4. Electrochemistry
A section of Chemistry, 0620
Listing 10 of 500 questions
Lead(bromide can be electrolysed using the apparatus shown below. heat + – graphite rod ceramic crucible lead(bromide graphite rod Choose one word from the list below which describes the graphite rods. Put a ring around the correct answer. cations electrodes electrons insulators metals State the name of the products formed during this electrolysis at the negative graphite rod. the positive graphite rod. Which of the following conduct electricity? Tick two boxes. ceramic crucible graphite rod molten lead(bromide solid lead(bromide The structure of lead(bromide is shown below. Br – Br – Br – Br – Pb2+ Pb2+ Pb2+ Pb2+ Br – Br – Br – Br – Br – Br – Br – Br – Pb2+ Pb2+ Write the simplest formula for lead(bromide. Lead(bromide is formed as a precipitate when aqueous solutions of lead(nitrate and potassium bromide are mixed. What do you understand by the term precipitate ? The formula of lead(nitrate is Pb(NO3)2. State the number of different types of atom present in this formula. State the total number of oxygen atoms present in this formula. Lead compounds are pollutants in the air. State one harmful effect of lead compounds on health.
0620_w10_qp_21
THEORY
2010
Paper 2, Variant 1
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The diagram shows a cell. This is a device which produces electrical energy. The reaction in a cell is a redox reaction and involves electron transfer. filter paper soaked in sodium chloride solution to complete the circuit solution of bromine Br2solution of iron(salt Fe2+inert electrode inert electrode V + – Complete the sentence. A cell will change energy into electrical energy. Draw an arrow on the diagram to show the direction of the electron fl ow. In the left hand beaker, the colour changes from brown to colourless. Complete the equation for the reaction. Br2 + → Is the change in oxidation or reduction? Give a reason for your choice. Complete the following description of the reaction in the right hand beaker. Fe2+ changes into . When a solution of bromine is replaced by a solution of chlorine, the voltage increases. When a solution of bromine is replaced by a solution of iodine, the voltage decreases. Suggest an explanation for this difference.
0620_w10_qp_31
THEORY
2010
Paper 3, Variant 1
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Chromium is a transition element. Predict two differences in the physical properties of chromium and sodium. Predict two differences in the chemical properties of chromium and sodium. Chromium is used to electroplate steel objects. The diagram shows how this could be done. lead anode + – object to be plated chromium(sulfateadd more chromium(sulfateGive two reasons why steel objects are plated with chromium. The formula of the chromium(ion is Cr3+ and of the sulfate ion is SO4 2–. Give the formula of chromium(sulfate. Write the equation for the reaction at the negative electrode . A colourless gas, which relights a glowing splint, is formed at the positive electrode . Name this gas. During electrolysis, it is necessary to add more chromium(sulfate but during copper-plating using a copper anode, it is not necessary to add more copper(sulfate. Explain.
0620_w10_qp_32
THEORY
2010
Paper 3, Variant 2
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The electrolysis of concentrated aqueous sodium chloride, between inert electrodes, is used to make four important chemicals. hydrogen chlorine sodium hydroxide sodium chlorate(The ions present in the electrolyte are Na+, H+, Cl – and OH –. Hydrogen ions are discharged at the negative electrode . Write an equation for this reaction. The hydrogen ions are from the water. H2O H+ + OH – Suggest an explanation why the concentration of hydroxide ions increases. When a dilute solution of sodium chloride is used, chlorine is not formed at the positive electrode , a different gas is produced. Name this gas. State an example of an inert electrode. State a use of hydrogen. Why is chlorine used to treat the water supply? Sodium chlorate(is made by the reaction between chlorine and sodium hydroxide. It is used as bleach but over time it decomposes. 2NaCl O→ 2NaCl + O2The rate of decomposition can be studied using the apparatus shown below. sodium chlorate(solution How could you measure the rate of decomposition of sodium chlorate(? Describe how you could show that the rate of decomposition of sodium chlorate(is a photochemical reaction.
0620_w10_qp_33
THEORY
2010
Paper 3, Variant 3
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Lead and lead compounds are common pollutants of the air. State one source of lead in the air. State one effect of lead on human health. Lead(oxide can be reduced by heating with carbon. heat PbO + C → Pb + CO Write a word equation for this reaction. Explain how you know that lead(oxide is reduced in this reaction. Explain why this reaction is described as endothermic. Lead nitrate solution reacts with sodium iodide solution. lead nitrate + sodium iodide → lead iodide + sodium nitrate Lead iodide is insoluble in water but the reactants and sodium nitrate are soluble. Draw a labelled diagram to explain how you can separate lead iodide from the rest of the reaction mixture. Complete the table below to show the number of protons, electrons and neutrons in the isotope of lead . number of protons number of electrons number of neutrons 204Pb
0620_w12_qp_23
THEORY
2012
Paper 2, Variant 3
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Lead is an excellent roofi ng material. It is malleable and resistant to corrosion. Lead rapidly becomes coated with basic lead carbonate which protects it from further corrosion. Lead has a typical metallic structure which is a lattice of lead ions surrounded by a ‘sea’ of mobile electrons. This structure is held together by attractive forces called a metallic bond. Explain why there are attractive forces in a metallic structure. Explain why a metal, such as lead, is malleable. Basic lead(carbonate is heated in the apparatus shown below. Water and carbon dioxide are produced. soda lime, carbon dioxide reacts here basic lead carbonate heat U-tube filled with silica gel to absorb water Silica gel absorbs water. Silica gel often contains anhydrous cobalt(chloride. When this absorbs water it changes from blue to pink. Suggest a reason. Soda lime is a mixture of sodium hydroxide and calcium oxide. Why do these two substances react with carbon dioxide? Name two substances formed when soda lime reacts with carbon dioxide. Basic lead(carbonate has a formula of the type xPbCO3.yPb(OH)2 where x and y are whole numbers. Determine x and y from the following information. PbCO3  PbO + CO2 Pb(OH)2  PbO + H2O When heated, the basic lead(carbonate gave 2.112 g of carbon dioxide and 0.432 g of water. Mass of one mole of CO2 = 44 g Mass of one mole of H2O = 18 g Number of moles of CO2 formed = Number of moles of H2O formed = x = and y = Formula of basic lead(carbonate is
0620_w13_qp_31
THEORY
2013
Paper 3, Variant 1
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Questions Discovered
500