Equilibrium Electrochemistry

Equilibrium Electrochemistry1ERT 108 Physical Chemistry Semester II Sidang 2010/20111ERT BBLee@UniMAPSubtopicsHalf-Reactions and ElectrodesVarieties of CellsThe Cell PotentialStandard PotentialsApplications of Standard PotentialsImpact on Biochemistry: Energy Conversion in Biological Cells2ERT 108 Physical Chemistry Semester II Sidang 2010/2011Equilibrium ElectrochemistryAn electrochemical cell consists: two electrodes (or metallic conductors)an electrolyte (an ionic conductor may be a solution, a liquid or a solid).An electrode & its electrolyte comprise an electrode compartment. two electrodes may share the same compartment if the electrodes are different, the two compartments may be joined by a salt bridge [a tube containing a concentrated electrolyte solution (potassium chloride in agar jelly)] that completes the electrical circuit & enables the cell to function.

ERT 108 Physical Chemistry Semester II Sidang 2010/20113Equilibrium ElectrochemistryA galvanic cell is an electrochemical cell that produces electricity as a result of the spontaneous reaction occurring inside it.

A electrolytic cell is an electrochemical cells in which a non-spontaneous reaction is driven by an external source of current.ERT 108 Physical Chemistry Semester II Sidang 2010/20114Half-Reactions and electrodesOxidation the removal of electrons from a species.Reduction the addition of electrons to a species.Redox reaction transfer of electrons from one species to another.Reducing agent (reductant) the electron donor.Oxidizing agent (oxidant) the electron acceptor.Any redox reaction (or even not redox reaction) may be expressed as the difference of two reduction half-reactions.Half-reactions conceptual reactions showing the gain of electrons. the reduced & oxidized species in half-reaction form a redox couple.ERT 108 Physical Chemistry Semester II Sidang 2010/20115Example 1Express the following reactions in terms of reduction half-reactions. The dissolution of silver chloride in water:

(Note: it is not a redox reaction.)The formation of H2O from H2 and O2 in acidic solution.

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Reaction quotient, QUseful to express the composition of an electrode compartment in terms of the reaction quotient, Q for the half reaction. The reaction quotient, Q has the form Q= activities of products/activities of reactants with each species raised to the power given by its stoichiometric coefficient.Q is defined as ERT 108 Physical Chemistry Semester II Sidang 2010/20117

Consider the reaction 2A + 3B C + 2D, in which case vA= -2, vB= -3, vC= +1 and vD= +2. the reaction quotient is then

Example:The reaction quotient for the reduction of O2 to H2O in acid solution O2(g) + 4H+ (aq) + 4e- 2H2O (l) is

The approximations used in 2nd step ate that the activity of water is 1 (because the solution is dilute) and the oxygen behaves as perfect gas, so aO2 pO2/p.

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Half-Reactions and electrodesAnode: the electrode at which the oxidation occurs. (-): removal of e-.Cathode:the electrode at which the reduction occurs. (+): addition of e-.ERT 108 Physical Chemistry Semester II Sidang 2010/20119

Half-Reactions and electrodesERT 108 Physical Chemistry Semester II Sidang 2010/201110

Varieties of cellsIn an electrolyte concentration cell- the electrode compartments are identical except for the concentrations of electrolytes.In an electrode concentration cell- the electrodes themselves have different conc, either because they are gas electrodes operating at different pressures or because they are amalgams (sol in mercury) with different concs. ERT 108 Physical Chemistry Semester II Sidang 2010/201111Varieties of CellsDaniel cell: the redox couple at one electrode is Cu2+/Cu and at the other is Zn2+/Zn.ERT 108 Physical Chemistry Semester II Sidang 2010/201112

Liquid junction potentialsLiquid junction potentials (Elj): an additional source of potential difference across the interface of the two electrolytes.E.g. In the Daniel cell (i) two different electrolyte solutions are in contact, (ii) different concentration of hydrochloric acid- At the junction, the mobile H+ ions diffuse into the more dilute solution. The bulkier Cl- ions follow, but initially do so more slowly- results in a potential difference at the junction. The potential then settles down to a value such that, after brief initial period, the ions diffuse at the same rates. The contribution of the liquid junction to the potential can be reduced by joining the electrolyte compartments through a salt bridge.

ERT 108 Physical Chemistry Semester II Sidang 2010/201113Liquid junction potentialsERT 108 Physical Chemistry Semester II Sidang 2010/201114

Galvanic cell without liquid junction.

Galvanic cell with liquid junction.

NotationPhase boundaries are denoted by a vertical bar.A liquid junction is denoted by Interface is denoted by a double vertical line ||. For which It is assumed that the junction potential has been eliminatedFig 1: Zn (s)|ZnSO4 (aq) CuSO4 (aq) |Cu (s)

Fig 2: Zn (s)|ZnSO4 (aq)||CuSO4 (aq) |Cu (s)ERT 108 Physical Chemistry Semester II Sidang 2010/201115

Fig 1Fig 2The cell potentialThe cell reaction corresponding to a cell diagram:1st : write the right hand half-reaction as a reduction (cathode) (Assumption: spontaneous reaction).2nd : subtract from it the left-hand reduction half- reaction. (By implication, the electrode is the site of oxidation)In the cell: Zn(s)|ZnSO4(aq)||CuSO4(aq)|Cu(s)

Right-hand electrode: Cu2+(aq)+2e- Cu(s)Left-hand electrode: Zn2+(aq)+2e- Zn(s)Overall cell reaction: Cu2+(aq)+ Zn(s) Cu(s) +Zn2+(aq)ERT 108 Physical Chemistry Semester II Sidang 2010/201116The Nernst equationA cell in which the overall cell reaction has not reached chemical equilibrium can do electrical work as the reaction drives electrons through an external circuit. the work that a given transfer of electrons can accomplish depends on the potential difference between the two electrodes.This potential differences is called the cell potential and is measured in volts, V (1 V = 1 JC-1 s).A cell in which the overall reaction is at equilibrium can do no work, & then the cell potential is zero.ERT 108 Physical Chemistry Semester II Sidang 2010/201117The Nernst equationWhen expressed in terms of a cell potential, the spontaneous direction of change can be expressed in terms of the cell emf. the reaction is spontaneous when E>0. the reverse reaction is spontaneous when E Table 9.1ERT 108 Physical Chemistry Semester II Sidang 2010/201130

Application of standard potentialsIn the Daniel cells:Cu2+(aq)+2e- Cu(s) E0=0.34VZn2+(aq)+2e- Zn(s) E0= - 0.76V

Zn spontaneously reduces Cu2+ to form Zn2+ and Cu.

Standard potential of a cell, Ecell Ecell = E(right)- E(left)

ERT 108 Physical Chemistry Semester II Sidang 2010/201132Impact on Biochemistry: Energy Conversion in Biological Cells.The whole of lifes activities depends on the coupling of exergonic & endergonic reactions, for the oxidation of food drives other reactions forward.In biological cells, the energy released by the oxidation of foods is stored in adenosine triphosphate (ATP). the essence of the action of ATP is its ability to lose its terminal phosphate group by hydrolysis & to form adenosine diphosphate (ADP).

where Pi denotes an inorganic phosphate group e.g. H2SO4.

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Impact on Biochemistry: Energy Conversion in Biological Cells.Examples: Glycolysis the oxidation of glucose to CO2 and H2O by O2 (the breakdown of foods is coupled to the formation of ATP in the cell).Glycolysis is the main source of energy during anaerobic metabolism, a form of metabolism in which inhaled O2 does not play a role.The citric acid cycle & oxidative phosphorylation are the main mechanisms for the extraction of energy from carbohydrates during aerobic metabolism (in which inhaled O2 does play a role).

ERT 108 Physical Chemistry Semester II Sidang 2010/201134Answer (Example 1)ERT 108 Physical Chemistry Semester II Sidang 2010/201135The two reduction half-reactions:

The redox couples are AgCl/Ag, Cl- & Ag+/Ag.The two reduction half-reactions:

The redox couples are H+/H2 & O2,H+/H2O

Answer (Example 2)ERT 108 Physical Chemistry Semester II Sidang 2010/201136For E = 0.01V,

= 1.476For E = 0.1V, K = 49.0For E = 1.0V, K = 8.02 x 1016