Redox Equilibria

Redox Equilibria

1. Redox Equations

   • Loss of electrons is called oxidation
     • An oxidising agent gets reduced
   • A gain of electrons is called reduction
     • A reducing agent gets oxidised
   • The oxidation state of an element tells you the total number of electrons it has donated or accepted

2. Electrode potentials and the electrochemical series

   • Electrochemical Cells
     • Electrochemical cells can be made from two different metals dipped in salt solutions of their own ions and connected by a wire
       • The solutions are connected by a salt bridge this allows ions to flow through and completes the circuit
     • Electrons flow through the wire from the most reactive metal to the least
       • A voltmeter in the external circuit shows the voltage between the two half-cells
         • This is the cell potential
   • Electrode Potentials
     • The metal that is oxidised has the more negative electrode potential
   • Drawing Electrochemical Cells
   • Calculating the Cell Potential
     • Cell potential will always be positive

3. Standard Electrode Potentials

   • The Standard Hydrogen Electrode
     • In the standard hydrogen electrode, hydrogen gas is bubbled into a solution of aqueous H+ ions.
       • A platinuim electrode is used as a platform for the oxidation/ reduction reactions
     • Everything needs to be done in standard conditions which are:
       • Any solutions of ions must have a concentration of 1.00 mol dm-3
       • The temperature must be 298K
       • The pressure must be 100KPa
   • The standard hydrogen electrode is always shown on the left of a cell diagram
   • The cell potential may be positive or negative

4. Electrochemical Series

   • An electrochemical series is a list of electrode potentials for different electrochemical half-cells
   • When two half equations are put together, the one with the more negative electrode potential goes in the direction of oxidation and the more positive electrode potential goes in the direction of reduction
   • More reactive metals have more negative standard electrode potentials
   • More reactive non-metals have more positive standard electrode potentials
   • You can use the standard electrode potentials to determine the direction of a reaction

5. Electrochemical Cells

   • Non-rechargeable cells
     • Use irreversible reactions
       • Reaction is irreversible because the metal anode forms the casing of the battery and becomes thinner as the metal is oxidised which could make it leak or expode
         • Also if they contain ammonium ions they will produce hydrogen gas which would escape from the battery so it can't react again
   • Rechargeable cells
     • Use reversible reactions
       • To recharge these batteries, a current is supplied to force the electrons to flow in the opposite direction around the circuit and reverse the reactions
         • This is possible because none of the substances in a rechargeable battery escape or are used up
   • Pros and Cons of Non-rechargeable Cells
     • They are cheaper to buy initially but have to be replaced so are more expensive in the long run
     • Work for longer but cannot be recharged
     • Cannot supply as much power
     • More are produced as they can only be used once so more resources are used up
     • Less likely to contain toxic metals which can leak in landfills
   • Fuel Cells
     • In a fuel cell the chemicals are stored seperately outside the cell and fed in when electricity is required
     • An example is the hydrogen-oxygen fuel cell
       • Hydrogen and oxygen gases are fed into two seperate platinium containing electrodes
         • These electrodes are made by coating a porous ceramic material with platinuim
           • Electrodes are sperated by an ion-exchange membrane that allows ions but not electrons through
             • The electrons flow from the negative electrode (hydrogen) through an external circuit to the positive electrode (oxygen)
           • Cheaper and larger surface area
     • Pros and Cons of Fuel Cells
       • They dont need electrical recharging
       • Only waste product is water
       • Needs energy to produce a supply of hygrogen and oxygen
         • Produced from electrolysis of water, electricity used generated from fossil fuels
       • Hydrogen is highly flammable so difficult to store and transport

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