AS & A Level Chemistry - 9701 Paper 2 2013 Winter Zone 2

Valence Shell Electron Pair Repulsion theory (VSEPR) is a model of electron-pair repulsion (including lone pairs) that can be used to deduce the shapes of, and bond angles in, simple molecules.

(a) Complete the table below by using simple hydrogen-containing compounds. One example has been included.

[Table_1]

| number of bond pairs | number of lone pairs | shape of molecule | formula of a molecule with this shape |
|--------------------|-------------------|----------------|------------------------------------|
| 3 | 0 | trigonal planar | $\text{BH}_3$ |
| 4 | 0 | | |
| 3 | 1 | | |
| 2 | 2 | | |

[3]

(b) Tellurium, Te, proton number 52, is used in photovoltaic cells.

When fluorine gas is passed over tellurium at 150°C, the colourless gas TeF$_6$ is formed.

(i) Draw a ‘dot-and-cross’ diagram of the TeF$_6$ molecule, showing outer electrons only.

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(ii) What will be the shape of the TeF$_6$ molecule?
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(iii) What is the F–Te–F bond angle in TeF$_6$?
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[3]

The molecular formula $C_3H_6$ represents the compounds propene and cyclopropane.



(a) What is the H–C–H bond angle at the terminal =CH₂ group in propene?
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(b) Under suitable conditions, propene and cyclopropane each react with chlorine.
(i) With propene, 1,2-dichloropropane, $CH_3CHClCH_2Cl$ is formed.
State fully what type of reaction this is.
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(ii) When cyclopropane reacts with chlorine, three different compounds with the molecular formula $C_3H_4Cl_2$ can be formed.
Draw displayed structures of each of these three compounds.

Chlorine gas is manufactured by the electrolysis of brine using a diaphragm cell.
(a) (i) Write half-equations, including state symbols, for the reactions occurring at each of the electrodes of a diaphragm cell.
anode ...........................................................
cathode ...........................................................
(ii) In the diaphragm cell, the anode is made of titanium and the cathode is made of steel.
Suggest why steel is never used for the anode.
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(b) Chlorine is very reactive and will form compounds by direct combination with many elements.
Describe what you would see when chlorine is passed over separate heated samples of sodium and phosphorus.
In each case write an equation for the reaction.
sodium ..........................................................
phosphorus ..........................................................

(c) Chlorine reacts with aqueous sodium hydroxide in two different ways, depending on the conditions used. In each case, water, sodium chloride and one other chlorine-containing compound are formed.
For each condition below, give the formula of the other chlorine-containing compound and state the oxidation number of chlorine in it.
[Table_1]
condition | formula of other chlorine-containing compound | oxidation number of chlorine in this compound
cold dilute NaOH(aq) ................................................
hot concentrated NaOH(aq) ........................................

(d) Magnesium chloride, $\text{MgCl}_2$, and silicon tetrachloride, $\text{SiCl}_4$, each dissolve in or react with water.
Suggest the approximate pH of the solution formed in each case.
$\text{MgCl}_2$ .................................
$\text{SiCl}_4$ .................................
Explain, with the aid of an equation, the difference between the two values.
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Compound R is a weak diprotic (dibasic) acid which is very soluble in water.
(a) A solution of R was prepared which contained 1.25 g of R in 250 $\text{cm}^3$ of solution. When 25.0 $\text{cm}^3$ of this solution was titrated with 0.100 mol $\text{dm}^{-3}$ NaOH, 21.6 $\text{cm}^3$ of the alkali were needed for complete reaction.
(i) Using the formula $H_2X$ to represent R, construct a balanced equation for the reaction between $H_2X$ and NaOH.
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(ii) Use the data above to calculate the amount, in moles, of $OH^-$ ions used in the titration.
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(iii) Use your answers to (i) and (ii) to calculate the amount, in moles, of R present in 25.0 $\text{cm}^3$ of solution.
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(iv) Calculate the amount, in moles, of R present in 250 $\text{cm}^3$ of solution.
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(v) Calculate $M_r$ of R.

(b) Three possible structures for R are shown below.
[Table_1]
(i) Calculate the $M_r$ of each of these acids.
$M_r$ of S = ...................... $M_r$ of T = ...................... $M_r$ of U = ......................
(ii) Deduce which of the structures, S, T or U, correctly represents the structure of the acid, R.
R is represented by ...........

(c) It is possible to convert S, T, or U into one another.
State the reagent(s) and essential conditions that would be used for the following conversions.
S into T
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S into U
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T into S
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(d) Give the structural formula of the organic product formed in each of the following reactions.
T reacting with an excess of Na
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U reacting with an excess of $Na_2CO_3$

(e) The acid S shows stereoisomerism. Draw structures to show this isomerism. Label each isomer.

(f) When one of the isomers of S is heated at 110°C in the absence of air, a cyclic compound V, with molecular formula $C_4H_2O_3$, is formed. The other isomer of S does not react at this temperature.
Suggest the displayed formula of V.

Propane, C_3H_8, and butane, C_4H_{10}, are components of Liquefied Petroleum Gas (LPG) which is widely used as a fuel for domestic cooking and heating.
(a) (i) To which class of compounds do these two hydrocarbons belong?
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(ii) Write a balanced equation for the complete combustion of butane.
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(b) When propane or butane is used in cooking, the saucepan may become covered by a solid black deposit.
(i) What is the chemical name for this black solid?
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(ii) Write a balanced equation for its formation from butane.
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(c) Propane and butane have different values of standard enthalpy change of combustion.
Define the term standard enthalpy change of combustion.
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(d) A 125 cm^3 sample of propane gas, measured at 20°C and 101 kPa, was completely burnt in air. The heat produced raised the temperature of 200 g of water by 13.8°C. Assume no heat losses occurred during this experiment.
(i) Use the equation $pV = nRT$ to calculate the mass of propane used.
(ii) Use relevant data from the Data Booklet to calculate the amount of heat released in this experiment.
(iii) Use the data above and your answers to (i) and (ii) to calculate the energy produced by the burning of 1 mol of propane.

(e) The boiling points of methane, ethane, propane, and butane are given below.
[Table_1]
| compound | CH_4 | CH_3CH_3 | CH_3CH_2CH_3 | CH_3(CH_2)_2CH_3 |
| boiling point / K | 112 | 185 | 231 | 273 |
(i) Suggest an explanation for the increase in boiling points from methane to butane.
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(ii) The isomer of butane, 2-methylpropane, (CH_3)_3CH, has a boiling point of 261 K. Suggest an explanation for the difference between this value and that for butane in the table above.
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