Enthalpy

• Exothermic


  • Energy released
  • Enthalpy change is negative
  • Making bonds
• Endothermic


  • Energy taken in
  • Enthalpy change is positive
  • Breaking bonds
• Enthalpy change = products - reactants
• Enthalpy

  Heat energy stored in a chemical system
• Activation energy

  MINIMUM energy required to break bonds and start a reaction
• Bond enthalpy

  Amount of energy required to break 1 mol of bonds
• What are the standard conditions?

  • 100kPa pressure
  • Stated temp - 298K
  • Concentration of 1 moldm-1
• Standard enthalpy change of reaction

  The enthalpy change that accompanies a reaction in the MOLAR QUANTITIES shown in a chemical equation under standard conditions, with all reactants and products in their standard states
• Example of the standard enthalpy change of reactions

  2 Mg + O2 -> 2MgO
• Standard enthalpy change of formation

  The enthalpy change that takes place when 1 MOLE of a compound is formed from its ELEMENTS under standard conditions, with all reactants and products in their standard states
• Example of standard enthalpy change of formation

  • Mg + 1/2 O2 -> MgO
  • 1/2 N2 + 3/2 H2 -> NH3
• Standard enthalpy change of combustion

  The enthalpy change that takes place when 1 MOLE of a substance reacts COMPLETELY with OXYGEN under standard conditions with all reactants and products in their standard states - Therefore MUST be complete combustion
• Example of enthalpy change of combustion

  Butane:

  C4H10 + 6 1/2 O2 -> 4 CO2 + 5 H2O
• Standard enthalpy change of neutralisation

  The enthalpy change that accompanies a reaction of an ACID by a BASE to form 1 MOLE of H2O under standard conditions, with all reactants and products in their standard states
• Example of standard enthalpy change of neutralisation

  H+ (aq) + OH- (aq) -> H2O (L) = -57 KJmol-1

  • Enthalpy change value is the same for ALL neutralisation reactions
• Equation that links energy change, mass of the liquid being heated, specific heat capacity and temp change

  q = mc/\T

  • q = Energy change (J)
  • m = Mass (g) -> Density of water = 1/gcm3
  • c = Specific heat capacity -> 4.18 Jg-1K-1
  • /\T = Temp change (celcius)
• Hess' Law

  If a reaction can take place by more than 1 route and the initial and final conditions are the same, the total enthalpy change is the same for each route
• Enthalpy change of formation cycle

  A

  2 SO2 + O2 --------> 2 SO3

  B C

  <----------Elements ---------> (Facing upwards to reactants + products from elements)
• Enthalpy change of combustion cycle

  A

  3 C + 4 H2 -----------> C3H8

  B C

  -----> Combustion products <----- (Facing downwards to combustion products from reactants + products)
• Why can the experimental of enthalpy change be different from the data book value?

  • Heat lost to surroundings
  • Incomplete combustion
  • Evaporation of fuel or water
  • Heat absorbed by appartus
  • Non-standard conditions used