Redox Electrode and Nernst Equation

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Questions and Answers

What is the cathode material in a dry cell?

NH4Cl and ZnCl2

Carbon (graphite) rod (correct)

Powdered MnO2 and carbon

Zn container

What is the function of Pb(s) and PbO2(s) in a lead storage battery?

They are used as electrolytes

They are used to connect multiple cells together

They are regenerated at the respective electrodes during recharging (correct)

They act as electrodes

What is the main difference between a fuel cell and an ordinary battery?

Fuel cells use nickel-cadmium instead of lead

Fuel cells are used in mobile devices

Fuel cells have a higher voltage

Reactants are externally supplied in fuel cells (correct)

What is the purpose of soaking the glass electrode in 0.1 HCl for 24 hours?

To prepare the electrode for pH measurement Signup and view all the answers

What is corrosion?

The loss of materials due to a chemical or electrochemical reaction Signup and view all the answers

What is the standard potential of a dry cell?

1.5 V Signup and view all the answers

What is the function of NH4Cl and ZnCl2 in a dry cell?

Electrolytes Signup and view all the answers

What is the voltage of a mercury cell?

1.35 V Signup and view all the answers

What is the characteristic of a redox electrode?

Two different oxidation states of the same metal are used in the same half cell. Signup and view all the answers

What is the effect of increasing the concentration of ions on the reduction potential?

It increases the reduction potential. Signup and view all the answers

What is the purpose of a salt bridge in a concentration cell?

To connect the two solutions of the same electrolyte having different concentrations. Signup and view all the answers

What type of batteries are those that cannot be reused once the active materials are consumed?

Primary batteries. Signup and view all the answers

What is the arrangement of galvanic cells connected in series called?

Battery. Signup and view all the answers

What is the equation that relates electrode potential with the concentration of ions?

Nernst equation. Signup and view all the answers

What is the type of cell where two electrodes of the same metal are dipped separately into two solutions of the same electrolyte having different concentrations?

Concentration cell. Signup and view all the answers

What is the reaction that occurs at the cathode during the electrolysis of molten sodium chloride?

Na+ + e- → Na. Signup and view all the answers

What is the reason for the formation of anodic and cathodic parts in a metal due to stress?

Due to bending, which produces strains Signup and view all the answers

What is the reaction that occurs at the anode in stress corrosion?

Fe → Fe2+ + 2e- Signup and view all the answers

What is the net reaction of stress corrosion?

Fe2+ + 2OH- → Fe(OH)2 Signup and view all the answers

What type of soil corrosion occurs in water-logged soils?

Micro-biological corrosion Signup and view all the answers

What is the primary factor influencing soil corrosion in gravel or sandy soils?

Moisture content Signup and view all the answers

How are soils classified based on particle size?

Into six types based on particle size Signup and view all the answers

What is the result of stress corrosion?

Formation of a crack in the metal Signup and view all the answers

What is the role of micro-organisms in soil corrosion?

They accelerate corrosion by producing acids Signup and view all the answers

What is the purpose of applying a primer to a steel surface?

To oxidize the steel surface and inhibit corrosion Signup and view all the answers

What is the result of heating steel in a furnace at high temperature after dipping it in a suspension of powdered glass?

The glass powder melts and coats the metal Signup and view all the answers

What is used to coat the inner side of steel pipelines carrying water or wastewater?

Cement coating Signup and view all the answers

What is the purpose of phosphating a steel surface?

To improve the bond between the metal and the paint Signup and view all the answers

What is the result of anodic oxidation of steel?

An oxide film coats the steel Signup and view all the answers

What is a common component of paint?

Inorganic oxide or metal powder Signup and view all the answers

What is the result of dipping steel in a solution containing phosphoric acid and zinc phosphate?

A layer of iron phosphate forms on the steel Signup and view all the answers

What is the purpose of cleaning a metal surface before applying paint?

To remove oxides and clean the surface Signup and view all the answers

What type of paint is suitable for severe marine and industrial atmospheres?

Epoxy resin paint Signup and view all the answers

What is the main component of lacquers?

Thermoplastic polymer Signup and view all the answers

What is the use of a 50% coal tar + 50% epoxy mixture?

To protect underground structures Signup and view all the answers

What is the purpose of a temporary coating?

To protect metallic structures during shipping and storage Signup and view all the answers

What is a corrosion inhibitor?

A chemical compound that decreases the corrosion rate of a metal Signup and view all the answers

How many substances are present in coal tar?

About 200 substances Signup and view all the answers

What type of corrosion inhibitor can work to slow the cathodic reaction down?

Cathodic inhibitor Signup and view all the answers

What are examples of cathodic inhibitors?

Sulfate and bisulfite ions Signup and view all the answers

Study Notes

Redox Electrode

A redox electrode uses two different oxidation states of the same metal in the same half cell.
Example: Fe2+ and Fe3+ are dissolved in the same container, with an inert electrode of platinum for electron transfer.

Nernst Equation

The Nernst equation relates electrode potential with the concentration of ions.
Reduction potential increases with the increase in the concentration of ions.
The equation is:
Substituting the values of R and F, we get:

Applications of Nernst Equation

The Nernst equation can be used to calculate the equilibrium constant for a Daniel cell.
At equilibrium, the Nernst equation can be applied to calculate the equilibrium constant.

Concentration Cells

Concentration cells are formed when two electrodes of the same metal are dipped separately into two solutions of the same electrolyte with different concentrations.
The solutions are connected through a salt bridge.
Example: Two electrodes of the same metal are dipped into two solutions of the same electrolyte with different concentrations.
There are two types of concentration cells:
- Electrode concentration cells
- Electrolyte concentration cells
The EMF of a concentration cell at 298 K is given by:

Cases of Electrolysis

Electrolysis of molten sodium chloride:
- The reactions occurring at the two electrodes are:
Electrolysis of an aqueous solution of sodium chloride:
- The reactions occurring at the two electrodes are:

Batteries

A battery is formed when Galvanic cells are connected in series to obtain a higher voltage.
Primary batteries:
- These batteries can be used until the active materials are present.
- Once they get consumed, the cell will stop functioning and cannot be reused.
- Examples: Dry cell or Leclanche cell and Mercury cell.
Dry cell:
- Anode: Zn container
- Cathode: Carbon (graphite) rod surrounded by powdered MnO2 and carbon.
- Electrolyte: NH4Cl and ZnCl2
- Reaction:
- The standard potential of this cell is 1.5 V and it falls as the cell gets discharged continuously.
Mercury cells:
- Anode: Zn – Hg Amalgam
- Cathode: Paste of HgO and carbon
- Electrolyte: Paste of KOH and ZnO
- The cell potential is approximately 1.35V and remains constant during its life.
Secondary batteries:
- These batteries can be recharged again and again for multiple uses.
- Examples: Lead storage battery and Ni – Cd battery.
Lead storage battery:
- To recharge the cell, it is connected with a cell of higher potential, and this cell behaves as an electrolytic cell.
- The reactions are reversed, and Pb(s) and PbO2(s) are regenerated at the respective electrodes.

Fuel Cells

A fuel cell differs from an ordinary battery in the sense that the reactants are not contained inside the cell but are externally supplied from an external reservoir.
Example: Fuel cell used in space vehicles, where the two gases are supplied from external storages.
In this cell, carbon rods are used as electrodes with KOH as the electrolyte.

Complete Cell for pH Measurement

The complete cell is set up to measure pH.
The glass electrode must be soaked in 0.1 HCl for 24 hours, followed by storing in distilled water.
The electrode results in the pH range 2-10, and up to 14 with a special glass.

Corrosion

Corrosion is the loss of materials as a result of chemical or electrochemical reaction with the environment.
The corrosion process can be classified on the basis of mechanisms.
Stress corrosion:
- It is an electrochemical corrosion.
- Presence of stress in a metal (due to bending) produces strains, which forms anodic part, while the remaining part acts as cathodic part.
- Corrosion reactions:
  - At Anode: Fe → Fe2+ + 2e-
  - At Cathode: O2 + H2O + 2e- → 2OH-
  - Net reaction: Fe2+ + 2OH- → Fe(OH)2
Soil or underground corrosion:
- Factors influencing soil corrosion:
  - Acidity of the soil
  - Moisture and electrolyte contents
  - Presence of micro-organisms and bacterias
  - Content of organic matter
  - Texture (physical properties) of the soil
- Classification of soil corrosion:
  - Based on the particle size, soils are classified into six types:
    1. Gravel or Sandy soils
    2. Water-logged soils
    3. Intermediate character soils
    - Oxidation (passivation):
      - Steel can be coated with an oxide film by:
        
        Heating at high temperature
        
        Chemical oxidation by treating steel with hot alkaline nitrate, or persulphate or perchlorate
        
        Anodic oxidation by making the steel structure an anode in an electrolytic cell

Corrosion Protection

Phosphating:
- Steel is coated with a layer of iron phosphate by dipping in a solution containing phosphoric acid and zinc phosphate.
- The iron phosphate film is not highly protective because it is porous, so it usually covered with paint.
Enamels:
- Enamels are glassy layers applied to the metal by dipping it in a suspension of powdered glass, and then the metal is heated in a stove (furnace) at high temperature.
Cement coating:
- It is used to coat the inner side of steel pipelines carrying water or wastewater.
Organic coating:
- Paints:
  - Paint consists of a film-forming substance, an organic solvent, and a pigment.
  - Before applying paint to a steel surface, the metal surface should be cleaned of oxides by sandblasting or acid pickling.
  - A primer is a paint containing a pigment such as lead oxide (Pb3O4 red lead) or zinc chromate which oxidizes the steel surface and inhibits its corrosion.
- Lacquers:
  - A lacquer consists of a thermoplastic polymer dissolved in an organic solvent.
  - Lacquers can be used to line steel tanks holding corrosive chemicals such as acids.
- Coal tar:
  - Coal tar is a brown or black liquid of extremely high viscosity.
  - It is used to protect underground structures.
- Temporary coating:
  - It is used to protect metallic structures during shipping and storage by coating the structure with a layer of lubricating oil which can be removed by an organic solvent when the structure is put to service.

Corrosion Inhibitors

A corrosion inhibitor is a chemical compound that can be added to liquids or gases to decrease the corrosion rate of a given material (usually a metal).
One method for the inhibition of corrosion is the addition of a coating on the surface of the metal which acts as a passivation layer and disallows access to the surface of the metal.
Types of corrosion inhibitors:
- Inorganic inhibitors:
  - Cathodic inhibitors:
    - Examples: Sulfite and bisulfite ions which can react with oxygen to form sulfates.
  - Anodic inhibitors:
  - Mixed inhibitors:
- Organic inhibitors:
- Green inhibitors:
- Vapor phase inhibitors:
- Volatile inhibitors:
- Contact inhibitors:
- Barrier inhibitors:
- Active inhibitors:
- Passive inhibitors:
- Cathodic protection:
  - It is a technique used to prevent corrosion by making the metal act as a cathode.
  - It is used to protect underground structures, ships, and other structures.

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Description

Learn about redox electrodes, where two different oxidation states of the same metal are used in the same half cell, and the Nernst Equation, which relates electrode potential with ion concentration.