Course: Ch 1209 - Chemistry tags: electro-chem

This study material provides a comprehensive breakdown of the fundamental terminology in electrochemistry as detailed in the sources.

1. Basic Concepts and Electrolytes

Electrochemistry is the branch of physical chemistry that examines the relationship between electrical energy and chemical change, specifically focusing on measurable electrical phenomena and identifiable chemical transformations,. These transformations occur via electrolytic cells, which convert electrical energy into chemical energy, and galvanic cells, which convert chemical energy into electrical energy.

An electrolyte is a substance that forms ions when dissolved in a solvent, allowing the resulting solution to conduct an electric current. Non-electrolytes are substances whose aqueous solutions do not conduct electricity, such as urea or glucose. Electrolytes are further subdivided into two categories:

• Strong Electrolytes: These substances, such as soluble salts (NaCl), strong acids (HCl), and strong bases (NaOH), are completely ionized when dissolved in water and conduct current very efficiently.
• Weak Electrolytes: These substances, such as acetic acid or ammonia, exhibit only a small degree of ionization in water, producing fewer ions and resulting in low current conduction,.

2. Mechanisms of Conduction

Conduction is categorized into two types based on the charge carrier:

• Electronic (Metallic) Conduction: Current is carried by the movement of electrons through a conductor without changing its chemical properties; notably, this conduction decreases as temperature increases.
• Electrolytic Conduction: Current is carried by the movement of ions through a solution or molten state,. This process involves a transfer of matter and is accompanied by chemical reactions at the electrode surfaces. Unlike metallic conduction, electrolytic conduction increases as temperature increases.

3. Electrolysis

Electrolysis is the process of using an external electric current to drive a chemical reaction within an electrolytic solution. During this process, a voltage is applied to two electrodes—metals in contact with the solution—causing cations (positive ions) to migrate toward the negative electrode (cathode) and anions (negative ions) to migrate toward the positive electrode (anode). The solution effectively acts as an electrical conductor through this ionic movement.

4. Conductance and its Types

Conductance ($C$) is defined as the reciprocal of electrical resistance ($1/R$) and is measured in units of $oh{m}^{-1}$, mho, or Siemens ($S$),. In electrochemistry, several specific types of conductance are used to describe the properties of solutions:

• Specific Conductance ($\kappa$): This refers to the conductance of a unit volume of a cell (specifically a $1\text{ cm}$ cube),. Its SI unit is $S{m}^{-1}$. While specific conductance decreases upon dilution because the number of ions per unit volume decreases, it increases with temperature,.
• Equivalent Conductance ($\Lambda$): This is defined as the conductance of a solution containing one gram-equivalent of an electrolyte dissolved in $V$ cc of solution. Mathematically, it is expressed as $\Lambda =\kappa \times \frac{1000}{c}$, where $c$ is the normality,.
• Molar Conductance ($\mu$): This is the conductance of a solution containing one mole of an electrolyte in $V$ cc of solution, calculated as $\mu =\kappa \times \frac{1000}{M}$, where $M$ is the molarity.

Both equivalent and molar conductance generally increase with dilution because the increase in volume ($V$) is greater than the decrease in specific conductivity ($\kappa$),.

5. Cell Constant

The cell constant ($K$) is a value derived from the physical dimensions and geometry of the conductivity sensor or probe,. It represents the relationship between the distance between the electrode plates ($l$) and the cross-sectional area of those plates ($a$), expressed as $K=l/a$,. For a standard cell where the plates are $1\text{ cm}$ apart and have an area of $1{\text{ cm}}^2$, the cell constant is $1{\text{ cm}}^{-1}$. A conductivity meter uses this constant to convert measured conductance into specific conductivity.

6. Modes of Mass Transfer (Ion Movement)

Mass transfer refers to the movement of material from one location in a solution to another, which can be driven by electrical potential, chemical potential, or physical movement,. The three primary modes are:

• Migration: The movement of a charged body (ion) under the influence of an electric field or a gradient of electrical potential. During migration, ions move toward the electrode with the opposite charge.
• Diffusion: The movement of a species under the influence of a chemical potential gradient, commonly referred to as a concentration gradient. An example is the Grotthuss Mechanism, where protons tunnel between water molecules via hydrogen bonding.
• Convection: The transport of material caused by the physical movement of the solution, such as stirring or hydrodynamic transport. This can occur naturally due to density gradients or be forced through mechanical means.