Energy Changes

• Endothermic reaction


  Reaction that feels like it gets colder
• Exothermic reaction


  Reaction that feels like it gets hotter
• Endothermic reaction


  Reaction that takes energy in
• Exothermic reaction


  Reaction that gives energy out
• Endothermic reactions


  • Electrolysis
• Exothermic reactions


  • Burning
  • Neutralization
• Calculating energy change for a reaction

  1. Write balanced equation

  2. List bond types and numbers

  3. Multiply bond energies

  4. Calculate energy change
• Exothermic reactions have a negative energy change
• Battery


  More than one cell
• Cell


  Single unit that produces a potential difference
• Non-rechargeable batteries


  • Chemical reaction that produces electricity is used up
  • Battery is then dead
• Rechargeable batteries

  • Reversible chemical reaction
  • Can be recharged by passing electricity through
• Hydrogen fuel cell


  Hydrogen gas reacts with oxygen gas to produce water and release energy
• Hydrogen fuel cells


  • Large amount of energy released
  • Only product is water, no carbon emissions
  • Problems with hydrogen production using fossil fuels
  • Difficulties with hydrogen storage and low temperature operation
• Hydrogen fuel cell reactions

  1. At negative electrode: Hydrogen gas - 2e- -> Hydrogen ions

  2. At positive electrode: Hydrogen ions + Oxygen gas + electrons -> Water
• Chemical reaction

  1. Bonds are made

  2. Bonds are broken
• Exothermic process

  Heat is given out, the surroundings get hotter
• Exothermic reactions

  • Combustion reactions
• Endothermic process

  Heat and energy is taken in, the surroundings get colder
• Endothermic reactions


  • Sports injury packs
  • Self-cooling drinks
• Exothermic reactions


  • Hand warmers (reusable and disposable)
  • Self-heating cans
• Measuring temperature change in a reaction


  1. Get reactants and place in polystyrene cup

  2. Take initial temperature reading

  3. Calculate change in temperature (final - initial)
• Reducing heat transfer in temperature change experiment


  • Use polystyrene cup
  • Put polystyrene cup in glass beaker
  • Wrap in insulating material
  • Put lid on
• Temperature increases


  Reaction is exothermic
• Temperature decreases

  Reaction is endothermic
• Variables in temperature change experiment

  • Independent: Reactants
  • Dependent: Temperature change
  • Control: Amount of reactants, amount of stirring
• Digital thermometer makes readings more accurate
• Repeat readings to make results more reliable
• Reaction profile

  Shows the progress of the reaction and how the energy is changing
• Activation energy

  Minimum energy required for the reaction to start
• Endothermic reaction

  Reactants have less energy than products
• Exothermic reaction


  Reactants have more energy than products
• Forming bonds is exothermic, breaking bonds is endothermic
• Using bond enthalpies to determine if a reaction is exothermic or endothermic

  1. Draw displayed formulas of reactants

  2. Identify bonds being broken (endothermic) and bonds being formed (exothermic)

  3. Calculate enthalpy change from bond enthalpies
• Exothermic

  Giving out heat
• Endothermic

  Taking in heat energy from the surroundings
• Deciding if a reaction is exothermic or endothermic from bond enthalpies

  1. Draw displayed formulas of reactants

  2. Identify bond breaking (endothermic) and bond making (exothermic)

  3. Calculate overall bond enthalpy
• If overall bond enthalpy is negative, the reaction is exothermic
• If overall bond enthalpy is positive, the reaction is endothermic
• Fuel cell

  Two metals of different reactivity as electrodes, with an electrolyte solution