Energetics

• Enthalpy Change (∆H o):

  • Bonds are broken and made in a reaction
  • Energy is taken in when bonds are broken
  • Energy is given out when bonds are formed
  • Enthalpy change is positive when energy is taken in
  • Enthalpy change is negative when energy is released
  • Overall enthalpy change can be calculated
• Measuring Enthalpy Change:

  • Enthalpy is measured under standard conditions of 100 kPa pressure and a temperature of 298 K
• Enthalpy of Formation (∆ f H o):

  • Enthalpy change when one mole of a substance is produced from its elements under standard conditions
• Enthalpy of Combustion (∆ c H o):

  • Enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
• Mean Bond Enthalpies:

  • Different covalent bonds require different amounts of energy to be broken
  • Bond enthalpy values calculated experimentally differ from data book values
  • Data book values are averaged values
• Calorimetry:

  • Experimental method for finding enthalpy change by measuring temperature change over time
  • Change in temperature is proportional to the energy change
  • Specific heat capacity is the energy required to raise 1g of substance by 1K without a change of state
  • Enthalpy change per mole of substance can be calculated
  • Enthalpy values found using calorimetry are not completely accurate
• Hess’s Law:

  • Overall enthalpy change for a reaction is the same regardless of the route taken
  • Hess’s Law is used to determine enthalpy changes for reactions that cannot be found directly using an experimental method
  • Triangular cycle method is used with an inbetween product
  • Enthalpies of Formation and Combustion can be calculated using Hess’s Law
• Bond Enthalpies:

  • Averaged value representing the energy required to break one mole of the stated bond in a gaseous state under standard conditions
  • Bond enthalpies can be used alongside Hess’s Law to find the overall enthalpy change for a reaction