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CED

9. Thermodynamics and electrochemistry

entropy increases when:

matter becomes more dispersed

phase change (solid to liquid or liquid to gas)

individual particles become freer to move and generally occupy a larger volume

increase in volume (at constant temperature)

gas molecules are able to move within a larger space

total number of moles of gas-phase products is greater than the total number of moles of gas-phase reactants

energy is dispersed (temperature increases)

according to kinetic molecular theory, the distribution of kinetic energy among the particles of a gas broadens as the temperature increases

entropy change (ΔS°) can be calculated from the absolute entropies of the species involved before and after the process occurs

⁠

Gibbs free energy (ΔG°)

standard state is necessary for Gibbs free energy

pure substances

solutions with a concentration of 1.0 M

gases at a pressure of 1.0 atm

standard Gibbs free energy change is a measure of thermodynamic favorability

ΔG° < 0: thermodynamically favored

also known as spontaneous

⁠

sometimes necessary to consider both enthalpy and entropy to determine if a process will be thermodynamically favored

e.g. freezing of water, dissolution of sodium nitrate

⁠

when ΔH° < 0 and ΔS° > 0, no calculation of ΔG° is necessary to determine that the process is thermodynamically favored (ΔG° < 0)

when ΔH° > 0 and ΔS° < 0, no calculation of ΔG° is necessary to determine that the process is thermodynamically unfavored (ΔG° > 0)

ΔH°

ΔS°

symbols

ΔG° < 0 (favored) at:

< 0

> 0

< >

all T

> 0

< 0

> <

no T

> 0

> >

high T

< 0

< <

low T

There are no rows in this table

⁠

under kinetic control: thermodynamically favored but does not proceed at a measurable rate

many processes that are thermodynamically favored do not occur to any measurable extent, or they occur at extremely slow rates

the fact that a process does not proceed at a noticeable rate does not mean that the chemical system is at equilibrium

if a process is known to be thermodynamically favored, and yet does not occur at a measurable rate, it is reasonable to conclude that the process is under kinetic control

thermodynamically favored (ΔG° < 0) means that the products are favored at equilibrium (K > 1) under standard conditions

⁠

connections between K and ΔG° can be made qualitatively through estimation

ΔG° is near zero: K is near 1

ΔG° is much larger or smaller than RT: K deviates strongly from 1

processes with ΔG° < 0 favor products (K > 1)

processes with ΔG° > 0 favor reactants (K < 1)

free energy change (ΔG°) for dissolution of a substance

multiple factors

breaking of intermolecular interactions holding the solid together

reorganization of the solvent around the dissolved species

interaction of the dissolved species with the solvent

possible to estimate the sign and relative magnitude of the enthalpic and entropic contributions to each of these factors

making predictions for the total change in free energy of dissolution can be challenging due to the cancellations among the free energies associated with the three factors

coupled reactions

using an external source of energy to make a thermodynamically unfavorable process occur

examples

electrical energy

drive an electrolytic cell

charge a battery

light

drive the overall conversion of carbon dioxide to glucose in photosynthesis

a desired product can be formed by coupling a thermodynamically unfavorable reaction that produces that product to a favorable reaction

e.g. conversion of ATP to ADP in biological system

individual reactions share common intermediate(s)

the sum of the individual reactions produces an overall reaction that achieves the desired outcome and has ΔG° < 0

electrochemical cells

each component plays a specific role in the overall functioning of the cell

electrodes

solutions in the half-cells

salt bridge

voltage/current measuring device

operational characteristics can be described at both the macroscopic and particulate levels

galvanic vs. electrolytic

direction of electron flow

electrode mass

evolution of a gas at an electrode

ion flow through the salt bridge

galvanic (voltaic) cell: involves a thermodynamically favored reaction

electrolytic cell: involves a thermodynamically unfavored reaction

visual representations of galvanic and electrolytic cells are tools of analysis to identify where half-reactions occur and in what direction current flows

oxidation occurs at the anode and reduction occurs at the cathode

electrochemistry: study of redox reactions that occur in electrochemical cells

thermodynamically favored: positive voltage

thermodynamically unfavored: negative voltage; requires externally applied potential for the reaction to proceed

standard cell potential can be calculated by identifying the oxidation and reduction half-reactions and their respective standard reduction potentials

⁠

positive E°: thermodynamically favored

negative E°: thermodynamically unfavored

in a real system under nonstandard conditions, cell potential will vary depending on the concentrations of active species

driving force toward equilibrium (the farther from equilibrium, the greater the magnitude of the cell potential)

equilibrium arguments (e.g. le Châtelier’s principle) do not apply to electrochemical systems because the system is not in equilibrium

standard cell potential E° corresponds to the standard conditions of Q = 1

as the system approaches equilibrium, the magnitude (absolute value) of the cell potential decreases, reaching zero at equilibrium (Q = K)

deviations from standard conditions

further from equilibrium than Q = 1: increase the magnitude of the cell potential relative to E°

closer to equilibrium than Q = 1: decrease the magnitude of the cell potential relative to E°

concentration cells: direction of spontaneous electron flow can be determined by considering the direction needed to reach equilibrium

Nernst equation

⁠

qualitative understanding of the effects of concentration on cell potential

Faraday’s laws: can be used to determine the stoichiometry of the redox reaction occurring in an electrochemical cell

with respect to:

number of electrons transferred

mass of material deposited on or removed from an electrode (as in electroplating)

current

time elapsed

charge of ionic species

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Gallery

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