Chemistry-Chemical Reactions, Cells and batteries, Energy, Electroysis

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PART 1-Energy from Chemical Reactions SEND Skills questions and past exam questions Complete the following Close Activity with suitable terms. a. During a chemical reaction energy changes occur as bonds are broken or 1 made. The energy level of a chemical system is often expressed by a . the heat content or ________________. quantity called b. A change in energy level or enthalpy of a chemical system is measured as the _______________ of reaction (symbol __________). H = energy level of _______________ energy level of reactants. c. Heat of reaction is measured in units of ____________ or kilojoules per mole and the value of H can be positive or _______________. d. In exothermic reactions the energy of the reactants is _____________than the energy of the products: H is _____________, heat is released, the temperature of the surroundings rises. e. In endothermic reactions the energy of the reactants is _____________than the energy of the products: H is _____________, heat is absorbed, the temperature of the surroundings drops. f. The energy change for a reaction can be represented by an ___________ profile diagram. In order for the reaction to proceed, a minimum amount of energy called the __________________ energy is required. g. A catalyst ____________ the activation energy needed for a reaction. h. consider the equation C (s) + O2 (g) CO2 (g) in which H = -393.5 kJmol-1 The H value for the reaction 2C (S) +2O2(g) ___________________ kJ mol-1. The H value for the reaction CO2 (g) ___________________ kJmol-1. 2CO2 (g) will be C(s) +O2(g) will be 2. Explain the following observations in terms of the collision model for particle behaviour. (a) Surfboard manufacturers find that fibreglass plastics set within hours in summer but may remain tacky for days in winter. ________________________________________________________ ________________________________________________________ ________________________________________________________ (b) A bottle of fine aluminium powder has a caution sticker warning that it is ‘highly flammable, dust explosion possible’. ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ (c) A potato cooks much more slowly in a billy of boiling water on a trekking holiday in Nepal than a potato boiled in a similar way in the Australian bush. Hint: at high altitudes pressure is lower so water boils at a lower temperature than at sea level. ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ Past Exam Questions: The following questions include multiple-choice and two extended written questions from past exam papers. The estimated time allowable for this group of questions in an exam is approximately 18 to 20 minutes. To help you develop an awareness of the time it takes to answer exam questions, we would like you to time yourself in this next group of questions. Please don’t rush your answers. It’s important that you understand the questions while you are still learning the topic. Note down the time you start and finish: Time started: Time finished: Multiple choice questions (1 mark each) 1. Hydrogen iodide (HI) is formed from the reaction of the elements hydrogen and iodine H2(g) + I2(g) → 2HI(g); ΔH = +52 kJ mol-1 When two moles of HI decompose A. 52 kJ of energy is released. B. 52 kJ of energy is absorbed. C. 104 kJ of energy is released. D. 104 kJ of energy is absorbed. 2. Which one of the following will not increase the rate of formation of hydrogen from the reaction of magnesium with hydrochloric acid? A. Add sodium chloride solution to the magnesium/acid mixture. B. Increase the temperature of the magnesium/acid mixture. C. Increase the concentration of the hydrochloric acid in contact with the magnesium. D. Use the metal in the form of finely powdered magnesium instead of as magnesium ribbon. 3. 4. The reaction A + B → C;ΔH negative involves a two-step process A + B → X;ΔH positive and X → C;ΔH negative Which one of the following diagrams best represents the energy changes during the course of the reaction? The following information is referred to in Question 4 The reaction 2SO2(g) + O2(g) 2SO3(g); ΔH = –197 kJ mol–1 is an important step in the industrial production of sulfuric acid when it occurs in the presence of a solid vanadium pentoxide catalyst. Which one of the following energy profiles best illustrates the energy change of the reaction both in the absence (solid lines) and presence (dotted lines) of a catalyst. 5. The best description of the effect of a catalyst on a chemical reaction is that it A.lowers the activation energy of the forward reaction without changing the activation energy of the reverse reaction. B. lowers the activation energy of the forward reaction and raises the activation energy of the reverse reaction. C. lowers the activation energy of both forward and reverse reactions by the same amount. D. lowers the activation energy of the reverse reaction without changing the activation energy of the forward reaction. Short-answer questions from past exams (marks shown at the end of each question) 6. a. The graph on the right shows the kinetic energy profile of particles at two different temperatures, 40C and 60C. Indicate the temperatures represented by graph A and graph B. Graph A:______________ Graph B:_______________ b. Draw a graph of number of particles versus kinetic energy that shows the effect of a catalyst on a reaction. Energy profiles at 400C and 600C c. Use the diagram you have drawn in part b to explain in terms of collision theory how a catalyst increases the rate of a reaction. (Total marks 2+4+3= 9) _______________________________________________________________ _______________________________________________________________ _______________________________________________________________ _______________________________________________________________ ___________ 7. A student was asked to design an experiment to investigate the effect of temperature using rhubarb stalks and potassium permanganate solution. In the experiment on the next page, rhubarbstalks, which contain oxalic acid, are used to reduce and decolourisepotassium manganate(VII) solution. The student proposed the following experimental design. The aim of the investigation is to determine the effect of the temperature on the rate of reaction between oxalic acid and potassium permanganate. Rhubarb contains oxalic acid (ethanedioic acid) which has the formula C2H2O4: The equation of the reaction is 2MnO4-(aq) + 5 H2C2O4(aq) + 6 H+(aq) H2O(l) 10 CO2(g) + 2 Mn2+(aq) + Students made up twosolutions as follows. SOLUTION A made up of: SOLUTION B made up of 100 ml of distilled water, Cut the stalks of rhubarb into thin slices (about 0.5 cm) and put them into the 250 mL beaker. Boil with distilled water and leave the rhubarb mixture to cool. When cool enough to handle, filter or strain the mixture and keep the filtrate (liquid). 50 ml of 0.02M potassium manganate and 50 mls of 2M sulphuric acid. Procedure: • • Pour solution A into a burette. Run 20 mls of solution A from the burette into a boiling tube. • • • • • • • • • Add 5 mls of solution B into a test tube. Heat solutions A and B in a water bath to a known temperature. Remove solution A from the water bath. Put the thermometer into solution A. Add 5mls of solution B to solution A. Record the temperature of the solution. Record the time taken for the purple colour to become colourless. Repeat at various temperatures .e.g room temperature, 30oC, 40oC, 50oC, 60oC, etc. Critically evaluate the student’s proposal. i. Will the experimental design enable a valid conclusion to be made about the effect to temperature on rate of reaction? Provide reasons for your answer. (2 marks) __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ _______________ ii. What changes, if any, should be made to improve the experimental design? Justify your suggestion.(3 marks) __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ _______________ Optional questions: 8. An energy profile is shown in the diagram for the following reaction. N2 (g) + 3H2 (g) → 2NH3 (g) a. Give the value and sign of the ∆H of the forward reaction. . ________________________________________________________________ . b. Give the value and sign of the activation energy of the forward reaction. _______________________________________________________________ c. Give the value and sign of the ∆H of the reverse reaction. _______________________________________________________________ d. Give the value and ( sign of the activation energy of the reverse reaction. d _______________________________________________________________ ) e. Catalyst A is removed ( and is replaced with catalyst B, a more effective catalyst that makes e the reaction go faster. ) Sketch on the blank graph below possible form of the energy profile for the same reaction in the presence of catalyst B. 1+1+1+1+3 = 7 marks 9. The graph below represents the energy changes over the course of a chemical reaction CO(g) + NO2(g) → CO2(g) + NO(g) a. Give the magnitude and sign of the ΔH for the forward reaction in kJ mol-1. (1 mark) ______________________________________________________________ _ b. Give the activation energy for the reverse reaction in kJ mol-1 .(1 mark) ______________________________________________________________ _ c. Give two reasons explaining why the rate of this reaction increases with increasing temperature.(2 marks) ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ____________ d. A suitable catalyst is discovered for the reaction. What would be the likely effect of the catalyst on i. the activation energy? Explain your answer. _________________________________________________________ _________________________________________________________ _________________________________________________________ _________________________________________________________ _________________________________________________________ _______________ ii. the ΔH? Explain your answer. _________________________________________________________ _________________________________________________________ _________________________________________________________ _________________________________________________________ _________________________________________________________ _______________ 1 + 1 = 2 marks Total 6 marks PART 2-Electricity from Chemical Reactions SEND Skills questions and past exam questions 1. Construct a passage by matching the beginning of each sentence in the table below with the correct ending. Write the sentences in logical order, using paragraphs if necessary and write a heading. Oxidation occurs at electrical energy Reactions in galvanic cells are the cathode The anode is spontaneous Galvanic cells convert chemical energy Connecting wire between the two half to cells An electric current can be produced by the anode Reduction occurs at redox reactions Electrons always go to salt bridge Redox reactions involve negative in a galvanic cell The cathode is separating the reaction into two half –cells. Electrical neutrality is maintained by the positive in a galvanic cell Reactions in galvanic cells are the cathode The external circuit is the the transfer of electrons ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ____________________________________ 2. A half-cell is constructed with a copper electrode in a 1.0 mol L−1 copper(II) sulfate solution. It is connected by a salt bridge to another half-cell containing an aluminium electrode in a 1.0 mol L−1 aluminium nitrate solution. Source: StudyOn Chemistry 2, Jacaranda (a) A voltmeter is connected into the external circuit. What would be the expected Eo value for this cell? _________________________________________________________ (b) Give two changes that might be observed in the copper half-cell if a large current flowed for many hours. _______________________________________________________________ _________________________________________________________________ (c) What is the polarity of the copper electrode? ________________________________________________________________ (d) Which arrow, a or b, shows the direction of negative ions in the salt bridge? _________________________________________________________________ (e) Write a balanced half-equation for the reaction occurring at the anode. _________________________________________________________________ 3. a. An electrochemical cell is created using gold and magnesium half-cells. (Au3+ + 3e- Au(s) +1.68V] i. Determine which half-cell will undergo oxidation and which will undergo reduction, identify anode and cathode, and calculate the voltage for the cell. You do not need to diagram the cell. ( 4 marks) Cathode:____________________________________________________ Anode:______________________________________________________ ii. If the mass of the magnesium electrode changes by 5.0 g, what will be the change in mass of the gold electrode, and will its mass increase or decrease? (2 marks) b. An alkaline cell that is used to provide electricity has electrodes which can be represented by the following half-equations. The zinc electrode is the negative electrode. Zn2+ (aq)+ 2e– Zn(s) E = –0.76 V – – MnO2 (aq)+ H2O(l) + e MnO(OH)(aq) + OH (aq) E is to be determined i. The e.m.f. of this cell is 1.60V. Calculate a value for the electrode potential of the manganese oxide electrode. ii. Deduce the oxidation state of manganese in MnO(OH). _____________________________________________________________ iii. Write an equation for the overall cell reaction. iv. Identify the oxidising agent and the reducing agent in this alkaline cell. _______________________________________________________________ _______________________________________________________________ v. Give one reason why the e.m.f. of the cell decreases to a very low value after the cell has been used for a long time. (2+2+2+2+2 =10 marks) ________________________________________________________________ ________________________________________________________________ Past Exam Questions: The following questions contain five multiple-choice questions from past exam papers. The estimated time allowable for this group of questions in an exam is approximately 5 to 6 minutes. To help you develop an awareness of the time it takes to answer exam questions, we would like you to time yourself in this next group of questions. Please don’t rush your answers. It’s important that you understand the questions while you are still learning the topic. Note down the time you start and finish: Time started: Multiple choice questions (1 mark each) Question 1 Half cell I II III Electrode Metal A Platinum Metal C Time finished: electrolyte A2+(aq) B2+(aq)and B3+aq) C+(aq) • When a galvanic cell is constructed from half cell I and half cell II, the electrode in half cell II is negative. • When a galvanic cell is constructed from half cell II and half cell III, the electrode in half cell III is negative. The strongest oxidant is a. A2+(aq) b. B2+(aq) c. B3+(aq) d. . C+(aq) Question 2 Questions 2 and 3 refer to the following information. A galvanic cell is constructed from the following half cells, at 25°C. half cell 1 half cell 2 electrode silver copper Half-cell solution (all concentration 1.0M) Colourless solution of AgNO3 Blue-coloured of CuCl2 The half cells are connected with a salt bridge and the electrodes are joined by a wire. Which one of the following is likely to occur? A. B. C. D. The copper electrode will increase in mass. Bubbles of gas will form at the copper electrode. The concentration of silver ions in solution will increase. The blue colour of the copper (II) chloride solution will become more intense. Question 3 When the current is flowing A. B. C. D. The anode is positive and the cathode is negative An oxidation reaction occurs at the positive electrode. Anions in the salt bridge move towards the negative electrode. Electrons travel in the external circuit from the cathode to the anode. Question 4 The following reactions occur spontaneously as written. 2Cr2+(aq) + Co2+(aq) →2Cr3+(aq) + Co(s) Co(s) + Pb2+(aq) →Co2+(aq) + Pb(s) Fe(s) + 2Cr3+(aq) →Fe2+(aq) + 2Cr2+(aq) Using this information, predict which one of the following pairs of reactants will react spontaneously. A. Co(s) + Fe2+(aq) B. Cr2+(aq) + Fe2+(aq) 2+ 2+ C. Cr (aq) + Pb (aq) D. Pb(s) + Co2+(aq) Question 5 Four half cells are constructed as follows. Half cell I: an electrode of metal P in a 1.0 M solution of P+(aq) ions Half cell II: an electrode of metal Q in a 1.0 M solution of Q+(aq) ions Half cell III: an electrode of metal R in a 1.0 M solution of R+(aq) ions Half cell IV: an electrode of Cu(s) metal in a 1.0 M solution of Cu2+(aq) ions The half cells are connected in pairs, as shown below, to form a series of galvanic cells For each cell, the polarity of the electrodes and the voltage generated are recorded. Half cell used Positive electrode Negative electrode Voltage (V) I and IV P Cu 0.46 II and IV Cu Q 0.57 III and IV Cu R 1.10 II and III Q R 0.53 Which one of the following alternatives lists the metals in order of increasing strength as reductants? A. R, Q, Cu, P B. Cu, P, Q, R C. P, Cu, R, Q D. P, Cu, Q, R Question 6 A galvanic cell is constructed from the following two half cells under standard conditions. Half cell 1: a nickel electrode in a solution of 1.0 M nickel nitrate Half cell 2: a cadmium electrode in a solution of 1.0 M cadmium nitrate A sketch of the cell is given below. i. Given that the standard reduction potential of Cd2+(aq)/ Cd(s) is –0.40V, show on the above sketch the direction in which electrons will flow in the external circuit of this galvanic cell. ii. Give the equation for the half reaction that takes place at the anode of this cell. iii. List two factors that need to be considered when selecting an appropriate substance for use in the salt bridge. ________________________________________________________________________ ________________________________________________________________________ ______ 1+1+2 = 4 marks PART 3-Electrochemical Cells and Batteries SEND Skills questions and past exam questions 1. Summarise the features, uses and give examples of these cells in the correct section of the Venn diagram. 2. When a Ni–Cad cell is converting chemical energy to electrical energy (discharge), the electrode reactions are best described as follows: positive electrode: NiOOH(s) + H2O(l) + e–→ Ni(OH)2(s) + OH–(aq) negative electrode: Cd(s) + 2OH–(aq) → Cd(OH)2(s) + 2e– a. Which metal, nickel or cadmium, is at the anode of this cell as it is discharging? b.Give the formula of a salt that might be expected to be found in the electrolyte paste of a Ni–Cad cell. c Ni–Cad cells are superior to alkaline cells as they can be recharged. Explain why cells such as the Ni–Cad can be recharged. _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ ____________ d. Write equations for the half reactions and the overall cell reaction occurring when a Ni– Cad cell is recharged. e. Describe the energy transformation that occurs when a Ni–Cad cell is recharged. 3,The methanol fuel cell generates electricity by consuming methanol and oxygen. The equations for the reactions occurring at the electrodes are: CH3OH (l) + H2O(l) → CO2(g) + 6H+(aq) + 6e– Equation 1 O2(g) + 4H+(aq) + 4e–→2H2O(l) Equation 2 a.Which equation represents the reaction occurring at the anode? b.Write an equation that represents the overall cell reaction. Past Exam Questions: The following questions contain three multiple-choice and one short-answer question from past exam papers. The estimated time allowable for this group of questions in an exam is approximately 12 minutes. To help you develop an awareness of the time it takes to answer exam questions, we would like you to time yourself in this next group of questions. Please don’t rush your answers. It’s important that you understand the questions while you are still learning the topic. Note down the time you start and finish: Time started: Multiple choice questions (1 mark each) Use the following information to answer Questions 1 and 2. Time finished: Fuel cells are electrochemical cells that convert chemical energy into electrical energy. The diagram below shows an alkaline hydrogen-oxygen fuel cell. Question 1 The alternative that gives the correct polarity of electrodes 1 and 2 and the name of reactants X and Y is Electrode 1 Electrode 2 Reactant X A. positive negative O2(g) B. positive negative H2(g) C. negative positive O2(g) D. negative positive H2(g) Reactant Y H2(g) O2(g) H2(g) O2(g) Question 2 The reactions at the anode and cathode are respectively Anode reaction Cathode reaction A. H2 (g) → 2H+(aq) + 2e– O2 (g) + 4H+(aq) + 4e–→ 2H2O (l) B. O2 (g) + 4H+(aq) + 4e–→ 2H2O (l) H2 (g) → 2H+(aq) + 2e– C. O2 (g) + 2H2O (l) + 4e–→ 4OH– (aq) H2 (g) + 2OH– (aq) → 2H2O(l) + 2e– D. H2 (g) + 2OH– (aq) → 2H2O(l) + 2e– O2 (g) + 2H2O (l) + 4e–→ 4OH– (aq) Question 3 Common fuels are substances that, under the right conditions, can undergo A. an exothermic oxidation process. B. an endothermic oxidation process. C. an exothermic reduction process. D. an endothermic reduction process. Question 4 The rechargeable nickel-cadmium cell is used to power small appliances such as portable computers. When the cell is being used, the electrode reactions are represented by the following equations. NiO2(s) + 2H2O(l) + 2e- → Ni(OH)2(s) + 2OH- (aq) Cd(s) + 2OH- (aq) → Cd(OH)2(s) + 2eWhich of the following occurs during the recharging of the nickel-cadmium cell? I. cadmium is deposited on the negative electrode II. the pH of the electrolyte increases III. the direction of electron flow in the external circuit is from the anode to the cathode A.I only B.I and II only C.II and III only D.I and III only Question 5 A fuel cell currently under development for powering small electronic devices is based on the reaction of methanol and oxygen using an acidic electrolyte. The reductant in the cell reaction and the half reaction at the anode are reductant A. methanol B. oxygen C. methanol D. oxygen anode reaction O2(g) + 4H+ (aq) + 4e-→ 2H2O(l) O2(g) + 4H+(aq) + 4e-→ 2H2O(l) CH3OH(g) + H2O(l) → CO2(g) + 6H+(aq) + 6eCH3OH(g) + H2O(l) → CO2(g) + 6H+(aq) + 6e- Short-answer question An Internet site reporting the latest developments in fuel cell technology describes a cell that uses a solid ceramic material as the electrolyte and hydrogen gas and oxygen gas as the reactants. Key features of this cell are • water is the only product from the cell reaction • the ceramic material allows the movement of oxide ions (O-2) • the reaction at the anode is H2(g) + O-2.(in ceramic) → H2O(l) + 2e-. • operation at very high temperatures of over 1000°C means that precious metal catalysts are not required. A representation of the cell providing electricity for an appliance is shown in the diagram below. (a) What distinguishes a fuel cell from a galvanic cell such as a dry cell or lead-acid battery? (1 mark) (b) On the diagram above (c) i. in circles A and B, indicate the polarity of the cathode and anode ii. show, by using an arrow, the direction of electron flow in the external circuit.(1+1 = 2 mark) Write an equation for each of the following reactions. You are not required to i. show states in these two equations. The overall cell reaction. ii The reaction at the cathode ( 1+1=2 marks) (b) Write a balanced overall equation of the cell reaction. (c) Briefly describe two important functions of the electrodes in this cell. Total = 5 marks (suggested time 7 minutes) PART 4 SEND Application Task 1: Determine Avogadro’s number from electrolysis using secondhand data Source: Heinemann Chemistry 2 Teacher’s resource book Avogadro’s number is an important chemical constant. There are a number of ways to determine its value by experiment. One of the more common methods is to use electrolysis. When a concentrated and slightly alkaline solution of sodium chloride is electrolysed using copper electrodes, copper metal is oxidised to Cu+ ions at the anode. Source: http://dl.clackamas.cc.or.us/ch10509/electrol1.htm The Cu+ ions react with OH– ions in the solution to form copper (I) oxide, Cu2O, which precipitates out of the solution. The mass of copper metal consumed can be determined by comparing the mass of the anode before and after the electrolysis. The Data Suppose you performed T the electrolysis described above, and recorded the following data. h • The initial mass of the copper anode was 186.38 g. e • The current flowed through the electrolytic cell for 10.0 minutes at an average of 2.27 amps.D a • The final mass of the anode after electrolysis was185.49 g. t Use this data to answer a the following questions. 1. Write an equationQfor the reaction occurring at the anode. 1 . 2. Write an equationQfor the precipitation of Cu+ ions as copper (I) oxide (Cu2O). 2 . 3. Calculate the quantity of electric charge that passed through the circuit during the electrolysis. 4. Given that the charge on an electron is 1.60  10–19C, calculate the number of electrons that passed through the external circuit of the cell. 5.Use the decrease in mass of the anode to calculate the amount of copper metal, in mol, consumed at that electrode during the electrolysis. 6. Use your answers to Questions 1, 3 and 5 to determine a value for Avogadro’s number. 7. Identify potential sources of error in the experiment. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ SEND Application Task 2: Fuel cells and Faraday’s laws Source: Heinemann Chemistry 2 Teacher’s resource book http://news.bbc.co.uk/2/low/uk_news/wales/41 54316.stm The alcohol concentration in a motorist’s breath can be determined using a fuel cell. In the cell, oxygen gas from the air is reduced to water, and ethanol in the breath is oxidised to ethanoic acid. The electrolyte is a solution of phosphoric acid, H3PO4, and a platinum/solver/plastic interface is used for each electrode. The voltage produced by the cell is proportional to the ethanol concentration. When the cell is calibrated, it can determine the amount of ethanol in the breath. Light-emitting diodes on the instrument show green for zero ethanol in the breath, amber for a small amount of ethanol, and red when above the limit. 1.Write half-cell equations for the reaction at each electrode Anode:__________________________________________________________________ Cathode:_________________________________________________________________ 2. Correctly label the diagram for this fuel cell, including the anode, cathode and electrolyte. − + Anode or cathode? Anode or cathode? Electrolyte Inlet for O2 (air) Inlet for breath Reaction products 3. Other current and future uses of fuel cells involve satellites, power stations and cars. An aluminium-air fuel cell has been proposed for electric cars using aluminium metal as the fuel and oxygen from the air as the oxidant. Potassium hydroxide solution is used as the electrolyte to produce a voltage of 1.00 V. (Remember an oxidant ‘causes oxidation’. In a redox reaction, the oxidant is reduced (gains electrons). a. Using the electrochemical series on page 418 of your textbook, write the anode and cathode reactions for this fuel cell. Anode: ________________________________________________________________ Cathode: _______________________________________________________________ b. Assuming a car was designed to carry 50.0 kg of aluminium, calculate the energy released (in kJ) by the cell before the car had to refuel. c. Assuming that the fuel cells are 80% efficient, how much of this energy could be used to move the car? d. List the energy changes that occur in a car fuelled by petrol and in an electric car powered by a fuel cell. ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______ e. List two advantages and two disadvantages of using fuel cells to power cars. ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ _____ SEND Electroplating cells- Analysis of second hand data Source: Heinemann Chemistry 2 work book Procedure: Four students investigated the principles of electrolysis by measuring the mass of metal deposited when electricity is passed through different electroplating cells. In an electroplating cell a layer of the metal is deposited on the surface of the cathode. The anode is usually made of the metal to be deposited and the electrolyte contains ions of the metal being electroplated. The results of the students are given in Tables below: Results for silver electroplating cell using AgNO3 (aq) electrolysis Experiment no Current(A) Time (s) Increase in Charge(C) mass of Q= I ×t cathode (g) 1 1.97 200 0.45 2 2.05 400 0.90 3 2.00 600 1.34 4 1.83 800 1.57 Amount (mol) Note: Relative atomic mass of silver is 107.9 g mol-1 Results for copper electroplating cell using Cu(NO3)2 (aq) electrolysis Experiment no Current(A) Time (s) 1 3.56 200 Increase in mass of cathode (g) 0.23 2 2.15 400 0.30 3 2.45 600 0.48 4 3.06 800 0.80 Charge(C) Q= I ×t Amount (mol) Note: Relative atomic mass of copper 63.54 g mol-1 Results for Chromium electroplating cell using Cr(NO3)3 (aq) electrolysis Experiment no Current(A) Time (s) 1 2 3 4 3.51 2.10 2.50 2.58 200 400 600 800 Increase in Charge(C) mass of Q= I ×t cathode (g) 0.11 0.17 0.26 0.39 Amount (mol) Note: Relative atomic mass of chromium is 52.00 g mol-1 Results for silver electroplating cell using Sn(NO3)2 (aq) electrolysis Experiment no Current(A) Time (s) 1 2.50 200 Increase in mass of cathode (g) 0.32 2 2.67 400 0.67 Charge(C) Q= I ×t Amount (mol) 3 2.65 600 1.15 4 2.34 800 1.17 Note: Relative atomic mass of tin is 118.7 g mol-1 Questions 1. Draw a diagram for Q a copper plating cell. Labelling the cathode, anode, polarity of electrodes. Direction 1 of electron flow in wires, direction of movement of the ions in the electrolyte and the. metal used for the anode. Q for the reaction occurring in each of the four cells. Write half- equations 2 Electroplating cell. Anode reaction Cathode reaction Silver copper Chromium Tin 3. As the reactions proceed, what happens to the concentration of the metal ions in the solution in each cell? ________________________________________________________________________ Q 3 . 4. Calculate the charge, Q, in each cell using the formula Q= it. Record your results in thetables above. Q 4 . Q 5 . 5. Draw and label graphs of the mass of metal deposited against charge for each of the four cells on the grid provided. Q 6 . 6. What is the relationship between the mass deposited and the charge for a particular cell? _______________________________________________________________________ Q 7 . 7. Draw and label graphs of the amount of metal deposited, in mol. against charge for each of the four cells. 8. a.Use the graphs you drew in Question 7 to find the charge, in C required to deposit 0.010 mol of each metal. Complete the table. Electroplating Charge on cell metal ion in electrolyte From half equations, amount (in mol) of electrons needed to deposit one mole of metal Charge (C) to deposit0.010mol of metal (from graph) Charge (c)to deposit one mole of metal b. Allowing for experimental error, what is the relationship between the change on the metal ion present in the electrolyte in a cell and the charge, in coulombs’, needed to deposit one mole of metal? ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ________ 9. Use your answers to questions 8 to deduce the charge of one mole of electrons, in coulomb. (The charge of one mole of electrons is called a Faraday) 10. The charge on an electron is 1.602 ×10-19C. Calculate the values of Avogadro’s number using the value of a Faraday you determine in Question 9. SEND Skills questions 1 1.Do Review question 27 on page 461 of the textbook. . 2. An electrochemical cell contains a mixture of metal ions in solution: 1M Ni(NO3)2, 1M Ag NO3, and 1 M Cu(NO3)2 The mixture is electrolysed using inert electrodes. a. Identify the reactants present and write down the possible half-cell reactions, including their E values. (Assume NO3−(aq) does not take part in the reaction). b. Identify the reductant from the reactions in part (a). Identify the oxidant that will react first. c. Identify the oxidant that will react second and third. d. Describe what will happen at the cathode if all oxidants are reacted. 2. 2. A lead-acid battery is made up of six cells connected in series. When the battery is providing energy, the reactions occurring at the electrodes of a single cell are Pb(s) + SO42– (aq) → PbSO4(s) + 2e– PbO2(s) + SO42– (aq) + 4H+(aq) + 2e–→ PbSO4(s) + 2H2O(l) a. (i) Give an equation for the net reaction that occurs while a lead-acid battery is providing energy. (ii) Give the formula of the oxidant and the formula of the reductant in the above reaction. Oxidant ……………………………………..... Reductant ……………………………………. b. A particular lead-acid battery delivers a current of 3.50 A for 2.00 minutes at a potential difference of 12.0 V. (1) What amount, in mole, of PbO2 will be used up within the whole battery of six cells? (ii) What mass, in gram, of PbSO4 will be deposited in each one of the six cells? (iii) How much energy, in joule, could be obtained in this situation? 4. The simplified diagram below shows the Hall Cell that is used for the industrial production of aluminium. (a) In circles X and Y in the diagram, show the polarity of the electrodes (b) In the Hall Cell, the electrolyte consists of alumina mixed with another compound. In box Z, write the name or the formula of this compound. (c).Write a half equation for the reaction that occurs at the i. Anode ii.Cathode d. Suppose the electrolyte was replaced with an aqueous solution of Al(NO3)3 at 25°C. Write an equation for the half reaction that would occur at the cathode.
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Sitejabber
4.4

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