22 4.6 Oxidation-Reduction Reactions

Practice questions

1. 1. Is this statement true? The following reaction is a redox reaction: [latex]\ce{2K{(s)} + Br_{2}{(l)} {\rightarrow} 2KBr {(s)}}[/latex]
   2. In the reaction [latex]\ce{Ca{(s)} + O_{2}{(g)} {\rightarrow} 2CaO}[/latex], indicate which element/compound has lost electrons and what element/compound has gained electrons.
   3. In the reaction [latex]\ce{2Li{(s)} + O_{2} {(g)} {\rightarrow} Li_{2}O_{2}{(s)}}[/latex], indicate which element/compound has been oxidised and which element/compound has been reduced.
   4. What is oxidation? Select all applicable answers.
      
      1. 1. The loss of electrons
         2. The gaining of electrons
         3. An increase in the oxidation number
         4. A decrease in the oxidation number
   5. What is reduction? Select all applicable answers.
      
      1. 1. Increase in oxidation number
         2. Decrease in the oxidation number
         3. Gain of electrons
         4. Loss of electrons
   6. Assign oxidation numbers to each atom in each substance:
      1. 1. [latex]\ce{P}[/latex] in [latex]\ce{P_{4}}[/latex]
         2. [latex]\ce{S}[/latex] and [latex]\ce{O}[/latex] in [latex]\ce{SO_{2}}[/latex]
         3. [latex]\ce{S}[/latex] and [latex]\ce{O}[/latex] in [latex]\ce{SO_{2}^{2-}}[/latex]
         4. [latex]\ce{Ca}[/latex], [latex]\ce{N}[/latex] and [latex]\ce{O}[/latex] in [latex]\ce{Ca{(NO_{3})}_{2}}[/latex]
   7. Assign oxidation numbers to each atom in each substance:
      1. 1. [latex]\ce{C}[/latex] and [latex]\ce{O}[/latex] in [latex]\ce{CO}[/latex]
         2. [latex]\ce{C}[/latex] and [latex]\ce{O}[/latex] in [latex]\ce{CO_{2}}[/latex]
         3. [latex]\ce{Ni}[/latex] and [latex]\ce{Cl}[/latex] in [latex]\ce{NiCl_{2}}[/latex]
         4. [latex]\ce{Ni}[/latex] and [latex]\ce{Cl}[/latex] in [latex]\ce{NiCl_{3}}[/latex]
   8. Identify what is being oxidised and reduced in the redox equation [latex]\ce{2NO + Cl_{2} {\rightarrow} 2NOCl}[/latex] by assigning oxidation numbers to the atoms. [latex]\ce{N}[/latex] is being ……………, and  [latex]\ce{Cl}[/latex] is being ………… .
   9. Identify what is being oxidised and reduced in the redox equation [latex]\ce{2KrF_{2} + 2H_{2}O {\rightarrow} 2Kr + 4HF + O_{2}}[/latex] by assigning oxidation numbers to the atoms. [latex]\ce{O}[/latex] is being ……………, and [latex]\ce{Kr}[/latex] is being ………… .
   10. Identify the oxidising and reducing agents in the following reactions:
       1. 1. [latex]\ce{Cu{(s)} + Pt^{2+}{(aq)} {\rightarrow} Cu^{2+}{(aq)} + Pt{(s)}}[/latex]
          2. [latex]\ce{2Mg{(s)} + CO_{2}{(g)} {\rightarrow} 2MgO{(s)} + C{(s)}}[/latex]
   11. Which of the following is the correct balanced net ionic equation for the reaction shown below? [latex]\ce{Cr_{2}O_{7}^{2-} + Fe^{2+} {\rightarrow} Cr^{3+} +  Fe^{3+}}[/latex]
       1. 1. [latex]\ce{14H^{+} + Cr_{2}O_{7}^{2-} + Fe^{2+} {\rightarrow} 2Cr^{3+} + 7H_{2}O + Fe^{3+}}[/latex]
          2. [latex]\ce{14H^{+} + Cr_{2}O_{7}^{2-} + 6Fe^{2+} {\rightarrow} 2Cr^{3+} + 14H_{2}O + 6Fe^{3+}}[/latex]
          3. [latex]\ce{14H^{+} + Cr_{2}O_{7}^{2-} + 6Fe^{2+} {\rightarrow} 2Cr^{3+} + 7H_{2}O + 6Fe^{3+}}[/latex]
          4. [latex]\ce{14H^{+} + Cr_{2}O_{7}^{2-} + 6Fe^{2+} {\rightarrow} 2Cr^{3+} + 7H_{2}O + 6Fe^{3+}}[/latex]
   12. Which of the following is the correct balanced net ionic equation for the reaction shown below? [latex]\ce{Pb{(NO_{3})}_{2}{(aq)} + 2KI{(aq)} {\rightarrow} 2KNO_{3}{(aq)} + PbI_{2}{(s)}}[/latex]
       1. 1. [latex]\ce{Pb^{2+}{(aq)} + I^{-}{(aq)} {\rightarrow} PbI_{2}{(s)}}[/latex]
          2. [latex]\ce{Pb^{2+}{(aq)} + 2NO_{3}^{-}{(aq)} + 2K^{+} + 2I^{-}{(aq)} {\rightarrow} 2K^{+} {(aq)} + 2NO_{3}^{-} {(aq)} + PbI_{2}{(s)}}[/latex]
          3. [latex]\ce{Pb^{2+}{(aq)} + 2I^{-}{(aq)} {\rightarrow} PbI_{2}{(s)}}[/latex]
          4. [latex]\ce{Pb{(NO_{3})}_{2} {(aq)} + 2KI{(aq)} {\rightarrow} 2KNO_{3}{(aq)} + PbI_{2}}[/latex]

Answers

1. 1. 1. 1. True
         2. Lost electrons –[latex]\ce{Ca}[/latex] and gained electrons – [latex]\ce{O_{2}}[/latex]
         3. Oxidised – [latex]\ce{Li}[/latex], Reduced – [latex]\ce{O_{2}}[/latex]
         4. a and c
         5. b and c
         6. Oxidation numbers are as follows:
            1. 1. [latex]\ce{P}[/latex] = 0
               2. [latex]\ce{S}[/latex] = +4 and [latex]\ce{O}[/latex] = -2
               3. [latex]\ce{S}[/latex] = +2 and [latex]\ce{O}[/latex] = -2
               4. [latex]\ce{Ca}[/latex] =+2, [latex]\ce{N}[/latex] = +5, and [latex]\ce{O}[/latex] = -2
         7. Oxidation numbers are as follows:
            1. 1. [latex]\ce{C}[/latex] = +2 and [latex]\ce{O}[/latex] = -2
               2. [latex]\ce{C}[/latex] = +4 and [latex]\ce{O}[/latex] = -2
               3. [latex]\ce{Ni}[/latex] = +2 and [latex]\ce{Cl}[/latex] = -1
               4. [latex]\ce{Ni}[/latex] = +3 and [latex]\ce{Cl}[/latex] = -1
         8. Oxidised, reduced
         9. Oxidised, reduced
         10. Reducing and oxidising agents are as follows:
             1. 1. Reducing agent [latex]\ce{Cu}[/latex] and oxidising agent [latex]\ce{Pt^{2+}}[/latex]
                2. Reducing agent [latex]\ce{Mg}[/latex] and oxidising agent [latex]\ce{CO_{2}}[/latex]
         11. d
         12. c