Chemistry 27: Electro-chemistry

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Chemistry 27 Electro-chemistry

• Christopher Chui

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Circuits and Conductivity

• A complete metal circuit allows electrons to flow
• A galvanometer measures electric current
• Electric current is the flow of charged particles
• A salt bridge is a U-tube containing ionic
  substance in solution
• The galvanometer registers the current flow
  between two different metals placed in a salt
  solution

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Conduction and Potential Difference

• Metallic conduction or electronic conduction
  refers to the movement of electrons through a
  metal
• Electric potential difference is measured in
  volts
• The rate of electron flow is measured in amperes
• Electrolytes are substances whose solutions
  conduct electricity
• Molten salts and solutions of acids, bases, and
  salts conduct electricity
• Conduction that takes place by the migration of
  ions is called electrolytic conduction
• Positive ions are called cations negative ions
  are anions

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Electrode Reactions

• Positive ions (cations) are attracted to the
  cathode
• At the cathode, positive ions are reduced by
  gaining electrons Na e- ? Na
• Negative ions (anions) are attracted to the anode
• At the anode, negative ions are oxidized by
  losing electrons 2 Cl- ? Cl2 2 e-
• In a cell, oxidation and reduction occur as
  separate half-reactions at the same time
• Electrolysis is the use of an electric current to
  produce a chemical change

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Electrolysis of a Salt Solution

• In the electrolysis of a concentrated NaCl
  solution, H2 is produced at the cathode, and Cl2
  is produced at the anode
• 2 NaCl 2 H2O ? 2 NaOH H2 Cl2
• Electronic conduction is the movement of
  electrons through a metal
• Electrolytic conduction is the migration of ions
  through the liquid between the electrodes

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Voltaic Cells

• A voltaic cell converts chemical energy into
  electric energy
• A voltaic cell consists of zinc metal in a
  solution of zinc ions connected with a salt
  bridge and an external circuit to copper metal in
  a solution of copper ions
• Reactions take place at separate electrodes.
  Electrons flow from the anode to the cathode in
  the external circuit
• When using the shorthand method of representing
  cells, place the anode on the left

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Redox and Standard Electrode Potentials

• The hydrogen (reference) half-cell is assigned
  zero potential 2 H(aq) 2 e- ?? H2 (g)
• The hydrogen half-cell consists of a sheet of Pt
  immersed in 1M solution of H ions. H2 gas is
  bubbled into the solution at a pressure of 101.3
  kPa. The H2 molecules are adsorbed on the Pt
  surface and form the electrode
• Potentials vary with temp T, pressure P, and
  conc.
• Half-cell potential tables are useful for
  predicting the direction of chemical reactions
• Electrons flow from donor (anode) to acceptor
  (cathode)

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Cell Potential

• Standard reduction potential is an intensive
  property
• A negative voltage for a redox reaction means the
  reaction is not spontaneous, and the reverse
  reaction will occur
• Magnesium has a higher potential than copper
• If a reaction yields a negative voltage, the
  reverse reaction yields a positive voltage
• The standard reduction potential indicates a
  tendency of the half-reaction to gain electrons.
  The greater (more ve) the voltage, the greater
  the tendency to gain electrons

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Electrolysis and Effects of Conditions on Cells
and Energy

• German chemist, Nernst, found
• EE0 (RT)/nF ln(products conc/reactants conc)
• R is the gas constant T in K F factor from volt
  to J 96485 ln is natural log n is the number
  of electrons transferred
• After simplification, EE0 - 0.05916/n
  log(prod/react)

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pH Meter, Energy and Electric Cells

• The Nernest equation can be applied to half-cells
  or to an entire cell EE0-2.3 RT log K/(nF)
• E is a linear function of pH
• The pH meter makes use of a hydrogen ion
  electrode and a reference electrode, or a
  combination electrode
• At equilibrium, E0, therefore, 2.3 RT log K
  nFE0
• DG0 -nFE0

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Quantitative Electrochemistry

• Cell voltage can be used to find 1) Gibbs free
  energy change 2) tendency to gain electrons 3)
  tendency to lose electrons 4) equilibrium
  constant, Keq
• A reaction tends to be spontaneous if Keq gtgt1,
  E0gt0, and DG lt 0
• 1 coul is the amount of electricity produced by 1
  amp flowing for 1 sec 96485 coul 1 mole of
  electrons
• Cu 2 2 e- ?Cu
• 2 moles of electrons will reduce 1 mole of Cu 2
  to Cu
• Cr in Cr2O7 2- has an oxidation number of 6