Electrochemistry Unit Review - Answer Key

Electrochemistry: Unit Review 1. What is the oxidation number of chlorine in HClO4? 1. –1 2. +1 3. +7 4. +8 2. What is the oxidation number of carbon in C2H6? 1. –6 2. –3 3. +3 4. +6 3. A battery is an example of: 1. An electrochemical cell 2. A electrolytic cell 4. Given the reaction: Mg + CuSO4  MgSO4 + Cu that takes place? 1. Mg+2 + 2e-  Mg 2. Mg  Mg+2 + 2e3. Cu+2 + 2e-  Cu 4. Cu  Cu +2 + 2e- 3. A pressure cell Which equation represents the oxidation 5. In the following reaction: Fe + Zn+2  Fe+2 + Zn A. Fe is reduced and Zn+2 is the oxidizing agent B. Fe is reduced and Zn+2 is the reducing agent C. Fe is oxidized and Zn+2 is the oxidizing agent D. Fe is oxidized and Zn+2 is the reducing agent 6. In which species does oxygen have an oxidation number of –1? A. H2O 2. Na2O2 3. Al2O3 4. O2 7. Elements that readily gain electrons tend to have A. High ionization energies and high electronegativities B. High ionization energies and low electronegativities C. Low ionization energies and high electronegativities D. Low ionization energies and low electronegativities 8. In an electrochemical cell, the cathode is 1.neutral 2. negative 3. positive 9. What is the purpose of a salt bridge in an electrochemical cell? 1.To act as a reducing agent 2. To act as an oxidizing agent 3. To provide electrons that flow between the two half cells and complete the electrical circuit 4. To provide ions that flow between the two half cells and complete the electrical circuit 10. What is NOT the same for an electrochemical cell and an electrolytic cell? A. An oxidation/reduction chemical reaction occurs within them B. They have two electrodes: a cathode and an anode C. They produce voltage and an electric current D. Electrons flow through an external circuit 11. Given the equations A, B, C, and D; which two equations represent redox reactions? A. AgNO3 + NaCl  AgCl + NaNO3 B. Cl2 + H2O  HClO + HCl C. CuO + CO  CO2 + Cu D. NaOH + HCl  NaCl + H2O 1. 12. A and B 2. B and C 3. C and A Given the unbalanced equation: ___ NO3- + 4H+ + ___ Ag  ____Ag+ + ____NO + 2H2O What is the coefficient in front of Ag when the equation is correctly balanced? (Where coefficients are given, these are correct and should not be changed) 1. 1 2. 2 3. 3 4. 4 _____________________________________________________________________ Use the diagram of the electrochemical cell below to answer the next four questions Pb(s ) Cu(s) Pb+2(aq) Cu+2(aq) Pb(s) + Cu+2(aq)  Pb+2(aq) + Cu(s) 13. When the switch is closed which electrode will become heavier? A. Pb B. Cu C. Both Pb and Cu will increase in mass D. Neither Pb nor Cu will increase in mass 14. Which direction will electrons flow? A. From Pb through the voltmeter to Cu B. From Cu through the voltmeter to Pb C. From Pb through the salt bridge to Cu D. From Cu through the salt bridge to Pb 15. 4. D and B Which metal ion will increase in concentration as the cell runs 1. Cu+2 only 2. Pb+2 only 3. Both Cu+2 and Pb+2 4. Neither Cu+2 nor Pb+2 16. What will increase the voltage and current from this cell? A. Increasing the level of the half cell liquids B. Increasing the length of the external circuit C. Increasing the length of the salt bridge D. Increasing the concentration of the electrode solutions 17. Electroplating is an example of what type of chemical cell? A. Electrolytic cell B. Electrochemical cell C. Voltaic cell D. Galvannic cell 18. In the following reaction, how many total electrons are transferred between chlorine and oxygen? 2 ClO2  Cl2 + 2O2 1. 1 19. 2. 2 3. 8 4. 4 In the following reaction what is the reducing agent? Mg + 2HNO3 -> Mg(NO3)2 + H2 1. Mg 2. H+ 3. O-2 4. N+5 20. In a chemical reaction what is conserved? 1. Charge 2. Mass 3. Neither charge and mass 4. Both charge and mass _________________________________________________________________ Answer the next four questions from the reaction below: Given the unbalanced reaction: _____Au3+(aq) + ____I-(aq) Au(s) + ____I2(s) 21. Write the reduction half reaction _______________________________________________________________________ 22. Write the oxidation half reaction _______________________________________________________________________ 23. Write the balanced reaction _______________________________________________________________________ 24. How many total electrons are transferred when the reaction is balanced _________ ___________________________________________________________________________________ The diagram below represents an electrochemical cell for the reaction: Mg(s) + Zn(NO3)2(aq)  Mg(NO3)2 + Zn(s) Mg(s) 25. Label the cathode 26. Label the anode 27. Write the half reaction which occurs at the cathode __________________________________________________________ 28. Write the half reaction which occurs at the anode 29. 30. 31. Show the direction of movement of Mg+2 ions on the diagram above Show the direction of Zn+2 ions ion the diagram above Label and show the direction of movement of electrons Zn(s) Mg+2(aq) Zn+2(aq) 32. A. The diagram at right shows a cell for silver plating jewelry, adding a thin coat of silver on top of a cheaper metal such as steel. What is the name of this type of cell? ___________________________ There are two electrodes in the diagram, the silver electrode and the copper wire that holds the object to be plated. Label correctly in the diagram with anode or cathode. Write the half reaction next to each electrode. In what two ways does this differ from the other type of cell you have studied? B. C. D. _________ ___________ Honors: 33. For the cell at right: A. Write the line notation, the shorthand, for the process which is occurring ___________________________ B. Determine the standard cell voltage _______ (use Old Table N from packet) C. What would be the voltage of this cell if the anode and cathode reactions were reversed? _____ D. Explain what would be necessary to make the reactions in # 3 above to occur Fe(s) Ca(s) Fe+2(aq) ____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Answers to Electrochemistry: Practice Questions 1. 7. 13. 19. 3 1 2 1 2. 8. 14. 20. 2 3 1 4 3. 1 9. 4 15. 2 4. 2 10. 3 16. 4 5. 3 11. 2 17. 1 6. 2 12. 3 18. 3 Given the unbalanced reaction: _____Au3+(aq) + ____I-(aq) Au(s) + ___I2(s) 21. Write the reduction half reaction Au+3 + 3 e-  Au(s) => 2Au+3 + 6e-  2Au(s) 22. Write the oxidation half reaction: 2I-  I2 (s) + 2e=> 6I-  3I2 (s) + 6e23. Write the balanced reaction 2Au+3 + 6I-  3I2 (s) + 2Au(s) 24. How many total electrons are transferred when the reaction is balanced ? 6 electrons (Not 12 electrons! The 6 electrons lost by 6I- are the same ones gained by 2Au+3– don’t double count them) Ca+2(aq) 31 25 29,30 cathode The diagram below represents an electrochemical cell for the reaction: Mg(s) Mg(s) + Zn(NO3)2(aq)  Mg(NO3)2 + Zn(s) Zn(s) anode 26 Mg+2(aq) Mg(s)  Mg+2 + 2e-1 Zn+2(aq) Zn+2(aq) + 2e-1Zn(s) 28 27 32. A. electrolytic cell D. requires power, requires electrical energy, the cathode and anode are all in same cell/compartment, does have a salt bridge anode cathode Ag(s)  Ag+(aq) + e-1 Ag+(aq) + e-1 Ag(s) 33. A. Ca/Ca+2//Fe+2/Fe B. Determine the standard cell voltage _______ (use Old Table N from packet) Ca  Ca+2 + 2e- = - (-2.87) V (flipped reaction since lower, will be the anode reaction) Fe+2 + 2 e-  Fe = -0.45 V (keep reaction as is, since higher, will be the cathode reaction) Total voltage = 2.87 V + (-0.45 V) = 2.42 V at standard conditions C. What would be the voltage of this cell if the anode and cathode reactions were reversed? If reversed, the reactions and voltage would flip and so total voltage = -2.42 V D. Explain what would be necessary to make the reactions in # 3 above to occur With a negative sign, the reaction is NOT spontaneous and would require the addition of an external power source such as used in an electrolytic cell.