AS & A Level Chemistry - 9701 Paper 2 2014 Summer Zone 3

(a) Define the term mole.
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(b) 10 cm$^3$ of a gaseous hydrocarbon, C$_x$H$_y$, was reacted with 100 cm$^3$ of oxygen gas, an excess.

The final volume of the gaseous mixture was 95 cm$^3$.

This gaseous mixture was treated with concentrated, aqueous sodium hydroxide to absorb the carbon dioxide present. This reduced the gas volume to 75 cm$^3$.

All gas volumes were measured at 298 K and 100 kPa.

(i) Write an equation for the reaction between sodium hydroxide and carbon dioxide.
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(ii) Calculate the volume of carbon dioxide produced by the combustion of the hydrocarbon.

volume of CO$_2$ produced = ................. cm$^3$ [1]

(iii) Calculate the volume of oxygen used up in the reaction with the hydrocarbon.

volume of O$_2$ used = ................. cm$^3$ [1]

(iv) Use your answers to (b)(ii) and (b)(iii), together with the initial volume of hydrocarbon, to balance the equation below.
............C$_x$H$_y$ + ............O$_2$ $ ightarrow$ ............CO$_2$ + zH$_2$O [2]

(v) Deduce the values of x, y and z in the equation in (iv).
x = ...............................
y = ...............................
z = ............................... [3]

(c) Another hydrocarbon, W, with the formula C$_4$H$_8$, reacts with hydrogen bromide, HBr, to give two products X and Y. X and Y are structural isomers of molecular formula C$_4$H$_9$Br.

Reaction of X with aqueous alkali produces an alcohol, Z, that has no reaction with acidified dichromate(VI).

(i) Give the structures and names of the compounds W, X, Y, and Z

W
.............................................................. X
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Y
.............................................................. Z
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(ii) When W reacts with hydrogen bromide, more X than Y is produced. Explain why.
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A sample of a hydrated double salt, Cu(NH₄)ₓ(SO₄)₂·6H₂O, was boiled with an excess of sodium hydroxide. Ammonia was given off. The ammonia produced was absorbed in 40.0 cm³ of 0.400 mol dm⁻³ hydrochloric acid. The resulting solution required 25 cm³ of 0.12 mol dm⁻³ sodium hydroxide to neutralise the excess acid.
(a) Write the ionic equation for the reaction between ammonium ions and hydroxide ions. [1]
(b) (i) Calculate the amount, in moles, of hydrochloric acid in 40.0 cm³ of 0.400 mol dm⁻³ solution. [1]
(ii) Calculate the amount, in moles, of sodium hydroxide needed to neutralise the excess acid. This will be equal to the amount of hydrochloric acid left in excess. [1]
(iii) Calculate the amount, in moles, of hydrochloric acid that reacted with ammonia. [1]
(iv) Calculate the amount, in moles, of ammonium ions in the sample of the double salt. [1]
(v) The sample contained 0.413 g of copper. Use this information and your answer to (iv) to calculate the value of x in Cu(NH₄)ₓ(SO₄)₂·6H₂O. [2]
(vi) Calculate the $M_r$ of Cu(NH₄)ₓ(SO₄)₂·6H₂O. [1]

Nitrogen dioxide, NO\textsubscript{2}, can enter the atmosphere in a variety of ways.
(a) (i) State one natural and one man-made source of atmospheric NO\textsubscript{2}.
natural ........................................................................................................................
man-made ..................................................................................................................
[1]
(ii) Write an equation to show how NO\textsubscript{2} leads to the formation of nitric acid in acid rain.
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[1]
(iii) Use equations to illustrate the catalytic role of NO\textsubscript{2} in the formation of sulfuric acid in acid rain.
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[3]

(b) Nitrogen dioxide exists in equilibrium with dinitrogen tetroxide, N\textsubscript{2}O\textsubscript{4}.
$$2\text{NO}\textsubscript{2(g)} \rightleftharpoons \text{N}\textsubscript{2}\text{O}\textsubscript{4(g)}$$
2.00 mol of dinitrogen tetroxide was sealed in a container at 350 K. After equilibrium had been established the total pressure was 140 kPa and the mixture of gases contained 1.84 mol of dinitrogen tetroxide.
(i) Give the expression for the equilibrium constant, K\textsubscript{p}, for this equilibrium.
$$K\textsubscript{p} =$$
[1]
(ii) Calculate the number of moles of NO\textsubscript{2} present at equilibrium.
[1]
(iii) Calculate the total number of moles of gas present at equilibrium and hence the mole fraction of each gas present at equilibrium.
[2]
(iv) Calculate the partial pressure of each gas present at equilibrium.
[2]
(v) Calculate the value of the equilibrium constant, K\textsubscript{p}, at 350 K. Give your answer to three significant figures and include the units.
$$K\textsubscript{p} = ....................$$
units = ............................
[2]

The halogens and their compounds have a wide variety of uses and the chemical and physical properties of the elements show regular patterns related to their positions in Group VII.
(a) Chlorine, bromine and iodine all react with hydrogen.
(i) State the trend in the reactivities of the halogens with hydrogen.
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(ii) Explain this trend in terms of bond energies.
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(b) In the laboratory it is not very convenient to prepare hydrogen halides from their elements. Hydrogen halides can be prepared from their salts.
(i) Write an equation for the reaction of calcium chloride, $\text{CaCl}_2$, with concentrated sulfuric acid.
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(ii) Explain why hydrogen iodide is not prepared in this way.
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(iii) When potassium bromide, $\text{KBr}$, reacts with concentrated sulfuric acid, sulfur dioxide, $\text{SO}_2$, is produced. State what you would see and write an equation for this reaction.
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(c) (i) Give the structures of the four structural isomers of $\text{C}_4\text{H}_9\text{Br}$ and identify each as primary, secondary or tertiary.
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(ii) Name the isomer of $\text{C}_4\text{H}_9\text{Br}$ that contains a chiral centre and draw the three-dimensional structures of the two optical isomers.
name ............................................................................................................................
structures
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(d) Aqueous silver nitrate solution was added to separate tubes containing chloroethane, bromoethane and iodoethane. The tubes were heated in a water bath. A yellow precipitate appeared first in the tube containing iodoethane, followed by a cream precipitate in the tube containing bromoethane and finally a white precipitate appeared in the tube containing chloroethane.
Explain these observations.
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(e) (i) Give the full name of the mechanism for the reaction between aqueous sodium hydroxide and bromoethane.
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(ii) Complete the diagram below to represent this mechanism. Include all necessary curly arrows, partial charges and lone pairs.
$$\begin{align*} &\text{H} &\text{H} &\text{H} &\text{H} \\ &| & &| & &| & &| \\ &\text{H—C—C—Br} &\rightarrow &\text{H—C—C—OH} &+ &\text{Br}^- \\ &| & &| & &| \\ &\text{H} & &\text{H}\\ &\text{H} &\\ &\text{HO}^- \end{align*}$$ [2]
(f) In the past, CFCs such as $\text{CF}_3\text{Cl}$ were widely used as refrigerants.
(i) State a property of CFCs which makes them suitable for use as refrigerants.
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(ii) State the damaging effect of CFCs in the upper atmosphere.
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Explain your answer.
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