Energetics

• Enthalpy change (ΔH°)


  The overall energy change of a reaction, depends on the energy taken in to break bonds and the energy released when new bonds are formed

  
• Positive enthalpy change


  Energy is taken in from the surroundings
• Negative enthalpy change


  Energy is released
• Enthalpy is measured under standard conditions of 100 kPa pressure and 298 K temperature
• Enthalpy of formation (ΔfH°)


  The enthalpy change when one mole of a substance is produced from its elements under standard conditions

  
• Enthalpy of combustion (ΔcH°)

  The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
• Mean bond enthalpies


  Values found experimentally using calorimetry, differ from data book values as they are not exact and vary in each situation

  
• Calorimetry


  1. Experimental method for finding enthalpy change by measuring temperature change over time

  2. Extrapolate data to give accurate value for temperature change

  3. Calculate energy change using q = mcΔT

  4. Calculate enthalpy change per mole

  
• Enthalpy changes found using calorimetry are never completely accurate as energy is easily lost from the system through conduction or convection and inaccuracies in measuring temperatures
• Hess's Law


  The overall enthalpy change for a reaction is the same, regardless of the route taken

  
• Using Hess's Law to determine enthalpy changes

  1. Use a triangular cycle method with an intermediate product

  2. Direction of arrows indicates whether values should be added or subtracted

  
• Using Hess's Law with enthalpies of formation

  Arrows point out from the central product as both reactants are formed from the elements at the central product

  
• Using Hess's Law with enthalpies of combustion

  Arrows point towards the central products (H2O and CO2) as both reactants burn to form the products
• Bond enthalpy data


  Averaged value representing the energy required to break one mole of the stated bond in a gaseous state, under standard conditions

  
• Using bond enthalpy data with Hess's Law

  Reaction arrows always point towards the central product