energy changes

• energy is conserved in chemical reactions.
• if a reaction transfers energy to the surroundings the product molecules must have less energy than the reactions by the amount transferred
• an exothermic reaction transfers energy to the surroundings so the temperature of the surroundings increases
• exothermic reactions include:

  • combustion
  • many oxidation reactions
  • neutralisation
• everday uses of exothermic reactions include self heating cans and hand warmers
• an endothermic reactions takes in energy from the surrounding so the temperature of the surrounding decreases
• endothermic reactions:

  • thermal decomposition
  • reaction of citric acid and sodium hydrocarbonate
• chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
• the minium amount of energy that particles must have to react is called activation energy
• reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction
• overall energy - the difference in energy level between products and reactants measures in kJ/mol
• a reaction is exothermic when the energy of products will be at a lower level than reactants
• a reaction is endothermic when the products have a higher energy level than the reactants
• the activation energy is the difference in energy between the reactants and the peak of the curve
• during a chemical reaction:

  • energy must be supplied to break bonds in the reactants
  • energy is released when bonds in the products are formed
• The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction.
• In an exothermic reaction:

  • the energy released from forming new bonds is greater than the energy needed to break existing bonds.
• In an endothermic reaction:

  • the energy needed to break existing bonds is greater than the energy released from forming new bonds.
• cells contain chemicals which react to produce electricity
• The voltage produced by a cell is dependent upon a number of factors including the type of electrode and electrolyte.
• A simple cell can be made by connecting two different metals in contact with an electrolyte.
• Batteries consist of two or more cells connected in series to provide a greater voltage.
• In non-rechargeable cells and batteries, the chemical reactions stop when one of the reactants has been used up.
• Alkaline batteries are non-rechargeable.
• Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.
• fuel cells are supplied by an external source of fuel (hydrogen) and oxygen or air
• the fuel is oxidised electrochemically within the fuel cell to produce a potential difference
• the overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water
• hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries
• hydrogen fuels:

  • will produce electricity for as long as you provide hydrogen
  • do not become less efficient the longer they run
  • can be a source of drinking
  • run on an explosive gas (hydrogen)
  • produce a low potential difference so serval are needed
• rechargeable batteries:

  • run out and need to be charged
  • can store less electricity the more charging cycles they go through and eventually need to be replaced
  • no dangerous fuels are required with rechargeable batteries
  • can catch fire if not manufactured correctly
  • can produce a higher pd than a hydrogen fuel cell
• half equation for the electrode reactions in the hydrogen fuel cell:

  • positive ( anode) - O2 + 4H + 4e --> 2H20
  • negative ( cathode) - 2H2 +O2 --> 2H20