energetics!

• Enthalpy Change (∆H)


  Heat energy change, represented by the symbol ∆H°
• Enthalpy


  • Measured under standard conditions of 100 kPa pressure and 298 K temperature
• Reaction


  1. Bonds are broken

  2. Bonds are formed
• Energy taken in from surroundings

  Enthalpy change is positive
• Energy released to surroundings

  Enthalpy change is negative
• Enthalpy change (∆H)


  Can be calculated as the difference between energy required to break bonds and energy released when new bonds are formed
• Energy level diagram


  • Indicates if reaction is endothermic or exothermic
• Endothermic reaction


  More energy is needed to break bonds than make new ones, so ∆H is positive and heat is taken in from surroundings
• Exothermic reaction


  More energy is released when making new bonds than is required to break existing ones, so ∆H is negative and heat is given out
• Enthalpy Change of Reaction (∆H)

  The enthalpy change when quantities of substances in standard states react completely under standard conditions
• Enthalpy Change of Formation (∆Hf)


  The enthalpy change when one mole of a substance is produced from its elements under standard conditions
• Enthalpy Change of Combustion (∆Hc)

  The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
• Enthalpy Change of Neutralisation (∆Hn)

  The enthalpy change when solutions of acid and alkali react together under standard conditions to produce one mole of water
• Calorimetry


  Experimental method for finding enthalpy change by measuring temperature change over time
• Calorimetry


  1. Data is extrapolated to give accurate value for temperature change

  2. Energy change is proportional to temperature change
• Specific heat capacity


  The energy required to raise 1g of substance by 1K without a change of state
• Enthalpy change values found using calorimetry are never completely accurate due to energy losses
• Sources of error in calorimetry

  • Heat loss due to conduction, convection or inaccuracies in temperature measurement
  • Specific heat capacity of solution assumed to be 4.18 kJ mol-1, which is the value for water and not the actual solution
  • Specific heat capacity of calorimeter not taken into account
• Hess's Law


  Energy in a reaction system must be conserved, so the overall enthalpy change for a reaction is the same regardless of the route taken
• Using Hess's Law


  1. Triangular cycle method with an intermediate product is used

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


  Hess's law can be used to calculate enthalpy change using given enthalpies of formation
• Enthalpies of Combustion


  Hess's law can be used to calculate enthalpy change using given enthalpies of combustion
• Bond enthalpy


  Averaged value representing the energy required to break one mole of the stated bond in a gaseous state under standard conditions
• Bond enthalpy values calculated experimentally often differ from databook values as they are not exact and vary in each situation</b>
• Mean bond enthalpy


  Tells you how much energy is required to break a particular bond, averaged out across the range of compounds containing that bond
• Bonds with lower bond enthalpy values


  Would be expected to break first in a chemical reaction
• Reaction requires breaking of many strong bonds or is endothermic overall


  Reaction is likely to occur slowly at room temperature
• Calculating overall enthalpy change using bond enthalpies

  Sum of bond enthalpies for bonds formed minus sum of bond enthalpies for bonds broken