Electrolytes and conductivity Knowledge-Based Study

Study notes

1. What Is an Electrolyte?

   An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. Electrical current in a solution consists of the movement of charged particles. Unlike metallic conductors, where electric current is carried by mobile electrons, electrical conduction in aqueous solutions requires freely mobile ions.

   When an ionic compound or a polar covalent substance ionizes or dissociates in water, the resulting cations and anions are free to move toward oppositely charged electrodes when an external electric potential is applied:

   • Cations (positively charged ions) migrate toward the negative electrode (cathode).
   • Anions (negatively charged ions) migrate toward the positive electrode (anode).

   If a substance dissolves without generating ions, it cannot conduct electricity in solution.

   Classification by Extent of Ionization

   Electrolytes are broadly classified based on the extent to which they generate ions in aqueous solution:

   • Strong Electrolytes: Solutes that dissociate or ionize essentially completely (100%) in aqueous solution. Because they produce a high concentration of mobile ions, their solutions exhibit high electrical conductivity.
     Examples: Soluble ionic salts such as $\text{NaCl}$ and $\text{KNO}_3$; strong acids such as $\text{HCl}$, $\text{HNO}_3$, and $\text{H}_2\text{SO}_4$; strong bases such as $\text{NaOH}$ and $\text{KOH}$.
     Dissociation equation: $$\text{NaCl(s)} \rightarrow \text{Na}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$$
   • Weak Electrolytes: Solutes that ionize only partially in water, establishing a dynamic chemical equilibrium between un-ionized molecules and dissolved ions. Because only a small fraction of dissolved particles exist as ions, their solutions conduct electricity poorly.
     Examples: Weak acids such as acetic acid ($\text{CH}_3\text{COOH}$) and carbonic acid ($\text{H}_2\text{CO}_3$); weak bases such as aqueous ammonia ($\text{NH}_3$).
     Ionization equilibrium: $$\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{CH}_3\text{COO}^-\text{(aq)} + \text{H}^+\text{(aq)}$$

   Non-electrolytes

   A non-electrolyte is a substance that dissolves in water to yield a neutral, molecular solution containing essentially no ions. Because there are no charged carriers available to transport electrical current, these solutions do not conduct electricity.

   Common non-electrolytes include molecular compounds such as sucrose ($\text{C}_{12}\text{H}_{22}\text{O}_{11}$), glucose ($\text{C}_6\text{H}_{12}\text{O}_6$), ethanol ($\text{C}_2\text{H}_5\text{OH}$), and urea ($\text{CO(NH}_2)_2$). When dissolved, their molecules disperse intact among water molecules without breaking into charged ions.

   Key Factors Affecting Solution Conductivity

   1. Ion Concentration: Conductivity is directly related to the total concentration of dissolved ions. Higher ion concentrations provide more charge carriers per unit volume.
   2. Magnitude of Ionic Charge: Multivalent ions (such as $\text{Mg}^{2+}$ or $\text{SO}_4^{2-}$) carry more charge per ion than monovalent ions (such as $\text{Na}^+$ or $\text{Cl}^-$), enhancing conductivity at comparable concentrations.
   3. Ionic Mobility: Smaller ions and ions with lower effective hydration radii generally move through the solvent with less resistance, contributing to greater conductivity.
   4. Temperature: Increasing temperature lowers the viscosity of the solvent and increases the average kinetic energy of ions, thereby increasing ionic mobility and electrical conductivity.

   Summary of Solution Behavior

   To determine whether an aqueous solution conducts electricity, consider the chemical nature of the solute:

   • Soluble Ionic Compounds: Strong electrolytes that conduct strongly.
   • Strong Acids and Bases: Molecular or ionic solutes that ionize completely to form strong electrolytes.
   • Weak Acids and Bases: Polar covalent solutes that ionize partially, forming weak electrolytes.
   • Molecular Covalent Compounds (non-acids/bases): Non-electrolytes that do not conduct.

   Common Misconception: High solubility does not necessarily mean high electrical conductivity. For example, sucrose dissolves in water to extremely high concentrations, but because it dissolves exclusively as neutral molecules, its solution exhibits zero electrolyte conductivity.