energetics 1.4

• Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant
• enthalpy change (∆H) units KJ mol^-1
• ΔH° = measurements were made under standard conditions (pressure and temp) with products and reactants in their standard states (gas, solid, liquid)
• conditions:

  • pressure = 100kpa
  • temperature = 298K
• exothermic reactions:

  • negative enthalpy change

  give out energy to their surroundings so temperature usually goes up
• endothermic reactions:

  • positive enthalpy change

  takes in energy from their surroundings so temperature usually goes down
• EXOTHERMIC:

  A) less

  B) activation

  C) negative

  
• ENDOTHERMIC:

  A) more

  B) positive

  
• standard enthalpy change of formation:

  is the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), and all reactants and products being in their standard states
• standard enthalpy change of combustion:

  when one mole of a substance is completely burned in oxygen under standard conditions (298K and 100kPa), with all reactants and products being in their standard states.
• standard enthalpy change of reaction:

  when a reaction occurs in the molar quantities shown in the chemical equation under standard conditions with all reactants and products in their standard states
• break bonds = put energy in = endothermic

  form bonds = release energy = exothermic
• MEASURING ENTHALPY CHANGES:

  energy change (J) = mass of solution (g) x specific heat capacity (J g-1K-1) x temperature change (K)
• Calculating the enthalpy change of reaction:

  1. Using q = m x c x T calculate energy change (change J ---> KJ)
  2. Work out the moles of the reactants used (n = m/Mr)
  3. Divide q by the number of moles of the reactant not in excess to give enthalpy change
• enthalpy change of neutralisation is the energy change when 1 mole of water is formed by the reaction of an acid and an alkali
• Hess's law is when the total enthalpy change for a reaction is independent of the route taken
• total enthalpy change of route 1 is the same as total enthalpy change for route 2
• The enthalpy change of formation of an element alone is always ZERO = it is in its standard elemental state
• a value may be different to standard enthalpy change of combustion as:

  • heat lost to surroundings
  • incomplete combustion of fuels
  • evaporation of fuel
• The Mean bond enthalpy is the enthalpy change needed to break the covalent bond into gaseous atoms, averaged over different molecules
• The enthalpy change of solution refers to the amount of heat that is released or absorbed during the dissolving process (at constant pressure)
• enthalpy of hydration is the enthalpy change when one mole of gaseous ions is dissolved in water to give one mole of aqueous ions and a solution of infinite dilution
• The lattice dissociation enthalpy is the enthalpy change needed to convert 1 mole of solid crystal into its scattered gaseous ions
• ion size has an impact on enthalpy of hydration:

  1. larger charge of the ion (eg. 2+ > 1+) has a larger charge density
  2. forces of attraction between the bigger ion and water is stronger
  3. bigger hydration enthalpy

  OR

  1. same size charge (eg. Br and Cl have 1-) but Cl is smaller ion
  2. smaller ionic radius so attraction between Cl ion and water stronger
  3. more exothermic