c7 - exothermic and endothermic reactions

• what is an exothermic reaction

  transfer energy from reacting molecules into the surroundings - temp of surroundings increases
• example of exothermic reaction

  combustion, oxidation and neutralisation
• how are energy changes shown

  energy profile diagram
• features of exothermic energy profile diagram

  • products have less energy than reactants because energy is transferred out
  • difference between energy of reactants and products tells how much energy has released into surroundings
• what can exothermic reactions be used for

  • hand warmers
  • self heating cans
• what is an endothermic reaction

  take in energy from surroundings - temperature of surroundings decreases
• example of exothermic reaction

  thermal decomposition
• features of exothermic energy profiles

  • products have more energy than reactants
  • difference in energy between reactants and products shows amount of energy taken in
• comparing energy profiles

  • both cases energy rises to a peak (activation energy)
• what is activation energy

  • reactions only occur when particles collide with sufficient energy
  • minimum amount of energy is called activation energy
  • this is shown from reactants to peak of the curve
• charge of energy transfer

  exothermic - negative because energy is transferred from chemicals

  endothermic - positive because energy is gained
• what are bond energies

  • when chemical bonds are broken it requires energy and is endothermic
  • making bond releases energy and is exothermic
  • every chemical bond has an energy value
  • tells us energy required to break the bond
• temperature required practical

  1. Measure 25cm' of hydrochloric acid into a polystyrene cup.
  2. Place the cup inside the beaker to make it more stable.
  3. Measure and record the temperature of the hydrochloric acid.
  4. Measure 5cm' of sodium hydroxide and add it to the polystyrene cup
  5. Quickly put a lid on the cup and gently stir the solution with the thermometer through the hole of lid.
  6. When the reading on the thermometer stops changing and becomes fairly constant, record the temperature.
  7. Repeat steps 4 and 5 to add further 5 cm' amounts of sodium hydroxide to the cup. A minimum total of 40 cm' needs to be added.
  8. Repeat steps 1-7 to ensure reliability of results.
  9. Calculate the mean maximum temperature reached for each of the sodium hydroxide volumes