Chemistry: Post your doubts here!

[Musab100]

cis trans

isomers

 

[snowbrood]

isomers

when is cis trans isomerism not possible i thought C=CR2 but the examiners says if r=h then it is possible am i right

 

[kalpaveen]

can sum1 plzzz give me a link for a set of gud section B short notes :/

 

[a.abid]

Hey guys, I always have problems in calculating the enthalpy change of reaction, combustion and formation ... I mean I mix up... I don't get which value to subtract and which to add... is there any easy way to learn it? I read the book and its quite confusing like for formation we have to consider the elemnts and the products. .. plz help me if u have any easier method of learning this. .

 

[a.abid]

Hey guys, I always have problems in calculating the enthalpy change of reaction, combustion and formation ... I mean I mix up... I don't get which value to subtract and which to add... is there any easy way to learn it? I read the book and its quite confusing like for formation we have to consider the elemnts and the products. .. plz help me if u have any easier method of learning this. .

 

[Afza Munir]

Hey guys, I always have problems in calculating the enthalpy change of reaction, combustion and formation ... I mean I mix up... I don't get which value to subtract and which to add... is there any easy way to learn it? I read the book and its quite confusing like for formation we have to consider the elemnts and the products. .. plz help me if u have any easier method of learning this. .

hey me to have problem in the same thin its so confusin well I N33D URGENT HELP HERE!!!!!!
how to calculate the enthalpy change of reaction for the following
equation: 2NaHCO3 -----> Na2CO3 + CO2 + H2O
a) delta standard enthalpy change of formation [NaHCO3] = -950.8 KJ/mol
b) " " " " " " " " [Na2CO3] = -1130.7 KJ/mol
c) " " " " " " [CO2] = -393.5 KJ/mol
d) " " " " " " [H2O] = -285.8 KJ/mol

how to do it need complete explanation hope any 1 of ya can help me.........

 

[Mushroom Juu]

Hi, can someone please help me answer a silly question? >.<
tresting for unsaturation for organic compounds (alkene) using BROMINE WATER OR BROMINE IN TETRACHLOROMETHANE/CYCLOHEXANE?
and what colour is bromine in water/organic solvent?
and also if anyone knows, what colours are halogens Cl2, Br2 and I2 in water and organic solvent.
it would be VERY much appreciated thanks! >.<

 

[Mushroom Juu]

hey me to have problem in the same thin its so confusin well I N33D URGENT HELP HERE!!!!!!
how to calculate the enthalpy change of reaction for the following
equation: 2NaHCO3 -----> Na2CO3 + CO2 + H2O
a) delta standard enthalpy change of formation [NaHCO3] = -950.8 KJ/mol
b) " " " " " " " " [Na2CO3] = -1130.7 KJ/mol
c) " " " " " " [CO2] = -393.5 KJ/mol
d) " " " " " " [H2O] = -285.8 KJ/mol

how to do it need complete explanation hope any 1 of ya can help me.........

NOTE:!!! i labelled wrongly!! delta f should be delta H.......
eq above is constructed and balanced.
enthalpy of formation of NaHCO3 is multiplied by 2 cuz 2mol is used according to equation.
hess's law stated that the enthalpy for a chemical reaction is independent of route taken provided initial and final conditions are same.
delta H = route from 2NaHCO3-----> 2Na + H2 +2C + 3O2------> Na2CO3 + CO2 + H2O (follow the arrows)
for the arrow pointing in the opposite direction from the route u are taking, change from + to - value OR from - to + value.

I hope this helps >.<

 

[AbbbbY]

Hey guys, I always have problems in calculating the enthalpy change of reaction, combustion and formation ... I mean I mix up... I don't get which value to subtract and which to add... is there any easy way to learn it? I read the book and its quite confusing like for formation we have to consider the elemnts and the products. .. plz help me if u have any easier method of learning this. .

Tis' easy.

CRAP. Remember the word.
C = r-p
F, the other way round.

Easy enough?

 

[Habooshi ;)]

asalam alaykum ,
guys please give me tips on how to learn the reactions in organic chemistry , i dont seem to remember any x_x

 

[Afza Munir]

View attachment 32664

NOTE:!!! i labelled wrongly!! delta f should be delta H.......
eq above is constructed and balanced.
enthalpy of formation of NaHCO3 is multiplied by 2 cuz 2mol is used according to equation.
hess's law stated that the enthalpy for a chemical reaction is independent of route taken provided initial and final conditions are same.
delta H = route from 2NaHCO3-----> 2Na + H2 +2C + 3O2------> Na2CO3 + CO2 + H2O (follow the arrows)
for the arrow pointing in the opposite direction from the route u are taking, change from + to - value OR from - to + value.

I hope this helps >.<

thanks alot buddy u r a life saver

 

[Mushroom Juu]

this is the 2nd time im posting pls help! >.<

Hi, can someone please help me answer a silly question? >.<
tresting for unsaturation for organic compounds (alkene) using BROMINE WATER OR BROMINE IN TETRACHLOROMETHANE/CYCLOHEXANE?
and what colour is bromine in water/organic solvent?
and also if anyone knows, what colours are halogens Cl2, Br2 and I2 in water and organic solvent.
it would be VERY much appreciated thanks! >.<

 

[smoothieoeek]

p42 MJ 11
Question 4(c) (v) can someone help?
Why is it molecular formula and cannot be empirical formula?
I thought if you have same molecular formula you literally should have the same empirical formula? correct me if im wrong

 

[KZW]

this is the 2nd time im posting pls help! >.<

Hi, can someone please help me answer a silly question? >.<
tresting for unsaturation for organic compounds (alkene) using BROMINE WATER OR BROMINE IN TETRACHLOROMETHANE/CYCLOHEXANE?
and what colour is bromine in water/organic solvent?
and also if anyone knows, what colours are halogens Cl2, Br2 and I2 in water and organic solvent.
it would be VERY much appreciated thanks! >.<

The test for unsaturated compounds is with bromine water.
Cl2(aq) is yellow, Br2(aq) is reddish brown, and I2(aq) is purple. I'm not too sure about dissolving them in organic solvent, but i suspect they will be the same colour.

p42 MJ 11
Question 4(c) (v) can someone help?
Why is it molecular formula and cannot be empirical formula?
I thought if you have same molecular formula you literally should have the same empirical formula? correct me if im wrong

I couldn't find your question, I think you typed it wrong. P42 MJ11 question 4c is on stereoisomerism, and there is no (v).

Molecular formula is not necessarily the same as empirical. Molecular is the exact number of each atom in a molecule, while empirical formula is the simplified ratio of the atoms in a molecule.

For example, C6H12O4
The molecular formula is C6H12O4. The empirical formula is C3H6O2 - The ratio of atoms is the same, but it has just been simplified.

Hope it helps, good luck

 

[Avik181]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w10_qp_22.pdf

no.1 C...

 

[100]

http://olevel.sourceforge.net/papers/9701/9701_s13_qp_23.pdf


how to solve question 2b? plzzzz help

 

[Mushroom Juu]

The test for unsaturated compounds is with bromine water.
Cl2(aq) is yellow, Br2(aq) is reddish brown, and I2(aq) is purple. I'm not too sure about dissolving them in organic solvent, but i suspect they will be the same colour.
K thx! >_<



I couldn't find your question, I think you typed it wrong. P42 MJ11 question 4c is on stereoisomerism, and there is no (v).

Molecular formula is not necessarily the same as empirical. Molecular is the exact number of each atom in a molecule, while empirical formula is the simplified ratio of the atoms in a molecule.

For example, C6H12O4
The molecular formula is C6H12O4. The empirical formula is C3H6O2 - The ratio of atoms is the same, but it has just been simplified.

Hope it helps, good luck

Thanks! >_<

The test for unsaturated compounds is with bromine water.
Cl2(aq) is yellow, Br2(aq) is reddish brown, and I2(aq) is purple. I'm not too sure about dissolving them in organic solvent, but i suspect they will be the same colour.



I couldn't find your question, I think you typed it wrong. P42 MJ11 question 4c is on stereoisomerism, and there is no (v).

Molecular formula is not necessarily the same as empirical. Molecular is the exact number of each atom in a molecule, while empirical formula is the simplified ratio of the atoms in a molecule.

For example, C6H12O4
The molecular formula is C6H12O4. The empirical formula is C3H6O2 - The ratio of atoms is the same, but it has just been simplified.

Hope it helps, good luck

 

[joannalovegood]

does anyone know where I can find chemistry 9701 mark scheme for summer 2002 paper 2? Xtremepaper doesn't have it.

 

[smoothieoeek]

The test for unsaturated compounds is with bromine water.
Cl2(aq) is yellow, Br2(aq) is reddish brown, and I2(aq) is purple. I'm not too sure about dissolving them in organic solvent, but i suspect they will be the same colour.



I couldn't find your question, I think you typed it wrong. P42 MJ11 question 4c is on stereoisomerism, and there is no (v).

Molecular formula is not necessarily the same as empirical. Molecular is the exact number of each atom in a molecule, while empirical formula is the simplified ratio of the atoms in a molecule.

For example, C6H12O4
The molecular formula is C6H12O4. The empirical formula is C3H6O2 - The ratio of atoms is the same, but it has just been simplified.

Hope it helps, good luck

Yea i did type it wrong -_- its Q4 (b) (v) What is the relationship between the repeat unit of polymer D and the repeat unit of PVA.

 

[KZW]

http://olevel.sourceforge.net/papers/9701/9701_s13_qp_23.pdf


how to solve question 2b? plzzzz help

In the question, they give you the initial number of moles of each substance present in the equilibrium. All of them should change by the same amount since the ratios are all 1:1, so lets say the change in the number of moles is x.

Therefore, the equilibrium concentrations are
(0.70–x) for CO2 and H2, and (0.30+x) for CO and H2O.

You know Kc is 1.44, and Kc = [products]/[reactants] yes? So pop those values in, and solve as a quadratic.
1.44= (0.30+x)^2 /(0.70-x)^2 etc, and solve for x.

It should give you x= 0.25, and so you substitute it back into the (0.70-x) for CO2 and H2, and (0.30+x) for CO and H2O.

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_22.pdf

no.1 C...

i) The 30cm^3 is the excess oxygen that wasn't reacted with A.
ii) CO2 is the only other gas that is produced, so it should be quite obvious.
iii) Total volume of gas left after combustion = 40cm^3. If 30cm^3 is oxygen, then 10cm^3 should be CO2.
iv) Original volume of oxygen=50cm^3, and there is 30cm^3 of oxygen left. Therefore volume of oxygen reacted is 20cm^3

Yea i did type it wrong -_- its Q4 (b) (v) What is the relationship between the repeat unit of polymer D and the repeat unit of PVA.

Basically they are referring to the number of each element in the monomers. They both have the same amount of each element, so you could say they're isomers, or they have the same molecular formula.