Enthalpy

Enthalpy

1. Enthalpy

   • Heat content stored in a chemical system
   • Law of conservation energy
     • No heat energy is lost. It is transferred from one place to another
   • Enthalpy change
     • Heat exchange with surroundings during a chemical reaction.
     • At constant pressure; ΔH = H(products) - H(reactants)
     • KJ mol-1
   • Exothermic reactions
     • Enthalpy of products = smaller than enthalpy of reactants
     • Heat loss
     • ΔH = -ve
   • Endothermic reactions
     • Enthalpy of products > enthalpy of reactants
     • Heat gain
     • ΔH = +ve

2. Exo and endothermic reactions

   • Exothermic
     • Oxidation of fuels by combustion
     • Enthalpy of products < reactants
     • Excess energy released as heat
   • Endothermic
     • Thermal decomposition
     • Energy taken in

3. Standard enthalpy changes

   • Standard conditions
     • Pressure = 100 kPa
     • Teperature = 298K
     • Concentration = 1 mol dm-3
   • Substances must be in standard state

4. To find enthalpy change

   • ΔH = mcΔT
   • 1) Find energy and temp change. Then heat gained and lost (mcΔT)
   • 2) Find amount, in mol, reacted
   • 3) Scale quantities to match molar quantities in equation

5. Enthalpy change of combustion

   • 1) Find enthalpy change
   • 2) Find amount, in mol, of substance reacted
   • 3) Work out heat loss
     • Divide heat loss by amount, in mol, burnt
   • Difference between standard
     • Incomplete combustion
     • Heat loss to surroundings

6. Bond enthalpies

   • Tell you how much energy needed to break bonds
   • Breaking bonds - endothermic
   • Forming bonds - exothermic