 9 possible products:
CH3 -CH2Br
CH2Br - CH2Br
CH2Br - CHBr2
CHBr2 - CHBr2
CHBr2 - CBr3
CBr3 - CBr3

CH3 - CHBr2
CH3 - CBr3
CH2Br - CBr3
Thats one difficult question though !


37.
Answer is A
1 is possible very simple !
2 is possible when two .C2H5 free radicals combine
3 is possible when the product from 2 combines with Cl. free radicals

They never said that they are reacting just 1 mol of ethane !

 

[hmlahori]

3.
Answer is C
These are gases at RTP: O2, CO2, SO2
Since there is a 1:3 ratio with O2, O2 used is 30cm3
That means 60 - 30 = 30cm3 is left,
CO2 and SO2 are each produced 10 cm3
So,
Total volume of gases left = 30 cm3 O2 + 10 cm3 CO2 + 10 cm3 SO2
=50 cm3

23.
Answer is D
These are 9 possible products:
CH3 -CH2Br
CH2Br - CH2Br
CH2Br - CHBr2
CHBr2 - CHBr2
CHBr2 - CBr3
CBr3 - CBr3

CH3 - CHBr2
CH3 - CBr3
CH2Br - CBr3
Thats one difficult question though !


37.
Answer is A
1 is possible very simple !
2 is possible when two .C2H5 free radicals combine
3 is possible when the product from 2 combines with Cl. free radicals

They never said that they are reacting just 1 mol of ethane !

hey thanks for explaining all those can u explain the rest too?
p.s. love your quote

 

[Muhammad Bin Anis]

Hey fellows need help with some paper 1 questions​

Here are the links and questions.....​

Thanks.......​

​

​

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s05_qp_1.pdf Q13,18,38​

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berryllium (B)
shud be the answer for 13
going back on periodic table decreases the e.n value and going up increases ... net effect = 0

for 18,
it shud be "A"
because
NH4 ammonium has charge +1 , anything attached to it shud be of charge -1
and then count the number of C,N,H,O in the products and reactants

for 38,
i think it shud be B but i am not sure

 

[hmlahori]

​

berryllium (B)

shud be the answer for 13
going back on periodic table decreases the e.n value and going up increases ... net effect = 0

for 18,
it shud be "A"
because
NH4 ammonium has charge +1 , anything attached to it shud be of charge -1
and then count the number of C,N,H,O in the products and reactants

for 38,
i think it shud be B but i am not sure

could you please explain Q 13 again.
i already know all that you explained
but why isnt the answer not magnesium because it is closest to aluminium in the periodic table
so that means its electronegativity must be most similar to aluminium.

and for question 38 the correct answer is A.
i just realized that option 3 (HOCH2CH2OH) which is a diol is formed when ethene is reacted with cold, dilute acidified potassium manganate.