Energy changes

• In chemical reactions, energy is not created or destroyed
• Energy is conserved (saved) and transferred between objects
• Exothermic reactions


  Energy from the reacting chemicals is transferred to the surroundings, which often increase in temperature
• Exothermic reactions


  • Combustion
  • Neutralisation
  • Oxidation
  • Hand warmers
• Endothermic reactions


  Energy from the surroundings is transferred to the reacting chemicals, causing the temperature of the surroundings to decrease
• Endothermic reactions


  • Thermal decomposition
  • Citric acid + baking soda
  • Sports injury packs
• Disposable hand warmers use the energy released by iron oxidation
• Reusable hand warmers source their energy from the crystallisation of salt solutions
• When squeezed forcefully, ammonium nitrate and water mix in the pack, resulting in instant cooling
• Requirements for a Reaction to Happen

  • Collisions
  • Activation energy
• Collisions


  Particles of the reactants have to collide
• Activation energy


  The threshold amount of energy required for collisions to result in a reaction
• All chemical bonds have a "bond energy" that measures the strength of chemical bonds
• Determining if a reaction is endothermic or exothermic

  Subtract total bond energies of products from total bond energies of reactants
• Exothermic reactions


  Energy released by making bonds > energy required to break bonds
• Endothermic reactions


  Energy released by making bonds < energy required to break bonds
• Voltage output of a cell

  Can be changed by changing the type of electrode, the type of electrolyte or both
• Type of electrode


  • A greater difference in reactivity between the two electrodes will result in the cell having a greater voltage
• Type of electrolyte


  • Changing the electrolyte changes the reactions that happen at the electrodes
• Cells and batteries


  • Rechargeable
  • Non-rechargeable
• In non-rechargeable cells and batteries, irreversible reactions take place at the electrodes
• Electricity can no longer be produced in non-rechargeable cells as soon as one of the reacting chemicals has run out
• In rechargeable cells and batteries, connection to an electric current reverses the reactions that happen at the electrodes
• Electricity can continue to be produced in rechargeable cells as long as there is access to this external electric current
• Fuel cells


  Produce electrical energy using a reaction between an external fuel source (often hydrogen) and oxygen
• Advantages of fuel cells

  • High efficiency
  • Less pollution than traditional methods
  • Compact and lightweight
  • Reliable
• The absence of moving parts in fuel cells means they are reliable
• Hydrogen fuel cells vs Batteries

  Hydrogen fuel cells can be used as alternatives to rechargeable batteries
• Reasons hydrogen fuel cells may be better than rechargeable batteries


  • Toxicity
  • Lifetime
  • Capacity
• Batteries are made from toxic metal compounds and their disposal can cause pollution
• Hydrogen fuel cells use hydrogen and oxygen and the only waste product is water
• Batteries must be replaced after a certain number of recharges
• Hydrogen fuel cells can be reused indefinitely by adding more hydrogen and oxygen
• Hydrogen fuel cells produce more energy per gram of hydrogen relative to petrol
• Batteries have a smaller capacity than hydrogen fuel cells, and so have to be charged more regularly
• Hydrogen has to be stored at a high pressure and hydrogen gas is explosive