topic c5- energy changes

• exothermic reaction

  • transfers energy to the surroundings- shown by a rise in temperature
  • include:
  • combustion
  • neutralisation
  • most oxidation reactions
  • uses- self heating cans and hand warmers
  • freezing is an exothermic process
  • bond forming- exothermic
• exothermic reaction

  
• endothermic reaction

  • takes in energy from the surroundings- shown by a fall in temperature
  • include:
  • thermal decomposition
  • reaction between citric acid and sodium hydrocarbonate
  • uses- sports injury packs
  • melting is an endothermic process
  • bond breaking- endothermic
• endothermic

  reaction

  
• activation energy

  minimum amount of energy that reactants need to react
• bond breaking- endothermic

  HCl ->(energy) H + H - bond broken
• bond forming- exothermic

  C + O -> CO (strong bond formed) + energy released
• bond energy calculations

  overall energy change= total energy needed to break bonds - total energy released by forming new bonds
• e.g. calculate the energy change for the reaction between H2 and Cl2 forming HCl

  H-H + Cl-Cl -> H-Cl H-H=436 H-Cl=431

  H-Cl Cl-Cl=242

  436+242=678

  2x431=862

  678-862=-184kj/ mol