www.hschemsolutions.com Acids Acids donate protons in aqueous solutions Redox Reactions 7.2 es HCl(aq) Æ H+(aq) + Cl-(aq) H2SO4(aq) Æ H+(aq) + HSO4-(aq) N ot REDOX Reactions in Acid and Basic Solutions ts 'L ec tu re Strong acid like HCl and H2SO4 experience ~100% dissociation in polar solvents, just like soluble ionic compounds. Ex) Redox Reactions in Acidic Solutions (I) H+ ions can oxidize many metals (X). Ex) Write the balanced net ionic equation for the reaction that occurs when a solid piece of magnesium is placed in a solution of hydrochloric acid? of St a H+(aq) + X(s) Æ H2(g) + X a+(aq) ud Reactions between metals and all strong acids except Nitric Acid en Redox Reactions in Acidic Solutions (I) C op y H+(aq) pulls electrons out of the metal to form H2(g). Ex1) Redox Reactions in Acidic Solutions (II) Reactions between metals and Nitric Acid. Ex1) A solid copper penny is placed in a solution of nitric acid. Sa m pl e Redox Reactions in Acidic Solutions (II) NO3- is a stronger oxidizing agent then H+. Thus, NO3- will oxidize metals that H+ cannot. Cu(s) + NO3-(aq) Æ NO(g) + Cu2+(aq) This must be balanced using a special procedure. NO3- (aq) + X (s) Æ NO (g) + X a+ (aq) © 2009 High School Chem Solutions. All rights reserved. 1 www.hschemsolutions.com Balancing REDOX Reactions in Acidic Solutions Ex1) The Reduction Half-Reaction Cu(s) + NO3-(aq) Æ NO(g) + Cu2+(aq) 1) Write the two unbalanced half-reactions. • Oxidation half-reaction • Reduction half-reaction es Step 1) For each of the half-reactions: Step 2) 2) Balance all atoms except for O and H. 3) Balance for O by adding H2O molecules. 4) Balance for H by adding H+ ions. 5) Balance the charge by adding electrons. 6) Cross multiply to cancel electrons. 7) Add half reactions and cancel things that are the same. N ot Step 3) Step 4) ts 'L ec tu re Step 5) Ex1) Step 6. Cross Multiply Ex1) The Oxidation Half-Reaction Cu(s) + NO3-(aq) Æ NO(g) + Cu2+(aq) en Reduction half-reaction [NO3-(aq) + 4 H+(aq) + 3 e- Æ NO(g) + 2 H2O(l)] ud Step 1) Step 2) St Step 3) Step 4) C op y of Step 5) Oxidation half reaction [Cu(s) Æ Cu2+(aq) + 2 e-] m pl e Ex1) Step 7. Add the Two ½ Reactions Sa 2 NO3-(aq) + 8 H+(aq) + 6 e- Æ 2 NO(g) + 4 H2O(l) 3 Cu(s) Æ 3 Cu2+(aq) + 6 e- Strong oxidizing agents that act in acidic solutions Nitrate: NO3-(aq) Æ NO(g) Permanganate: MnO4-(aq) Æ Mn2+(aq) Dichromate: Cr2O72-(aq) Æ Cr3+(aq) All of the above oxidize p and d-block metals, sulfite ions, peroxides, and substances that have a lower (less positive) oxidation state than usual in acidic solutions. © 2009 High School Chem Solutions. All rights reserved. 2 www.hschemsolutions.com Oxidation of Hydrogen Peroxide SO32- Æ SO42- H2O2(aq) Æ O2(g) The oxidation number of sulfur increases from +4 to +6 The oxidation number on oxygen increases from -1 to 0. Substances with Lower (less positive) Oxidation States than Usual. Substances with Lower (less positive) Oxidation States than Usual. St ud • In C2O42-, Carbon has an oxidations state of +3. • As Carbon is in Group 4A, it would rather have an oxidation state of +4. • It will oxidize (lose electrons) to raise its oxidation state. en ts 'L ec tu re N ot es Oxidation of Sulfite of CO2(g) This happens when hydrochloric acid is combined with a strong oxidizing agent. e.g.) Chlorine Ion, Cl- 2 Cl-(aq) Æ Cl2(g) C op y C2O42-(aq) Æ Some non-metals can raise their oxidation states by bonding with like elements. m pl e What if you have a choice of two species that can be oxidized? Sa When compounds contain p-block or d-block metals and non-metals with less positive than usual oxidation states… Ex2) Redox Reactions in Acidic Solutions (II) Ex2) An acidic solution containing tin (II) chloride is mixed with a solution of potassium permanganate. The metal will be oxidized!! e.g.) SnCl2 Sn2+ will be oxidized to Sn4+ © 2009 High School Chem Solutions. All rights reserved. 3 www.hschemsolutions.com Ex2) The Oxidation Half-Reaction MnO4- (aq) + Sn2+(aq) Æ Mn2+(aq) + Sn4+(aq) MnO4- (aq) + Sn2+(aq) Æ Mn2+(aq) + Sn4+(aq) Step 2) Step 2) Step 3) Step 3) Step 4) Step 4) Step 5) Step 5) N ot Step 1) ts 'L ec tu re Step 1) es Ex2) The Reduction Half-Reaction Ex2) Step 6. Cross Multiply Ex2) Step 7. Add the Two ½ Reactions Reduction half-reaction [MnO4-(aq) + 8H+(aq)+ 5 e- Æ Mn2+(aq) + 4 H2O(l)] en 2 MnO4-(aq) + 16 H+(aq)+ 10e- Æ 2 Mn2+(aq) + 8 H2O(l) ud 5 Sn2+(aq) Æ 5 Sn4+(aq) + 10 e- 2 MnO4- + 5 Sn2+ + 16 H+Æ 2 Mn2+ + 5 Sn4+ + 8 H2O The balanced redox reaction C op y of St Oxidation half reaction [Sn2+(aq) Æ Sn4+(aq) + 2 e-] Sa m pl e Basic Solutions Strong oxidizing agent that acts in Basic Solutions Permanganate: • Basic solutions have a high concentration of hydroxide ions [OH-]. MnO4-(aq) Æ MnO2 (aq) Permanganate will oxidize p or d-block metals, sulfite ions, and substances that have a lower (less positive) oxidation state than usual in basic solutions. © 2009 High School Chem Solutions. All rights reserved. 4 www.hschemsolutions.com Ex) Redox Reactions in Basic Solutions Ex) The Reduction Half-Reaction MnO4-(aq) + SO32-(aq) Æ MnO2(aq) + SO42-(aq) Ex) A basic solution containing magnesium sulfite is mixed with a solution of potassium permanganate. Step 1) es Step 2) N ot Step 3) Step 4) ts 'L ec tu re Step 5) Ex) The Oxidation Half-Reaction Ex) Step 6. Cross Multiply MnO4-(aq) + SO32-(aq) Æ MnO2(aq) + SO42-(aq) en Reduction half-reaction [MnO4-(aq) + 4 H+(aq) + 3e- Æ MnO2(aq) + 2 H2O(l)] x2 Step 1) ud Step 2) St Step 3) Step 4) [SO32-(aq) + H2O(l) Æ SO42-(aq) + 2 H+(aq) + 2e-] x3 C op y of Step 5) Oxidation half reaction m pl e Ex) Step 7. Add the Two ½ Reactions Sa 2 MnO4- + 8 H+ + 6e- Æ 2 MnO2 + 4 H2O 3 SO32- + 3 H2O Æ 3 SO42- + 6 H+ + 6e- Ex) Step 8. Add OH- to Eliminate H+ 2MnO4- + 3 SO32- + 2 H+ Æ 2 MnO2 + 3 SO42- + H2O + 2 OH- + 2 OH- © 2009 High School Chem Solutions. All rights reserved. 5