Energy Changes

• Exothermic: when a reaction releases energy into the surroundings

  Endothermic: when a reaction absorbs energy from the surroundings

  Reaction Profile: diagrams that show the energies of the reactions and products in a reaction and how the energy changes over the course of the reaction
• Activation Energy: the minimum amount of energy the reactants need to collide with each other and react

  Electrochemical Cell: a basic system made up of two different electrodes in contact with an electrolyte

  Fuel Cell: electrical cell supplied with a fuel and oxygen and the reaction between them produces electrical energy
• • Overall energy change = energy needed to break bonds of reactants – energy released forming bonds in products

  o In exothermic reactions, the energy from forming new bonds is greater then energy needed to break bonds in the reactants

  o In endothermic reactions, the energy to break bonds is greater than the energy released from forming new bonds in the products
• Bendo Mex

  Breaking is endothermic

  Making is exothermic
• Cells & batteries

  • Cells contain chemicals that react to produce electricity

  • A simple cell is made by connecting two different metals (electrodes)

  connected with an electrolyte (liquid containing ions which react with

  the electrodes)

  • Voltage produced by a cell depends on

  o Type of electrodes - bigger difference in reactivity between

  electrodes causes a bigger potential difference

  o Electrolyte - different ions will react differently with the

  metal of the electrodes

  • Batteries consist of two or more cells connected in series to provide a greater voltage (voltage adds up)
• • The more reactive metal is the negative electrode, the less reactive is the positive electrode

  • The more reactive metal donates electrons to the less reactive metal

  o Electrons flow from the negative electrode to the positive electrode

  o This produces a positive potential difference

  o If the negative electrode is less reactive than the positive electrode, then the potential difference produced is negative
• • In non-rechargeable batteries the chemical reactions stop when the reactants are used up

  o E.g. alkaline batteries

  • Rechargeable batteries & cells 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 (e.g. hydrogen) & oxygen or air. The fuel is oxidised

  electrochemically within the fuel cell to produce a potential difference
• Advantages of hydrogen fuel cells

  • Don’t produce the more harmful pollutants e.g.

  greenhouse gases- only by-products are water &

  heat whereas electric cars also don’t produce

  pollutants, but the batteries are made of toxic

  metal compounds

  • Batteries are more expensive & have a limit to the

  amount of time they can be recharged

  • Batteries store less energy
• Disadvantages of hydrogen fuel cells

  • Hydrogen is a gas so takes up more space than a rechargeable battery

  • Hydrogen is explosive when mixed with air – so has to be stored safely

  • Hydrogen fuel is often made by hydrocarbons (from fossil fuels) or by electrolysis of water which uses electricity

  o This electricity may be form non - renewable, polluting sources