Energy changes (Endo+exothermic)

• Exothermic reaction


  Energy is given out to the surroundings, surroundings temp increases
• Endothermic reaction


  Energy is taken from the surroundings, surrounding temp decreases
• Exothermic examples


  Self-heating cans

  hand warmers

  combustion reactions
• Endothermic examples 


  Instant ice packs

  decomposition reactions
• Endothermic - energy enters
• Exothermic - energy exits
• Energy level diagram show if a reaction is endothermic or exothermic
• Exothermic diagram


  overall charge is negative

  products have less energy than reactants so energy is released
• Endothermic diagrams 


  Overall charge is positive

  products have more energy that reactants so energy is taken in
• Energy is transferred when bonds are Brocken or formed
• Bond energy


  the amount of energy needed to break one mole of a particular covalent bond
• Calculating energy change


  Energy in - energy out = energy change
• Fuel cells


  Produce a voltage continuousLYY as long as they are supplied with:

  • a constant supply of a suitable fuel

  • oxygen eg. from air
• In fuel cells, the fuel is oxidised and energy is released as electrical energy
• Hydrogen fuel cell: A fuel cell that uses hydrogen as the reactant.
• Alkaline cell/batteries pros

  Cheap to manufacture
• Alkaline cell/batteries Cons

  May end up in landfill

  recycling is expensive
• Rechargeable cells pros

  Can be recharged and reused

  reduces use of recourses
• Rechargeable cells cons

  Costs more to manufacture
• Hydrogen fuel cell pros

  Easy to maintain - no moving parts

  small size

  water is only chemical product so sustainable
• Hydrogen fuel cell cons

  very expensive to manufacture

  needs a constant supply of hydrogen fuel which is a flammable gas