0620_s10_qp_33
A paper of Chemistry, 0620
Questions:
8
Year:
2010
Paper:
3
Variant:
3
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The hydrolysis of complex carbohydrates to simple sugars is catalysed by enzymes called carbohydrases and also by dilute acids. They are both catalysts. How do enzymes differ from catalysts such as dilute acids? Explain why ethanol, C2H6O, is not a carbohydrate but glucose, C6H12O6, is a carbohydrate. Draw the structure of a complex carbohydrate, such as starch. The formula of a simple sugar can be represented by . HO OH For Examiner’s Use Iodine reacts with starch to form a deep blue colour. In the experiment illustrated below, samples are removed at intervals and tested with iodine in potassium iodide solution. pipette to remove samples hot water aqueous starch and a few drops of saliva (saliva contains enzymes) Typical results of this experiment are shown in the table. time / min colour of sample tested with iodine in potassium iodide solution deep blue pale blue colourless Explain these results. If the experiment was repeated at a higher temperature, 60 °C, all the samples stayed blue. Suggest an explanation.
11. Organic chemistry  →  11.7. Carboxylic acids
6. Chemical reactions  →  6.2. Rate of reaction
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For Examiner’s Use The following are examples of redox reactions. Bromine water was added to aqueous sodium sulfi de. Br2+ S2–→ 2Br –+ SDescribe what you would observe when this reaction occurs. Write a symbol equation for this reaction. Explain, in terms of electron transfer, why bromine is the oxidant (oxidising agent) in this reaction. Iron and steel in the presence of water and oxygen form rust. Fe2+ goes into solution oxygen dissolved in water electrons move in metal colourless gas forms The reactions involved are: reaction 1 Fe → Fe2+ + 2e– The electrons move through the iron on to the surface where a colourless gas forms. reaction 2 Fe2+ + 2OH– → Fe(OH)2 from water reaction 3 Fe(OH)2 + O2 + H2O → Fe(OH)3 The water evaporates to leave rust. For Examiner’s Use What type of reaction is reaction 1? Deduce the name of the colourless gas mentioned in reaction 1. What is the name of the iron compound formed in reaction 2? Balance the equation for reaction 3. Fe(OH)2 + O2 + H2O → Fe(OH)3 Explain why the change Fe(OH)2 to Fe(OH)3 is oxidation. Explain why iron in electrical contact with a piece of zinc does not rust.
6. Chemical reactions  →  6.4. Redox
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But-1-ene is a typical alkene. It has the structural formula shown below. CH3 CH2 CH2 CH The structural formula of cyclobutane is given below. C C C C H H H H H H H H These two hydrocarbons are isomers. Defi ne the term isomer. For Examiner’s Use Draw the structural formula of another isomer of but-1-ene. Describe a test which would distinguish between but-1-ene and cyclobutane. reagent result with but-1-ene result with cyclobutane Describe how alkenes, such as but-1-ene, can be made from alkanes. Name the product formed when but-1-ene reacts with: bromine, hydrogen, steam.
11. Organic chemistry  →  11.2. Naming organic compounds
11. Organic chemistry  →  11.5. Alkenes
11. Organic chemistry  →  11.1. Formulae, functional groups and terminology
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For Examiner’s Use Fuel cells are used in spacecraft to produce electrical energy. hydrogen oxygen anode electrolyte cathode How is oxygen obtained from liquid air? Hydrogen and oxygen react to form water. 2H2 + O2 → 2H2O Give an example of bond breaking in the above reaction. Give an example of bond forming in the above reaction. Is the change given in exothermic or endothermic? Give two reasons why hydrogen may be considered to be the ideal fuel for the future. Suggest a reason why hydrogen is not widely used at the moment.
4. Electrochemistry  →  4.2. Hydrogen–oxygen fuel cells
5. Chemical energetics  →  5.1. Exothermic and endothermic reactions
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For Examiner’s Use Thallium is a metal in Group III. It has oxidation states of +1 and +3. Give the formula for the following thallium compounds. thallium(sulfi de thallium(chloride Thallium(chloride is insoluble in water. Complete the description of the preparation of a pure sample of this salt. Step 1 Mix a solution of sodium chloride with thallium(sulfate solution. A white precipitate forms. Step 2 Step 3 Step 4 When thallium(chloride is exposed to light, a photochemical reaction occurs. It changes from a white solid to a violet solid. Name another metal halide which changes colour when exposed to light. Give the major use of this metal halide. name use For Examiner’s Use A piece of paper coated with thallium(chloride is exposed to a bright light. paper coated with thallium(chloride lamp Suggest two ways of increasing the time it takes for the violet colour to appear. Thallium(hydroxide is an alkali. It has similar properties to sodium hydroxide. Complete the following word equation. thallium(hydroxide ammonium sulfate + + + → Complete the equation. Tl OH + H2SO4 → + Aqueous thallium(hydroxide was added to aqueous iron(sulfate. Describe what you would see and complete the ionic equation for the reaction. observation equation Fe2+ + OH– →
12. Experimental techniques and chemical analysis  →  12.5. Identification of ions and gases
9. Metals  →  9.4. Reactivity series
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For Examiner’s Use Aluminium was fi rst isolated in 1827 using sodium. Al Cl 3 + 3Na → Al + 3NaCl Aluminium, obtained by this method, was more expensive than gold. Suggest an explanation why aluminium was so expensive. The modern method for extracting aluminium is the electrolysis of a molten electrolyte, aluminium oxide dissolved in cryolite. The aluminium oxide decomposes. 2Al 2O3 → 4Al + 3O2 Both electrodes are made of carbon. Give two reasons why the oxide is dissolved in cryolite. Complete the ionic equation for the reaction at the anode. O2– → O2 + e– Why do the carbon anodes need to be replaced frequently? The electrolysis of a molten electrolyte is one method of extracting a metal from its ore. Other methods are the electrolysis of an aqueous solution and the reduction of the oxide by carbon. Explain why these last two methods cannot be used to extract aluminium. electrolysis of an aqueous solution using carbon
9. Metals  →  9.6. Extraction of metals
9. Metals  →  9.4. Reactivity series
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For Examiner’s Use Nitrogen dioxide is a brown gas. It can be made by heating certain metal nitrates. 2Pb(NO3)2 → 2PbO + 4NO2 + O2 Name another metal whose nitrate decomposes to give the metal oxide, nitrogen dioxide and oxygen. Complete the word equation for a metal whose nitrate does not give nitrogen dioxide on decomposition. metal nitrate → + oxygen At most temperatures, samples of nitrogen dioxide are equilibrium mixtures. 2NO2N2O4dark brown pale yellow At 25 °C, the mixture contains 20 % of nitrogen dioxide. At 100 °C this has risen to 90 %. Is the forward reaction exothermic or endothermic? Give a reason for your choice. Explain why the colour of the equilibrium mixture becomes lighter when the pressure on the mixture is increased. For Examiner’s Use A 5.00 g sample of impure lead(nitrate was heated. The volume of oxygen formed was 0.16 dm3 measured at r.t.p. The impurities did not decompose. Calculate the percentage of lead(nitrate in the sample. 2Pb(NO3)2 → 2PbO + 4NO2 + O2 Number of moles of O2 formed = Number of moles of Pb(NO3)2 in the sample = Mass of one mole of Pb(NO3)2 = 331 g Mass of lead(nitrate in the sample = g Percentage of lead(nitrate in sample =
6. Chemical reactions  →  6.3. Reversible reactions and equilibrium
10. Chemistry of the environment  →  10.3. Air quality and climate
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