Chemistry Solutions 9701/11/M/J/15

Ref: 9701/11/M/J/15 #3
Use of the Data Booklet is relevant to this question.

1.00 g of carbon is combusted in a limited supply of oxygen. 0.50 g of the carbon combusts to form CO2 and 0.50 g of the carbon combusts to form CO.
The resultant mixture of CO2 and CO is passed through excess NaOH (aq) and the remaining gas is then dried and collected.
What is the volume of the remaining gas? (All gas volumes are measured at 25 degree Celsius and 1 atmosphere pressure.)


A. 1 dm3
B. 1.5 dm3
C. 2 dm3
D. 3 dm3


Solution
A. 1 dm3

Form the balanced equation for the combustion of carbon. 4C+3O2--->2CO+2CO2
The RAM of carbon is 12. Hence 1g of carbon= 1/12=0.08333 mol. From the equation, 4 mols of carbon produce 2 mols of CO and 2 mols of CO2 respectively. Hence, 0.08333 mol of carbon will produce 0.041667 of CO and CO2 respectively.
NaOH absorbs acidic gases such as CO2 and SO2. Since there is limited amount of oxygen (not in excess), only CO remains once CO2 is absorbed by NaOH. At room temperature and pressure, 1 mol of gas occupies 24 dm3. Hence, 0.041667 mol of CO occupies 0.041667 x 24= 1 dm3.



Ref: 9701/11/M/J/15 #6

One mole of phosphorus(V) chloride, PCl5, is heated to 600 K in a sealed flask of volume 1 dm3. Equilibrium is established and measurements are taken.



PCl5(g)--->< PCl3(g)+Cl2(g)

The experiment is repeated with one mole of phosphorus (V) chloride heated to 600K in a sealed flask of volume 2dm3.

How will the measurements vary?

A. The equilibrium concentrations of PCl3 (g) and Cl2 (g) are higher in the second experiment.

B. The equilibrium concentration of PCl5 (g) is lower in the second experiment.

C. The equilibrium concentrations of all three gases are the same in both experiments.

D. The value of the equilibrium constant is higher in the second experiment.

Solution

B

Pressure is inversely proportional to volume. When the volume of the flask is increased from 1dm3 to 2dm3, the equilibrium pressure decreases and hence the equilibrium concentrations of PCl5, PCl3 and Cl2 are subsequently less than in the first experiment.

The equilibrium constant remains the same as the temperature remains the same. Hence D is incorrect.

Though there is an increase in the number of moles of PCl3 and Cl2 when pressure is decreased, the value of the equilibrium concentration is decreased due to an increase in volume from 1 dm3 to 2 dm3. Hence A is incorrect and the answer is B.




Ref: 9701/11/M/J/15 #10
The double bond between the two carbon atoms in an ethane molecule consists of one pi bond and one sigma bond.
Which orbitals overlap to form each of these bonds?

Solution
A. sp2-sp2 and p-p


In an ethene, C2H4 molecule, each carbon atom is surrounded by 2 hydrogen atoms and forms a double bond with the other carbon atom. The double bond consists of a sigma bond, and the pi bond which is formed after the sigma bond. Since each carbon atom is surrounded by 3 other atoms, they are both sp2 hybridised. Thus the sigma bond is formed between two sp2 hybrid orbitals. C and D are incorrect.
Pi bonds are only formed through the overlapping of p orbitals, hence B is incorrect. The answer is A.
* If you would like to increase your understanding on hybridisation, head over to Chemguide. Click on the bonding of methane and ethane.




Ref: 9701/11/M/J/15 #13
The three minerals below are obtained from mines around the world. Each one behaves as a mixture of two carbonate compounds. They can be used as fire retardants because they decompose in the heat, producing CO2. This gas smothers the fire.

 barytocite BaCa(CO3)2

dolomite CaMg(CO3)2

huntite Mg3Ca(CO3)4

What is the order of effectiveness of fire retardant, from best to worst?

Solution 

D

The effectiveness of the compounds as fire retardants is based on their ability to decompose under heat to produce CO2. Going down Group II, the stability of the metal carbonates increases, hence lower down the group, the more difficult and the more heat energy required for the metal carbonates to decompose. Barytocite, dolomite and huntite all have Ca. Barytocite has Ba, which is lower in the group than Mg (present in dolomite and huntite), hence barytocite decomposes least easily among all three, making it the worst fire retardant.

Although dolomite and huntite both have Ca and Mg, huntite has a higher proportion of Mg compared to dolomite. Thus huntite will decompose more under the same conditions, making it the best fire retardant.

Ref: 9701/11/M/J/15 #26

The presence of a halogen in an organic compound may be detected by warming the organic compound with aqueous silver nitrate.

Which compound would be the quickest to produce a precipitate?

Solution

C.

Warming the compounds would cause the bonds in the compounds to break, releasing the halide ions which then react with the silver ions in AgNO3 to form precipitates. The smaller the bond energy, the weaker the bond, the more quickly it breaks and the faster a precipitate is produced.

Bond energy depends on two factors: Bond length and Bond polarity. The greater the bond length, the weaker the bond. The less polar the bond, the weaker the bond.

Among F, Cl, Br and I, I has the largest atomic radius and the least electronegativity, hence the bond with I is the longest and least polar, thus the weakest bond. Compound C is the only compound with I in it, hence the bond containing I in compound C would be the first to break. Thus when heated, compound C produces a precipitate (yellow for AgI) most quickly.

Ref: 9701/11/M/J/15 #28

In 1869 Ladenburg suggested a structure for benzene, C6H6, in which one hydrogen atom is attached to each carbon atom.

A compound C6H4Cl2 could be formed with the same carbon skeleton as the Ladenburg structure.
How many structural isomers would this compound have?

A. 3

B. 4

C. 5

D. 6

Solution

A. 3

This question requires visualisation. The Ladenburg structure consists of two triangular faces and three rectangular faces.

The three structural isomers are as follows :

1. Two Cl atoms attaching to two adjacent carbon atoms on the triangular face

2. Two Cl atoms attaching to two adjacent carbon atoms on the rectangular face

3. Two Cl atoms attaching to two carbon atoms diagonally across the rectangular face

This 3-D model of the Ladenburg structure might help you to see it more clearly.

Ref: 9701/11/M/J/15 #29

The citrus flavour of lemons is due to the compound limonene, present in both the peel and the juice.

What is the mole ratio of carbon dioxide to water produced when limonene is completely burnt in oxygen?

Solution

B

When burnt in excess oxygen, hydrocarbons will produce CO2 (with the C atoms) and H2O (with the H atoms). By counting, you will obtain 10 carbon atoms and 16 hydrogen atoms. 10 mols of C atoms will produce 10 mols of CO2. 16 mols of H atoms will produce 8 mols of H2O (watch out for the '2' in the compound, hence 8 mols of H2O and not 16).

Thus the mole ratio of CO2 to H20 is 10:8=5:4.

Ref: 9701/11/M/J/15 #37

The diagram shows the structure of an alkene molecule.

Which statements about this molecule are correct?

1 All the carbon atoms are in the same plane

2 It has geometrical isomers

3 It is optically active

Which statements about this molecule are correct?

Solution

C 2 and 3 only

The carbon atoms can only be in the same plane if bonds are sp2-sp2 or sp3-sp2. They cannot be in the same plane if the bonds are sp3-sp3. Since there are sp3-sp3 bonds between carbon atoms (carbon atoms which form bonds with four other atoms), not all the carbon atoms are in the same plane. Hence statement 1 is incorrect.

The two C atoms in the double bond each have two different groups attached to them, hence they display cis-trans/ geometrical isomerism. Statement 2 is correct.

The compound has a chiral carbon atom, hence it is optically active. Statement 3 is correct.