Electrochemistry

Question 1

Electrochemical Cells

Answer 1

• Three Types: Galvanic (Voltaic) Cell, Electrolytic Cell, Concentration Cell.
• AN OX & RED CAT: Anode is the site of Oxidation, and Cathode is the site of Reduction. Electrons flow from Anode to Cathode. Current flows from Cathode to Anode.
• Galvanic cells and concentration cells are spontaneous (-ΔG) with positive EMF, while Electrolytic cells are nonspontaneous (+ΔG) with negative EMF. Free energy change and electromotive force always have opposite signs.

Question 2

Galvanic (Voltaic) Cells

Answer 2

• Spontaneous (-ΔG) with Positive EMF. Used as household batteries as they supply energy.
• Anode (-) and Cathode (+).
• Electrodes placed in separate Half Cells containing Electrolyte solutions are connected via a conductive wire and Salt Bridge consisting of inert salt.
• Anions from salt bridge diffuse into anode solution to react with Zn²⁺. Cations from salt bridge diffuse into cathode solution to react with sulfate ions left in solution from the Galvanization of copper onto the cathode (from Cu²⁺ ions to solid Cu).
• Cell Diagram for Daniell Cell shows: Zn(s) | Zn²⁺ (1M) || Cu²⁺ (1M) | Cu(s).
• Left to Right: Anode to Cathode.

Question 3

Electrolytic Cells

Answer 3

• Nonspontaneous (+ΔG) with Negative EMF. Require input of energy to proceed and is used for Electrolysis (decomposition of compounds, such as breaking down water into oxygen and hydrogen).
• Anode (+) and Cathode (-) corresponds to battery terminals because Anode attracts anions and Cathode attracts Cations.

Question 4

Concentration Cells

Answer 4

• Special type of Galvanic Cell in which the electrodes are the same and the current is generated due to the concentration gradient (one solution has a higher concentration of the same solute than the other).
• Movement of electrons stops when the concentration of ionic species in the half-cells are equal.
• Resting membrane potential is example of concentration cell.

• Nernst Equation:
E(cell) = E°(cell) - (0.06/n)logQ,
where Q = [Zn²⁺]/[Cu²⁺].

Question 5

Reduction Potential

Answer 5

• Defined relative to Standard Hydrogen Electrode (SHE) which is given potential of 0V.
• Standard Reduction Potential E°(red) is measured under standard conditions (298K, 1atm, 1M).
• More positive E°(red) means greater relative tendency for reduction to occur. Oxidation occurs in species with lower E°(red).
• In Galvanic Cells, cathode has higher E°(red).
• In Electrolytic Cells, external voltage source forces electrode with higher E°(red) to be oxidized, and oxidation occurs in anode. This is why reaction is nonspontaneous with +ΔG.

Question 6

Standard Electromotive Force

Answer 6

• Standard Electromotive Force Equation: E°(cell) = E°(red, cathode) - E°(red, anode).

Question 7

Thermodynamics of EMF

Answer 7

• ΔG° = -n F E°(cell), therefore ΔG° and E°(cell) always have opposite signs.
• ΔG° = -R T ln(Keq).
• n F E°(cell) = R T ln(Keq).
• Electrolytic cells have K(eq) < 1, where equilibrium state favors reactants, so ln(Keq) is negative, and so is E°(cell). Hence, electrolytic cells are nonspontaneous.
• Galvanic cells have K(eq) > 1, where equilibrium state favors products, so ln(Keq) is positive, and so is E°(cell). Hence, galvanic cells are spontaneous.