Energetic definitions

• Endothermic


  A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants
• Exothermic


  A reaction in which the enthalpy of the products is less than the enthalpy of the reactants
• Exothermic reactions give out energy
• Standard conditions


  298K and 100kPa
• Standard enthalpy change


  Change in heat energy at constant pressure, measured at standard conditions
• Standard enthalpy of combustion

  The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions
• Standard enthalpy of formation

  The enthalpy change when one mole of a compound is formed from its elements under standard conditions
• Standard enthalpy of neutralisation

  The enthalpy change when one mole of water is produced in a neutralisation reaction under standard conditions
• Energy cannot be created or destroyed but it can change from one form to another
• The enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same
• Average bond enthalpy


  The energy required to break one mole of a given bond averaged over many compounds
• All chemical reactions involve

  • Making bonds (exothermic)
  • Breaking bonds (endothermic)
• The relationship between chemical reactions and heat changes is referred to as thermochemistry
• Activation energy


  The minimum energy required to break the bonds in the reactants
• Exothermic reactions


  • Chemical energy is converted into heat energy and the temperature of the system rises
• Exothermic reactions


  • Neutralisation reaction NaOH + HCL NaCl + H₂O
  • Combustion reaction CH₄ + 2O₂ CO₂ + 2H₂O
• Endothermic reactions


  • Heat energy is converted into chemical energy and the temperature of the system falls
• Endothermic reactions


  • Thermal decomposition CaCO₃
  • Photosynthesis