AS Chemistry - Enthalpy Changes

AS Chemistry - Enthalpy Changes

1. Exothermic and Endothermic

   • Endothermic
     • The absorption of energy to the surroundings
     • Delta H is positive
     • Themal Decomposition
     • Photosynthesis
   • Exothermic
     • The release of energy to the surroundings
     • Delta H is negative
     • Combustion
     • Respiration

2. Types of enthalpy change

   • Standard Enthalpy Change of Reaction
     • The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states
   • Standard Enthalpy Change of Combustion
     • The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard state
   • Standard Enthalpy Change of Formation
     • The enthalpy change that takes place when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products being in their standard states

3. Determining the enthalpy change

   • q= m x c x delta t
   • q - enthalpy change (Joules)
   • m - mass of surroundings involved in heat exchange
   • c - specific heat capacity
   • delta t - temperature change of surroundings

4. Bond Enthalpies

   • the enthalpy change that takes place when breaking by homolytic fission 1 mole of a given bond in the molecules of a gaseous species
   • Bond breaking - ENDOTHERMIC - energy is needed to break bonds
   • Bond making - EXOTHERMIC - energy is released when making bonds
   • Using average bond enthalpies to determine enthalpy changes
     • Delta H = the sum of the bond enthalpies of bonds broken -- the sum of the bond enthalpies of bonds made