Redox and electrolysis

Acidified potassium dichromate solution 同 sodium sulphite solution 既reaction :
我唔明 potassium dichromate 既reduction half equation 點解要係
Cr2O7 (2-) + 14H+ + 6e ---> 2Cr3+ + 7H2O, 而唔係
K(2+) + 2e- ---> K, 明明兩隻都係oxidizing agent~

仲有, dilute zinc nitrate electrolyzed, electrodes 係platinum(cathode), 同zinc(anode). 我明點解cathode果度出hydrogen, 但我唔明nitrate, 同樣係oxidizing agent, 佢點解會去anode 果邊?

回答後仍請定時留意我的補充.