Cells & Batteries

• Chemical reactions in a cell produce electricity
• An electrochemical cell is a basic system made up of two different electrodes in contact with an electrolyte

  
• The 2 electrodes must be able to conduct electricity and so are usually metals

  
• The electrolyte is a liquid that contains ions which react with the electrodes

  
• The chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes. If the electrodes are then connected by a wire, the charge is able to flow and electricity is produced. A voltmeter can also be connected to the circuit to measure the voltage of the cell

  
• Different metals will react differently with the same electrolyte - this is what causes the charge difference, or the voltage, of the cell. So the type of electrodes used will affect the voltage of the cell
• The bigger the difference in reactivity of the electrodes, the bigger the voltage of the cell
• You can predict what the voltage of a cell might be from information about the voltages or other cells
• Example

  The voltages of 2 simple cells with the same electrolyte & different electrodes are shown in the table. If the metal used for Electrode A is less reactive than the other metal electrode, the V is positive & vice versa. Calculate the V of a cell where E. A is lead & E. B is tin.

  > The V of both cells are negative, so iron is more reactive than tin & lead. The V for the iron/lead cell is more negative than the iron/tin cell, so lead is less reactive than tin. The difference in V of these two cells is 0.01 V. So, for a cell where Electrode A is lead & Electrode B is tin, the V is +0.01 V

  
• The electrolyte used in a cell will also affect the size of the voltage since different ions in solution will react differently with the metal electrodes used

  
• A battery is formed by connecting two or more cells together in series. The voltages of the cells in the battery are combined so there is a bigger voltage overall

  
• In non-rechargerable batteries the reactants get used up
• In some cells, the chemical reactions that happen at the electrodes are irreversible
• Over time the reacting particles - the ions in the electrolyte and metal ions on the electrode - get used up and turned into the products of the reaction
• In an irreversible reaction once any of the reactants is used up, the reaction can't happen and so no electricity is produced. The products can't be turned back into the reactants, so the cell can't be recharged
• Non-rechargeable batteries, e.g. alkaline batteries, contain cells which use irreversible reactions. Once one of the reactants is used up, they don't produce any more charge and you have to replace them
• In a rechargeable cell, the reaction can be reversed by converting it to an external electric current