#1 Using oxidation states and half-reactions. Balance the following equations: K

Question

#1 Using oxidation states and half-reactions. Balance the following equations:

K2CrO4 + Na2SO3 + HCl------- KCl + Na2SO4 + CrCl3 + H2O

#2 Balance each redox reaction ocurring in acidic aqueous solution

Part A

Zn (s) + Sn 2+ (aq)  ---------- Zn2+(aq) + Sn (s)

Part B

Zn(s) + Na+(aq) ------------- Zn2+(aq) + Na(s)

Part C

Cl-(aq) + 2NO3-(aq) ---------------- NO(g) + Cl2(g)

#3 Balance each redox reaction occurring in basic aqueous solution

Part A

MnO-4(aq) + Br-(aq) ------------- MnO2(s) + BrO3-(aq)

#4 A Cu/Cu2+ concentration cell has a voltage of 0.24 V at 25 degree C. The concentration of Cu2+ in one of the half-cells is 1.6e-3. What is the conentration of Cu+2 in the other half-cell?

Explanation / Answer

K2CrO4 + Na2SO3 + HCl -----> KCl + Na2SO4 + CrCl3 + H2O

Cr goes from +6 in K2CrO4 to +3 in CrCl3; Cr gains 3e- (reduced)
S goes from +4 in Na2SO3 to +6 in Na2SO4; S loses 2e- (oxidized)

To balance the electrons, take the Cr compounds times 2 and the S compounds times 3 to get 6 electrons gained and lost. This gives:

2K2CrO4 + 3Na2SO3 + HCl -----> KCl + 3Na2SO4 + 2CrCl3 + H2O

Next, balance the oxygens by inspection (17 oxygens on each side).

2K2CrO4 + 3Na2SO3 + HCl -----> KCl + 3Na2SO4 + 2CrCl3 + 5H2O

Balance the hydrogens (10 hydrogens on each side)

2K2CrO4 + 3Na2SO3 + 10HCl -----> KCl + 3Na2SO4 + 2CrCl3 + 5H2O

Balance the chlorines (10 Cl on each side)
2K2CrO4 + 3Na2SO3 + 10HCl -----> 4KCl + 3Na2SO4 + 2CrCl3 + 5H2O

2)

Zn = Zn2+ + 2e-
Sn2+ + 2e- = Sn
Zn + Sn2+ = Zn2+ + Sn


2 Cl- = Cl2 + 2e- ) x 3
NO3- + 4 H+ + 3e- = NO + 2 H2O ) x 2
6 Cl- + 2 NO3- + 8 H+ = 3 Cl2 + 2 NO + 4 H2O



Zn = Zn2+ + 2e-
Na+ + 1e- = Na ) x 2
Zn + 2 Na+ = Zn2+ + 2 Na

3)

2 MnO4{-}(aq) + Br{-}(aq) + 2 H{+}(aq) ? 2 MnO2(s) + BrO3{-}(aq) + H2O(l)

or

2 MnO4{-}(aq) + Br{-}(aq) + H2O(l) ? 2 MnO2(s) + BrO3{-}(aq) + 2 OH{-}(aq)

In either case, the coefficient for water is 1.

4) E = (0.0591 / 2) * log (1.6*10^-2 / [Cu2+]) = 0.24

[Cu2+] =1.2*10^-10 M

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