Chemistry: Post your doubts here!

[hussamh10]

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w07_qp_2.pdf Q2 of hessis law cannot do it through vector way or any other plzzz guide me !!

 

[oldfashionedgirl]

in the addition of a -CN to a halogenoalkane, ethanolic KCN is used right? but what always confuses me is that whether HCN, KCN or NaCn is added to aldehydes and alkenes.....some help? whats the difference? mark schemes are really confusing since even in them the answers are different everywhere

 

[oldfashionedgirl]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w07_qp_2.pdf Q2 of hessis law cannot do it through vector way or any other plzzz guide me !!

look, for finding heat of formation when enthalpy changes of combustion are given, the formula to use is pretty simple:
heat of combustion of reactant elements - heat of combustion of product

so if u substitute the values given u'll get the answer like this:

2(-393.7)+ 2(285.9) - (-1411) = +51.8 KJ/mol

 

[leosco1995]

in the addition of a -CN to a halogenoalkane, ethanolic KCN is used right? but what always confuses me is that whether HCN, KCN or NaCn is added to aldehydes and alkenes.....some help? whats the difference? mark schemes are really confusing since even in them the answers are different everywhere

You are getting confused between 2 different reactions. In nucleophilic substitution, this happens:

- KCN in an ethanoic solution is added to a a halogenoalkane.
- It happens with heat under reflux and the resulting compound contains an extra carbon atom.

Like this:

CH3CH2Br + CN- -> CH3CH2CN + Br-

In nucleophilic addition, HCN in the presence of CN- ions (like from NaCN) is added to an aldehyde or ketone. The C=O bond breaks, CN is added to the carbon atom and hydrogen is added to the oxygen atom, and CN- ions are regenerated like this:

CH3COCH3 + CN- -> CH3CH(OH)CNCH3 + CN-

You need to know the mechanisms for both of these reactions too, I think you should check that up on chemguide or something.

 

[hussamh10]

look, for finding heat of formation when enthalpy changes of combustion are given, the formula to use is pretty simple:
heat of combustion of reactant elements - heat of combustion of product

so if u substitute the values given u'll get the answer like this:

2(-393.7)+ 2(285.9) - (-1411) = +51.8 KJ/mol

Oh thanks igot it .....

 

[hmlahori]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s11_qp_21.pdf
http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s11_ms_21.pdf
For question 2 (c i) why cant SO3 have been formed because the ethanethiol is burned in excess oxygen?

 

[Jaf]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s11_qp_21.pdf
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s11_ms_21.pdf
For question 2 (c i) why cant SO3 have been formed because the ethanethiol is burned in excess oxygen?

Because the oxidation of SO2 to SO3 requires a catalyst.

 

[hmlahori]

Because the oxidation of SO2 to SO3 requires a catalyst.

didnt event think of that thanks!

 

[Mobeen]

anybody got notes for paper 4 ?

 

[00tanveer]

What's the H-S-H bond angle in a H2S molecule? Please provide an answer with an explanation. Thank you.

 

[JD REBORN]

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??

 

[JD REBORN]

What's the H-S-H bond angle in a H2S molecule? Please provide an answer with an explanation. Thank you.

S and O are in the same group.In H2O O has two lone pair of electrons.Similarly in H2S S has two lone pairs of electrons which repel the bond and bond angle is 104.5 degrees exactly

 

[Jaf]

What's the H-S-H bond angle in a H2S molecule? Please provide an answer with an explanation. Thank you.

Sulfur is in Group VI. It is bonded to two hydrogens.
This means Sulfur is left with 2 lone pairs. So the bond angle would be the same as that of water (104.5).

 

[00tanveer]

If you happen to have the Chemistry coursebook, you should check the end-of-chapter question 7.b of chapter 4. The answer (in CD-ROM) is shown to be 92degrees to be exact. The range of values was given as 90-102degrees. :| My answer was the same as yours.

 

[Jaf]

If you happen to have the Chemistry coursebook, you should check the end-of-chapter question 7.b of chapter 4. The answer (in CD-ROM) is shown to be 92degrees to be exact. The range of values was given as 90-102degrees. :| My answer was the same as yours.

Well if you want to be scientifically correct, 104.5 is NOT the bond angle it is much lesser due to several factors (size of sulfur atom, electronegativity of sulfur, orbital hybridization). But we don't need to learn that for AS (at least I think we don't).

 

[00tanveer]

But why did the COURSEBOOK have to give that answer considering what you had said? I'am not challenging you. I'm just confused, bro. So do I stick to the standard value of 104.5degrees for these kinds of molecules? Is it gonna be safe?

 

[Jaf]

But why did the COURSEBOOK have to give that answer considering what you had said? I'am not challenging you. I'm just confused, bro. So do I stick to the standard value of 104.5degrees for these kinds of molecules? Is it gonna be safe?

I think examiners will avoid putting such questions into the paper. Are you aware of any pastpaper question(s) that ask for the bond angle of H2S?

 

[00tanveer]

I'll have to revise them. Haven't done it yet. I'll post again when I bump into questions of this type in the question papers.

 

[angelicsuccubus]

What's the H-S-H bond angle in a H2S molecule? Please provide an answer with an explanation. Thank you.

its like H2O so 104.5.. The H2S molecule has two lone pairs. These exert a greater repulsion than one lone pair and the hydrigen atoms are squeezed together to give n angle of 104.5

 

[angelicsuccubus]

I think examiners will avoid putting such questions into the paper. Are you aware of any pastpaper question(s) that ask for the bond angle of H2S?

yeh I think Ive seen one .. thats how I remember it being 104.5 as the answer..