0620_w14_qp_33
A paper of Chemistry, 0620
Questions:
8
Year:
2014
Paper:
3
Variant:
3
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2
Compound X is a colourless liquid at room temperature. A sample of pure X was slowly heated from –5.0 °C, which is below its melting point, to 90 °C, which is above its boiling point. Its temperature is measured every minute and the results are represented on the graph. temperature time A B C D E F t °C 90 °C –5 °C Complete the equation for the equilibrium present in the region BC. X What is the signifi cance of temperature t °C? What is the physical state of compound X in the region EF? What would be the difference in the region BC if an impure sample of X had been used? Compound X is a hydrocarbon. It contains 85.7% of carbon. The mass of one mole of X is 84 g. What is the percentage of hydrogen in the compound ? Calculate the empirical formula of X. Show your working. empirical formula = What is the molecular formula of compound X?
3. Stoichiometry  →  3.1. Formulae
3. Stoichiometry  →  3.3. The mole and the Avogadro constant
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3
In 1985 the fullerenes were discovered. They are solid forms of the element carbon. The structure of the C60 fullerene is given below. In the C60 fullerene, how many other carbon atoms is each carbon atom bonded to? Another fullerene has a relative molecular mass of 840. How many carbon atoms are there in one molecule of this fullerene? Fullerenes are soluble in liquid hydrocarbons such as octane. The other solid forms of carbon are insoluble. Describe how you could obtain crystals of fullerenes from soot which is a mixture of fullerenes and other solid forms of carbon. A mixture of a fullerene and potassium is an excellent conductor of electricity. Which other form of solid carbon is a good conductor of electricity? Explain why metals, such as potassium, are good conductors of electricity. The mixture of fullerene and potassium has to be stored out of contact with air. There are substances in unpolluted air which will react with potassium. Name two potassium compounds which could be formed when potassium is exposed to air.
2. Atoms, elements and compounds  →  2.5. Simple molecules and covalent bonds
11. Organic chemistry  →  11.5. Alkenes
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4
A fuel cell produces electrical energy by the oxidation of a fuel by oxygen. The fuel is usually hydrogen but methane and methanol are two other fuels which may be used. A diagram of a hydrogen fuel cell is given below. cathode anode V e– e– H2 H2O O2 When the fuel is hydrogen, the only product is water. What additional product would be formed if methane was used? Write the equation for the chemical reaction that takes place in a hydrogen fuel cell. At which electrode does oxidation occur? Explain your choice. Write an ionic equation for the reaction at this electrode. Fuel cells are used to propel cars. Give two advantages of a fuel cell over a gasoline-fuelled engine.
4. Electrochemistry  →  4.2. Hydrogen–oxygen fuel cells
4. Electrochemistry  →  4.1. Electrolysis
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5
Sodium chlorate(decomposes to form sodium chloride and oxygen. The rate of this reaction is very slow at room temperature provided the sodium chlorate(is stored in a dark bottle to prevent exposure to light. 2NaCl O → 2NaCl + O2 The rate of this decomposition can be studied using the following experiment. sodium chlorate(solution oxygen collects in syringe Sodium chlorate(is placed in the fl ask and 0.2 g of copper(oxide is added. This catalyses the decomposition of the sodium chlorate(and the volume of oxygen collected is measured every minute. The results are plotted to give a graph of the type shown below. volume of oxygen time Explain why the gradient of this graph decreases with time. Cobalt(oxide is a more effi cient catalyst for this reaction than copper(oxide. Sketch, on the grid, the graph for the reaction catalysed by cobalt(oxide. All other conditions were kept constant. What can you deduce from the comment that sodium chlorate(has to be shielded from light? Explain, in terms of collisions between particles, why the initial gradient would be steeper if the experiment was repeated at a higher temperature. The ions present in aqueous sodium chloride are Na+, Cl –, H+and OH–. The electrolysis of concentrated aqueous sodium chloride forms three products. They are hydrogen, chlorine and sodium hydroxide. Explain how these three products are formed. Give ionic equations for the reactions at the electrodes. If the solution of the electrolyte is stirred, chlorine reacts with sodium hydroxide to form sodium chlorate(, sodium chloride and water. Write an equation for this reaction. Cl 2 + NaOH → + +
6. Chemical reactions  →  6.2. Rate of reaction
4. Electrochemistry  →  4.1. Electrolysis
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6
Rubidium and strontium are very reactive metals at the top of the reactivity series. Because their ions have different charges, their compounds behave differently when heated. The formulae of the ions of these two elements are Rb+ and Sr2+. Explain why these metals, which are in different groups, form ions which have different charges. Strontium carbonate is similar to calcium carbonate. It is insoluble in water and it decomposes when heated. Rubidium carbonate is soluble in water and does not decompose when heated. Describe a method to prepare a pure sample of the insoluble salt, strontium carbonate, by precipitation. Complete the equation for the decomposition of strontium carbonate. SrCO3 → + Metal nitrates decompose when heated. Rubidium nitrate decomposes as follows: 2RbNO3 → 2RbNO2 + O2 What is the name of the compound RbNO2? The nitrates of most other metals decompose in a different way. Complete the equation for the decomposition of strontium nitrate. Sr(NO3)2 → + 4NO2 +
12. Experimental techniques and chemical analysis  →  12.5. Identification of ions and gases
9. Metals  →  9.4. Reactivity series
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7
Butane is oxidised to a mixture of carboxylic acids by oxygen in the presence of a catalyst. The acids formed are methanoic acid, ethanoic acid and propanoic acid – the fi rst three members of the carboxylic acid homologous series. Give the name and structural formula of the fourth member of this series. name structural formula showing all the atoms and bonds State three characteristics of a homologous series. All members of this series are weak acids. What is meant by the term weak acid ? Carboxylic acids react with alcohols to form esters. Ethanol reacts with ethanoic acid to form the ester ethyl ethanoate, CH3COOCH2CH3. Give the name and formula of the ester which is formed from methanol and propanoic acid. name formula What is the name of the ester which has the formula CH3COOCH3? Complete the equation for the oxidation of butane to propanoic acid. 3C4H10 + O2 → 4C2H5COOH + H2O Name another compound which can be oxidised to propanoic acid.
11. Organic chemistry  →  11.7. Carboxylic acids
11. Organic chemistry  →  11.2. Naming organic compounds
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8
Describe how cobalt chloride paper can be used to test for the presence of water. Complete the description of the preparation of crystals of the soluble salt, cobalt(chloride-6-water, CoCl 2.6H2O, from the insoluble base, cobalt(carbonate. CoCO3+ 2HCl → CoCl 2+ CO2+ H2O50 cm3 of dilute hydrochloric acid, concentration 2.2 mol / dm3, was heated and cobalt(carbonate was added in small amounts until 6.31 g of cobalt(chloride-6-water crystals were obtained. Calculate the percentage yield to 1 decimal place. number of moles of HCl in 50 cm3 of acid, concentration 2.2 mol / dm3 = maximum number of moles of CoCl 2.6H2O which could be formed = mass of 1 mole of CoCl 2.6H2O = 238 g maximum yield of CoCl 2.6H2O = g percentage yield = %
10. Chemistry of the environment  →  10.1. Water
12. Experimental techniques and chemical analysis  →  12.5. Identification of ions and gases
7. Acids, bases and salts  →  7.3. Preparation of salts
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