Electrochem group 1 new (1)


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Electrochem group 1 new (1)

  3. 3. INTRODUCTION An electrochemical cell is a device in which electron transfer in a redox reaction are made to pass through an electric circuit. Oxidation process – loss of electron, the substance oxidized is the reducing agent. Reduction process – gain of electron, the substance reduced is the oxidizing agent. Two types of cell :  Galvanic cell / voltaic cell  Electrolytic cell
  4. 4. •A galvanic cell is anelectrochemical cellthat produces electricityas a result of thespontaneous reaction.•Also called as voltaiccell
  5. 5. Component of Galvanic cell The 2 metals are connected by a wire The 2 containers are connected by a salt bridge A voltmeter is used to detect voltage generated example: i- Zn metal in an aqueous solution of Zn2+ ii- Cu metal in an aqueous solution of Cu2+
  6. 6. Galvanic cell
  7. 7. What happens at zinc electrode? Zn is more electropositive than Cu Zn has a tendency to release electron Zn(s) Zn2+(aq) + 2e- Zn dissolves Oxidation occurs at Zn electrode Zn2+ ions enter ZnSO4 solution Zn is the negative electrode (anode)
  8. 8. What happens at copper electrode? Cu2+(aq) + 2e- Cu(s) The electron move from negatives to positive terminal Cu2+ ions from the solution accept electrons and the blue colour of copper(II) solution fades Cu is deposited Reduction occurs at the Cu electrode Cu is the positive electrode (cathode)
  9. 9. Cell NotationAnode: Zn(s) Zn2+(aq) + 2e-Cathode: Cu2+(aq) + 2e- Cu(s) Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)Also can be represented as: Zn(s) Zn2+(aq) Cu2+(aq) Cu(s)
  10. 10.  An electrolytic cell is an electrochemical cell in which a non- spontaneous reaction occur.
  11. 11.  It is made up of two electrodes immersed in an electrolyte A direct current is passed through the electrolyte from an external source Molten salt and aqueous solution are commonly used as electrolytes
  12. 12. Differences between Electrolytic and Galvanic cell Characteristic Electrolytic cell Galvanic cellEnergy change Electrical energy Chemical energy Chemical energy Electrical energyElectric current and Electric current results in a Chemical reaction producesreaction chemical reaction an electric currentCathode : Negative terminal Positive terminalAnode: Positive terminal Negative terminalNegative terminal Cation receives electrons Electrons are released at the from the cathode negative terminalPositive terminal Anions release electrons to Electrons are received by the the anode positive terminal
  13. 13.  Include the working electrode, reference electrode, and the auxiliary electrode. The three electrodes are connected to the power source, which is a specially designed circuit for precise control of the potential applied to the working electrode and often called a potentiostat or polarograph. This electrode system is important in voltammetry.  Voltammetry is an electrochemical technique in which the current-potential behaviour at an electrode surface is measured.
  14. 14. Auxiliary Electrode Counter or Auxiliary electrode : electrode in the cell that completes the current path. All electrochemistry experiments (with non-zero current) must have a working – counter pair. Auxiliary electrode makes sure that current does not pass through the reference cell. It makes sure the current is equal to that of the working electrodes current.
  15. 15. Reference electrode Serve as experimental reference points. Specifically they are a reference for the potential (sense) measurements. Reference electrodes should hold a constant potential during testing. Example: Saturated Calomel, Silver/Silver Chloride, Mercury/Mercury (mercurous) Oxide, Mercury/Mercury Sulfate, Copper/Copper Sulfate, and more.
  16. 16. Working Electrode Working electrode is the designation for the electrode being studied.  In corrosion experiments, this is likely the material that is corroding.  In physical echem experiments, this is most often an inert material— commonly gold, platinum or carbon— which will pass current to other species without being affected by that current.
  17. 17. ELECTROLYTEElectrochemical reactions occur in a medium, a solvent containing a supporting electrolyte which is mobile and support current flow. A medium containing mobile ions must exist between the electrodes in an electrochemical cell to allow for measurement of the electrode potential. Electrolyte provides the pathway for ions to flow between and among electrodes in the cell to maintain charge balance.
  18. 18. Liquid Electrolytes - Include molten salts and appropriate solventsElectrolytes Solid Electrolytes - Solids and some of those are crystalline solids
  19. 19. Liquid ElectrolytesMolecular Liquids Ionic Liquids Atomic Liquids Aqueous (water) Molten salts and usually used at Super Atomic Mixed aqueous Electrolyte (SPE) relatively high (water and temperatures Metallic mercury cosolvent) Blend of a solvating Nonaqueous (organic Mixtures of organic polymer and a salt or or inorganic solvent) a nonaqueous halides with electrolyte solution aluminium trichloride Exhibit various liquid electrolytes properties
  20. 20.  Choice-solubility of the analyte , its redox activity, and by solvent properties(electrical conductivity, electrochemical activity, and chemical activity) The solvent should not react with the analyte (or products) and should not undergo electrochemical reactions over a wide potential range.
  21. 21. PROPERTIES OF SOLVENTS Physical Chemical Boiling point  Acidity Melting point  Basicity Vapor pressure Heat of vaporization Relative permittivity
  22. 22. EFFECT OF SOLVENT PROPERTIES ON CHEMICAL REACTION Solvents with WEAK ACIDITY Solvents with STRONG ACIDITY• Solvation to small anions is difficult • Solvation to small anions is easy -Small anions are reactive -Small anions are nonreactive• Proton donation from solvent is difficult • Proton donation from solvent is easy -pH region is wide on the basic side -pH region is narrow on the basic side -Strong bases are differentiated -Strong bases are leveled -Very weak acids can be titrated -Very weak acids cannot be titrated• Reduction of solvent is difficult • Reduction of solvent is easy -Potential region is wide on negative -Potential region is narrow on negative side side -Strong reducing agent is stable in the -Strong reducing agent is unstable in solvent the solvent -Strong oxidizing agent is stable in the -Strong oxidizing agent is unstable in solvent the solvent -Substances difficult to reduce can be -Substances difficult to reduce cannot reduced be reduced
  23. 23. Solvents with WEAK BASICITY Solvents with STRONG BASICITY• Solvation to small cations is difficult • Solvation to small cations is easy -Small cations are reactive -Small cations are nonreactive• Proton acceptance by solvent is difficult • Proton acceptance by solvent is easy -pH region is wide on the acidic side -pH region is narrow on the acidic side -Strong acids are differentiated -Strong acids are leveled -Very weak bases can be titrated -Very weak bases cannot be titrated• Oxidation of solvent is difficult • Oxidation of solvent is easy -Potential region is wide on positive -Potential region is narrow on positive side side -Strong oxidizing agent is stable in -Strong oxidizing agent is unstable in the solvent the solvent -Substances difficult to oxidize can be -Substances difficult to oxidize cannot oxidized be oxidized
  24. 24. 1. A large number of the ions of one species should be mobile. This requires a large number of empty sites, either vacancies or accessible interstitial sites.  Empty sites are needed for ions to move through the lattice.2. The empty and occupied sites should have similar potential energies with a low activation energy barrier for jumping between neighboring sites.  High activation energy decreases carrier mobility, very stable sites (deep potential energy wells) lead to carrier localization.3. The structure should have solid framework, preferable 3D, permeated by open channels.  The migrating ion lattice should be ―molten‖, so that a solid framework of the other ions is needed in order to prevent the entire material from melting.4. The framework ions (usually anions) should be highly polarizable.  Such ions can deform to stabilize transition state geometries of the migrating ion through covalent interactions.
  25. 25. Liquid Electrolytes VS. Solid Electrolytes Liquid electrolytes show generally better leveling capabilities for both temperature and concentration discontinuities and allow for small volume changes due to chemical or electrochemical reactions. Liquid electrolytes maintain a permanent interfacial contact at the electrolyte or electrode interface and have generally higher conductivities. Liquid electrolytes is capable to dissolve the reaction products; they may hence be used in electro synthesis as reaction media. Liquid electrolytes are potential gassing and leakage problems in cells, and the higher effort in assembling cells. Solid electrolytes often offer cationic or anionic transport in contrast to liquid electrolyte, where anions and cations are contributing to the conductivity. Avoids the need for a separator. However, their electronic conductivity may be detrimental in some applications
  26. 26. What to consider in choosing electrolytes? Conductivity Mobility of active species Temperature Chemical thermal stability Electrochemical stability Solubility Viscosity
  27. 27. Supporting Electrolyte An electrolyte containing chemical species that are not electroactive (within the range of potentials used) which has an ionic strength and conductivity much larger than those due to the electroactive species added to the electrolyte. Inert electrolyte / inactive electrolyte The typical concentration of the supporting electrolyte is 0.1 to 1.0 mol/kg
  28. 28. Maintain constant ionic strength and constant pH ↑↓ resistance conductivity Functions of the solution eliminate the contribution of the analyte to the migration current & ↓transport number of electroactive species
  29. 29. Change metal ions in the sample to the metal-ion complexes with different electrochemical properties Functions Determine the useable potentialMaintain constant of range of the activity polarographic &coefficients and the voltammetricdiffusion coefficients measurement.
  30. 30. Example of supporting electrolyteAcids HCl, HNO3, H2SO4, H3PO4, Citric acidBases NaOH, KOH, TBAOH, NH4OHBuffers Citrate, Tartate, Acetate, Phosphate, BorateNon- Alcohols, Acetonitrile, DMF,aqueous DMSO-containing dissolvedSolvents salts for conductivity
  31. 31. Thank you!!