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Lecture

Electrochemistry Lecture Notes (Part II)

6 Pages
60 Views

Department
Chemistry
Course Code
CHEM 112
Professor
John Carran

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Description
Non standard Conditions • Concentration not always 1M or pressure not always 1atm o Delta E cell NOT = delta E degree cell Nernst Equation • ∆Ecell ∆Eocell– (RT)/(nF) lnQ Nernst Equation *** n = moles o f electrons transferred in redox) • Refer to slide 5 and 6 for special cases for Nernst Equation • If every solution in cell is 1M, deltaE cell = deltaE degree cell o Cell dead = reached equilibrium o Meaning Delta E cell = 0 Tying in thermochem, electrochem and equilibrium! Concentration Cells • Identical half cells = cell voltage of 0 • Possible to produce a potential difference other than 0 when using same half cells with one minor difference: concentrations of electrolytes in two half cells are different • On slide 11m , the reaction of slide 10 deltaE cell NOT = delta E degree cell. Refer to these slides for other information • Remember spontaneous (∆E > 0V, ∆G < 0J), therefore both electrons and ions migrating from the cell with low concentration to cell with high concentration • Concentration cell useful for measuring extremely low concentrations such as those required in Ksp calculations Measurement of Ksp • Ksp = solubility product constant • Ksp is equilibrium constant of slightly soluble salt in equilibrium with its ions (so it can dissociate) • Refer to slide 14 for diagram and example • Small Ksp value = not much favor for aqueous side Primary Cells • Not reversible • When reactants have been converted to products, cell is dead since electrical potential energy has been spent • Cell is then disgarded or recycled Secondary Cell • Cell reaction is reversible through charging • Can be used for hundreds of cycles of discharging and recharging Flow batteries or fuel cells • Reactants, products and electrolytes pass through the battery • Battery converts chemical into electrical energy Desirable properties of cells and batteries • Portable, safe, provide instant current long shelf life, rechargeable *** focus on stuff above. Below is less important*** Leclanche or Zinc – Carbon Cell • Most familiar primary cell • Often called drycell as no free liquid electrolyte present • Moist ammonium chloride paste (acidic/corrosive) – salt bridge • Zinc shell • Steal case prevents leakage once zinc anode begins to disintegrate • Con: ion concentrations change over time, voltage of cells falls as it is used Note: on slide 21, pay attention to states of the species in the overall cell reaction. In Nernst equation, you have aq, therefore the concentrations of ions will not stay constant over time Alkaline Dry Battery • Ammonium chloride is replaced by base, potassium hydroxide. Hence it is an alkaline cell • Products are all solid, therefore the ion concentrations do not change • Pro: longer life since hydroxide ions are regulated as fast as they are consumed • Voltage does not drop (no ions or gases in overall reaction Not possible to Recharge Alkaline Batteries • Zn(OH)2(s) and Mn2O3(s) do not stick to electrodes and hence not possible to reverse reaction by applying external voltage • Hydrogen gas may be produced, causing an explosion Rechargeable Batteries • Original electrodes can be regenerated by imposing an external potential (voltage) across them that reverses direction of current flow through cell • Primary batteries: doesn’t occur, as H2 (g) instead of original electrodes is ofte
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