Balancing Redox Reactions Questions Key Questions The Model: Balancing a Redox R

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Balancing Redox Reactions Questions

Key Questions The Model: Balancing a Redox Reaction in an Acidic Solution 1. The substances oxidized and reduced may not be obvious in the following redox reaction: Example 1. Copper(I) chloride and arsenic precipitate out of solution when copper metal is added to tetraarsenic hexoxide in an acidic solution of chloride. A. Write unbalanced reaction: Cu + As(), + Cl. CuCl As (Acidieeondition: when balanced, H. will be reactant or a product.) Write the oxidation numbers above each atomic symbol on the left and right sides of the reaction above. a. B. Assign oxidation numbers and determine what's oxidized and reduced: b. Does the oxidation number of any atom increase? Is that species the oxidizing agent or the reducing agent? oxidation reduction Reducing agent: Cu. Oxidizing agent: As O6 c. Does the oxidation number of any atom decrease? Is that species the oxidizing agent or the C. Determine the half-reactions: Oxidation: Cu + Cl. CuCl Reduction: D. Balance all elements except O & H: Reduction: E. Use H20 to balance oxygen atoms Reduction: Reduction: As,06 4 As + 6H20 d. Which species is being oxidized? F. Use H. to balance hydrogen atoms: 12 H. AsO6 4 As + 6 H2O . Which species is being reduced? Oxidation: Reduction: 12 e. + 12H. + As4O6 4 As + 6H2O G. Use e-to balance charge: Cu + Cl. CuCl + e Exercises H. 12 Cu + 12 CI. 12 CuCl 12 e Conserve # of e (# e' lost-4e gained) oxidation = reduction = 12 e. + 12H. + As,06 4 As + 6H20 2. Write the oxidation number above each symbol in the following compounds or ions. L Combine the ½-reactions a. KBr b. BrF3 c. HBrO3 d. CBra 12 Cu + As4Os + 12 Cl. + 12 H. + 12 e. 12 CuCl + 4 As + 6H2O + 12e J. Simplify algebraically to get the simplest whole number ratio and check to see if balanced by both charge and mass. e. MnO4 f. Mn2O3 g. KMnO 3. 12 Cu + As406 + 12 CT + 12H. 12 CuCI + 4 As + 6H20 Assign oxidation numbers to each atom in the following reaction and then identify the oxidizing agent, reducing agent, the substance oxidized and the substance reduced. Key Questions 4. In step A of the model why was the product written as CuCl and not Cu+CI? According to the Model, how is the number of oxygen atoms balanced in each half-reaction? How is the number of hydrogen atoms balanced in each half-reaction? How is charge balanced in each half-reaction? a. b. Oxidizing agent: Reducing agent: c. Substance oxidized: Substance reduced: d. Page 2 of6 Page 3 of 6

Explanation / Answer

1. For the reaction

a. Oxidation state

8H+(+1) + Cr2(+6)O7(-2)^2- + 3S(+4)O3(-2)- -----> 2Cr3+(3+) + 3S(+6)O4(-2)^2- + 4H2(+1)O(-2)

b. Oxidation state of S is increasing from +4 to +6. It is a reducing agent

c. Oxidation state of Cr is reducing from +6 to +3. It is an oxidizing agent

d. species oxidized = SO3^2-

species reduced = Cr2O7^2-

2. Oxidation number

a. K(+1)Br(-1)

b. Br(+3)F(-1)3

c. H(+1)Br(+5)O(-2)3

d. Mn(+7)O(-2)4^-

e. Mn(+3)2O(-2)3

f. K(+1)Mn(+7)O(-2)4

3. Reaction,

N(+5)O(-2)3^- + 4Zn(0) + 7O(-2)H(+1)^- + 6H(+1)O(-2)2 ---> 4Zn(+2)(O(-2)H(+1))4^-2 + N(+3)H(+1)3

Oxidizing agent = NO3-         Reducing agent = Zn

Substance oxidized = Zn            Substance reduced = NO3-

5. a. The reaction is multiplied to balance electrons in both the reaction.

b. Oxidation and reduction reaction occur suimultaneously = True

Both the eletron loss and acceptance must occur together to have an stable reaction.

6. Balance reaction

Oxidation half cell,

C2O4^2- ---> CO3^2-

Balance C, and O

C2O4^2- + 2H2O ---> 2CO3^2-

Balance hydrogen and add e-,

C2O4^2- + 2H2O ---> 2CO3^2- + 4H+ + e- ---(1)

reduction half cell,

MnO4^- ---> MnO2

Balance O and H

MnO4^2- + 3e- ---> MnO2 + 2H2O ---(2)

Multiply (1) with 3 and add to (2),

MnO4^2- + 3C2O4^2- + 4H2O ---> MnO2 + 3CO3^2- + 8H+

is balanced equation.

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