ELECTRO

• Batteries used on satellites

  1. Provide power at "night" when satellite passes behind the earth and is no longer illuminated by the Sun

  2. In the "day" phase, energy is produced by solar panels, which recharge the batteries
• Battery issue with Boeing!
• • Since entering service, Boeing 787 lithium-ion batteries, each with eight cells, have logged more than 2.2 million cell-hours on the ground and in the air during more than 50,000 flight-hours
  • No battery-related incidents occurred before January 2013
• When a zinc metal strip and a copper metal strip were each applied to the opposite ends of a nerve in a frog leg soaked in brine, the leg would move
• • What was actually happening was an oxidation-reduction reaction between the zinc metal and hydrogen ions in the brine
  • This discovery paved the way for the development of electrical batteries

  
• All electrochemical processes involve the movement of electrons from one chemical species to another in an oxidation-reduction (redox) reaction
• Oxidation


  • One reactant loses electrons
  • Reducing agent is oxidized
  • Oxidation number increases
• Reduction


  • Other reactant gains electrons
  • Oxidizing agent is reduced
  • Oxidation number decreases
• • Zinc loses electrons
  • Zinc is the reducing agent and becomes oxidized
  • The oxidation number of Zn increases from 0 to +2
  • Hydrogen ion gains electrons
  • Hydrogen ion is the oxidizing agent and becomes reduced
  • The oxidation number of H decreases from +1 to 0

  
• Acceptance of electron results reduction, and release of electron results oxidation
• • In the reaction of zinc metal with copper(II) ion, the zinc metal loses two electrons and is oxidized
  • The copper(II) ion gains the two electrons and is reduced
  • Oxygen is the oxidizing agent and the zinc metal is the reducing agent

  
• Oxidation states


  The charges on the species in an oxidation-reduction reaction, which tell how many electrons have been gained or lost from the neutral state of the elements
• • The electrolyte conducts electricity due to its ability to dissociate in solution
  • The electrolyte solution is held in the salt bridge by dissolving material and plugging the ends with glass wool
  • The purpose of the salt bridge is to allow the movement of ions between the two solutions in order to prevent an excess of either positive or negative charge from building up in either container as the reaction proceeds

  
• As Zn2+(aq) ions are produced in the anode half-cell, they migrate through the salt bridge to the cathode half-cell

  
• How to design or set-up a system to utilize this spontaneous electron transfer

  1. Oxidation of an ion or molecule on the surface of an electrode releases electrons into the electrode, known as anode

  2. These electrons travel through external circuit to another electrode and being accepted by another ion or molecule, i.e. getting reduced

  3. The electrode where reduction happens, known as cathode

  4. The solution conducts electricity through the movement of ions, known as ionic conduction

  5. Cations travel towards cathode and anions travel towards anode (to balance the charge!!)

  
• If there is electron release, then released electron(s) must be accepted! If there is oxidation, there must be reduction
• Overall reaction in a galvanic cell

  1. Oxidation half-reaction: Zn(s) -> Zn2+(aq) + 2e-

  2. Reduction half-reaction: Cu2+(aq) + 2e- -> Cu(s)

  3. Overall cell reaction: Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s)

  
• Gaining of weight of Cu (0) and losing the weight of Zn (0)
• The sign of electrodes!! Anode is negative (-) and cathode is positive (+)
• Galvanic cell


  An electrochemical cell, where spontaneous chemical change is used to produce electric current
• Electrolytic cell


  An electrochemical cell, where electric current is used to carry out a non-spontaneous chemical reaction
• • The reaction at a particular electrode is called half reaction
  • The reduced state and oxidized state of substance taking part in the half reaction form a redox couple
• • The salt bridge is crucial in a galvanic cell
  • By allowing ions to flow into each half-cell, the bridge closes the circuit and allows current to flow
  • A wire can carry a current of electrons, but it cannot transport the ions needed to complete the circuit

  
• Cu2+


  Copper(II) ions

  
• Ag+


  Silver ions
• Salt bridge


  Allows ions to flow into each half-cell, closing the circuit and allowing current to flow
• Voltmeter


  Measures the cell voltage
• Cl-


  Chloride ions
• NH4+


  Ammonium ions
• If a wire connects the half-cells


  No voltage is measured. The wire cannot transport ions to close the circuit.
• If the wire is replaced with a salt bridge


  The release of ions at both ends of the bridge closes the circuit. Current can flow, and the cell voltage is measured.
• The salt bridge is crucial in a galvanic cell. By allowing ions to flow into each half-cell, the bridge closes the circuit and allows current to flow. A wire can carry a current of electrons, but it cannot transport the ions needed to complete the circuit.

  
• Without a salt bridge to close the circuit, local charges will build up around both electrodes. Neither electrode reaction can proceed to any significant extent, so no cell voltage can be measured.
• Copper(II) ions begin to enter solution

  Resulting in a positive charge and an electromotive potential.
• Electrons from the silver electrode react with Ag+ ions


  Leaving nitrate anions in solution.
• Energy density


  Measure of the maximum amount of stored energy per unit weight or volume
• Mnemonic


  "OIL RIG" - oxidation is loss (of electrons) and reduction is gain (of electrons)
• Components of some common electrochemical cells

  • Negative electrode
  • Positive electrode
  • Electrolyte
  • Nominal voltage
• Oxidation


  Loss of electrons, increase in oxidation state
• Reduction


  Gain of electrons, decrease in oxidation state