Chemistry (Electrochemistry)

Question 1

How to balance redox reactions

Answer 1

1) separate half reactions
2) Balance all atoms. H and O with H20 and H+(or OH-) when necessary.
3) Balance the charges (e-) on 2 half reactions
4) Add the half reactions.

Question 2

Oxidation and Reduction

Answer 2

Oxidation: lose e-, gain oxygen.
-> oxidizing agent: oxidize other materials, but it itself is reduced.

Reduction: gain e-, lose oxygen, gain hydrogen
-> reducing agent: reduce other materials, but it itself is oxidized.

Question 3

Galvanic cells

Answer 3

Zinc electrode oxidizing in Zn(NO₃) ₂
And a copper electrode reducing in Cu(NO₃) ₂

• Electrons move from (-) to (+) electrode. Anode is (-) and undergoes oxidation. Cathode is (+) and undergoes reduction.
• Zinc electrode has negative charge and because electrons leave becomes more positive. This means an increase of Zn ²+ ions deposited in solution compared to NO₃-. Solution has positive charge.
• Copper electrode has positive charge electrons come towards it making it more negative. This means a lot of Cu ²+ ions attracted to electrode and increase in NO₃- ions in solution. Solution has a negative charge.
• Salt bridge of KNO₃ adds NO₃- ions in solution containing Zn ²+ ions to compensate for lack of positive charge and K+ ions added in solution containing Cu ²+ ions to compensate for lack of negative charge.
• zinc electrode eaten away and copper electrode build up
• galvanic cells are spontaneous: ΔG is negative
• Cell notation:
  Anode | Anode solution | Cathode solution | Cathode

Question 4

Electrolytic Cells

Answer 4

Electrolytic cell with molten NaCl as it’s medium

1) Cl- attracted to positive electrode and Na+ attracted to negative electrode.
2) Electrons move from anode to cathode. Anode is positive and cathode is negative
3) solid sodium is created and chlorine gas.
4) non spontaneous: Δ G is positive.

Question 5

Reduction potential

Answer 5

2 species in an electrochemical cell=> which one is oxidized? Reduced?

E°red (reduction potential) => lowest (oxidized), highest (reduced)

E°ox (highest, oxidized), E°red ( lowest, reduced)

• After balancing redox reaction DON’T multiply stoichiometric coefficients.

Question 6

Thermodynamics with respect to E cell

Answer 6

ΔG ° = -nFE°cell ∴ ΔG ∝ - E°cell

• > n= moles
• > F= faradays constant

E for galvanic cell: E°cell = (+) ∴ ΔG = (-)
E for Electrolytic cell: E°cell = (-) ∴ ΔG = (+)