Chemistry Solutions 9701/11/M/J/14

Ref: 9701/11/M/J/14 #14
What is the order of increasing melting point of the four chlorides shown?
CCl4       HCl      MgCl2     PCl5

Solution
B

MgCl2 is an ionic compound with some covalent character. The other three compounds are covalent compounds. MgCl2 will have a significantly higher melting point than the rest due to its ionic bonding. The melting points of the other three compounds depend on the total number of electrons in the molecule. The order of increasing number of electrons is HCl - CCl4 - PCl5. The greater the number of electrons, the higher the melting point.

Ref: 9701/11/M/J/14 #15
When calcium is burnt in oxygen, what colour is the flame?
A. green
B. red
C. white
D. yellow

Solution
B. red

Flame colours of group 2 elements burnt in oxygen are as follows:
Mg - white
Ca - red
Sr - red
Ba - green

Ref: 9701/11/M/J/14 #28
Compound X has the molecular formula C4H10O2. X has an unbranched carbon chain and contains two OH groups.
On reaction with an excess of hot acidified, aqueous manganate(VII) ions, X is converted into a compound of molecular formula C4H6O4.
To which two carbon atoms in the chain of X are the two OH groups attached?
A. 1st and 2nd
B. 1st and 3rd
C. 1st and 4th
D. 2nd and 3rd

Solution
C

Upon realization that there is an increase of two oxygen atoms after oxidation, we can deduce that the new molecule contains two carboxylic acid groups. (It is not possible that the oxidation product is ketone or aldehyde because the number of oxygen atoms should remain the same.) In order to form ONLY carboxylic acid from alcohol through oxidation, the alcohol has to be a primary alcohol. So, the OH groups should be attached to the first and last carbon atoms.

Ref: 9701/11/M/J/14 #29
How many geometrical (cis-trans) isomers are there of hex-2,4-diene, CH3CH=CHCH=CHCH3?
A. none; hex-2,4-diene does not show geometric isomerism
B. 2
C. 3
D. 4

Solution
C. 3

This question is a special question. Usually, we use the formula of 2^n, n being the number of double bonds. In this case, however, if the first double bond has a cis arrangement, and the second double bond has a trans arrangement, it is actually the same as the first double bond having a trans arrangement, and the second double bond having a cis arrangement. (Please refer to diagram, they are actually the same molecule flipped.) So, the total number of isomers would be 2^2 - 1 = 4 - 1 = 3.

Ref: 9701/11/M/J/14 #31
Use of the Data Booklet is relevant to this question.
When the liquid N2F4 is heated, it decomposes into a single product, X.
Which statements are correct?
1. N-F bonds are broken during this decomposition.
2. The enthalpy change when N2F4 decomposes into X is approximately +160 kJ mol-1.
3. Molecules of X are non-linear.

Solution
C. 2 and 3 only

From the question, we need to deduce the following equation:
N2F4 -> 2NF2
From the equation, we can see that only N-N bonds are broken. So 1 is incorrect.
Based on Data Booklet, the bond energy of N-N bond is +160 kJ mol-1, so 2 is correct because that is the only bond broken in the reaction.
3 is correct because there are lone pair(s) on the NF2 molecule, so it has to be non-linear. (Please take note that we are not entirely sure of the nature of this ion/molecule as well, but the knowledge you will need to solve this question is that there are lone pairs on the nitrogen atom.)