Practice Test Redox. Page 1

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1 1. What is the oxidation state of nitrogen in the compound NH4Br? (1) 1 (2) +2 (3) 3 (4) What is the oxidation number of sulfur in Na2S2O3? (1) 1 (2) +2 (3) +6 (4) During which process does an atom gain one or more electrons? (1) transmutation (2) reduction (3) oxidation (4) neutralization 4. Which half-reaction correctly represents reduction? (1) Mn 4+ Mn 3+ + e (2) Mn 4+ Mn e (3) Mn 4+ + e Mn 3+ (4) Mn e Mn Which change in oxidation number indicates oxidation? (1) 1 to +2 (2) 1 to 2 (1) Fe 2+ Fe 3+ + e (2) Fe e Fe (3) +2 to 3 (4) +3 to Which half-reaction equation represents the reduction of an iron(ii) ion? (1) +2 to 0 as electrons are transferred (2) +2 to 0 as protons are transferred (3) +3 to 0 as electrons are transferred (4) +3 to 0 as protons are transferred (3) Fe 3+ + e Fe 2+ (4) Fe Fe e 7. Given the balanced equation representing a reaction: Fe2O3 + 2Al Al2O3 + 2Fe During this reaction, the oxidation number of Fe changes from 8. Given the redox reaction: 11. Which metal reacts spontaneously with a solution containing zinc ions? (1) magnesium (2) nickel (3) copper (4) silver 12. A voltaic cell spontaneously converts chemical energy to (1) electrical energy (2) geothermal energy 13. Reduction occurs at the cathode in (3) mechanical energy (4) nuclear energy (1) electrolytic cells, only (2) voltaic cells, only (3) both electrolytic cells and voltaic cells (4) neither electrolytic cells nor voltaic cells 14. Which statement is true about oxidation and reduction in an electrochemical cell? (1) Both occur at the anode. (2) Both occur at the cathode. (3) Oxidation occurs at the anode and reduction occurs at the cathode. (4) Oxidation occurs at the cathode and reduction occurs at the anode. 15. Which statement describes one characteristic of an operating electrolytic cell? (1) It produces electrical energy. (2) It requires an external energy source. (3) It uses radioactive nuclides. (4) It undergoes a spontaneous redox reaction. 16. The diagram below shows a key being plated with copper in an electrolytic cell Cr 3+ + Al Cr + Al 3+ As the reaction takes place, there is a transfer of (1) electrons from Al to Cr 3+ (2) electrons from Cr 3+ to Al (3) protons from Al to Cr 3+ (4) protons from Cr 3+ to Al 9. In which reaction are electrons transferred from one reactant to another reactant? (1) 2Ca(s) + O2 (g) 2CaO(s) (2) AgNO3 (aq) + KCl(aq) AgCl(s) + KNO3 (aq) (3) HCl(aq) + NaOH(aq) NaCl(aq) + H2O( ) (4) H3O + (aq) + OH (aq) 2H2O( ) 10. Which reaction occurs spontaneously? (1) Cl2(g) + 2NaBr(aq) Br2( ) + 2NaCl(aq) (2) Cl2(g) + 2NaF(aq) F2(g) + 2NaCl(aq) (3) I2(s) + 2NaBr(aq) Br2( ) + 2NaI(aq) (4) I2(s) + 2NaF(aq) F2(g) + 2NaI(aq) Given the reduction reaction for this cell: Cu 2+ (aq) + 2e - Cu(s) This reduction occurs at (1) A, which is the anode (2) A, which is the cathode (3) B, which is the anode (4) B, which is the cathode Page 1

2 17. Base your answer to the following question on the information and equation below. Human blood contains dissolved carbonic acid, H2CO3, in equilibrium with carbon dioxide and water. The equilibrium system is shown below. H2CO3(aq) CO2(aq) + H2O( ) What is the oxidation number of carbon in H2CO3(aq)? 18. Which energy transformation occurs when an electrolytic cell is in operation? (1) chemical energy electrical energy (2) electrical energy chemical energy (3) light energy heat energy (4) light energy chemical energy Base your answers to questions 19 through 21 on the information below. The diagram below represents an operating voltaic cell at 298 K and 1.0 atmosphere in a laboratory investigation. The reaction occurring in the cell is represented by the balanced ionic equation below. 19. Write a balanced half-reaction equation for the reduction that occurs in this cell. 20. Determine the total number of moles of Ni 2+ (aq) ions produced when 4.0 moles of Ag + (aq) ions completely react in this cell 21. Identify the anode in this cell. Page 2

3 Base your answers to questions 22 through 24 on the information below. A voltaic cell with magnesium and copper electrodes is shown in the diagram below. The copper electrode has a mass of 15.0 grams. When the switch is closed, the reaction in the cell begins. The balanced ionic equation for the reaction in the cell is shown below the cell diagram. After several hours, the copper electrode is removed, rinsed with water, and dried. At this time, the mass of the copper electrode is greater than 15.0 grams. 22. Explain, in terms of copper ions and copper atoms, why the mass of the copper electrode increases as the cell operates. Your response must include information about both copper ions and copper atoms. 23. State the purpose of the salt bridge in this cell. 24. State the directions of electron flow through the wire between the electrodes when the switch is closed. Base your answers to questions 25 and 26 on the diagram of the voltaic cell below. 25. Based on the given equation, write the balanced half-reaction that occurs in half-cell 1. Page 3

4 26. When the switch is closed, state the direction that electrons will flow through the wire. Page 4

5 Answer Key #2 - REDOX or Ag + + e - Ag or 2Ag + + 2e - 2Ag mol. 21. Ni(s) or the nickle electrode 22. copper ions from the solution are reduced to copper atoms at the electrode, increasing the mass of the electrode; Copper ions become copper atoms; The number of copper ions decreases, and the number of copper atoms increases. 23. The salt bridge allows ions to flow between the half-cells; Preventing polarization. 24. The electrons flow from the Mg electrode to the Cu electrode; From anode to cathode. 25. Allow credit for Pb(s) Pb 2+ (aq) + 2e even if the labels (s) and (aq) are not included. 26. Acceptable responses: from Pb electrode to Ag electrode, left to right, cell 1 cell 2, Do not allow credit for a response that indicates that electrons flow through the salt bridge. Page 5