🧤Physical Chemistry I Unit 10 – Electrochemistry: Cells and Reactions

Fundamentals of Electrochemistry

• Electrochemistry studies the interconversion of electrical and chemical energy through redox reactions
• Involves the transfer of electrons between chemical species in a solution or at an electrode-solution interface
• Redox reactions are the foundation of electrochemistry where oxidation (loss of electrons) and reduction (gain of electrons) occur simultaneously
• Electrochemical processes are driven by the difference in electrochemical potential between the oxidized and reduced species
• Faraday's laws of electrolysis relate the amount of chemical change to the quantity of electricity passed through an electrochemical cell
  • First law states the mass of a substance altered at an electrode is directly proportional to the quantity of electricity transferred
  • Second law states the mass of a substance altered at an electrode is directly proportional to its equivalent weight
• Electrochemical systems have wide-ranging applications in energy storage (batteries), energy conversion (fuel cells), and chemical analysis (sensors)

Electrochemical Cells: Structure and Components

• An electrochemical cell consists of two half-cells connected by an external circuit and an electrolyte or salt bridge
• Each half-cell contains an electrode (anode or cathode) immersed in an electrolyte solution containing the redox species
• The anode is the electrode where oxidation occurs and electrons are released into the external circuit
• The cathode is the electrode where reduction occurs and electrons are consumed from the external circuit
• The electrolyte is a solution containing ions that facilitate charge transfer between the electrodes
• A salt bridge or porous membrane separates the half-cells while allowing the flow of ions to maintain charge balance
  • Prevents the mixing of the electrolyte solutions and maintains electrical neutrality
• The external circuit allows the flow of electrons from the anode to the cathode, generating an electric current
• The direction of electron flow determines the polarity of the electrochemical cell (galvanic or electrolytic)

Redox Reactions and Half-Cells

• Redox reactions involve the transfer of electrons between chemical species
• Oxidation is the loss of electrons and an increase in oxidation state
  • Occurs at the anode in an electrochemical cell
  • Example: $Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$
• Reduction is the gain of electrons and a decrease in oxidation state
  • Occurs at the cathode in an electrochemical cell
  • Example: $Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$
• Half-cells represent the individual oxidation or reduction reactions occurring at each electrode
• The oxidation half-cell is written as an oxidation reaction, while the reduction half-cell is written as a reduction reaction
• The overall redox reaction is obtained by combining the half-cell reactions and canceling out the electrons
• The standard cell notation describes the components of an electrochemical cell
  • Example: $Zn(s)|Zn^{2+}(aq)||Cu^{2+}(aq)|Cu(s)$

Electrode Potentials and Standard Reduction Potentials

• Electrode potential is the measure of the tendency of an electrode to gain or lose electrons in a redox reaction
• Standard electrode potential $E^0$ is measured under standard conditions (1 M concentration, 1 atm pressure, 25°C)
• Standard reduction potentials are tabulated for various half-cell reactions relative to the standard hydrogen electrode (SHE)
  • SHE has an assigned potential of 0.00 V and serves as a reference
• The half-cell with a more positive reduction potential undergoes reduction, while the half-cell with a more negative reduction potential undergoes oxidation
• The difference in standard reduction potentials determines the direction and magnitude of the cell potential
• The electrochemical series arranges elements based on their standard reduction potentials
  • Elements with more positive potentials are stronger oxidizing agents and are reduced more easily
  • Elements with more negative potentials are stronger reducing agents and are oxidized more easily

Nernst Equation and Cell Potential

• The Nernst equation relates the cell potential to the standard cell potential and the concentrations of the reactants and products
  • $E_{cell} = E^0_{cell} - \frac{RT}{nF} \ln Q$
  • $E_{cell}$ is the cell potential under non-standard conditions
  • $E^0_{cell}$ is the standard cell potential
  • $R$ is the universal gas constant (8.314 J/mol·K)
  • $T$ is the temperature in Kelvin
  • $n$ is the number of electrons transferred in the redox reaction
  • $F$ is Faraday's constant (96,485 C/mol)
  • $Q$ is the reaction quotient, the ratio of product concentrations to reactant concentrations
• The Nernst equation predicts the cell potential based on the concentrations of the reactants and products
• At equilibrium, $E_{cell} = 0$ and $Q = K$, where $K$ is the equilibrium constant
• The cell potential can be used to determine the spontaneity and direction of a redox reaction
  • If $E_{cell} > 0$, the reaction is spontaneous in the forward direction
  • If $E_{cell} < 0$, the reaction is spontaneous in the reverse direction
  • If $E_{cell} = 0$, the reaction is at equilibrium

Types of Electrochemical Cells

• Galvanic (voltaic) cells generate electrical energy from spontaneous redox reactions
  • Example: Daniell cell (Zn-Cu cell) used in early telegraphs and batteries
  • Anode undergoes oxidation, and cathode undergoes reduction
  • Electrons flow from anode to cathode through the external circuit
• Electrolytic cells use electrical energy to drive non-spontaneous redox reactions
  • Example: Electrolysis of water to produce hydrogen and oxygen gases
  • Anode undergoes oxidation, and cathode undergoes reduction
  • Electrons flow from an external power source to the cathode and then to the anode
• Concentration cells generate electrical energy from the difference in concentrations of a redox species between two half-cells
  • Example: Concentration cell with two hydrogen electrodes in solutions of different H+ concentrations
• Fuel cells generate electrical energy from the oxidation of a fuel (e.g., hydrogen) and the reduction of an oxidant (e.g., oxygen)
  • Example: Hydrogen fuel cell used in vehicles and power generation
  • Anode: $H_2(g) \rightarrow 2H^+(aq) + 2e^-$
  • Cathode: $\frac{1}{2}O_2(g) + 2H^+(aq) + 2e^- \rightarrow H_2O(l)$

Applications in Energy Storage and Conversion

• Batteries are electrochemical cells that store electrical energy in chemical form
  • Primary batteries (e.g., alkaline batteries) are non-rechargeable and have irreversible redox reactions
  • Secondary batteries (e.g., lithium-ion batteries) are rechargeable and have reversible redox reactions
• Fuel cells convert chemical energy directly into electrical energy through the oxidation of a fuel and the reduction of an oxidant
  • Hydrogen fuel cells use hydrogen as the fuel and oxygen as the oxidant, producing water as a byproduct
  • Methanol fuel cells use methanol as the fuel and have potential applications in portable devices
• Solar cells (photovoltaic cells) convert light energy into electrical energy through the photovoltaic effect
  • Dye-sensitized solar cells use a photoactive dye to absorb light and generate electrons
• Supercapacitors store electrical energy in the electric double layer formed at the electrode-electrolyte interface
  • Offer high power density and fast charge-discharge cycles compared to batteries
• Electrochemical sensors detect and measure the concentration of specific analytes based on their redox behavior
  • Example: Glucose sensor used in blood glucose monitoring for diabetes management

Electrochemical Techniques and Measurements

• Potentiometry measures the cell potential under zero current conditions
  • Used in pH meters and ion-selective electrodes for determining the concentration of specific ions
• Voltammetry studies the current-potential relationship in an electrochemical cell
  • Cyclic voltammetry (CV) applies a cyclic potential sweep and measures the resulting current
  • Used to study the redox behavior of electroactive species and determine their formal potentials
• Amperometry measures the current at a fixed potential
  • Used in electrochemical sensors and detectors for quantitative analysis
• Coulometry measures the total charge passed during an electrolysis reaction
  • Used to determine the amount of substance electrolyzed based on Faraday's laws
• Electrochemical impedance spectroscopy (EIS) measures the impedance of an electrochemical system as a function of frequency
  • Provides information about the kinetics and mechanisms of electrochemical processes
• Scanning electrochemical microscopy (SECM) uses a microelectrode to probe the local electrochemical behavior of a surface
  • Enables high-resolution imaging and mapping of electrochemical activity
• Electrochemical quartz crystal microbalance (EQCM) combines electrochemical measurements with mass changes on a quartz crystal resonator
  • Allows the study of mass changes associated with electrochemical processes, such as film deposition or adsorption