3.1.4 Energetics

• Hess's law states if a reaction can happen by more than one route the overall enthalpy change is independent of the route taken.
• The standard enthalpy of formation is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
• The standard enthalpy of combustion is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions and all substances are in their standard states.
• Standard enthalpy change of a reactions using enthalpy of formation:

  ΔH° = (sum of products) - (sum of reactants)
• Standard enthalpy change of a reactions using enthalpy of combustion:

  ΔH° = (sum of reactants) - (sum of products)
• Equation of enthalpy change using bond enthalpies:

  ΔH° = (Sum of bonds broken)-(Sum of bonds formed)
• The mean bond enthalpy is the enthalpy change when 1 mole of a specific covalent bond is broken, averaged over different compounds
• Using Enthalpy of formation and combustion to work out the enthalpy change is more accurate the using mean bond enthalpy as it is an average value from a range of values
• Exothermic energy level diagram

  A) Activation energy

  B) Enthalpy change

  C) Reactants

  D) Products

  E) Energy

  
• Endothermic energy level diagram:

  A) Activation energy

  B) Enthalpy change

  C) Reactants

  D) Products

  E) Energy

  
• Equation for calculating energy change:

  q=mcΔT ΔH=q/mol

  q- energy (Joules)

  m- mass of water (g)

  c- specific heat capacity of water

  ΔT- change in temperature

  ΔH- enthalpy of combustion (kj/mol)
• Why is the enthalpy of combustion calculated Lower than the actual value?

  A lot of heat energy is lost from the simple calorimeter to the surroundings and also some incomplete combustion occurs
• The standard enthalpy of formation is the enthalpy change when one mole of a substance is formed from its elements, and all products and reactants are in their standard states and under standard conditions