Unit 2.1 - Thermochemistry

• Hess’s Law

  • Hess’s law states that the enthalpy change of a reaction is independent of the route of the reaction.
• Enthaply change of formation

  • It is the enthaply change when one mole of a compound is produced from its elements in their standard states.
• Enthalpy change of combustion

  • It is the enthalpy change when one mole of a substance is completely in excess oxygen under standard conditions.
• Enthalpy changes

  • All enthalpy changes are measured in standard conditions: 25 degrees or 298K
  • Exothermic enthalpy changes have a negative sign and Endothermic enthalpy changes have a positive sign.
• Exothermic reactions

  • Exothermic - heat (energy) is given out to the surroundings (ex=exit).
  • The reactants have more energy than the products.
  • Energy is released in the process and the surroundings get warmer.
  • Common exothermic reactions. - The burning or combustion of fuels. - Explosions are caused by VERY fact exothermic reactions.
• Endothermic reactions

  • Endothermic - heat (energy) is taken in from the surroundings (en=entrance).
  • The reactants have less energy than the products.
  • Energy is absorbed in The process and the surroundings get colder.
  • Common endothermic reactions. - Melting ice cubes - Baking Bread - Photosynthesis