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Lecture 9

CHMB16Fall2012 Lecture 9 Notes.docx

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Kagan Kerman

`CHMB16Fall2012 Lecture 9: Fundamentals of Electrochemistry What is electrochemistry? o Studies the dynamics of electrical parameters in relation to specific chemical properties o Use of electrical measurements to draw info from a chemical system o Use of electricity to drive a chemical reaction o Use of a chemical reaction to produce electricity Oxidation and Reduction Reaction (Redox reactions) o Oxidation/reduction reactions involve the transfer of electrons and a change in oxidation state o A reducing age (reductant) is an electron donor o An oxidizing agent (oxidant) is an electron acceptor Rules for Determining Oxidation Numbers o Oxidation number of an element is itself is 0 o H is +1 unless it is in a metal hydride where H is -1 o O is -2 unless it is in peroxide where it is -1 o Alkali metals (Na, Li, K etc) are +1 o Alkaline earth metals (Be, Mg, Ca etc) are +2 o Halogens (F, Cl, Br, I) are -1 o The sum of oxidation numbers of each atom in a molecule MUST be equal to the charge of that molecule Balancing Redox Reactions o Assign oxidation numbers o o break the reactions into its 2 half reactions Balancing Redox Reactions Balancing Redox Reactions o o balance the atoms being oxidized/reduced for each half reaction  in this example the atoms are already balanced o balance the electrons for each half reaction +2 ++7 +7 +3 +3 +22 Balancing Redox Reactions STEPo5. Balance theoxygen atoms for each half-reaction by2ddingH O STEP 5. Balance the oxygen atoms for each half-reaction byaddingH O o balance the oxygen atoms for each half reaction by 2dding H2O STEPo. Balance thehydrogen atomsfoeeach half-reaction o balance the hydrogen atoms for each half reaction STEP6. Balance thehydrogen atoms foe each half-reaction o o multiply to balance and add the half reactions together to get the net equation  this example is balancing in acidic ST*Now both half-reactions should bebalanced!gether to get the net equation 9 *Now both half-reactions should be balanced! o 9 o if you have to balance in basic conditions, add OH- equal to H+ on both sides of the reaction Balanced! 10 1 o Charge 11 o q=nF where F = 9.649 x 10 C/mol e- o studies the dynamics of electrical parameters (current and potential) in relation to specific chemical properties o electric current: the amount of charge flowing past a point in a circuit each second  measured in A (1A = 1C/s) o electrical work: the electric potential (E) between 2 points is the work needed when moving charge between 2 points o this potential is measured in V o when moving a charge q through a potential difference E the work done is: o W =Eq Free Energy Change o Electrochemical Cells o Galvanic cell : does work, provides E through spontaneous electrochemical reaction o Electrolytic cell: requires work, E must be externally applied to drive the chemical reaction o Both types of cells can be also divided into reversible and irreversible cells o Reversible: reversing the direction of electron flow causes the direction of the electrochemical reactions to reverse o Irreversible: reversing the direction of electron flow causes different half reactions to occur at one or both electrodes o Electrons flow from the anode to the cathode o Oxidation occurs at the anode and reduction occurs at the cathode o Salt Bridges o salt bridge maintains electroneutrality throughout the cell o nitrate ions move into the cell at the anode and potassium ions move into the cell at the cathode o they balance the charges there, so there is no charge buildup o ex: since electrons
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