Chemistry: Post your doubts here!

[princess Anu]

Can u link that papers here.
True.

I haven't even started P5
Can someone guide me on what I should do
what are the specific p5 topics that I must cover?
pls help
Guide me for physics & bio p5 too

qwertypoiu

 

[awesomaholic101]

Can u link that papers here.
True.

I am not sure I have them. But I'll search and let you know.

 

[qwertypoiu]

I haven't even started P5
Can someone guide me on what I should do
what are the specific p5 topics that I must cover?
pls help
Guide me for physics & bio p5 too

qwertypoiu

You should attempt a few paper 5s and compare your answers with the mark schemes. Most of the questions are logical and only require basic chemistry concepts.
In question 1 you'll be asked a few questions then asked to devise an experiment. You just need to be careful t mention the independent, dependent, and control variables. You must make clear how you'll change the independent variable. You must specify all the apparatus to be used. You must explain how the control variables will be controlled. They'll guide you on these points. Then they'll ask you a few more questions like precautions. You just have to think logically.
In question 2, you'll be given data. You just have to draw graphs and analyse. Even simpler.

For physics it's mainly same. In question 2 there is uncertainties involved and you'll need to learn to draw "line of worst Fit". Again a few mark schemes and examiner reports will make it very clear what they want.

Bio is also same.

 

[The Sarcastic Retard]

You should attempt a few paper 5s and compare your answers with the mark schemes. Most of the questions are logical and only require basic chemistry concepts.
In question 1 you'll be asked a few questions then asked to devise an experiment. You just need to be careful t mention the independent, dependent, and control variables. You must make clear how you'll change the independent variable. You must specify all the apparatus to be used. You must explain how the control variables will be controlled. They'll guide you on these points. Then they'll ask you a few more questions like precautions. You just have to think logically.
In question 2, you'll be given data. You just have to draw graphs and analyse. Even simpler.

For physics it's mainly same. In question 2 there is uncertainties involved and you'll need to learn to draw "line of worst Fit". Again a few mark schemes and examiner reports will make it very clear what they want.

Bio is also same.

Hats off man! ^_^

 

[pundadesh]

Hi guys can I ask if

CH3CHClCHClCH3 is the same as CH3CH(Cl)CH(Cl)CH3, cause the mark scheme specifies CH3CH(OH)CH(OH)CH3 for the -OH functional group

 

[The Sarcastic Retard]

Hi guys can I ask if

CH3CHClCHClCH3 is the same as CH3CH(Cl)CH(Cl)CH3, cause the mark scheme specifies CH3CH(OH)CH(OH)CH3 for the -OH functional group

You can write without brackets as well in this case, no problem.

 

[pundadesh]

You can write without brackets as well in this case, no problem.

Would CIE accept though?

 

[The Sarcastic Retard]

Would CIE accept though?

To be at safer side include brackets. In my As level I did without brackets. ^_^

 

[Eugene99]

So precipitate with aquous silver nitrate means that could be any of the halogens but what does solubility with aquous ammonia tell about?

 

[qwertypoiu]

View attachment 60111


So precipitate with aquous silver nitrate means that could be any of the halogens but what does solubility with aquous ammonia tell about?

The silver ions will form the following precipitates:
AgCl - white ppt
AgBr - creamy ppt
AgI - yellow ppt

These ppt will dissolve as follows:
If dilute ammonia is added to AgCl, it dissolves.
If concentrated ammonia is added to AgBr, it dissolves.
Even conc. NH3 is not able to dissolve AgI.

Therefore, in your question, since they said dilute ammonia cannot dissolve the ppt, we know that X cannot be a chloro-compound.

 

[Eugene99]

The silver ions will form the following precipitates:
AgCl - white ppt
AgBr - creamy ppt
AgI - yellow ppt

These ppt will dissolve as follows:
If dilute ammonia is added to AgCl, it dissolves.
If concentrated ammonia is added to AgBr, it dissolves.
Even conc. NH3 is not able to dissolve AgI.

Therefore, in your question, since they said dilute ammonia cannot dissolve the ppt, we know that X cannot be a chloro-compound.

thank u so much I get that...here is another
Question is :
Compound A is a liquid which does not react with 2,4-DNPH reagent or with aqueous bromine. Suggest two structural formula of A.
(We found the molecular formula in previous part which was C2H4O2) Here is what the markscheme says

It's (d)
I know ethanoic acid is formed but isn't HCO2CH3 just another way of writing CH3CO2H?

 

[qwertypoiu]

thank u so much I get that...here is another
Question is :
Compound A is a liquid which does not react with 2,4-DNPH reagent or with aqueous bromine. Suggest two structural formula of A.
(We found the molecular formula in previous part which was C2H4O2) Here is what the markscheme says
View attachment 60113
It's (d)
I know ethanoic acid is formed but isn't HCO2CH3 just another way of writing CH3CO2H?

No!
HCO2CH3 is in fact an ester!
The name is methylmethanoate.
It can be formed by esterification of methanoic acid (HCOOH) and methanol (CH2OH).
If we wanted to write ethanoic acid with the acid group on the left, we'd write:
HO2CCH3.

Because otherwise an R1-CO2-R2 is interpreted as an ester.

 

[Eugene99]

No!
HCO2CH3 is in fact an ester!
The name is methylmethanoate.
It can be formed by esterification of methanoic acid (HCOOH) and methanol (CH2OH).
If we wanted to write ethanoic acid with the acid group on the left, we'd write:
HO2CCH3.

Because otherwise an R1-CO2-R2 is interpreted as an ester.

wow, you just cleared my concept. thank you! Okay, just one more please


We have to draw the Hess's law here. In b(iii), we found enthalpy change of K2CO3, it was -32.6 kJ/mol and in c(iii), we found enthalpy change of KHCO3, it was +23.19 kJ/mol , can you please draw the Hess's cycle for me??

 

[Iris Dylan Lane]

Solids W, X, Y and Z are compounds of two different Group II metals. Some of their applications are described below. Compound W is used as a refractory lining material in kilns. Compound X is used as a building material. It can also be heated in a kiln to form compound Y. When Y is hydrated, it forms compound Z which is used agriculturally to treat soils.
Which statements about these compounds are correct?

1 Adding W to water has less effect on pH than adding Y.
2 Adding Z to soil increases the pH of the soil.
3 The metallic element in Y reacts with cold water more quickly than the metallic element in W.

Answer A.
My Name? How please?

 

[Rizwan Javed]

Solids W, X, Y and Z are compounds of two different Group II metals. Some of their applications are described below. Compound W is used as a refractory lining material in kilns. Compound X is used as a building material. It can also be heated in a kiln to form compound Y. When Y is hydrated, it forms compound Z which is used agriculturally to treat soils.
Which statements about these compounds are correct?

1 Adding W to water has less effect on pH than adding Y.
2 Adding Z to soil increases the pH of the soil.
3 The metallic element in Y reacts with cold water more quickly than the metallic element in W.

Answer A.
My Name? How please?

For this question, first you need to identify what those w, x, y and z compounds are.
By looking at their applications, you can figure out that w is MgO (since it has a high mp, there it used as a refractory material)
Compound x is CaCO3 (it is used in marbles etc)
When CaCO3, that is X is heated, it produces Y which is CaO like this:

CaCO3 ----> CaO + CO2

Adding Y, to water produces Ca(OH)2 which is alkaline solution.

Now you know what these mystery compounds are.
When W(MgO) is added to water, it does not cause much change of pH since it has very little solubility in water (solubility with water increases down the group), whereas Y which is CaO is soluble, thus causes an increase in pH. - Statement 1 correct.

Z is Ca(OH)2 is an alkali. So it will raise the pH of the soil. - Statement 2 is correct.

The metallic element in Y is Ca. I think you know that Ca reacts readily with cold water, while Mg does not react readily with cold water but it reacts with Steam. - Hence Statement 3 is also correct.

So the answer is A.

 

[Rizwan Javed]

Can you please give the whole question?
Enthalpy changes of K2CO3 & KHCO3 you mentioned refer to enthalpy changes of formation or combustion for these compounds?

wow, you just cleared my concept. thank you! Okay, just one more please
View attachment 60114

We have to draw the Hess's law here. In b(iii), we found enthalpy change of K2CO3, it was -32.6 kJ/mol and in c(iii), we found enthalpy change of KHCO3, it was +23.19 kJ/mol , can you please draw the Hess's cycle for me??

 

[Iris Dylan Lane]

For this question, first you need to identify what those w, x, y and z compounds are.
By looking at their applications, you can figure out that w is MgO (since it has a high mp, there it used as a refractory material)
Compound x is CaCO3 (it is used in marbles etc)
When CaCO3, that is X is heated, it produces Y which is CaO like this:

CaCO3 ----> CaO + CO2

Adding Y, to water produces Ca(OH)2 which is alkaline solution.

Now you know what these mystery compounds are.
When W(MgO) is added to water, it does not cause much change of pH since it has very little solubility in water (solubility with water increases down the group), whereas Y which is CaO is soluble, thus causes an increase in pH. - Statement 1 correct.

Z is Ca(OH)2 is an alkali. So it will raise the pH of the soil. - Statement 2 is correct.

The metallic element in Y is Ca. I think you know that Ca reacts readily with cold water, while Mg does not react readily with cold water but it reacts with Steam. - Hence Statement 3 is also correct.

So the answer is A.

W.O.W.
Thanks a lot

 

[ashcull14]

 

[The Sarcastic Retard]

View attachment 60115

Can I get the diagram of electrochemical cell?

 

[ashcull14]

Can I get the diagram of electrochemical cell?

oh sorry