Reactivity 3.2 - Electron Transfer Reactions (redox)

• What happens when electrons are transferred in a chemical reaction?

  Oxidation and reduction occur, involving changes in oxidation states.
• What are the definitions of oxidation and reduction?

  • Oxidation: Gain of oxygen or loss of hydrogen or loss of electrons or increase in oxidation state.
  • Reduction: Loss of oxygen or gain of hydrogen or gain of electrons or decrease in oxidation state.
• What are the oxidized and reduced species in a chemical reaction?


  The oxidized species loses electrons, while the reduced species gains electrons.
• What is the role of oxidizing agents in a chemical reaction?

  Oxidizing agents cause other substances to be oxidized and are reduced in the process.
• What is the role of reducing agents in a chemical reaction?


  Reducing agents cause other substances to be reduced and are oxidized in the process.
• How can oxidation states help classify oxidation and reduction in reactions?


  Oxidation states indicate whether an element has lost or gained electrons.
• What is the oxidation state of each atom in a pure element?


  Zero
• What is the oxidation state of an atom in a monatomic ion?


  The charge on the ion.
• What is the oxidation state of oxygen in most compounds?


  • 2
• What is the oxidation state of hydrogen in most compounds?


  +1
• What happens to the oxidation state of an element when it is oxidized?


  It increases.
• What happens to the oxidation state of an element when it is reduced?


  It decreases.
• How are variable oxidation states indicated in naming compounds?


  By showing the oxidation number as a Roman numeral attached to the name of the element.
• Write the balanced chemical equation for the reaction between zinc and sulfur to form zinc sulfide.

  Zn + S → ZnS
• What is the KOHES method used for in redox reactions?


  It is used to write half-equations in acidic conditions.
• What is the first step in the KOHES method?


  Balance the key element undergoing oxidation or reduction.
• How do you balance oxygen in the KOHES method?


  Add H2O to the opposite side of the equation.
• How do you balance hydrogen in the KOHES method?


  Add H+ to the opposite side of the equation.
• How do you balance the charge in the KOHES method?


  Add electrons to the more positive side of the equation.
• What must be included in both half and overall equations?


  States of the substances involved.
• What is the overall redox equation for the reaction of permanganate ions with sulfur dioxide?


  2MnO4– (aq) + 5SO2 (g) + 2H2O (l) → 2Mn2+ + 5SO42– (aq) + 4H+ (aq)
• Identify the half-equations for the reduction of VO3- by H2S.


  Reduction: VO3- + 6H+ + 3e- → V2+ + 3H2O; Oxidation: H2S → S + 2H+ + 2e-
• What is the oxidation state of sulfur in H2S?


  • 2
• What is the oxidation state of nitrogen in NO2?


  +4
• What is the oxidation state of sulfur in H2SO4?


  +6
• What is the oxidation state of manganese in KMnO4?


  +7
• What is the oxidation state of aluminum in Al?


  0
• What is the oxidation state of oxygen in H2O2?


  • 1
• What is the oxidation state of phosphorus in PO43–?


  +5
• What is the oxidation state of chromium in CrO42–?


  +6
• What is the oxidation state of carbon in C2O42–?


  +3
• What is the oxidation state of tin in SnCl4?


  +4
• What is the oxidation state of carbon in CO32–?


  +4
• What is the oxidation state of sodium in NaH?


  +1
• What is the oxidation state of copper in CuSO4?


  +6
• What is the oxidation state of chromium in K2Cr2O7?


  +6
• What is the oxidation state of iodine in I2?


  0
• What is the oxidation state of sulfur in S2O52–?


  +4
• What is the oxidation state of sodium in Na2O2?


  +1
• What is the oxidation state of manganese in MnO2?


  +4