Chemistry: Post your doubts here!

[aiskw1]

Edit and place the MS.
i'm going to give a try..
For the part a :
Firstly you need to learn all the reactions of organic chemistry to solve this question:

• B- CH3CHO ---> CH3CH(OH)CN [When aldehyde reacts with HCN+NaCN at a temp of 10-20 degree C gives that product, Showing an increaase in the no.of carbon atoms and its a Nucleopilic addition rxn]
• When product B reacts with dilute H2SO4or HCl when heated it gives a -COOH in place of -CN and -OH remains the same. So its going to be like.. CH3CH(OH)CN ---> CH3CH(OH)COOH
• E- CH3CHO ---> CH3COO- [An aldehyde CH3CHO ----->CH3COO- + Ag when reacted with Tollens reagent [Ag(NH3)2]+ ]

• C- CH3=CHCOOH ----> OHCH2CH(OH)COOH [When alkene reacts with Cold KMnO4 when the double bond is broken two -OH are added, eg: CH2=CH2 ---> CH2(OH)CH2(OH) and its an electrophilic addition reaction. The -COOH remains the same.
• When C reacts with Cr2O7 2-/H+ the COOH at the right remains as it is.. the secondary alcohol changes to a Ketone and Primary alcohol to -COOH, so it has two reactions happening at the same time.. that is the rxn of Primary and secondary alcohol.
• D- aldehyde changes to primary alcohol on reduction. The -CHO changes to -OH.
  
  For part B i don't really know well but..
  (i) The carboxilic acid when reacted with a CH3CH2OH with concentrated H2SO4 (heat) gives an ester.
  (ii) Probably someone else could help u out cuz i'm not sure.
  
  Hope this will help you in someway and if i've made any mistake please correct me cuz even i'm still a learner. ​

For Part B (ii) I think the answer would still be an ester formation

Heres what I thought for B(ii) :

the compound cam be drawn for C as

CH2OH
|
CHOH
|
COOH

therefore only the top and second chain will react with CH3COOH to form an ester

CH2OOCCH3
|
CHOOCCH3
|
COOH

I don't know why it is written as this though as they both essentially mean the same thing, as if the 2nd carbon on the top chain was an ether, and joined by single bond to O then it would not have a full octet shell (8 electrons)

CH2OCOCH3
|
CHOCOCH3
|
CO2H

I hope someone could explain why it's written as it is like that

 

[aiskw1]

can a tertiary alcohol react with carboxyllic acid forming an ester??

Yes it can react with carboxylic acid.

It would look something like

CH2CH3
|
COOCCH3 or CHOCOCH3 (it says in the marking scheme like this but I don't know why)
|
CH2CH3

It's easier to think about it if u draw it sideways

 

[NouranAyman]

Guys hows preparation for chemistry paper 2?? any tipss :"( paper in a week

 

[M.Kabir Nawaz]

All elements have a first ionisation energy - even atoms which don't form positive ions in test tubes. The reason that helium (1st I.E. = 2370 kJ mol-1) doesn't normally form a positive ion is because of the huge amount of energy that would be needed to remove one of its electrons.

I don't understand the meaning of this statement... whether helium doesn't form positive ion or only at 2370kJ?

 

[M.Kabir Nawaz]

What is meant by 1st Ionisation energy?

 

[M.Kabir Nawaz]

Making the solution more alkaline. as it would increase the amount of OH- and the equilibrium will proceed towards left.

Anyone explain these two questions to me please? Thanks first! :Yahoo!:
FYI, the answer given for Q10 is D whereas for Q11 is D.

 

[DumbIdiot]

Q1 (e) of http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s07_qp_2.pdf
What is the reaction between hydrazine (N2H4) and HCl like, the ms says it is a neutralisation reaction and a dative bond forms but can anyone explain how and where the bond forms and also the structure? also tell about the reaction between N2H5Cl and HCl (and structure).

 

[DumbIdiot]

What is meant by 1st Ionisation energy?

Energy needed to remove 1st electron from an atom

 

[DumbIdiot]

In Q1 (e) of http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w07_qp_2.pdf
What formula is used? Shouldn't it be deltaH(products)-deltaH(reactants)? I don't think that's the formula they used in the ms. Please explain

 

[ismailmz]

What is meant by 1st Ionisation energy?

1st ionisation energy of an element is the energy needed to remove one electron from each atom in one mole of atoms of the element in the gaseous state to form one mole of gaseous 1+ ions....
For example = Ca (g) ----> Ca^+ (g) +e-

 

[Kumkum]

Please help me with question 2 b(i),(ii),(iii).
I understand the question wants diagrams for these compounds but how do i explain the three points. Please Help!!!!

 

[yashi30]

In Q1 (e) of http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w07_qp_2.pdf
What formula is used? Shouldn't it be deltaH(products)-deltaH(reactants)? I don't think that's the formula they used in the ms. Please explain

Make use of Hess's Law
deltaHf of C2H4 + deltaHcombustion of C2H4= 2* deltaHcombustion of carbon + 2*deltaH of hydrogen.
and then you solve for the unknown enthalpy.

deltaHformation of C2H4 + (-1411)= (2*-393.7)+(2*-285.9)
delta H formation of C2H4=(2*-393.7)+(2*-285.9) -(-1411)= +51.8kJ/mol

 

[yashi30]

Q1 (e) of http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s07_qp_2.pdf
What is the reaction between hydrazine (N2H4) and HCl like, the ms says it is a neutralisation reaction and a dative bond forms but can anyone explain how and where the bond forms and also the structure? also tell about the reaction between N2H5Cl and HCl (and structure).

Neutralisation occurs between a base and an acid.
Hydrazine acts as a base, that is, a proton acceptor because of its lone pair of electrons. (think about NH3 acting as a base)
HCl is an acid and is thus a proton donor, it donates it's H+ to hydrazine and the dative bond is from the lone pair on one of the nitrogen in hydrazine to the hydrogen ion donated by HCl.

Ethene is an unsaturated hydrocarbon with a Pi cloud of electrons
An electrophilic addition occurs when ethene reacts with HCL
H----Cl is a polar molecule with Cl being more electronegative than the H.
The Pi cloud of electrons on ethene attack the delta+ H of HCl, forming the carbocation C2H5+
The delta- Cl forms a bond with the carbocation giving C2H5Cl

No further reaction can occur between C2H5Cl and HCl because there are no more C=C.

 

[yashi30]

All elements have a first ionisation energy - even atoms which don't form positive ions in test tubes. The reason that helium (1st I.E. = 2370 kJ mol-1) doesn't normally form a positive ion is because of the huge amount of energy that would be needed to remove one of its electrons.

I don't understand the meaning of this statement... whether helium doesn't form positive ion or only at 2370kJ?

Any atom can lose electron(s) to form positive ions.
However, it depends on the amount of energy available.
For example, Helium requires 2370kJ/mol of energy to form He+, so if this amount of energy is supplied Helium can form a monopositive ion.
It doesn't normally form such an ion due to the large energy requirement.

hope it helped.

 

[yashi30]

Jazak Allah so much....

Yes got is ...it is C......

Because like ethanol mole should be found and then find mass of ehanoic acid...and then take 75% ....i got it....Jazak Allah

Another Doubt WHy is the marked answer is right while others not ? :SView attachment 16497

Number 38:
The molecule consists of the following functional groups: alcohol, ketone.
A.On addition of sodium metal, any alcohol give a salt + Hydrogen.
B. On addition of 2,4DNPH, an orange precipitate of a hydrazone is formed with the ketone group.
C. A positive test with Tollens' reagent or Fehling's reagent is only with the presence of an aldehyde group. Since there is no aldehyde group(-RCHO), the molecule will not give any observable result.

Number 39:
Propanoic acid is CH3CH2COOH
A. when you acid hydrolyse CH3CH2CO2CH3, you get CH3CH2COOH and CH3OH
B. when you acid hydrolyse CH3CH2CH2CN, you get CH3CH2CH2COOH (which is butanoic acid)
C. hydrolysis of a halogenoalkane yields an alcohol.

Number 40:
Since the precipitate formed is insoluble in dilute ammonia, it has to be either iodoalkane or bromoalkane, because AgCl is soluble in dilute ammonia.

 

[yashi30]

Hello there, can anyone respond to these 3 questions on chemical energetics and explain the answer to me please:

(1) Which of the following elements in its crystalline form will have the lowest enthalpy change of vapourisation? [ answer is A, why is it not B?]
(A) Argon
(B) Chlorine
(C) Phosphorus
(D) Silicon
(E) Sulfur

To vaporise a compound, you need to overcome intermolecular forces of attraction. since Argon is monoatomic , it consists of lesser forces of attraction as compared to chlorine which is diatomic.


(2) The gaseous oxides of nitrogen have positive enthalpy changes of formation. Which one of the following factors is likely to make significant contribution to these enthalpy changes? [answer is D, why is it not E?]
(A) the tendency of oxygen to form oxide ions O2-
(B) the high electron affinity for oxygen atoms
(C) the high electron affinity of nitrogen atoms
(D) the high bond energy of the nitrogen molecule, N2
(E) the similarity of the electronegativities of oxygen and nitrogen.

To form a nitrogen oxide, you have to break the triple bond in nitrogen molecule and O=O in O2.
so the enthalpy changes of formation depends on the amount of energy required to break theses bonds, esp the triple bond which has a high value(994kJmol-1). Similarity in electronegativities only gives idea of polarity of bonds.



(3) The standard enthalpy changes for two reactions are given by the following equations: [answer is D]

2Fe (s) + 3/2 O2(g) → Fe2O3 ▲H = -822 Kjmol-1​

C(s) + 1/2 O2 (g) → CO (g) ▲H=-110 Kjmol-1​

What is the standard enthalpy change for the following reaction?

Fe2O3 (s) + 3C (s) → 2Fe (s) + 3CO (g)​

(A) -932 Kjmol-1
(B) -712 Kjmol-1
(C) -492 Kjmol-1
(D) +492 Kjmol-1
(E) +712 Kjmol-1

 

[PhyZac]

Number 38:
The molecule consists of the following functional groups: alcohol, ketone.
A.On addition of sodium metal, any alcohol give a salt + Hydrogen.
B. On addition of 2,4DNPH, an orange precipitate of a hydrazone is formed with the ketone group.
C. A positive test with Tollens' reagent or Fehling's reagent is only with the presence of an aldehyde group. Since there is no aldehyde group(-RCHO), the molecule will not give any observable result.

Number 39:
Propanoic acid is CH3CH2COOH
A. when you acid hydrolyse CH3CH2CO2CH3, you get CH3CH2COOH and CH3OH
B. when you acid hydrolyse CH3CH2CH2CN, you get CH3CH2CH2COOH (which is butanoic acid)
C. hydrolysis of a halogenoalkane yields an alcohol.

Number 40:
Since the precipitate formed is insoluble in dilute ammonia, it has to be either iodoalkane or bromoalkane, because AgCl is soluble in dilute ammonia.

Thank You so Much....Jazak Allah May Allah reward you for that help.

I Alhamdulilah now get all ....Thanks alot!

 

[NouranAyman]

guys please tell me hows preparation going for chemistry AS ?? its on wednesday

 

[aalmuhannadi]

Which solid contains more than one kind of bonding?
A. Iodine
B. Silicon dioxide
C. Sodium chloride
D. Zinc

The answer is A but I don't understand really why. I know that it has covalent bonds and van der waals forces, but don't silicon dioxide and sodium chloride also have those?

Which element is likely to have an electronegativity similar to that of aluminium?
A. barium
B. beryllium
C. magnesium
D. strontium
I have no clue as to how to figure this one out

Suggest an explanation for the existence of IF7 and for the non-existence of ClF7? The answer says because I is a bigger atom than Cl but I need a more thorough explanation as I didn't understand anything from that answer.

Thank you

 

[yashi30]

Which solid contains more than one kind of bonding?
A. Iodine
B. Silicon dioxide
C. Sodium chloride
D. Zinc

The answer is A but I don't understand really why. I know that it has covalent bonds and van der waals forces, but don't silicon dioxide and sodium chloride also have those?

Which element is likely to have an electronegativity similar to that of aluminium?
A. barium
B. beryllium
C. magnesium
D. strontium
I have no clue as to how to figure this one out

Suggest an explanation for the existence of IF7 and for the non-existence of ClF7? The answer says because I is a bigger atom than Cl but I need a more thorough explanation as I didn't understand anything from that answer.

Thank you

Iodine consists of covalent bonds and VDW forces
Silicon dioxide consists of only strong covalent forces.(draw the macromolecular structure and you'll get it)
NaCl contains only ionic bond(electrostatic force of attraction between Na+ and Cl- -------------> Again by drawing the giant lattice of NaCl, you'll get it, no VDW)

The answer to the second question is beryllium by the theory of Diagonal relationship. but we do not need to know much about it.
Electronegativity of Al is similar to that of Be likewise Al2O3 is amphoteric and BeO is amphoteric
Also, there is a diagonal relationship between Mg and Li.