AS & A Level Chemistry - 9701 Paper 4 2013 Winter Zone 3

(a) The nitrate ion, NO$_3^-$, contains a dative covalent bond.
Complete the following ‘dot-and-cross’ diagram of the bonding in the nitrate ion. For each of the four atoms show all the electrons in its outer shell. Three electrons have already been included.
Use the following code for your electrons.
• electrons from oxygen
x electrons from nitrogen
□ added electron(s) responsible for the overall negative charge

[3]
(b) (i) Write an equation showing the action of heat on magnesium nitrate, Mg(NO$_3$)$_2$.
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(ii) Describe and explain the trend that is observed in the thermal stabilities of the Group II nitrates.
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[4]
(c) When concentrated nitric acid, HNO$_3$, is added to copper turnings, a brown gas is evolved. Use data from the Data Booklet to construct an ionic equation for this reaction.
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[2]

(a) State two assumptions of the kinetic theory of gases, as applied to ideal gases.
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(b) (i) State the conditions of temperature and pressure under which real gases behave least like an ideal gas.
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(ii) Explain why real gases do not behave ideally under these conditions.
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(c) Gaseous aluminium chloride is dimeric at low temperatures, but the dimer dissociates on heating.
$$\text{Al}_2\text{Cl}_6(g) \rightleftharpoons 2\text{AlCl}_3(g)$$

(i) State whether this dissociation is endothermic or exothermic. Explain your answer.
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(ii) Choose one reaction in organic chemistry that is catalysed by $\text{AlCl}_3$, and write the structural formulae of the reactants and products in the boxes below.


[3]

(a) Write equations, with state symbols, to define the following.
(i) the C–Br bond energy in CH₃Br
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(ii) the Al–Cl bond energy in AlCl₃
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(b) (i) Describe and explain the trend in bond energies of the bonds in Cl₂, Br₂ and I₂.
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(ii) Fluorine, F₂, does not follow this trend. Suggest a possible reason why.
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(c) (i) Use data from the Data Booklet to calculate the enthalpy change of the following reaction.
$$H₂(g) + X₂(g) \rightarrow 2HX(g)$$
when $X = Cl$
$$\Delta H = \text{..................} \text{kJ mol}^{-1}$$
when $X = I$
$$\Delta H = \text{..................} \text{kJ mol}^{-1}$$
(ii) Use these results to describe and explain the trend in the thermal stabilities of the hydrides of Group VII.
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(d) Bromine reacts with hot NaOH(aq) to give a solution which on cooling produces white crystals of compound A.
A has the following percentage composition by mass: Na, 15.2; O, 31.8; Br, 53.0.
The remaining solution contains mostly NaBr, with a little of compound A.
(i) Calculate the empirical formula of A.
(ii) Construct an equation for the reaction between Br₂ and hot NaOH(aq).
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(a) The electrical conductivities of some Group IV elements are given below.

[Table_1]

From a consideration of the structures, suggest reasons for the following.
(i) The electrical conductivity of silicon is less than that of graphite.
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(ii) The electrical conductivity of tin is more than that of germanium.
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(b) Using data from the Data Booklet where appropriate, write equations for the following reactions of compounds of Group IV elements.
(i) the action of heat on PbO₂(s)
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(ii) PbO₂(s) + HCl(aq)
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(iii) SnO(s) + NaOH(aq)
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(iv) GeCl₄(l) + H₂O(l)
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(a) Bromine reacts with a variety of organic compounds. For each of the following reactions,
• complete and balance the equation, including the structural formula of the organic product,
• state the specific conditions (if any) under which the reaction takes place and the type of reaction that occurs.

(i) ..........Br_2 + !(inline_image)
reaction conditions ........................................................
type of reaction ........................................................

(ii) ..........Br_2 + !(inline_image)
reaction conditions ........................................................
type of reaction ........................................................

(iii) ..........Br_2 + !(inline_image)
reaction conditions ........................................................
type of reaction ........................................................

(b) When hydrocarbon B is heated with concentrated manganate(VII) ions, three organic compounds, C, D and E, are formed.

!(inline_image)
(i) Suggest the identities of compounds C, D and E, drawing their structures in the boxes above.

(ii) Use the relevant letter, C, D or E, to identify which of your compounds will react with each of the following reagents.
Each reagent may react with more than one of C, D and E, in which case state all the compounds that may react with each reagent.

• 2,4-dinitrophenylhydrazine ....................................
• alkaline aqueous iodine ....................................
• aqueous sodium hydroxide ....................................

Naturally-occurring $\alpha$-amino acids, RCH(NH_2)CO_2H, can be classified as amphiprotic substances. An amphiprotic substance is one which can act as both a Brønsted-Lowry acid and base.

[Table_1]

(a) What is the Brønsted-Lowry definition of an acid?
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(b)
(i) All $\alpha$-amino acids are soluble in water since they can form hydrogen bonds with water molecules and can also exist as zwitterions. Draw diagrams to show how the carboxylic acid and amino groups of alanine can form hydrogen bonds with water molecules.


(ii) Draw the structure of the zwitterionic form of glycine. [5]

(c) The amino acid alanine can be formed by the reaction of $CH_3CHC//(CO_2H)$ with an excess of ammonia. Outline a mechanism for this reaction using curly arrows. [3]

(d) Amino acids can form different ions at different pH values. Suggest the structures of the ions formed from the $\alpha$-amino acids below at the respective pH value.

[Table_2]
[2]

(e)
(i) How many different dipeptides is it possible to synthesise, each containing two of the three amino acids alanine, serine and lysine?
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(ii) Write the structural formula of one of these dipeptides incorporating serine and alanine. [3]

(f) Most naturally-occurring amino acids have a chiral centre and exhibit stereoisomerism.
(i) Define the term stereoisomerism.
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There are four optical isomers of threonine.
Some of these optical isomers are drawn below.


(ii) Which of the structures G, H or J is identical to structure F? ....................................................
(iii) The other two of the structures G, H or J represent two of the three other possible optical isomers of threonine.
Complete the following partial structure of the fourth optical isomer.


[3]

(a) Enzymes are particular types of proteins that catalyse chemical reactions. The efficiency of enzymes can be reduced by the presence of other substances known as inhibitors.
(i) State one example of a substance that can act as a non-competitive inhibitor in enzyme reactions.
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(ii) For the inhibitor you have identified, explain why it is a non-competitive inhibitor.
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(iii) The graph shows the rate of an enzyme-catalysed reaction against the substrate concentration in the absence of an inhibitor.

On the same axes, sketch a graph showing the rate of this reaction if a competitive inhibitor was present.

(b) DNA is responsible for encoding the amino acid sequence to produce proteins.
Ribosome, tRNA and mRNA are all involved in the process of protein synthesis.
(i) Write ribosome, tRNA and mRNA in the boxes below to show the correct sequence in which they are involved.

(ii) Sequences of three bases code for specific amino acids. The code UGA however does not usually code for an amino acid. Suggest its use.
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(c) Much of the energy used in biochemical reactions is provided by the hydrolysis of the molecule ATP.
(i) What are the breakdown products of the hydrolysis of ATP?
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(ii) Give two uses for the energy released by ATP hydrolysis in cells.
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(a) Both techniques use part of the electromagnetic spectrum. Which technique uses radiation with the longer wavelength, and in which part of the spectrum is it found? ..........................................................................................................................................................................................

(b) NMR spectroscopy provides detailed information about protons, but X-ray crystallography is unable to detect them. Explain these facts. .......................................................................................................................................................................................... .......................................................................................................................................................................................... ..........................................................................................................................................................................................

(c) The protein found in hair contains the amino acid cysteine, $C_3H_7SNO_2$. Crystalline cysteine was examined using X-ray crystallography. State which atom produced the strongest reflection, explaining your answer. .......................................................................................................................................................................................... ..........................................................................................................................................................................................

(d) Compound $P$ is an alcohol that can be converted into compound $Q$ in the following reaction sequence. $$P \rightarrow C_xH_6O \rightarrow Q$$ Spectral analyses of $P$ and $Q$ were carried out. (i) The mass spectrum of $P$ shows an $M:M+1$ peak ratio of 4.5 : 0.15. Calculate the number of carbon atoms in $P$. The NMR spectra of $P$ and $Q$ are shown below. (ii) In the spectrum of $P$, clearly label the peak due to the $-OH$ group with an $X$. (iii) State how many different proton environments are present in compound $Q$. ....................................... (iv) What evidence is there in these spectra that $P$ is a primary rather than a secondary alcohol? .......................................................................................................................................................................................... .......................................................................................................................................................................................... .......................................................................................................................................................................................... (v) Draw a structure for $Q$.

Until 1985, carbon was thought to exist in only two structural forms or allotropes. In 1985 another form, buckminsterfullerene, was discovered, in which the carbon exists as spherical molecules.

(a) The other two forms of carbon have very different structures.

(i) Name these two forms.

(ii) Give three differences in physical properties between these two forms.  

(b) The diagram shows the structure of buckminsterfullerene.

The molecule of buckminsterfullerene contains 60 carbon atoms. Suggest a reason why buckminsterfullerene reacts with hydrogen under suitable conditions and give a formula for the product. 

(c) In 2010, two scientists from the University of Manchester were awarded the Nobel Prize for Physics for their work on graphene, a new structural form of carbon.

Graphene is one of the new ‘nano-materials’ being developed for commercial uses in the next 10 years.

(i) Graphene is in the form of sheets of carbon one atom thick. Calculate the number of carbon atoms present in a sheet of graphene with a mass of one thousandth of a gram (0.001 g).

The number of hexagons in a large sheet of graphene can be assumed to be one half of the number of carbon atoms. Each hexagon has an area of 690 nm².

(ii) Calculate the area of the sheet of graphene in (i).

(iii) Would you expect samples of graphene and buckminsterfullerene to be electrical conductors? Explain your answers.