Electrochemistry #2

Chemistry 3202

Name:

Balancing half-reactions (Complete p. 732 - #20)

1. Balance each half reaction under acidic conditions. (a)

8 e- + 10 H+ + NO3-(aq) → NH4+(aq) + 3 H2O

(b)

H2C2O4(s) → 2 CO2(g) + 2 H+ + 2 e-

(c)

20 H2O + As2S3(s) → 2 H2AsO4-(aq) + 3 SO42-(aq) + 36 H+ + 28 e-

(d)

4 H2O + Zn(s) → Zn(OH)42- + 4 H+ + 2 e-

(e)

2 e- + 4 H+ + SO42- → SO2 + 2 H2O

Electrochemistry #2

2.

Chemistry 3202

Name:

Electrochemistry #2

Chemistry 3202

Name:

Electrochemistry #3

Chemistry 3202

Name:

1. Balance each half reaction under basic conditions. (a)

Balance like acidic 8 e- + 10 H+ + NO3-(aq) → NH4+(aq) + 3 H2O Add the same number of OH- as H+ to both sides. 10 OH- + 8 e- + 10 H+ + NO3-(aq) → NH4+(aq) + 3 H2O + 10 OHThe 10 OH- combines with the 10 H+ to form 10 water molecules 10 H2O + 8 e- + NO3-(aq) → NH4+(aq) + 3 H2O + 10 OHRemove 3 H2O FROM EACH SIDE for the final equation. 7 H2O + 8 e- + NO3-(aq) → NH4+(aq) + 10 OH-

(b)

Acidic:

H2C2O4(s) → 2 CO2(g) + 2 H+ + 2 e-

Basic:

2 OH- + H2C2O4(s) → 2 CO2(g) + 2 H+ + 2 e- + 2 OH2 OH- + H2C2O4(s) → 2 CO2(g) + 2 H2O + 2 eNo water to remove in this example!!

(c)

Acidic:

20 H2O + As2S3(s) → 2 H2AsO4-(aq) + 3 SO42-(aq) + 36 H+ + 28 e-

Basic:

36 OH- + 20 H2O + As2S3(s) → 2 H2AsO4-(aq) + 3 SO42-(aq) + 36 H+ + 28 e- + 36 OH36 water in the products → remove 20 from each side 36 OH- + As2S3(s) → 2 H2AsO4-(aq) + 3 SO42-(aq) + 16 H2O + 28 e-

(d)

Acidic:

4 H2O + Zn(s) → Zn(OH)42- + 4 H+ + 2 e-

Basic:

4 OH- + 4 H2O + Zn(s) → Zn(OH)42- + 4 H+ + 2 e- + 4 OH4 water in the products → remove 4 from each side -

4 OH + Zn(s) → Zn(OH)42-

(e)

Acidic:

+ 2 e-

2 e- + 4 H+ + SO42- → SO2 + 2 H2O 2 H2O + 2 e- + SO42- → SO2 + 4 OH-

Electrochemistry #3

Chemistry 3202

Name:

2. Balance each redox reaction under basic conditions.

MUST BALANCE EACH HALF-REACTION FIRST → THEN COMBINE!! MnO4-(aq) + CN-(aq) → MnO2(s) + OCN- (aq)

(a) Acidic:

3 e- + 4 H+ + MnO4- → MnO2(s) + 2 H2O H2O + CN- → OCN- + 2 H+ + 2 e-

multiply by 2by Basic:

3 e- + 2 H2O + MnO4- → MnO2(s) + 4 OH2 OH- + CN- → OCN- + 2 e- + H2O

multiply by 3by 6 e- + 4 H2O + 2 MnO4- → 2 MnO2(s) + 8 OH6 OH- + 3 CN- → 3 OCN- + 6 e- + 3 H2O H2O + 2 MnO4- + 3 CN- → 2 MnO2(s) + 2 OH- + 3 OCN-

(b)

Ag2O + Si → Ag + SiO32-

multiply by 2by

2 e- + H2O + Ag2O → 2 Ag + 2 OH6 OH- + Si → SiO32- + 3 H2O + 4 e2 OH- + 2 Ag2O + Si → SiO32- + 4 Ag + H2O

(c)

SO2 + Cr2O72- → Cr3+ + SO42-

multiply by 3by 4 OH- + SO2 →

SO42- + 2 H2O + 2 e-

6 e- + 7 H2O + Cr2O72- → 2 Cr3+ + 14 OHCr2O72- + H2O + 3 SO2 → 3 SO42- + 2 Cr3+ + 2 OH-