Chemistry: Post your doubts here!

[Soulgamer]

Delta H reaction=Bond Energy Of reactants - B.E of Products

You don't need to worry about exo or endo. Try to solve it with this.

 

[Vinnyvinvin]

Any1 help me with mj 22 2011, q 1b)ii)
i dont understand why n(NaOH)=n(Hcl)
can any1 clarify for me pleasE? Thanks!

 

[littlecloud11]

Any1 help me with mj 22 2011, q 1b)ii)
i dont understand why n(NaOH)=n(Hcl)
can any1 clarify for me pleasE? Thanks!

NaOH + HCl = NaCl + H2O
that's the neutralization reaction. 1 mole of NaOH reacts with 1 mole of HCl. So their ratio is 1:1

 

[DragonCub]

Any1 help me with mj 22 2011, q 1b)ii)
i dont understand why n(NaOH)=n(Hcl)
can any1 clarify for me pleasE? Thanks!

HCl + NaOH = NaCl + H2O
1 mole of hydrochloric acid requires (at least) 1 mole of sodium hydroxide for complete reaction.
So a certain amount of hydrochloric acid needs the same amount of sodium hydroxide to react completely.

 

[alim]

wats the bond angle and shape of a compnd with 2 bond pairs and 1 lonepair

 

[Utsav.]

how is increase in solubility makes the reaction endothermic??
plz expalin!!

 

[shan5674]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_23.pdf

4b(ii) --> how does the elimination reaction produce "ethane" doesn't elimination produce alkene? :/

 

[oldfashionedgirl]

I just wanted to know that when we are asked to draw a chiral compound,would the marks be deducted if I didnt draw it exactly as the way in marking scheme.Cuz almost all the times the position of 4 groups around chiral carbon in my structure are different from the structure given in marking scheme.And generally when writing structural formulae i cannot write them exactly as mentioned in marking scheme??

u can make the four groups around the chiral carbon in any plane, it doesn't really matter and no marks will be deducted.

 

[shan5674]

wats the bond angle and shape of a compnd with 2 bond pairs and 1 lonepair

3 bond pairs with 1 lone pair would be triangular pyramidal - 107. I havent come across 2 bond pairs with 1 lone pair as 2 bond pairs and 1 lone pair would give 6 electrons not a complete 8

 

[Saad (سعد)]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_23.pdf

4b(ii) --> how does the elimination reaction produce "ethane" doesn't elimination produce alkene? :/

That's a typo in the Mark Scheme. They meant to write Ethene; they've written the formula of Ethene, C2H4. Ethane is C2H6.

 

[shan5674]

That's a typo in the Mark Scheme. They meant to write Ethene; they've written the formula of Ethene, C2H4. Ethane is C2H6.

oh yea, didnt notice that lol thanks

 

[raamish]

It forms a double bond with both..

One more question when we will do a complete combustion of N2 will N2+o2->NO be formed and then NO will oxidise with NO2 or will NO2 be formed directly from first reaction: N2+O2->NO2

 

[leosco1995]

NO is first formed in the car engine, and then it reacts with 1/2 mole of oxygen in the air to form NO2.

 

[leosco1995]

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s09_qp_2.pdf

Question 5 part (f)

How do we draw the structure of S?

 

[littlecloud11]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s09_qp_2.pdf

Question 5 part (f)

How do we draw the structure of S?

CH3CH(OH)CH2CH2CHO
the C=O bond on the first aldehyde is broken and an O-H bond is formed between the O in the first aldehyde and the hydrogen from the CH3 in the second aldehyde.
basically, it's an addition reaction.

 

[littlecloud11]

3 bond pairs with 1 lone pair would be triangular pyramidal - 107. I havent come across 2 bond pairs with 1 lone pair as 2 bond pairs and 1 lone pair would give 6 electrons not a complete 8

SO2 has 2 bond pairs and a lone pair. The angle will be 117.5 for 2 bond pairs and a lone pair. and the shape is bent.

 

[leosco1995]

CH3CH(OH)CH2CH2CHO
the C=O bond on the first aldehyde is broken and an O-H bond is formed between the O in the first aldehyde and the hydrogen from the CH3 in the second aldehyde.
basically, it's an addition reaction.

o_o But how are we supposed to figure this out? I am still kind of confused.. even after looking at step I again and again..

 

[littlecloud11]

o_o But how are we supposed to figure this out? I am still kind of confused.. even after looking at step I again and again..

It might seem complicated but it's easier if you work backwards. the question says that T has an OH group. and U is formed from T.
U has a double bond, which was formed when T was dehydrated and the alcohol group was removed. As the = is between the second and the third carbon the -OH group also has to be on either the second or third carbon. but as T was formed from 2 aldehydes it be on the third because otherwise you would have 2 aldehyde groups in T. Therefore the formula of T was CH2CH(OH)CH2CHO.
you know that T was formed form two aldehyde molecules. CH3CHO and CH3CHO
when you see the formula of T you can easily guess that the C=O bond in one of the aldehyde has been broken and the two molecules re joined by O with the initial C-O bond still in place and a newly formed O-H bond with hydrogen atom of the CH3 part of the second aldehyde. The same happens while predicting the structure of S.

PS. It helps if you draw the displayed formula of the aldehydes and T if you're confused.

 

[leosco1995]

It might seem complicated but it's easier if you work backwards. the question says that T has an OH group. and U is formed from T.
U has a double bond, which was formed when T was dehydrated and the alcohol group was removed. As the = is between the second and the third carbon the -OH group also has to be on either the second or third carbon. but as T was formed from 2 aldehydes it be on the third because otherwise you would have 2 aldehyde groups in T. Therefore the formula of T was CH2CH(OH)CH2CHO.
you know that T was formed form two aldehyde molecules. CH3CHO and CH3CHO
when you see the formula of T you can easily guess that the C=O bond in one of the aldehyde has been broken and the two molecules re joined by O with the initial C-O bond still in place and a newly formed O-H bond with hydrogen atom of the CH3 part of the second aldehyde. The same happens while predicting the structure of S.

PS. It helps if you draw the displayed formula of the aldehydes and T if you're confused.

Thanks, I get it now. Drawing the displayed formula of the aldehyde reaction to make T really did help. Still, I think this is a pretty tricky question.

 

[littlecloud11]

Thanks, I get it now. Drawing the displayed formula of the aldehyde reaction to make T really did help. Still, I think this is a pretty tricky question.

you're welcome.
And it wouldn't be called the As without a few tricks.