AS & A Level Chemistry - 9701 Paper 4 2011 Summer Zone 1

Taken together, nitrogen and oxygen make up 99% of the air. Oxygen is by far the more reactive of the two gases, and most of the substances that react with air combine with the oxygen rather than with the nitrogen.

(a) State one reason why the molecule of nitrogen, $N_2$, is so unreactive.   

Despite the apparent lack of reactivity of $N_2$, nitrogen atoms have been found to form bonds with almost all of the elements in the Periodic Table. Lithium metal reacts with nitrogen gas at room temperature to give lithium nitride, $Li_3N$. Magnesium produces magnesium nitride, $Mg_3N_2$, as well as magnesium oxide, when heated in air.

(b) Calculate the lattice energy of magnesium nitride using the following data, in addition to relevant data from the Data Booklet.  

(c) Lithium reacts readily with nitrogen, and because of this $Li_3N$ has been considered as a possible intermediate in the ‘fixing’ of nitrogen to make ammonia-based fertilisers.

$N_2(g) + Li \rightarrow Li_3N + H_2O \rightarrow NH_3 + A$

(i) Construct an equation for the reaction between $Li_3N$ and $H_2O$, and hence identify compound A.

(ii) Using your knowledge of the Haber process, consider one advantage and one disadvantage of using lithium as a means of fixing nitrogen, rather than the Haber process.   

(d) Another possible advantage of $Li_3N$ is that it contains a large percentage by mass of nitrogen. Another fertiliser that contains a large percentage by mass of nitrogen is urea, $NH_2CONH_2$.

(i) Calculate and compare the percentages by mass of nitrogen in $Li_3N$ and $NH_2CONH_2$.    

(ii) What class of organic compound is urea?

(iii) Write an equation for the production of ammonia by the reaction between urea and water.

(iv) Urea can be applied directly to the soil either before or during the growing of crops.

What would be a major disadvantage of using lithium nitride in this way?    

(a) State briefly what is meant by the following terms.
(i) reversible reaction
..................................................................................................................
(ii) dynamic equilibrium
..................................................................................................................
..................................................................................................................

(b) Water ionises to a small extent as follows.
$$ \text{H}_2\text{O(l)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \quad \Delta\text{H} = +58 \text{kJ mol}^{-1} $$
(i) Write an expression for $K_c$ for this reaction.
..................................................................................................................
(ii) Write down the expression for $K_w$, the ionic product of water, and explain how this can be derived from your $K_c$ expression in (i).
..................................................................................................................
..................................................................................................................
(iii) State and explain how the value of $K_w$ for hot water will differ from its value for cold water.
..................................................................................................................
..................................................................................................................

(c) $K_w$ can be used to calculate the pH of solutions of strong and weak bases.
(i) Use the value of $K_w$ in the *Data Booklet* to calculate the pH of $0.050\text{ mol dm}^{-3}$ NaOH.

pH = ..........................................................
Ammonia ionises slightly in water as follows.
$$ \text{NH}_3\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{NH}_4^+\text{(aq)} + \text{OH}^-\text{(aq)} $$
The following expression applies to this equilibrium.
$$ [\text{H}_2\text{O}] \times K_c = [\text{NH}_4^+][\text{OH}^-]/[\text{NH}_3] = 1.8 \times 10^{-5}\text{ mol dm}^{-3} $$
(ii) Calculate [OH$^-$ (aq)] in a $0.050\text{ mol dm}^{-3}$ solution of NH$_3$. You may assume that only a small fraction of the NH$_3$ ionises, so that [NH$_3$] at equilibrium remains at $0.050\text{ mol dm}^{-3}$.
[OH$^-$ (aq)] = ..................................................
(iii) Use the value of $K_w$ in the *Data Booklet*, and your answer in (ii), to calculate [H$^+$ (aq)] in $0.050\text{ mol dm}^{-3}$ NH$_3$(aq).
[H$^+$ (aq)] = ..........................................................
(iv) Calculate the pH of this solution.
pH = ..........................................................

(a) State and explain the variation in the oxidation numbers of the chlorides of the elements Na, Mg, Al and Si. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................

(b) Describe the reaction of phosphorus(V) chloride with water, and write an equation for the reaction. ........................................................................................................................................ ........................................................................................................................................

(c) When microwave radiation is passed through phosphorus(III) chloride, $\text{PCl}_3$, at low pressure, a new chloride of phosphorus, B, is formed. B contains 69.6% by mass of chlorine and 30.4% by mass of phosphorus, and its $M_r$ is approximately 200.
(i) Calculate the empirical and molecular formulae of B. ........................................................................................................................................
(ii) Assuming phosphorus and chlorine show their typical valencies, draw the displayed formula of B, showing all bonds and lone pairs. ........................................................................................................................................
(iii) Calculate the oxidation number of phosphorus in B. ........................................................................................................................................
(iv) One mole of B reacts with four moles of water. Suggest the structure of the phosphorus-containing product of this reaction. ........................................................................................................................................

The combustion of fuels in motor vehicles, trains, aeroplanes and power stations produces the pollutant gas $NO_2$.
(a) Write an equation to show how $NO_2$ is formed in these situations.
.............................................................................................................................[1]

(b) (i) How is the $NO_2$ removed from the exhaust gases of motor vehicles?
.............................................................................................................................
(ii) Write an equation for this process.
.............................................................................................................................[2]

(c) Suggest whether the production of the pollutant $NO_2$ would be reduced if fossil fuels were replaced by hydrogen as a fuel for combustion. Explain your answer.
.............................................................................................................................
.............................................................................................................................[1]

(d) In the atmosphere, $NO_2$ acts as a catalyst for the oxidation of $SO_2$ to $SO_3$.
\[SO_2(g) + \frac{1}{2}O_2(g) \overset{NO_2}{\rightarrow} SO_3(g)\]
(i) What is the environmental significance of this reaction?
.............................................................................................................................
.............................................................................................................................
The oxidation takes place in two steps. The initial reaction is that between $NO_2$ and $SO_2$:
\[\text{reaction 1 } NO_2(g) + SO_2(g) \rightleftharpoons NO(g) + SO_3(g) \quad \Delta H = -168 \text{ kJ mol}^{-1}\]
(ii) Write an equation to show how the $NO_2$ is regenerated in the second step of the oxidation.
.............................................................................................................................
(iii) Write an expression for the equilibrium constant, $K_p$ for \text{reaction 1}, stating its units.
\[K_p =\]units ......................................................
(iv) If equal amounts of $NO_2(g)$ and $SO_2(g)$ are allowed to react at room temperature, it is found that 99.8% of the gases have been converted into products at equilibrium. Calculate a value for $K_p$.
\[K_p =\]......................................................
(v) The temperature of the atmosphere decreases with height. How will this affect the position of the equilibrium in \text{reaction 1}? Explain your answer.
.............................................................................................................................
.............................................................................................................................[7]

(a) There are several ways of introducing chlorine atoms into organic molecules. State the reagents and conditions necessary to carry out the following transformations.

[Table_1]

(b) (i) When treated with concentrated $HNO_3$ + $H_2SO_4$ at 55°C, benzene produces nitrobenzene. Outline the mechanism of this reaction. You should include all charges, and use curly arrows to represent the movement of electron pairs.

In aromatic substitution of monosubstituted benzenes, the orientation of an incoming group depends on the nature of the group already attached to the ring. For example, using the same reagents and conditions as in (i), methylbenzene and benzoic acid produce the following nitro compounds.


(ii) Using this information as an aid, suggest a structure for compound C in the following synthesis of 3-bromobenzoic acid.


(iii) Suggest reagents and conditions for steps 1 and 2.
[Table_2]

(a) The reaction producing tri-iodomethane (iodoform) can be used as a test for the presence of certain groups within a molecule.
(i) State the reagents and conditions used for this reaction.
......................................................................................................................................
(ii) Write the structural formula of one functional group that would give a positive result with this iodoform reaction.
..................................................................................................................................
(iii) What do you observe in a positive test?
..................................................................................................................................
..................................................................................................................................
(iv) In the following table place a tick (✓) in the column against each compound that would give a positive result with this test, and a cross (✗) against each compound that would give a negative result.

Enzymes are a special group of protein molecules present in large amounts in living organisms. Enzymes behave as catalysts but, unlike inorganic catalysts, they generally catalyse only one particular reaction.
(a) Inorganic catalysts often work better on heating, but enzymes rarely work at temperatures much above 45°C. Explain why this is the case.
..................................................................................................................................................................
..................................................................................................................................................................
.................................................................................................................................................................. [2]

(b) Using the shape below to represent an enzyme, sketch how an enzyme is specific to the breakdown of a particular substrate molecule

............................................................................................................................................................... [3]

(c) Describe the effects of a competitive, and of a non-competitive inhibitor on the interaction between enzyme and substrate.
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
.................................................................................................................................................................. [2]

(d) (i) The diagram shown illustrates an enzyme-catalysed reaction. On the diagram sketch the graph that would be obtained if the same reaction was carried out in the presence of a non-competitive inhibitor.


(ii) Explain why a non-competitive inhibitor has this effect on the reaction.
..................................................................................................................................................................
.................................................................................................................................................................. [3]

(a) Paper and gas chromatography rely on partition to separate the components in a mixture, whereas thin-layer chromatography uses adsorption.
Explain what is meant by (i) partition and (ii) adsorption, in the context of chromatography.
(i) partition ...........................................................................................................................
...............................................................................................................................
...............................................................................................................................
(ii) adsorption .........................................................................................................................
...............................................................................................................................
...................................................................................................................[2]

(b) In paper or thin-layer chromatography, better separation may be achieved by running the chromatogram in one solvent, then turning the paper at right angles and running it in a second solvent. The chromatogram below was produced in this way.

(i) Ring the spot which was insoluble in solvent 1.
(ii) Label as A and B the spots which were not resolved using solvent 1. [2]

(c) The mass spectrum shown was obtained from a compound of formula $C_pH_qX$, where $X$ represents a halogen atom.

(i) Deduce the identity of $X$, giving a reason.
$X$ is .........................................................................................................................
.............................................................................................................................
(ii) If the relative heights of the M and M+1 peaks are 9 and 0.3 respectively, calculate the value of $p$. Use this value and the $m/e$ value of the molecular ion to calculate the value of $q$, and hence the molecular formula of the compound. Show your working.
(iii) Suggest a formula for the ion responsible for the peak labelled R.
............................................................ [4]

(d) In the fragmentation of alcohols which occurs in a mass spectrometer, small stable, neutral molecules are sometimes produced. Suggest the identity of two such molecules, each with an $M_r$ less than 30.
(i) ..................................................... (ii) ...................................................... [2]

(a) (i) To form a polymer, what is the minimum number of functional groups that the monomer must possess?
.....................................................

(ii) Illustrate your answer to (i) with the structure of a possible monomer.
.....................................................

(b) State two differences between addition and condensation polymerisation.

(i) ............................................................................................................................
............................................................................................................

(ii) ...........................................................................................................................
...........................................................................................................

(c) The polymer formed from the co-polymerisation of the two monomers shown is known as Terylene.



(i) The two monomers react by condensation polymerisation. What other molecule is formed in this reaction?
.....................................................

(ii) Draw the structure of one repeat unit of Terylene.
.....................................................

(iii) What is the name given to polymers containing the same functional group as Terylene?
.....................................................

(d) The monomers ethene and but-1-ene can also co-polymerise to form a polyalkene, but this does not produce a regular alternating structure like Terylene. Explain why this is the case, drawing diagrams if you wish.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................