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Iron pyrite, FeS$_2$, has a yellow colour that makes it look like gold metal. The compound contains the ions Fe$^{2+}$ and S$_2^{2-}$.
(a) (i) Give the full electronic configuration of Fe$^{2+}$.
1s$^2$ .................................................................................................................................................. [1]
(ii) Calculate the oxidation number of sulfur in the S$_2^{2-}$ ion. Assume that each sulfur atom in the ion has the same oxidation number.
oxidation number of sulfur in the S$_2^{2-}$ ion = ............................... [1]
(b) Describe the metallic bonding in gold.
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(c) Iron pyrite is often called _fool's gold_ because of its appearance. Impure samples of iron pyrite often contain a small amount of gold.
The gold can be obtained from impure iron pyrite. The impure iron pyrite is roasted in oxygen, to produce iron(III) oxide and sulfur dioxide. Gold does not react with oxygen.
(i) The sulfur dioxide produced during roasting would cause environmental consequences if released into the atmosphere. State and explain one of these environmental consequences.
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(ii) Complete the equation to show the roasting of iron pyrite in oxygen.
4FeS$_2$ + .................................... $\rightarrow$ 2Fe$_2$O$_3$ + .................................... [2]
(iii) A sample of impure iron pyrite was roasted in oxygen. The composition of the mixture of solid products is shown.
$$\begin{align}
\text{solid product} & \text{mass/g} \\
\hline
\text{Fe}_2\text{O}_3 & 33.18 \\
\text{Au} & 0.37 \\
\end{align}$$
Calculate the mass of FeS$_2$ present in the sample of impure iron pyrite. Assume that all the FeS$_2$ was converted to Fe$_2$O$_3$ during the roasting process.
(M$_r$: FeS$_2$, 120.0; Fe$_2$O$_3$, 159.6)
mass of FeS$_2$ = ................................. g [2]
(iv) Use your answer to (iii) to calculate the percentage by mass of gold in this sample of impure iron pyrite. Assume that gold is the only impurity in this sample of impure iron pyrite.
Give your answer to two significant figures.
(If you were unable to calculate an answer to (iii), use 55.00 g as the mass of FeS$_2$ in this calculation. This is not the correct answer.)
percentage by mass of gold = ................................. % [1]
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(a) Nitrogen, $N_{2}$, is an inert gas that makes up 78% of the Earth's atmosphere.
(i) Explain why nitrogen is inert.
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(ii) Draw a 'dot-and-cross' diagram of a nitrogen molecule. Show outer electrons only.
[1]
(b) Nitrogen, $N_{2}$, and oxygen, $O_{2}$, react together in the air during lightning strikes to form nitrogen monoxide, NO.
(i) Explain why the reaction of $N_{2}$ and $O_{2}$ occurs during lightning strikes.
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(ii) Write two equations to suggest how the NO formed reacts further to create nitric acid, $HNO_{3}$.
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2 .............................................................................................................................. [2]
(c) Nitrate fertilisers are used to provide nitrogen for plant growth. Uncontrolled use of these can cause a reduction in animal and plant life in natural water supplies.
Explain how uncontrolled use of nitrate fertilisers can cause this problem.
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(d) Some soils have compounds such as ammonium nitrate, calcium carbonate and calcium hydroxide added to them.
(i) Suggest why calcium hydroxide is added to some soils.
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(ii) When calcium hydroxide reacts with compounds containing the ammonium ion, $NH_{4}^{+}$, a gas is produced.
State the identity of this gas and explain why the reaction occurs.
gas ..............................................................................................................................
explanation ..............................................................................................................................
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(iii) Another fertiliser, calcium ammonium nitrate, is formed when solid calcium carbonate is added to a mixture of aqueous ammonium nitrate and dilute nitric acid.
Suggest what would be observed in this reaction.
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(iv) Calcium nitrate decomposes at a higher temperature than calcium ammonium nitrate.
Write an equation for the thermal decomposition of calcium nitrate.
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Trihalomethanes are organic molecules in which three of the hydrogen atoms of methane are replaced by halogen atoms, for example CHCl_3.
(a) CHCl_3 is a colourless liquid with a high vapour pressure.
(i) Explain what is meant by high vapour pressure.
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(ii) An important reaction of CHCl_3(g) is the manufacture of CHClF_2(g), using the following reversible reaction.
CHCl_3(g) + 2HF(g) ⇌ CHClF_2(g) + 2HCl(g)
Use the data to calculate the enthalpy change of reaction, $\Delta H_r$, for the formation of CHClF_2(g) as shown in the equation.
[Table_1]
$$\begin{array}{|c|c|}\hline \text{compound} & \text{enthalpy change of formation, } \Delta H_f' \text{ kJ mol}^{-1} \\ \hline \text{CHCl}_3(g) & -103.2 \\ \text{CHClF}_2(g) & -482.2 \\ \text{HF}(g) & -273.3 \\ \text{HCl}(g) & -92.3 \\ \hline \end{array}$$
enthralpy change of reaction, $\Delta H_r = \text{..................... kJ mol}^{-1}$ [3]
(iii) The reaction in (ii) is carried out using a heterogeneous catalyst.
Explain fully the meaning of the terms heterogeneous and catalyst.
heterogeneous ..............................................................................................................................
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catalyst ..............................................................................................................................
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(b) CHClF_2 was used as an alternative to chlorofluorocarbons (CFCs). CHClF_2 should no longer be used because it was found to contribute to the enhanced greenhouse effect.
(i) Give the meaning of the term enhanced greenhouse effect.
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(ii) Explain how CHClF_2(g) may contribute to this effect.
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(iii) Suggest another environmental problem associated with the use of CHClF_2.
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(c) CHClF_2 is also used to produce the monomer tetrafluoroethene, C_2F_4.
This monomer can be used to produce poly(tetrafluoroethene), PTFE.
(i) State the type of polymerisation that occurs during the production of PTFE.
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(ii) Draw the repeat unit of PTFE.
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(iii) Suggest why PTFE is used as a coating for cooking pans.
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(iv) Waste disposal can cause litter problems.
State two other difficulties associated with the disposal of PTFE.
1 ..............................................................................................................................
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2 ..............................................................................................................................
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(a) Complete the table to show what you would observe when an aqueous solution of glycolic acid is added separately to each of the reagents. If a reaction occurs, state the functional group of glycolic acid that is responsible for the reaction.
[Table]
| reagent | observation with glycolic acid | does a reaction occur? ✓ / X | functional group |
|---|---|---|---|
| $Na_2CO_3(aq)$ | |||
| 2,4-DNPH | |||
| acidified $Cr_2O_7^{2-}$ |
[4]
(b) Two reaction sequences to make glycolic acid are shown.
sequence A: HCHO\( \xrightarrow{\text{HCN and NaCN, reaction 1}} \) X \( \xrightarrow{\text{reaction 2}} \text{CH}_2(\text{OH})\text{CO}_2\text{H} \)
sequence B: $CH_3CO_2H \xrightarrow{\text{Br}_2, \text{reaction 3}} \text{CH}_2\text{BrCO}_2H \xrightarrow{\text{reaction 4}} \text{CH}_2(\text{OH})\text{CO}_2H$
(i) Draw the structure of X. [1]
(ii) Name the reagent for reaction 2. [1]
(iii) Name the mechanism of reaction 3. [1]
(iv) Suggest the essential condition for reaction 3. [1]
(v) Reaction 4 occurs via an $S_N2$ mechanism. Complete the diagram for the mechanism for reaction 4. Include all relevant charges, partial charges, curly arrows and lone pairs.[2]
(c) Glycolic acid can also be made by reacting glyoxylic acid with $NaBH_4$.
(i) State the role of $NaBH_4$ in this reaction. [1]
(ii) Write an equation for this reaction using molecular formulae. Use [H] to represent $NaBH_4$. [2]
(d) When glycolic acid is heated in the presence of a sulfuric acid catalyst, a new compound, Y, \( C_4H_4O_4 \), is formed. The equation for the reaction is given. $2CH_2(OH)CO_2H \rightarrow C_4H_4O_4 + 2H_2O$
(i) The infra-red spectrum of Y is shown.
State how this spectrum differs from an infra-red spectrum of glycolic acid. Explain your answer with particular reference to the peaks within the range $1500−4000 \text{ cm}^{−1}$. [2]
(ii) Suggest a structure for Y. [2]
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