2.3.1 Enthalpy Changes

2.3.1 Enthalpy Changes

1. exothermic reaction

   • gives out heat
   • Delta H = -ve
   • temperature of surroundings ^^^
   • e.g combustion

2. Calculating enthalpy change directly

   • expt.
     • Q=MCAT

3. Calculating enthalpy changes indirectly

   • Combustion = arrows up
   • Formation = arrows down

4. Definitions

   • Activation energy = minimum energy req. to break bonds & initiate reaction
   • Standard conditions = 100 degrees 1 atmosphere
   • DeltaH of reaction = the enthalpy change when the products as specified in an equation react together under standard conditions
   • DeltaH of formation = The enthalpy change that occurs when one mole of a compound is formed from it's constituent elements in their standard states under standard conditions
   • DeltaH of Combustion = the enthalpy change when one mole of a substance reacts completely with 1 mole of oxygen under standard conditions
   • Average Bond Enthalpy = average enthalpy change that takes place when breaking by homiletic fission, 1 mole of a given type of bond in the molecules of a gaseous species
   • Hess's Law = states that if there is more than one route for a reaction and the initial and final conditions are the same then the total enthalpy change for each route is the same

5. Endothermic Reaction

   • Takes in heat
   • Delta H = +ve
   • Temp of surroundings = down
   • e.g Thermal decomposition