Electrochemistry

Topics covered

1. Electrochemical cells
2. Galvanic cells
3. Nernst Equation
4. Conductance of electrolytic solutions
5. Electrolytic cells and electrolysis
6. Batteries
7. Fuel Cells
8. Corrosion

Differences between Electrochemical cell and electrolytic cell

Electrochemical Cell

• Also called Galvanic or Voltaic cell
• Device converting chemical energy into electrical energy.
• Based on the redox reaction.
• The change in Gibb’s free energy is negative.

• Two electrodes are set up in two separate beakers.
• The electrolytes taken in the two beakers are different. Catholyte in cathode compartment. Anolyte in anode compartment.
• The electrodes are of different materials.
• At anode oxidation occurs. (Negative pole)
• At cathode reduction occurs. (positive pole)
• To connect the electrodes, a salt bridge is used.

1. Anode (oxidation occurs) – copper electrode
2. Cathode (reduction occurs) – silver electrode
3. Salt bridge
4. Voltmeter

Electrolytic cell

1. Device converting electrical energy into chemical energy.
2. The redox reaction is non-spontaneous. It takes place only when electrical energy is supplied. Change in free energy is positive.
3. Both electrodes are suspended in the solution or molten electrolyte in the same beaker.
4. Single electrolyte is taken.
5. The electrodes taken may be of the same or different materials.
6. Cathode (negative electrode) – connected to the negative terminal of the battery.
7. Anode (postive electrode) – connected to postive terminal of the battery.
8. No salt bridge is needed.

1. Battery
2. Silver anode
3. Cathode ( object to be electroplated)
4. Electrolyte – aqueous solution of silver electrolyte

CELL TERMINOLOGY

• Current – flow of electrons in one direction through a wire or conductor
• Electrode – a metallic rod or bar or strip conducting electrons into and out of electrolytic solution.
• Electrode is dipped in an electrolyte ( M/M^n+)
• Anode – electrode where oxidation occurs
• Cathode – reduction occurs
• Electrolyte – acid or base or salt solution conducting current in solution form or in molten form.
• Anodic compartment – contains anode and anolyte.
• Cathodic compartment – contains cathode and catholyte.
• Half cell – The half of an electrochemical cell in which either oxidation or reduction occurs.

CELL NOTATION

The negative sign of cell emf shows that the cell is not feasible.

SINGLE ELECTRODE POTENTIAL

• The potential of a single electrode in a half-cell is known as single electrode potential.
• It depends on concentration of ions in solution.
• Depends on tendency to form ions.
• Depends on the temperature of the cell bath.

Standard emf of a cell

The standard conditions – concn 1M, T = 298 K.

SHE (Standard Hydrogen Electrode)

1. High resistance voltmeter
2. Platinum wire
3. Platinum black coated platinum electrode
4. 1 M HCl solution
5. Hydrogen gas at 1 atm pressure

• It is a gas-ion electrode.
• represented by Pt, H2 (1 atm) / H^+ (1M)
• Electrode potential of SHE is zero.
• When SHE is placed on the right hand side of the zinc electrode, the hydrogen electrode reaction is given by the following reactions. It acts as cathode. Reduction occurs (electronation)

• When SHE is placed on the LHS of the cell, the SHE reaction is given by

• This indicates oxidation (anodic reaction or de-electronation)

STANDARD REDUCTION POTENTIAL AT 298 K

Predicting the Cell EMF

Predicting the feasibility of a reaction

If the cell emf value is positive, the reaction is feasible, if it is negative, it is not feasible.

Predict whether the reaction given below is feasible or not.

At anode, oxidation occurs.

At cathode, reduction occurs.

Cell emf = cathode potential – anode potential

Cell emf is negative and hence the reaction is not feasible.

Next example

Determine the feasibility of the reaction given below.

Calculate the cell emf and change in free energy of the cell given below.