Chemical reactions and electrolysis

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1 Chemical reactions and electrolysis Higher Revision Questions Name: Class: Date: Time: 95 minutes Marks: 95 marks Comments: Page of 29

2 (a) Magnesium metal is shaped to make magnesium ribbon. Explain why metals can be shaped (2) (b) Magnesium sulfate is a salt of magnesium. It can be prepared by the reaction of magnesium metal with an acid. The equation for the reaction of magnesium with this acid is: Mg(s) + H 2 SO 4 (aq) MgSO 4 (aq) + H 2 (g) Name the acid used to make magnesium sulfate.... acid () Page 2 of 29

3 Use the equation to help you to describe what you would observe when magnesium reacts with the acid. (2) (iii) The magnesium sulfate is in solution. How could you obtain solid magnesium sulfate from this solution? () (Total 6 marks) 2 Neutralisation reactions can be used to make salts. (a) Write an ionic equation for a neutralisation reaction, including state symbols.... (2) (b) Ammonium nitrate is a salt used as a fertiliser. Ammonium nitrate is made by mixing two solutions. Name these solutions.... and... () Page 3 of 29

4 Hazard information about ammonium nitrate states: it is not itself a fire hazard (does not burn); it must not be allowed to come into contact with combustible materials such as fuels because it can cause these to catch fire. Suggest why ammonium nitrate helps other substances to burn () (Total 4 marks) 3 Explain, in terms of ions and molecules, what happens when any acid reacts with any alkali (Total 3 marks) 4 Bordeaux Mixture controls some fungal infections on plants. A student wanted to make some Bordeaux Mixture. Page 4 of 29

5 (a) The student knew that calcium oxide could be made by heating limestone. Limestone contains calcium carbonate, CaCO 3. Write the word equation for this reaction.... () What type of reaction is this?... () (b) The student knew that copper sulphate, CuSO 4, could be made by the following general reaction. acid + base salt + water What type of reaction is this?... () The base used is copper oxide. Name and give the chemical formula of the acid used. Name. Chemical formula... (2) (c) The student wrote about how the copper sulphate was made. Some of the acid was warmed. Copper oxide was added. The mixture was stirred. More copper oxide was added until no more would react. The mixture was then filtered. Why was the acid warmed? () Copper oxide was added until no more would react. Explain why (2) Page 5 of 29

6 (iii) The filtration apparatus is shown. Describe and explain what happens as the mixture is filtered (2) (Total 0 marks) 5 A student tried to make some magnesium sulphate. Excess magnesium was added to dilute sulphuric acid. During this reaction fizzing was observed due to the production of a gas. Complete and balance the chemical equation for this reaction H 2 SO (3) At the end of the reaction the solution remaining was filtered. Why was the solution filtered?... () Page 6 of 29

7 (iii) The filtered solution was left in a warm place. Explain why the filtered solution was left in a warm place (2) (Total 6 marks) 6 The diagram shows one way of producing iron. Iron oxide reacts with aluminium to produce iron. The symbol equation for the reaction is: Fe 2 O Al 2 Fe + Al 2 O 3 (a) Complete the word equation for this reaction. iron oxide + aluminium iron +... () The magnesium ribbon is lit to start the reaction. Why does the burning magnesium ribbon start the reaction? () Page 7 of 29

8 (b) In industry, iron is produced in the blast furnace when iron oxide is heated with carbon. The iron from the blast furnace is called cast iron. Cast iron contains carbon. The diagrams show the structure of pure iron and cast iron. Pure iron Cast iron Use the diagrams to help you answer the questions. Draw a ring around the correct answer to complete the sentence. Pure iron is an element because pure iron contains only one sort of atom. is magnetic. is a metal. () Suggest why cast iron is harder than pure iron. (2) (c) Aluminium is extracted by electrolysis using the ionic compound aluminium oxide. Molten aluminium Page 8 of 29

9 Aluminium cannot be extracted by heating aluminium oxide with carbon. Suggest why. () Why is aluminium oxide dissolved in molten cryolite? () (iii) Aluminium metal is produced at the negative electrode (cathode). Complete the half equation for the process. Al e Al () (iv) Use the half equation to state why Al 3+ ions are reduced. () (v) Explain why the positive electrodes (anodes) burn away. Use your knowledge of the products of electrolysis to help you. (4) (Total 3 marks) Page 9 of 29

10 7 This question is about iron and aluminium. (a) Iron is extracted in a blast furnace. Figure is a diagram of a blast furnace. Calcium carbonate decomposes at high temperatures. Complete the word equation for the decomposition of calcium carbonate. calcium carbonate (2) Carbon burns to produce carbon dioxide. The carbon dioxide produced reacts with more carbon to produce carbon monoxide. Balance the equation. C(s) + CO 2 (g)... CO(g) () Page 0 of 29

11 (iii) Carbon monoxide reduces iron(iii) oxide: Fe 2 O 3 (s) + 3 CO(g) 2 Fe(s) + 3 CO 2 (g) Calculate the maximum mass of iron that can be produced from 300 tonnes of iron(iii) oxide. Relative atomic masses (A r ): O = 6; Fe = 56 Maximum mass =... tonnes (3) (b) Aluminium is extracted by electrolysis, as shown in Figure 2. Figure 2 Why can aluminium not be extracted by heating aluminium oxide with carbon? () Page of 29

12 Explain why aluminium forms at the negative electrode during electrolysis. (3) (iii) Explain how carbon dioxide forms at the positive electrodes during electrolysis. (3) (Total 3 marks) 8 This question is about potassium. (a) Humphrey Davy was a professor of chemistry. In 807 Davy did an electrolysis experiment to produce potassium. Davy first tried to electrolyse a solid potassium salt to produce potassium. Explain why this electrolysis did not work. (2) Page 2 of 29

13 Humphrey Davy was the first person to produce potassium. Humphrey Davy s experiment to produce this new element was quickly accepted by other scientists. Suggest why. () (b) A student dissolved some potassium chloride in water. The student tried to electrolyse the potassium chloride solution to produce potassium. The apparatus the student used is shown in the diagram. The student expected to see potassium metal at the negative electrode, but instead saw bubbles of a gas. Name the gas produced at the negative electrode. Explain why this gas was produced at the negative electrode and why potassium was not produced. The reactivity series of metals on the Chemistry Data Sheet may help you to answer this question (3) Page 3 of 29

14 (c) The student tried to electrolyse molten potassium chloride to produce potassium. Potassium metal was produced at the negative electrode. Describe how potassium atoms are formed from potassium ions. (2) Complete and balance the equation for the reaction at the positive electrode....cl Cl () (iii) Complete the diagram to show the electronic structure of a chloride ion (Cl ). () (Total 0 marks) Page 4 of 29

15 9 This question is about magnesium and magnesium chloride. (a) Magnesium chloride contains magnesium ions (Mg 2+ ) and chloride ions (Cl ). Describe, in terms of electrons, what happens when a magnesium atom reacts with chlorine atoms to produce magnesium chloride (4) (b) Magnesium chloride can be electrolysed. The diagram below shows two experiments for electrolysing magnesium chloride. Explain why magnesium chloride must be molten or dissolved in water to be electrolysed (2) Page 5 of 29

16 Explain how magnesium is produced at the negative electrode in Experiment (3) (iii) In Experiment 2 a gas is produced at the negative electrode. Name the gas produced at the negative electrode.... () (iv) Suggest why magnesium is not produced at the negative electrode in Experiment () (v) Complete and balance the half equation for the reaction at the positive electrode.... Cl Cl () (c) Magnesium is a metal. Explain why metals can be bent and shaped (2) (Total 4 marks) Page 6 of 29

17 0 The electrolysis of sodium chloride solution is an important industrial process. Three useful substances are produced: chlorine gas is formed at the positive electrode; hydrogen gas is formed at the negative electrode; an alkali is left in the solution. The reactions which take place at the electrodes are represented by the equations shown below: 2Cl 2e Cl 2 2H + + 2e H 2 (a) Name the important alkali which is left in the solution.... () (b) State why chloride ions move towards the positive electrode.... () (c) Why is the formation of chlorine at this electrode said to be an oxidation reaction?... () (Total 3 marks) Sando-K is a medicine. It is given to people whose bodies contain too little of a particular element. Sando-K is a mixture of two compounds. The formulae of the two compounds are given below. KHCO 3 KC (a) Which metal do people given Sando-K need?... () (b) Sando-K contains the ion, CO 3 2. Which gas would be produced if a dilute acid was added to Sando-K? (The Data Sheet may help you to answer this question.)... () Page 7 of 29

18 (c) The compounds in Sando-K contain ions. Complete the two sentences below. Atoms change into positive ions by... one or more.... Atoms change into negative ions by... one or more.... (4) (d) Electricity can be used to show that an aqueous solution of Sando-K contains ions. Draw a diagram of an apparatus that you could use to prove that Sando-K contains ions. (4) Explain, as fully as you can, what would happen when the electricity is switched on (3) (Total 3 marks) Page 8 of 29

19 Mark schemes (a) made of layers / rows (atoms / ions / particles) ignore free / delocalised electrons which can slide / slip (over each other) or reference to incorrect particles / covalency / intermolecular forces = max particles / ions / atoms can slide over each other ignore malleable / ductile / weak bonds (b) sulfuric accept sulphuric ignore formula ignore hydrogen sulfate any two from: list principle applies for incorrect observations (hydrogen) gas produced (or any indication of a gas such as bubbles etc.) ignore just hydrogen produced ignore cloudiness / colour changes magnesium / solid disappears / goes into solution accept magnesium / magnesium sulfate / solid / it dissolves accept forms a liquid / solution gets hot allow exothermic ignore floats 2 Page 9 of 29

20 (iii) crystallisation accept detailed answers such as: evaporate to half volume and then allow the solution to crystallise. or evaporation / heating / boiling / cooling ignore any references to filter [6] 2 (a) H + (aq) + OH (aq) H 2 O(l) or H 3 O + (aq) + OH (aq) H 2 O(l) mark for correct equation mark for state symbols any other symbols = 0 marks accept correct spectator ions e.g. Na + (aq) + OH (aq) + H + (aq) + Cl (aq) Na + (aq) + Cl (aq) + H 2 O(l) (b) nitric acid and ammonia (solution) HNO 3 NH 3 / NH 4 OH mark for both accept ammonium hydroxide / NH 4 OH instead of ammonia do not accept ammonia hydroxide do not accept hydrogen nitrate solution accept correct formulae 2 provides oxygen or oxidising (agent) or oxidant do not accept it contains oxygen alone or rich in oxygen [4] Page 20 of 29

21 accept CaO 3 CaO + CO 2 3 hydrogen ions (from acid) or protons / H + react with hydroxide ions (from alkali) / OH to produce water H + OH H 2 O gains all 3 marks ignore state symbols molecules of hydrogen ions and molecules of hydroxide ions produce water = 2 marks if they fail to get any of the above marks they can get mark for neutralisation / product neutral [3] 4 (a) calcium carbonate calcium oxide + carbon dioxide (thermal) decomposition accept endothermic accept reversible (b) neutralisation accept exothermic sulphuric (acid) H 2 SO 4 2 (c) to speed up the reaction accept to increase the rate of reaction or to increase the number or rate of collisions do not accept dissolves copper oxide faster Page 2 of 29

22 all acid reacts accept there will be no acid left or acid used up acid is neutralised (for 2 marks) do not accept to form a concentrated or saturated solution (excess) copper oxide collects in filter paper accept larger particles (of copper oxide) cannot pass through filter paper copper sulphate solution passes through the filter paper accept dissolved copper sulphate passes through filter paper or smaller particles (of copper sulphate) in solution (liquid) pass through filter paper accept (black) solid collects in filter paper and filtrate or soluble solid or (blue) solution (liquid) passes through filter paper for mark only [0] 5 Mg + (H 2 SO 4 ) MgSO4 + H 2 deduct mark if not balanced only if all three correct accept alternative metal of similar reactivity for example Zn or Fe candidate would not then be awarded first mark for Mg then error carried forward deduct mark if not balanced only if all three correct to remove the (excess) magnesium accept separate accept insoluble substances or solids or residue do not accept unreactive substances or impurities or remove magnesium from sulphuric acid Page 22 of 29

23 do not accept to leave MgSO 4 (iii) to evaporate (some of the water or solution) to form crystals or crystallise accept to form a saturated solution or concentrated solution [6] 6 (a) aluminium oxide ignore (III) after aluminium (because it provides) heat / energy (to overcome activation energy) (b) contains only one sort of atom the atoms (in cast iron) are different sizes any mention of molecules, maximum mark accept layers are distorted or structure is disrupted which prevents the layers / rows sliding accept an answer in terms of pure iron being softer than cast iron for both marks (c) because aluminium is more reactive than carbon or it = aluminium must be a comparison between the elements because aluminium is above carbon in the reactivity series do not accept any comparison of the reactivity of aluminium and iron reduces / lowers the temperature for the process or lowers the operating temperature or allows ions to move ignore any temperature values allow reduces the (effective) melting point (of Al 2 O 3 ) (iii) 3 accept multiples Page 23 of 29

24 accept 2O 2 O 2 + 4e (iv) electrons are gained (by Al 3+ ) ignore any numbers ignore any reference to oxygen (v) electrodes are made of carbon allow graphite / coke oxygen is produced (at the positive electrode / anode) so the electrodes react with the oxygen / are oxidised producing carbon dioxide (gas) accept C + O 2 CO 2 for marking points 3 and 4. [3] 7 (a) calcium oxide in either order carbon dioxide accept correct formulae (iii) C(s) + CO 2 (g) 2CO(g) allow multiples 20 (tonnes) award 3 marks for the correct answer with or without working allow ecf for arithmetical errors if answer incorrect allow up to 2 marks for any of the steps below: / or moles Fe 2 O 3 =.875 ( 0 6 ) or 300 / 60 moles of Fe = 3.75 ( 0 6 ) or 2 moles Fe 2 O 3 mass Fe = moles Fe (tonnes) scores 2 (missing :2 ratio) 420 (tonnes) scores 2 taken M r of iron as 2 3 Page 24 of 29

25 (b) aluminium is more reactive than carbon or carbon is less reactive than aluminium must have a comparison of reactivity of carbon and aluminium accept comparison of position in reactivity series. (iii) (because) aluminium ions are positive ignore aluminium is positive and are attracted / move / go to the negative electrode / cathode where they gain electrons / are reduced / Al e Al accept equation or statements involving the wrong number of electrons. (because) the anodes or (positive) electrodes are made of carbon / graphite oxygen is produced (at anode) which reacts with the electrodes / anodes do not accept any reference to the anodes reacting with oxygen from the air equation C + O 2 CO 2 gains mark (M3) [3] 8 (a) current / charge couldn t flow allow could not conduct (electricity) because the ions / particles couldn t move do not accept electrons/ molecules / atoms or (salt) needs to be molten / () dissolved (to conduct electricity) so that the ions / particles can move () do not accept electrons / molecules / atoms he had status accept he had authority or experience or he had evidence / proof accept the experiment could be repeated Page 25 of 29

26 (b) hydrogen / H 2 the ions are positive do not allow hydrogen ions accept because opposite (charges) attract potassium is more reactive (than hydrogen) (c) gain electron(s) accept potassium ions are less easily discharged (than hydrogen) or potassium ions are less easily reduced (than hydrogen) accept fully balanced correct equation for 2 marks one electron if no other marks awarded allow (potassium ions) reduced for mark 2 Cl Cl 2 + 2e (iii) 2, 8, 8 must be completely correct, including charge on electron accept correct multiples accept any combination of dots, crosses, e or any other relevant symbol ignore any charges if given [0] 9 (a) magnesium loses two electrons and chlorine gains one electron accept magnesium loses electrons and chlorine gains electrons for mark ignore oxidation and reduction one magnesium and two chlorines 2 accept MgCl 2 Page 26 of 29

27 noble gas structure or eight electrons in the outer shell accept full outer shell (of electrons) or (electrostatic) attraction between ions or forms ionic bonds do not accept covalent bonds reference to incorrect particles or incorrect bonding or incorrect structure = max 3 (b) because ions can move ignore ions attracted do not accept molecules / atoms moving do not accept incorrect reference to electrons moving (and ions move) to the electrodes or (iii) (and ions) carry charge accept converse for solid magnesium (ions) attracted (to the electrode) so magnesium ions gain electrons accept magnesium ions are reduced ignore oxidised 2 electrons accept a correct half equation for 2 nd and 3 rd marking points hydrogen allow H 2 Page 27 of 29

28 (iv) magnesium is more reactive than hydrogen accept converse allow magnesium is high in the reactivity series or magnesium is very/too reactive. do not accept magnesium ions are more reactive than hydrogen ions (v) 2 Cl - Cl 2 + 2e - must be completely correct (c) layers (of particles/atoms/ions) (particles/atoms/ions/layers) can slide any mention of intermolecular / weak bonds/forces = max [4] 0 (a) sodium hydroxide / caustic soda / NAOH for mark (b) negative ions move to the positive electrode etc. /because it is negative /opposite charges attract for mark (c) loss of electrons for mark [3] (a) potassium / K for mark (b) carbon dioxide / CO 2 for mark (c) losing electrons gaining electrons for mark each 4 Page 28 of 29

29 (d) power supply, (not mains) beaker containing solution, (inert) electrodes and circuit ammeter or bulb/ (or see bubbling etc. at electrodes written by drawing) for mark each 4 reading on ammeter/bulb lights / (solution) conducts (electricity) bubbling / gas produced hydrogen produced chlorine / oxygen produced ions move to electrodes (must be linked to ions move) negative ions move to the positive electrode and/or positive ions move to the negative electrode negative ions lose electrons and/or positive ions gain electrons any 3 for mark each 3 [3] Page 29 of 29