chemistry prelab question

Question

Potassium permanganate (KMnO4) is the oxidizing agent and sodium oxalate (Na2C2O4) is the reducing agent.
a. Manganese (Mn) is reduced.
b. Carbon (C) is oxidized.

 

2KMnO4(aq) + 5Na2C2O4(aq) + 8H2SO4(aq)  2MnSO4(aq) + 10CO2(g) + 8H2O(l) + 5Na2SO4(aq) + K2SO4(aq)
1) Predict the oxidation state on the manganese and carbon before and after the reaction.
2) Provide the electron balanced half reactions for oxidation and reduction. Also provide the complete balanced redox reaction.

 

(I am unsure how to do #1, and #2 above)

Answer 1

(1) Mn before the reaction is 7+ as detailed by LaRita.  Carbon before the reaction has an oxidation state of 3+.

After the reaction, Mn has an oxidation state of 2+ and C has an oxidation state of 4+.  

 

(2) MnO4^- + 8H⁺ + 5e- ==> Mn²⁺ + 4H₂O  reduction half reaction

C₂O₄^2- ==> 2CO₂ + 2e-  oxidation half reaction

Multiply reduction half reaction by 2 and oxidation half reaction by 5 to balance electrons and cancel electrons

2MnO₄^- + 16H⁺ + 5C₂O₄^2- ===> 2Mn²⁺ + 8H₂O + 10CO₂   Balanced for mass and charge

Answer 2

1) Before the reaction manganese is in the compound KMnO_4. To derive the oxidation state of Mn, we have to use the fixed oxidation states of the other two elements in the compound and remember that they all have to add up to equal zero (the charge on this neutral molecule). We know that oxygen (when not in a peroxide) keeps a fixed oxidation state of -2, and there are four oxygen atoms in this compound for a total charge of -8. We know that potassium keeps a fixed oxidation state of +1, and there is one K atom in the compound. 

K + Mn +(O x 4) = 0

1 + Mn + (-8) = 0

Mn = +7

After the reaction, Mn is in the compound MnSO₄. To derive it's oxidation state, we again use our knowledge of the fixed oxidation states of the other elements in the compound. Since the sulfate ion here has a fixed charge of -2 that means the Mn its bonded with must have a charge of +2, and since Mn is monatomic, that charge is also its oxidation state.

So Mn goes from +7 to +2