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Balancing Redox Reactions

Balancing Redox Reactions. Unit 5: Electrochemistry. Balancing Simple Equations Using Half-Reactions. Write a balanced half-reaction based on the ionic equation below: Cd (s) + 2Ag + ( aq )  Cd 2+ ( aq ) + 2Ag (s) Write a NIE for the following equation:

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Balancing Redox Reactions

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  1. Balancing Redox Reactions Unit 5: Electrochemistry

  2. Balancing Simple Equations Using Half-Reactions • Write a balanced half-reaction based on the ionic equation below: Cd(s) + 2Ag+(aq) Cd2+(aq) + 2Ag(s) • Write a NIE for the following equation: Au(NO3)3 (aq)+ 3Ag(s)  3AgNO3 (aq) + Au(s)

  3. Balancing Equations using half-reactions with differing numbers of electrons • Write a NIE given the following half equations: K(s) K+(s) + e- Cl2 (g) + 2e-  2Cl-(s)

  4. Balancing Equations for Reactions in Acidic or Basic Solutions • Sulfur is oxidized by nitric acid in an aqueous solution, producing sulfur dioxide, nitrogen monoxide, and water as shown by the equation. Use the half-reaction method to balance the following equation: S(s) + HNO3(aq) SO2(g) + NO(g) + H2O(l)

  5. Oxidation Number Method • 1. Write an unbalanced chemical equation if it is not given • 2. assign an oxidation number to each atom in the equation to determine if it is redox. • 3. If it is redox, identify the atoms undergoing oxidation and reduction. • 4. determine the number of electrons gained and lost. • 5. Find the LCM of the number of electrons lost by the reducing agent (increase in oxidation number) and the number gained by the oxidizing agent (decrease in oxidation number) • 6. apply your LCM • 7. balance the atoms in the reaction unless it is an acidic or basic reaction where you balance everything except O and H. • 8. For reactions in acidic solution, balance any oxygen atoms by adding water. Then balance hydrogen by adding hydrogen ions. • 9. For reactions in basic solution, balance like step 8 only afterwards, add hydroxide ions to both sides to neutralize hydrogen ions. Cancel any water molecules that are on both sides of the equation.

  6. Balancing Equations for Reactions in Acidic or Basic Solutions • The unbalanced equation fo the reaction between permanganate ions, MnO4-(aq) and oxalate ions, C2O42-(aq), in an acidic solution is show below. Spectator ions have been omitted. Use the half-reaction method to balance the equation: MnO4-(aq) + C2O42-(aq)  Mn2+(aq) + CO2(g)

  7. Balancing Equations for Reactions in Acidic or Basic Solutions • Cyanide, CN-(aq), is oxidized by permanganate, MnO4-(aq), in a basic solution, as shown in the following equation. Use the half-reaction method to balance the equation: CN-(aq) + MnO4-(aq) CNO-(aq) + MnO2(s)

  8. Disproportionation Reactions • When some atoms of an element undergo oxidation and other atoms of the same element undergo reduction in a single reaction Eg. Cu2O(aq) + H2SO4(aq) Cu(s) + CuSO4(aq) + H2O(l) Ionic Equation: 2Cu+(aq) + O2-(aq) + 2H+(aq) + SO42-(aq)  Cu(s) + Cu2+(aq) + SO42-(aq) + H2O(l) NIE: 2Cu+(aq) + O2-(aq) + 2H+(aq)  Cu(s) + Cu2+(aq) + H2O(l)

  9. Redox Reactions with Molecular Compounds • In these cases we do not use the half-reaction method but instead use the Oxidation Number Method • We went over this yesterday • Non-Integer oxidation Numbers • Using Lewis Structures to Assign Oxidation Numbers • Determine the oxidation numbers of each atom in acetone (H6C3O(l))

  10. Using Lewis Structures to Assign Oxidation Numbers • Draw the Lewis structure • Determine and record the electronegativities of each atom • Circle the electrons that ‘belong’ to each atom • Count the electrons that are inside each circle • Subtract the number of electrons in the circle from the number of valence electrons for each neutral atom. These results are the oxidation numbers of the atoms.

  11. Try This • The dichromate ion reacts with ethanol in an acidic solution to produce the chromium(III) ion and carbon dioxide. Write a balanced equation for the reaction using the oxidation number method.

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