AS & A Level Chemistry - 9701 Paper 2 2017 Summer Zone 3

Combustion data can be used to calculate the empirical formula, molecular formula and relative molecular mass of many organic compounds. Combustion data cannot distinguish between different structural isomers.
(a) Define the term structural isomers.
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(b) $P$ is a hydrocarbon, $C_xH_y$. A gaseous sample of $P$ occupied a volume of $25 \text{ cm}^3$ at $37\degree C$ and $100 \text{kPa}$.
The sample was completely burned in $200 \text{ cm}^3$ of oxygen (an excess).
The final volume, measured under the same conditions as the gaseous sample (so that the water produced is liquid and its volume can be ignored), was $150 \text{ cm}^3$.
Treating the remaining gaseous mixture with concentrated alkali, to absorb carbon dioxide, decreased the volume to $50 \text{ cm}^3$.
The equation for the complete combustion of $P$ can be represented as shown.
$$ C_xH_y + \left(x + \frac{y}{4}\right)O_2 \rightarrow xCO_2 + \frac{y}{2}H_2O $$
(i) Use the data given to calculate the value of $x$.
$x = \text{.....................}$ [1]
(ii) Use the data given to calculate the value of $(x + \frac{y}{4})$.
$(x + \frac{y}{4}) = \text{.....................}$ [1]
If you were unable to calculate values in (b)(i) and (b)(ii) then use the data in this box for the remaining parts of this question. These are not the correct values.
$x = 6\phantom{abc}(x + \frac{y}{4}) = 9$
(iii) Give the molecular formula and the empirical formula of $P$.
molecular formula of $P$ ...............................................................................................................
empirical formula of $P$ ................................................................................................................
[2]
(iv) $P$ is unbranched.
Give the skeletal formulae for two possible structures of $P$ that are positional isomers of each other.


[2]
(v) Use the general gas equation to calculate the mass of $P$ present in the original $25 \text{ cm}^3$ gaseous sample, which was measured at $37\degree C$ and $100 \text{kPa}$.
Give your answer to three significant figures.
mass = .............................. g [3]

(a) (i) Give the colours and states of chlorine, bromine and iodine at room temperature and pressure.

[Table_1]

[2]

(ii) The halogens become less volatile down the group.
Explain this trend in volatility.

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(b) The halogens are oxidising agents.
State and explain the trend in oxidising power of the halogens.

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(c) Concentrated sulfuric acid reacts with solid sodium halides.
(i) State any observations that would be made on addition of concentrated sulfuric acid to:
• solid sodium chloride, .................................................................................................................
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• solid sodium iodide. ......................................................................................................................
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(ii) Give reasons for the difference in the observations in (i).
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(iii) The addition of concentrated sulfuric acid to solid sodium bromide, NaBr, produces brown fumes and an acidic gas that decolourises acidified potassium manganate(VII) solution. This acidic gas is a significant contributor to acid rain.
Write the equation for the reaction of concentrated sulfuric acid with sodium bromide.
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(d) An aqueous solution, Z, contains a mixture of sodium chloride and sodium iodide.
(i) Excess aqueous silver nitrate is added to Z in a test-tube. A yellow precipitate forms.
Explain the colour of this precipitate.
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(ii) Aqueous ammonia is then added to the test-tube in (i). The mass of precipitate decreases.
Explain this observation.
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(d) The cyanate ion, NCO⁻, can act as a monodentate ligand.
(i) State what is meant by the terms monodentate, ..........................................................................
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ligand. ........................................................................................................................................
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Silver ions, Ag⁺, react with cyanate ions to form a linear complex.
(ii) Suggest the formula of this complex, including its charge.
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(e) When heated with HCl(aq), organic isocyanates, RNCO, are hydrolysed to the amine salt, RNH3Cl, and CO2.
$$\text{RNCO} + \text{H}_2\text{O} + \text{HCl} \rightarrow \text{RNH}_3\text{Cl} + \text{CO}_2$$
A 1.00 g sample of an organic isocyanate, RNCO, was treated in this way, and the CO2 produced was absorbed in an excess of aqueous Ba(OH)2 according to the equation shown. The solid BaCO3 precipitated weighed 1.66 g.

$$\text{Ba(OH)}_2(\text{aq}) + \text{CO}_2(\text{g}) \rightarrow \text{BaCO}_3(\text{s}) + \text{H}_2\text{O(l)}$$
(i) Calculate the number of moles of BaCO3 produced.
moles of BaCO3 = .......................... [1]

(ii) Hence calculate the Mr of the organic isocyanate RNCO.
$$\text{M}_r \text{ of RNCO} = ..........................$$ [1]

The R group in RNCO and RNH3Cl contains carbon and hydrogen only.
(iii) Use your Mr value calculated in (ii) to suggest the molecular formula of the organic isocyanate RNCO.
molecular formula of RNCO .................................................................................. [1]

(iv) Suggest a possible structure of the amine RNH2, which forms the amine salt, RNH3Cl.
[1]

(a) The enthalpy change of formation of $SO_2$, $\Delta H_f SO_2(g)$, is $-296.8\, \text{kJ mol}^{-1}$.

(i) Define the term enthalpy change of formation.
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(ii) Use the data to calculate the enthalpy change of formation of $SO_3(g)$.
\( \Delta H_f SO_3(g) = \text{....................... kJ mol}^{-1} \) [2]

(b) The Contact process is usually carried out at a temperature of approximately $700\, K$, a pressure of approximately $150\, \text{kPa}$ and in the presence of a vanadium(V) oxide catalyst, $V_2O_5$.

The Boltzmann distribution for a mixture of $SO_2$ and $O_2$ at $700\, K$ is shown. $E_{\text{a cat}}$ represents the activation energy for the reaction in the presence of the catalyst.

(i) Add a labelled mark, $E_{\text{a uncat}}$, to the diagram to indicate the activation energy in the absence of the catalyst. [1]
(ii) State the benefit of using a catalyst in this reaction. Explain how it achieves this effect.
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(iii) State and explain how an increase in pressure would affect both the rate of reaction and the yield of $SO_3$ in the Contact process.
rate ..............................................................................................................................................................................................................................................................
yield ........................................................................................................................................................................................................................................................... [4]

(c) At a pressure of $1.50 \times 10^5\, \text{Pa}$, $1.00\, \text{mol}$ of sulfur dioxide gas, $SO_2$, was mixed with $1.00\, \text{mol}$ of oxygen gas, $O_2$. The final equilibrium mixture formed was found to contain $0.505\, \text{mol}$ of $O_2$.
\( 2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \)
(i) Calculate the amount, in mol, of $SO_2$ and $SO_3$ in the equilibrium mixture.
$SO_2 = \text{.......................... mol}$
$SO_3 = \text{.......................... mol}$ [1]
(ii) Calculate the partial pressure of oxygen gas, $p_{O_2}$, in the equilibrium mixture.
$p_{O_2} = \text{.......................... Pa}$ [2]

(d) In another equilibrium mixture formed from different starting amounts of $SO_2$ and $O_2$, the partial pressures of $SO_2$, $O_2$ and $SO_3$ were as shown.
$p_{SO_2} = 8.42 \times 10^2\, \text{Pa}$
$p_{O_2} = 6.00 \times 10^4\, \text{Pa}$
$p_{SO_3} = 9.10 \times 10^4\, \text{Pa}$
(i) Write the expression for the equilibrium constant, $K_p$, for the production of $SO_3$ from $SO_2$ and $O_2$.
$K_p =$ [1]
(ii) Calculate the value of $K_p$ for this reaction and state the units.

$K_p = \text{..........................}$
units = \text{..........................} [2]

A, B and C all have the formula $C_4H_8$. They all decolourise bromine and are structural isomers of each other.
(a) State the name of the process by which A, B and C could be obtained from $C_{10}H_{22}$. [1]
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(b) Draw the structures of these three structural isomers. [1]

(c) Only A shows geometrical isomerism.
(i) Explain the meaning of the term geometrical isomerism. [2]
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(ii) Draw the displayed formula of A and use it to show the mechanism of the reaction of A with HBr. Include all necessary charges, dipoles, lone pairs and curly arrows. [4]

(d) B does not show geometrical isomerism. B reacts with HBr to form a mixture of two structural isomers, X and Y.
$\text{B + HBr} \rightarrow X \text{ (has a chiral centre and is produced in higher yield than Y)}$
$\rightarrow Y \text{ (does not have a chiral centre)}$
(i) State the meaning of the term chiral centre. [1]
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(ii) Name B. [1]
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(iii) X exists as a pair of optical isomers. Draw these isomers using the conventional three-dimensional representation. [2]

(iv) Explain why X is produced in higher yield than Y. [2]
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(e) C does not show geometrical isomerism. C reacts with HBr to form a mixture of two structural isomers, neither of which has a chiral centre.
(i) Name C. [1]
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(ii) Draw the displayed formula of each of the structural isomers produced by the reaction of C with HBr. [2]