Chemistry of Reactions: Redox (#) Candidates should be able to: a) define oxidation and reduction (redox) in terms of oxygen/hydrogen gain/loss b) define redox in terms of electron transfer and changes in oxidation state c) identify redox reactions in terms of oxygen/hydrogen gain/loss, electron gain/loss and changes in oxidation state (*) d) describe the use of aqueous potassium iodide and acidified potassium manganate (VII) in testing for oxidizing and reducing agents from the resulting color changes * not in combined Science syllabus # not in N level Science syllabus 1
What is oxidation? Oxidation occurs when, after a reaction, a substance: - gains oxygen; or - loses hydrogen; or - loses electrons; or - increases its oxidation state What is reduction? Reduction occurs when, after a reaction, a substance: - loses oxygen; or - gains hydrogen; or - gains electrons; or - decreases its oxidation state What is a redox reaction? A redox reaction occurs when oxidation and reduction take place together. There can be no oxidation without reduction and vice versa. What is a reducing agent? A reducing agent is a substance that causes another substance to be reduced. The reducing agent itself is oxidized during the reaction. What is an oxidizing agent? An oxidizing agent is a substance that causes another substance to be oxidized. The oxidizing agent itself is reduced during the reaction. Describe a simple test for the presence of an oxidizing agent using aqueous potassium iodide, KI. Add a few drops of aqueous potassium iodide to a solution containing an unknown agent. If an oxidizing agent is present in the solution, aqueous potassium iodide will turn from colorless to brown. In the presence of an oxidizing agent, a redox reaction occurs, turning the iodide ion in KI (which is colorless) to iodine, I 2 (which is brown). Describe a simple test for the presence of a reducing agent using acidified potassium manganate(vii) solution, KMnO 4. Add a few drops of acidified potassium manganate(vii) solution to a solution containing an unknown agent. If a reducing agent is present in the solution, the KMnO 4 solution will turn from purple to colorless. In the presence of a reducing agent, a redox reaction occurs, turning the Mn 7+ ion in KMnO 4 (which is purple) to Mn 2+ ion (which is colorless or slightly pinkish). Give examples of reducing agents. List at least 6 examples. - carbon (C) - carbon monoxide (CO) - hydrogen (H 2) - hydrogen sulfide (H 2S) - metals - potassium iodide (KI) - sulfur dioxide (SO 2) - ammonia (NH 3) - hydrogen peroxide (H 2O 2) (can act as oxidizing agent as well) Give examples of oxidizing agents. List at least 6 examples. - fluorine (F 2) - chlorine (Cl 2) - bromine (Br 2) - iodine (I 2) - concentrated sulfuric acid (H 2SO 4) - nitric acid (HNO 3) - oxygen (O 2) - potassium manganate(vii) (KMnO 4) - potassium dichromate(vi) (K 2Cr 2O 7) - hydrogen peroxide (H 2O 2) (can act as reducing agent as well) (It is stronger as an oxidizing agent, especially in acidic solutions) 2
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Format for explaination in terms of oxygen gain / loss (The reactant substance) gained / lost oxygen to form (the product substance). 1. 4Na (s) + O 2 (g) 2Na 2O (s) Sodium has been oxidized to sodium oxide. Sodium gained oxygen to form sodium oxide. 2. CuO (s) + H 2 (g) Cu (s) + H 2O (g) Copper (II) oxide has been reduced to copper. Copper (II) oxide lost oxygen to form copper. 3. FeO (s) + CO (s) Fe (s) + CO 2 (g) Iron(II) oxide has been reduced to iron. Iron(II) oxide lost oxygen to form iron. Format for explaination in terms of hydrogen gain / loss (The reactant substance) lost / gained hydrogen to form (the product substance). 1. H 2S (g) + Cl 2 (g) 2HCl (g) + S (s) Hydrogen sulfide has been oxidized to sulfur. Hydrogen sulfide lost hydrogen to form sulfur. 2. Cl 2 (g) + H 2 (g) 2HCl (g) Chlorine has been reduced to hydrogen chloride. Chlorine gained hydrogen to form hydrogen chloride. 3. 2NH 3 (g) + 3CuO (s) N 2 (g) + 3Cu (s) + 3H 2O (g) Ammonia has been oxidized to nitrogen. Ammonia lost hydrogen to form nitrogen. 5
Format for explaination in terms of oxidation state The oxidation state of (the element) increased / decreased from (o.s. no.) in (the reactant substance) to (o.s. no.) in (the product substance). 1. ZnO (s) + C (s) Zn (s) + CO (g) Zinc oxide has been reduced to zinc. The oxidatation state of zinc decreased from +2 in zinc oxide to 0 in zinc. 2. PbS (s) + 4H 2O 2 (aq) PbSO 4 (s) + 4H 2O (l) Lead(II) sulfide has been oxidized to lead(ii) sulfate. The oxidation state of sulfur increased from -2 in lead(ii) sulfide to +6 in lead(ii) sulfate. 3. PbO (s) + H 2 (g) Pb (s) + H 2O (g) Lead(II) oxide has been reduced to lead. The oxidation state of lead decreased from +2 in lead(ii) oxide to 0 in lead. 4. 2FeCl 2 (aq) + Cl 2 (g) 2FeCl 3 (aq) Fe 2+ has been oxidized to Fe 3+. The oxidation state of iron increased from +2 in iron(ii) chloride to +3 in iron(iii) chloride. Format for explaination in terms of electron gain / loss (The reactant atom) lost / gained electrons to form (the product ion). 1. 2Na(s) + Cl 2 (g) 2NaCl (s) Sodium has been oxidized to sodium chloride. Sodium atom lost electrons to form sodium ion. Na (s) Na + (s) + e 2. Fe (s) + Cu 2+ (aq) Fe 2+ (aq) + Cu (s) Cu 2+ ions has been reduced to copper. Cu 2+ ions gained electrons to form copper atoms. Cu 2+ (aq) + 2e Cu (s) 3. Fe (s) + 2H + (aq) Fe 2+ (aq) + H 2 (g) Iron has been oxidized to Fe 2+. Iron atom lost electrons to form Fe 2+ ions. Fe (s) Fe 2+ (aq) + 2e Explain, in terms of oxidation state, why the following reaction is a redox reaction. PbS (s) + 4H 2O 2 (aq) PbSO 4 (s) + 4H 2O (l) PbS has been oxidized to PbSO 4. The oxidation state of sulfur increased from -2 in PbS to +6 in PbSO 4. H 2O 2 has been reduced to H 2O. The oxidation state of oxygen decreased from -1 in H 2O 2 to -2 in H 2O. Since oxidation and reduction occurred simultaneously in the reaction, it is a redox reaction. 6