C7: energy changes

• Chemical reactions

  Involve energy being transformed from the surroundings and also into the surroundings because there is bond making and bond breaking going on
• Energy of bond breaking is more

  Reaction is endothermic and energy is taken into the surroundings
• Energy of bond making is more

  Reaction is exothermic and energy is given out to the surroundings
• Exothermic


  Energy is exiting, temperature increases
• Endothermic

  Temperature decreases
• Endothermic reactions


  • Usually associated with bond making
• Exothermic reactions

  • Usually associated with bond breaking
• Reaction profile for exothermic reaction

  1. Energy of products is less than energy of reactants

  2. Looks like a humpback shape
• Reaction profile for endothermic reaction

  1. Energy of products is higher than energy of reactants

  2. Difference in height between reactants and products is the energy taken in from surroundings
• Activation energy

  Energy required for reactants to get going
• Exothermic reactions

  • Combustion
  • Respiration
• Endothermic reactions

  • Cooking
  • Photosynthesis
• Exothermic applications

  • Self-heating cans
• Endothermic applications


  • Cooling packs
• Measuring exothermic/endothermic reactions

  1. Take initial temperature

  2. Control variables like heat from surroundings

  3. Record temperature every few seconds

  4. Temperature increases = exothermic, temperature decreases = endothermic
• Bond energy

  Energy in each bond, measured in kJ/mol
• Calculating exothermic/endothermic reactions using bond energies

  1. List bond breaking on left side

  2. List bond making on right side

  3. Compare energy of bond breaking vs bond making

  4. If bond making energy is greater, reaction is exothermic

  5. If bond breaking energy is greater, reaction is endothermic

  6. Calculate energy change by subtracting bond making from bond breaking
• Fuel cells

  • Use two different metals of different reactivity in a solution containing ions
  • Create a potential difference/voltage
• Factors affecting fuel cell voltage

  • Reactivity of metals used
  • Arrangement of metals (more/less reactive on left/right)
• Hydrogen fuel cells

  • Hydrogen gas is pumped in
  • Hydrogen atoms lose electrons, H+ ions travel through electrolyte, electrons travel around circuit
  • Combine with oxygen to form water as only product
• Advantages of hydrogen fuel cells

  • Quiet operation
  • Small size for energy produced
  • Produce drinking water in spacecraft
• Disadvantages of hydrogen fuel cells

  • Hydrogen is highly flammable
  • Reliability concerns compared to established petrol/diesel technology
  • Changing public perception