10.1 Electrolytes and ionic conductivity

Electrolytes and ionic conductivity are key players in electrochemistry. These substances dissociate into ions when dissolved, creating conductive solutions that enable charge transfer between electrodes. Understanding their properties is crucial for designing effective electrochemical cells and batteries.

Strong electrolytes fully dissociate, while weak ones only partially do. Factors like concentration, temperature, and solvent properties affect conductivity. This knowledge helps optimize electrolyte performance in various applications, from batteries and fuel cells to electroplating and sensors.

Electrolytes and their properties

Dissociation and conductivity

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• Electrolytes dissociate into ions when dissolved in a solvent forming an electrically conductive solution
• Dissociation depends on the nature of the solvent and the strength of the electrolyte
• The presence of electrolytes is essential for the functioning of electrochemical cells, batteries, and electrolysis processes
• Properties of electrolytes (solubility, conductivity, ion mobility) determine their suitability for specific electrochemical applications

Role in electrochemistry

• Electrolytes facilitate the transfer of charge between electrodes enabling redox reactions to occur
• Electrolytes can be acids, bases, or salts (HCl, NaOH, NaCl)
• In batteries, electrolytes enable ion transfer between electrodes during charge and discharge processes
• In fuel cells, electrolytes transport ions between electrodes and convert chemical energy into electrical energy
• Electrolytes are used in electroplating to deposit thin metal layers onto substrates

Ionic conductivity in solutions

Measuring and defining conductivity

• Ionic conductivity measures the ability of an electrolyte solution to conduct electric current through the movement of ions
• Molar conductivity is the conductivity of a solution containing one mole of the electrolyte dissolved in a specific volume of the solvent
• The Kohlrausch law describes the relationship between molar conductivity and concentration for strong electrolytes molar conductivity decreases with increasing concentration due to ionic interactions

Ion movement and electric fields

• When an electric field is applied, cations (positively charged ions) move towards the negative electrode (cathode), while anions (negatively charged ions) move towards the positive electrode (anode)
• The conductivity of an electrolyte solution depends on the concentration of ions, their charge, and their mobility
• Higher ion concentration and mobility generally lead to higher conductivity
• Ions with higher charge and smaller size tend to have higher conductivity due to their greater mobility

Strong vs weak electrolytes

Degree of dissociation

• Strong electrolytes completely dissociate into ions in solution resulting in a high degree of ionization and high conductivity
• Examples of strong electrolytes strong acids (HCl, H2SO4), strong bases (NaOH, KOH), soluble salts (NaCl, KNO3)
• Weak electrolytes only partially dissociate into ions in solution resulting in a lower degree of ionization and lower conductivity compared to strong electrolytes
• Examples of weak electrolytes weak acids (acetic acid, carbonic acid), weak bases (ammonia, amines), slightly soluble salts (AgCl, CaCO3)

Dissociation constants

• The degree of dissociation of weak electrolytes is governed by their dissociation constant (Ka for acids, Kb for bases)
• The dissociation constant determines the equilibrium between the undissociated and dissociated forms of the electrolyte
• A higher dissociation constant indicates a greater degree of dissociation and higher conductivity
• The pH of a weak acid or base solution can be calculated using the dissociation constant and the initial concentration of the electrolyte

Factors influencing conductivity

Concentration effects

• As electrolyte concentration increases, conductivity generally increases due to the higher number of ions available for charge transport
• At very high concentrations, conductivity may decrease due to ion pairing and reduced ion mobility
• The relationship between conductivity and concentration is often non-linear, especially for weak electrolytes

Temperature dependence

• Conductivity increases with increasing temperature because higher temperatures lead to increased ionic mobility and faster ion transport
• The Arrhenius equation describes the relationship between conductivity and temperature $\kappa = A e^{-E_a/RT}$, where $\kappa$ is conductivity, $A$ is a pre-exponential factor, $E_a$ is the activation energy, $R$ is the gas constant, and $T$ is the absolute temperature
• The temperature dependence of conductivity can be used to determine the activation energy for ion transport in the solution

Solvent effects

• The conductivity of an electrolyte solution is influenced by the properties of the solvent, such as its dielectric constant and viscosity
• Solvents with high dielectric constants (water) favor ion dissociation, while solvents with low dielectric constants (hexane) hinder dissociation
• Solvents with low viscosity (water) enable faster ion mobility, while high viscosity solvents (glycerol) slow down ion transport
• The choice of solvent can significantly impact the conductivity and performance of electrochemical systems

Applications of electrolytes and ionic conductivity

Electrochemical power sources

• In batteries, electrolytes facilitate ion transfer between electrodes during charge/discharge (Li-ion batteries use LiPF6 in organic solvents)
• Fuel cells rely on electrolytes to transport ions and convert chemical energy into electricity (proton exchange membrane fuel cells use Nafion as the electrolyte)
• The conductivity of the electrolyte directly affects the power output, efficiency, and lifetime of these devices

Electrolytic processes

• Electroplating uses electrolytes to deposit thin metal layers (copper plating uses CuSO4 electrolyte)
• Electrolysis drives non-spontaneous redox reactions using electrical energy (water electrolysis produces H2 and O2 using KOH electrolyte)
• The conductivity of the electrolyte influences the rate, efficiency, and quality of the electrolytic process

Electrochemical sensors

• pH meters and ion-selective electrodes rely on the conductivity of electrolyte solutions to measure analyte concentration
• The conductivity is related to the activity of the target ion (H+ for pH, specific ions for ISEs)
• The performance and sensitivity of these sensors depend on the properties of the electrolyte and the electrode materials used