Chemistry Solutions 9701/13/M/J/15

Ref: 9701/13/M/J/15 #6
Which statement can be explained by intermolecular hydrogen bonding?
A. Ethanol has a higher boiling point than propane.
B. Hydrogen chloride has a higher boiling point than silane, SiH4.
C. Hydrogen iodide forms an acidic solution when dissolved in water.
D. Propanone has a higher boiling point than propane.


Solution
A
Hydrogen bonds are formed when a H atom of a molecule, attached to a very electronegative atom (Fluorine, Oxygen, Nitrogen) accepts a lone pair from another very electronegative atom (FON) from another molecule. A is correct because ethanol molecules have OH groups and are able to form hydrogen bonds with one another, unlike propane which only has Van der Waal's forces between molecules.
B, C and D are incorrect because the compounds do not have any intermolecular hydrogen bonds.
If you have covered organic chemistry, it is worth noting that carboxylic acids and alcohols are able to form intermolecular hydrogen bonds whereas carbonyl compounds are unable to. This is because carboxylic acids and alcohols have -OH groups whereas carbonyl compounds do not.
Carboxylic acids have higher melting and boiling points than alcohols because they have the additional polar C=O groups.




Ref: 9701/13/M/J/15 #14
The compound (CH3)3NAlCl3 has a simple molecular structure. Which statement about (CH3)3NAlCl3 is correct?
A. (CH3)3NAlCl3 molecules attract each other by hydrogen bonds.
B. The Al atom has an incomplete valence shell of electrons.
C. The bonds around the Al atom are planar.
D. The molecules contain coordinate and covalent bonding.


Solution
D. The molecules contain coordinate and covalent bonding

A is incorrect because the H atoms in the molecule are not attached to very electronegative atoms (Fluorine, Oxygen, Nitrogen) hence it is not possible for the molecule to form hydrogen bonds.
B is incorrect. The Al atom originally has three valence electrons, two in the 3s orbital and 1 in the 3px orbital. After hybridisation, the Al atom has three sp2 hybrid orbitals and is able to form three polar covalent bonds with the three Cl atoms. The Al atom has an empty 3pz orbital, and it then accepts the lone pair of electrons from the nitrogen atom, forming a complete valence shell of electrons.
C is incorrect. Al is attached to three Cl atoms and 1 N atom, forming a tetrahedral structure, not planar.
D is correct as covalent bonding exists between the C and H atoms and a coordinate/dative bond is formed between the Al and N atom.


Ref: 9701/13/M/J/15 #25
Compound X, C2H12O, is oxidised by acidified sodium dichromate (VI) to compound Y.
Compound Y reacts with butan-2-ol in the presence of a little concentrated sulfuric acid to give liquid Z.
What could be the formula of Z?
A. CH3(CH2)3CO2CH(CH3)CH2CH3
B. CH3(CH2)3CO2(CH2)3CH3
C. CH3(CH2)2CO2CH(CH3)CH2CH3
D. (CH3)2CHCH2CO2C(CH3)3


Solution
A. CH3(CH2)3CO2CH(CH3)CH2CH3
Compound X is an alcohol since it has the general molecular formula CnH2n+2O. It is fully oxidised
by acidified sodium dichromate (VI) to pentanoic acid, compound Y. Compound Y then undergoes esterification with butan-2-ol to form CH3(CH2)3CO2CH(CH3)CH2CH3.



Ref: 9701/13/M/J/15 #26
Alkane X has molecular formula C4H10.
X reacts with Cl2(g) in the presence of sunlight to produce only two different monochloroalkanes, C4H9Cl. Both of these monochloroalkanes produce the same alkene Y, and no other organic products, when they are treated with hot ethanolic KOH.
What is produced when Y is treated with hot concentrated acidified KMnO4?
A. CO2 and CH3CH2CO2H
B. CO2 and CH3COCH3
C. HCO2H and CH3COCH3
D. CH3CO2H only


Solution
B CO2 and CH3COCH3

Alkane X is 2-methylpropane. It undergoes free radical substitution with Cl2 in sunlight to produce 1-chloro-2-methylpropane and 2-chloro-2-methylpropane. Both of these compounds, after undergoing dehydrohalogenation, produce the same alkene, 2-methylprop-1-ene.
Alkane X has to be a branched compound in order for the same alkene to be produced from two different monochloroalkanes. If alkane X were not branched, there would be two possible alkenes produced.

The above diagram shows an example of an unbranched alkane X forming two possible monochloroalkanes and then two different alkenes. Two different alkenes would also be produced if the 2nd and 3rd hydrogen atoms were substituted by chlorine instead of the 1st and 2nd. Thus the compound must be branched to obtain the same alkene from the monochloroalkanes.

The alkene produced undergoes vigorous oxidation with hot concentrated acidified KMnO4 to produce CO2 and CH3COCH3. Thus the answer is B.

Ref: 9701/13/M/J/15 #34

Which physical properties are due to hydrogen bonding between water molecules?

1 Water has a higher boiling point than H2S

2 Ice floats on water

3 The H-O-H bond angle in water is approximately 104'

Solution

B 1 and 2 only

Hydrogen bonding between water molecules causes water to have a higher boiling point than H2S, which does not have intermolecular hydrogen bonding. Intermolecular hydrogen bonding leads to the solid state of water, ice, being less dense than its liquid state. Hence statements 1 and 2 are correct.

Statement 3 is incorrect as the bond angles in molecules or ions depend on the magnitude of electron pairs repulsion and not hydrogen bonding.

*Hydrogen bonding in molecules leads to higher viscosity, higher surface tension and greater solubility in water. They can also cause some molecules to dimerise (molecules join together to form one larger molecule called a dimer), so that the apparent Mr is greater than expected.

Ref: 9701/13/M/J/15 #35

Under atmospheric conditions, in which transformations is sulfur dioxide involved as either a reagent or a catalyst?

1 NO2 to NO

2 NO to NO2

3 CO to CO2

Solution

D 1 only

Nitrogen monoxide acts as a catalyst for the formation of SO3 from SO2. The reactions are as follow:

2NO+O2--->2NO2

NO2+SO2---> NO+SO3

Thus it can be seen that sulphur dioxide acts as a reagent in transformation 1.

Transformation 2 only involves atmospheric oxygen and not SO3. Transformation 3 does not occur under atmospheric conditions. Hence the answer is D.



Ref: 9701/13/M/J/15 #38

Which changes in bonding occur during the reaction of propanal and hydrogen cyanide?

1 A carbon-hydrogen bond is broken.

2 An oxygen-hydrogen bond is formed.

3 A carbon-carbon bond is formed.

Solution

A 1,2 and 3

Propanal is an aldehyde and it undergoes nucleophilic addition with hydrogen cyanide, HCN. This reaction involves cyanide ions as a catalyst and produces 2-hydroxybutanenitrile.

A carbon-carbon bond is formed due to the nucleophilic attack of the cyanide ions on the slightly positive carbon atom in the polar C=O bond in propanal. 3 is correct.

A carbon-hydrogen bond is broken between the H+ and CN- ions in HCN, after which the H+ ion accepts a lone pair of electrons from the negative O2- ion. Thus an oxygen-hydrogen bond is formed. 1 and 2 are correct.

Ref: 9701/13/M/J/15 #39

Which pairs of reagents will react together in a redox reaction?

1 CH3CHO+ Fehling's reagent

2 C2H4+ Br2 (aq)

3 CH4 + Cl2 (g)

Solution

A 1,2 and 3

1 is correct as CH3CHO is an aldehyde (ethanal), and will be oxidised by Fehling's solution to produce ethanoic acid. Fehling's solution is reduced by ethanal, so this is a redox reaction.

2 is correct as the oxidation state of each carbon atom in C2H4 is -2. With aqueous bromine, electrophilic addition occurs across the double bond and the oxidation number of carbon increases from -2 to -1 whereas the bromine atoms are each reduced from 0 to -1 respectively.

3 is correct as the oxidation number of carbon in CH4 was originally -4, and then increases to -2 when a H atom is substituted by a Cl atom through free radical substitution to form CH3Cl. The oxidation number of the chlorine atom decreases from 0 to -1, so this is a redox reaction.

From the diagram, it can be seen that a difference in electronegativity will cause C to have an oxidation state of -1 in  C-H bond and +1 in a C-Br bond. The "net" oxidation state of the carbon atom in each compound is the addition of the oxidation numbers from each bond it forms.