Topic 5

• Exothermic reaction


  a chemical reaction in which heat is released to the surroundings
• exothermic reaction profile


  Products are at a lower energy than the reactants

  
• Endothermic reaction


  A reaction in which energy is absorbed from surrounding
• endothermic reaction profile


  Products are at a higher energy than the reactants

  
• Bond energies


  Bond breaking = endothermic

  Bond forming = exothermic
• Bond energy calculations


  negative energy change = exothermic (as shown on reaction profile)

  
  

  positive energy change = endothermic (as shown on reaction profile)
• Chemical reactions in a cell


  - produce ELECTRICITY

  - 2 different electrodes in contact with an electrolyte (liquid that contains ions that react with the electrodes)

  -the chemical reaction between electrodes and electrolytes form a CHARGE DIFFERENCE

  - if the electrodes are connected to a wire then the electricity produced can flow.
• What effects the voltage of a cell

  - type of electrodes

  - the difference of reactivity of electrodes (the bigger difference = bigger voltage)

  - electrolyte used
• Non-rechargeable batteries


  - the chemical reactions at the electrodes are irreversible

  - eventually one of the reactants is used up the the reaction stops

  SO

  - electricity production stops

  - battery needs to be replaced
• what is used in FUEL CELLS

  FUEL and OXYGEN to produce electrical energy
• Hydrogen and oxygen fuel cells

  - involve redox reaction

  
  

  1) hydrogen goes into anode compartment

  1.5) oxygen goes into cathode compartment

  2) at anode hydrogen loses electrons, produces H+ ions - oxidation

  3) H+ ions move to cathode

  4) at cathode oxygen gains electrons and reacts with H+ ions to make water

  5) electrons flow through an external circuit from anode to cathode

  6) OVERALL REACTION is HYDROGEN + OXYGEN = WATER
• Hydrogen Fuel cell pros

  - not as many pollutants produced

  - less expensive than batteries

  - easier to dispose of than batteries

  - store more energy then batteries
• Hydrogen Fuel cell cons

  - Hydrogen is gas so harder to store

  - hydrogen is explosive when mixed with air so have to make sure it's securely stored

  - often formed from hydrocarbons (fossil fuels) or by electrolysis which uses electricity (generated by fossil fuels)