Topic 8 - Energetics I

• Chemical energy is made up of kinetic energy and potential energy
• Enthalpy is the measure of the total energy of a system
• Exothermic is when energy is transferred from the system to the surroundings (negative)
• Endothermic is when energy is transferred from the surroundings to the system (positive)
• Standard conditions are 100kPa and 298K
• Standard enthalpy change of reaction is the enthalpy change when one mole of a substance reacts at standard conditions
• Enthalpy change diagrams are similar to reaction profiles but only show enthalpy change
• The standard enthalpy change of combustion is the enthalpy change from one mole of a substance being completely burned in oxygen at standard condtions
• Experimental values may be different to data book values because:

  • Energy is lost to surroundings
  • Incomplete combustion
  • Not standard conditions
  • Not standard states
• The standard enthalpy of neutralisation is the enthalpy change when one mole of water is produced by the neutralisation of an acid and alkali at standard conditions
• Standard enthalpy change of formation is the enthalpy change from one mole of a substance being formed from its elements at standard states at standard conditions
• Hess' law states that the enthalpy change of a reaction is independant to the path taken in converting reactants into products, provided that intial and final conditions are the same
• Doing Hess' law calculations:

  1. List all the elements below and balance
  2. Draw arrows - up if combustion, down if formation
  3. Using the values, calculate route 2 and equal it to route 1 + x (REMEMBER BIG NUMBERS!!!)
  4. Find x
• Mean bond enthalpy is only approximate
• Bond enthalpy shorthand is written as E(C-C)
• When calculate enthalpy change of a reaction using bond enthalpies do sum of bonds broken - sum of bonds formed
• Enthalpy change is the amount of heat energy taken in or give out during any change in a system, providing the pressure is constant
• In an exothermic reaction, ∆H is negative
• In an endothermic reaction, ∆H is positive]
• Standard enthalpy change is formation is the enthalpy change when 1 mole of a compound is formed from its elements at standard states at standard conditions (298K and 100kPa)
• The standard enthalpy change of combustion is the enthalpy change when 1 mole of a substance is combusted completely in oxygen from standard states, at standard conditions (298K and 100kPa)
• Enthalpy change of reaction is the enthalpy change when the number of moles of reactants as specified in the balanced equation react together
• The enthalpy change of neutralisation is the enthalpy change when solutions of acid and alkali react under standard conditions to form 1 mole of water
• Hess's law states that the total enthalpy change for a reaction is independant of the route by which the chemical change takes place
• Hess law cycles are used when an enthalpy change can't be measure experimentally (ie thermal decomposition)
• energy change = mass of solution x specific heat capacity x change in temperature
• Errors in the calorimetre method:

  • energy losses from calorimetre
  • Incomplete combustion of fuel
  • Evaporation of fuel after weighing
  • Heat capacity of calorimetre not included
  • Measurements not carried out from standard conditions
• The mean bond enthalpy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules
• change in enthalpy = sum of bond enthalpies broken - sum of bond enthalpies formed
• Calculating change is enthalpy from bond enthalpies is less exact as bond enthalpies are an average