POTENTIALS AND THERMODYNAMICS OF CELLS (1) POTENTIALS AND THERMODYNAMICS OF CELLS (1)

BASIC ELECTROCHEMICAL THERMODYNAMICS Chemical Reversibility

Electromotive Force (emf) The potential difference between the anode and cathode in a cell is called the electromotive force (emf). It is also called the cell potential, and is designated E cell. Cell Potential Cell potential is measured in volts (V). 1 V = 1 JCJC

Free Energy  G for a redox reaction can be found by using the equation Under standard conditions  G = −nFE where n is the number of moles of electrons transferred, and F is a constant, the Faraday. 1 F = 96,485 C/mol = 96,485 J/V-mol

Nernst Equation Remember that  G =  G  + RT ln Q This means −nFE = −nFE  + RT ln Q

Nernst Equation Dividing both sides by −nF, we get the Nernst equation: E = E  − RT nF ln Q or, using base-10 logarithms, E = E  − RT nF ln Q

Nernst Equation At room temperature (298 K), Thus the equation becomes E = E  − n ln Q RT F = V

Nernst Equation At room temperature (298 K), Thus the equation becomes E = E  − n ln Q RT F = V

RECAP

It is critically important that you learn how to read, interpret, understand, and properly use this table. Voltage (or potential) represents the driving force for “pushing” the electrons from one location to another (for example, from a reducing agent to an oxidizing agent). The higher the voltage the more strongly the electrons will be pushed through the wire (or solution) It is critically important that you learn how to read, interpret, understand, and properly use this table. Voltage (or potential) represents the driving force for “pushing” the electrons from one location to another (for example, from a reducing agent to an oxidizing agent). The higher the voltage the more strongly the electrons will be pushed through the wire (or solution)

A positive cell potential (voltage) indicates a spontaneous electrochemical reaction. A negative cell potential (voltage) indicates a non- spontaneous reaction. Note that the sign notation here is opposite that we learned for ΔG!

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How ion-selective electrodes work Respond selectively to one ion. Do not involve redox processes. Features a thin membrane capable of binding only the intended ion. The electric potential difference across the ion- selective membrane is measured with two reference electrodes. Respond selectively to one ion. Do not involve redox processes. Features a thin membrane capable of binding only the intended ion. The electric potential difference across the ion- selective membrane is measured with two reference electrodes. Ion-Selective Electrodes (a)Solid-State Membranes (B) Liquid and Polymer Membranes

(a)Solid-State Membranes  Solid state membranes are electrolytes having tendencies toward the preferential adsorption of certain ions on their surfaces. E.g. the single-crystal LaF 3 membrane, which is doped with EuF 2 to create fluoride vacancies that allow ionic conduction by fluoride.  Its surface selectively accommodates F - to the virtual exclusion of other species except OH -.  Other devices are made from precipitates of insoluble salts, such as AgCl, AgBr, Agl, Ag 2 S, CuS, CdS, and PbS.  For example, the Ag 2 S membrane responds to Ag +, S 2-, and Hg 2+. Likewise, the AgCl membrane is sensitive to Ag +, Cl -, Br -, I -, CN -, and OH -.

(b) Liquid and Polymer Membranes An alternative structure utilizes a hydrophobic liquid membrane as the sensing element. The liquid is stabilized physically between an aqueous internal filling solution and an aqueous test solution by allowing it to permeate a porous, lipophilic diaphragm

ION-SELECTIVE ELECTRODES (ISE) - Also known as indicator electrodes - Respond directly to the analyte - Used for direct potentiometric measurements - Selectively binds and measures the activity of one ion (no redox chemistry) Examples pH electrode Calcium (Ca 2+ ) electrode Chloride (Cl - ) electrode

Three groups of ISEs - Glass electrodes-- Responsive to univalent cations Employs thin ion-selective glass membrane - Liquid electrodes - Solid electrodes

pH GLASS ELECTRODE - The most widely used - For pH measurements (selective ion is H + ) - Response is fast, stable, and has broad range - pH changes by 1 when [H + ] changes by a factor of 10 - Potential difference is V when [H + ] changes by a factor of 10 For a change in pH from 3.00 to 6.00 (3.00 units) Potential difference = 3.00 x V = Thin glass membrane (bulb) consists of SiO 4 - Most common composition is SiO 2, Na 2 O, and CaO Glass membrane contains - dilute HCl solution - inbuilt reference electrode (Ag wire coated with AgCl)

pH GLASS ELECTRODE Glass Electrode Response at 25 o C (potential across membrane with respect to H + ) ΔpH = pH difference between inside and outside of glass bulb β ≈ 1 (typically ~ 0.98) (measured by calibrating electrode in solutions of known pH) K = assymetry potential (system constant, varies with electrodes)

pH GLASS ELECTRODE - Equilibrium establishes across the glass membrane with respect to H + in inner and outer solutions - This produces the potential, E - Linearity between pH and potential - Calibration plot yields slope = 59 mV/pH units - Electrode is prevented from drying out by storing in aqueous solution when not in use

pH GLASS ELECTRODE Sources of Error - Standards used for calibration - Junction potential - Equilibration time - Alkaline (sodium error) - Temperature - Strong acids - Response to H + (hydration effect)