Chemistry: Post your doubts here!

[Ibtehaj Munir]

If you check the threshold grades 32-33 is already an A (Depends on how hard the paper is) Just a couple of grades man! GO FOR IT!

That was a HELL LOT OF MOTIVATION

I understand.
First, to be able to do well in any subject, you will have to clear your negative mindset about it. Same goes with chemistry. Think of it as a challenge instead of a burden.
Having said that, for chemistry you will first have to understand each concept very well to go any further. It's best if you can have a good teacher who can help you out with that. So if you do, you need to pay attention in class. Taking notes during the lecture helps you stay awake and grasp things. Then, study each chapter from the textbook (highlight important points as you study if that helps you concentrate). Once you're done with a single chapter, do questions from around 5 - 10 past papers. Repeat that with every chapter.
Then in the end you will have to revise and do paper-wise. More the practice, better will be your marks.
For paper 1 specifically, doing the questions from last to first helps you finish sooner. (Don't ask how, but it helped me when I had a problem with time-management. ) But considering the fact that you are already scoring a 30, the only thing that will help you score more is practice. All the best! Above 30 is perfectly achievable.

Thanks for the tips. I was already thinking of going for topicals once again to improve my concepts

 

[Dark Destination]

Where does an A start in Chemistry usually? Like how much percentage marks do I need to get in all 5 papers combined to get an A?

 

[The Sarcastic Retard]

How do you do question 24??
Ans. A

Heyy! Can you please explain how to do this?
Answer is B

-3006 - ( - 2877) = -129 kj/mol

 

[drowning-in-studies]

Where does an A start in Chemistry usually? Like how much percentage marks do I need to get in all 5 papers combined to get an A?

Here is the threshold grades for Syllabus 9701 (Chemistry) in the May/June 2015 examination. It varies slightly each time according to how tough the papers were.

 

[The Sarcastic Retard]

Where does an A start in Chemistry usually? Like how much percentage marks do I need to get in all 5 papers combined to get an A?

Here is the threshold grades for Syllabus 9701 (Chemistry) in the May/June 2015 examination. It varies slightly each time according to how tough the papers were.

Moreover prepare at ur best, automatically u will get an A or A*.
Good luck

 

[drowning-in-studies]

-3006 - ( - 2877) = -129 kj/mol
View attachment 59574

Thank you!!!!

 

[My Name]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_s11_qp_41.pdf
question 6 part a part iv how to tell if result will be positive or negative?

 

[qwertypoiu]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s11_qp_41.pdf
question 6 part a part iv how to tell if result will be positive or negative?

Well firstly the iodoform test has been removed from the new syllabus, so you don't have to worry about this
[EDIT: Not removed just moved ]

But to answer your question the iodoform test is used to test for presence of:

• alcohol group with methyl group next to it [ CH3CH(OH) ]
• or carbonyl group (aldehyde and ketone only) with methyl group next to it [CH3CO]

The links contain easy to follow information.

So to your question:

CH3OH is negative, because there is no methyl group next to the carbon that OH group is attached to. (Any compound with only one C atom cannot give positive result because of this)

CH3CH2OH gives a positive result because the OH is attached to a carbon which is next to methyl group. (btw Ethanol is the only primary alcohol that can give positive result for this test)

CH3CHO gives a positive result since the C=O group is next to methyl group. (again ethanal is the only aldehyde to give positive result for this test)

CH3CO2H might look like it could give positive result but the iodoform test only works for aldehyde and ketones. (just like 2,4-DNPH)

C6H5 - CHO will give negative result. There is no CH3 group next to C=O group.

C6H5 - COCH3 will give positive result. The ketone group is next to a methyl group.

 

[qwertypoiu]

Hess's law?? Can someone please explain how you get D??
And what exactly are we supposed to look up in the data booklet?

First you have Ca(s)
You want to end up with Ca2+ (aq)

How do we do it? Simple:

1. Atomize. Ca(s) --> Ca(g) [+177]
2. Remove one electron. Ca(g) - e ---> Ca+(g) (This is first ionization energy. [+590]
3. Remove another electron. Ca+(g) - e ---> Ca2+(g) (This is second ionization energy) [+1145]
4. Hydrate it. Ca2+(g) + aq ----> Ca2+(aq) [-1565]

Add all the numbers up: answer = +177 +590 +1145 -1565 = +347kJ/mol

 

[My Name]

Well firstly the iodoform test has been removed from the new syllabus, so you don't have to worry about this

But to answer your question the iodoform test is used to test for presence of:

• alcohol group with methyl group next to it [ CH3CH(OH) ]
• or carbonyl group (aldehyde and ketone only) with methyl group next to it [CH3CO]

The links contain easy to follow information.

So to your question:

CH3OH is negative, because there is no methyl group next to the carbon that OH group is attached to. (Any compound with only one C atom cannot give positive result because of this)

CH3CH2OH gives a positive result because the OH is attached to a carbon which is next to methyl group. (btw Ethanol is the only primary alcohol that can give positive result for this test)

CH3CHO gives a positive result since the C=O group is next to methyl group. (again ethanal is the only aldehyde to give positive result for this test)

CH3CO2H might look like it could give positive result but the iodoform test only works for aldehyde and ketones. (just like 2,4-DNPH)

C6H5 - CHO will give negative result. There is no CH3 group next to C=O group.

C6H5 - COCH3 will give positive result. The ketone group is next to a methyl group.

Not REMOVED.Just MOVED. It is there in the AS for the new syllabus. :/
Thanks!

 

[drowning-in-studies]

First you have Ca(s)
You want to end up with Ca2+ (aq)

How do we do it? Simple:

1. Atomize. Ca(s) --> Ca(g) [+177]
2. Remove one electron. Ca(g) - e ---> Ca+(g) (This is first ionization energy. [+590]
3. Remove another electron. Ca+(g) - e ---> Ca2+(g) (This is second ionization energy) [+1145]
4. Hydrate it. Ca2+(g) + aq ----> Ca2+(aq) [-1565]

Add all the numbers up: answer = +177 +590 +1145 -1565 = +347kJ/mol

OOOH! Pretty simple, Thank you so much! I didn't even know how to think like that! wish i had a better teacher

 

[drowning-in-studies]

In which reaction does a single nitrogen atom have the greatest change in oxidation number?
A 4NH3 + 5O2 --> 4NO +6H20
B 3NO2 + H2O -->2HNO3 +NO
C 2NO +O2 --> 2NO2
D 4NH3 +6NO -->5N2 + 6H2O

The answer is A.
I'm confused at B where 2 products have N in them. Are we supposed to add their oxidation states (HNO3 AND NO)? Similarly at D where both reactants have N.
Please help, thank you!

 

[Zoha Ali]

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

Can anyone please explain Question 8!!?

 

[Saad the Paki]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s07_qp_1.pdf

Can anyone please explain Question 8!!?

The answer is B.
The enthalpy change of formation is when ONE mole of a compound is formed from its elements under standard states.
In this reaction TWO moles of compound are given. Therefore the enthalpy given is -1648, so the enthalpy change of formation (of ONE mole) will be -1648/2 = -824

 

[Saad the Paki]

In which reaction does a single nitrogen atom have the greatest change in oxidation number?
A 4NH3 + 5O2 --> 4NO +6H20
B 3NO2 + H2O -->2HNO3 +NO
C 2NO +O2 --> 2NO2
D 4NH3 +6NO -->5N2 + 6H2O

The answer is A.
I'm confused at B where 2 products have N in them. Are we supposed to add their oxidation states (HNO3 AND NO)? Similarly at D where both reactants have N.
Please help, thank you!

The question asks for a SINGLE nitrogen atom. So no u don't add them up, you have to compare them separately, first compare NO2 with HNO3 then compare NO2 with NO, but neither of them give the largest change separately. Same way for D

 

[drowning-in-studies]

The question asks for a SINGLE nitrogen atom. So no u don't add them up, you have to compare them separately, first compare NO2 with HNO3 then compare NO2 with NO, but neither of them give the largest change separately. Same way for D

Thank you!!

 

[Copy Cat]

 

[Awesome12]

View attachment 59582

24) The molecule could either be an ALCOHOL or an ETHER. It cannt be an ALDEHYDE or any other molcule with a carbonyl or carboxylic group. Thus (1) is not possible as the reaction can only happen if the molecule was an aldehyde, which it isnt.
(2) this could be possible in the case of an ether or TERTIARY ALCOHOL. It does not show a positive test with DNPH or TOLLENS REAGENT. It can also not be oxidized.
(3) this could be possible in the case of an PRIMARY OR SECONDARY ALCOHOL. It could be oxidized to an aldehyde and ketone respectively.

25) (1) An Aldehy will react with Fehlings Reagent. The Cu+2 ions are reduced whilst the aldehyde is oxidized to carboxylate ion
(2) this is a free radical substituion reaction. No oxidation and reduction whatsoever
(3) Reaction does not even take place

 

[Rizwan Javed]

(2) this is a free radical substituion reaction. No oxidation and reduction whatsoever

The reaction of CH4 and Cl2 is a redox reaction.
Consider this equation: CH4 + Cl2 ---> CH3Cl + HCl
In this equation, Cl goes from 0 to -1 oxidation state, and C goes from -4 to -2. So it IS a redox reaction.

Thus both 1, 2 are correct.

 

[Awesome12]

The reaction of CH4 and Cl2 is a redox reaction.
Consider this equation: CH4 + Cl2 ---> CH3Cl + HCl
In this equation, Cl goes from 0 to -1 oxidation state, and C goes from -4 to -2. So it IS a redox reaction.

Thus both 1, 2 are correct.

Oh yeah lol. I was thinking of it as the reactions taking place in the intermediate steps of substitution of Alkanes