Energetics

• Define term enthalpy of combustion

  • The enthalpy change when 1 mol of a substance is burned completely in oxygen.
  • With all reactants and products in standard conditions those conditions being 100kPa and 298K
• Define term enthalpy change of formation

  • The enthalpy change when 1 mol of a substance is formed from its constituent elements
  • With all products and reactants in standard state/conditions
• Hess's law states that the overall enthalpy change for reaction is independent of the route the reaction takes at standard pressure

  
• To calculate standard enthalpy change you do the total sum of enthalpy of reactants - total enthalpy of products

  
• Enthalpy changes are always negative as energy is released during reactions.
• Exothermic reactions and bonds

  • Where more energy is released from making new bonds in products than compare to the energy taken in breaking bonds in reactants. So overall energy change is always negative
• Endothermic reaction and bonds

  • Where more energy is used to break bonds in reactants compared to the energy released when bonds are formed in products
  • Overall energy charge is positive
• Definition of mean bond enthalpy 


  Enthalpy energy change needed in breaking covalent bonds. Averaged for that type of bond over a range of compounds
• Bond enthalpy

  bond enthalpy = total bond energy of reactants - total bond energy of products

  
• Maxwell-Boltzmann

  On the X axis, energy is labelled while the y axis is labelled the number of particles with a given energy
• There's two reasons why standard enthalpy is different from data book values

  • Heat is loss to the surroundings
  • Heat absorbed by equipment
• If a standard enthalpy of formation of an element is not 0 then it's not in its standard state
• Enthalpy change equation

  q = m × c × ∆T

  q is heat energy in joules

  m is mass of substance being heated in grams

  c is the specific heat capacity of water measured in joules per Kelvin per gram

  ∆T change in temperature (both Kelvin and °C can be used )

  ∆H = -q ÷ n

  ∆H is enthalpy change measured joules per mol

  q is heat energy

  N is moles