3B

• Enthalpy


  Measure of the heat energy in a chemical system
• Reactants and products have different bonds and structures


  They contain different amounts of energy
• System


  The reactants and products
• Surroundings


  The rest of the universe (e.g. equipment, laboratory)
• Exothermic


  Energy is transferred from system to the surroundings (-ve)
• Endothermic


  Energy is transferred from surroundings into the system (+ve)
• Exothermic reaction


  Temperature increase (-ve)
• Endothermic reaction


  Temperature decrease (+ve)
• Activation energy


  The minimum energy required to start a reaction by the breaking of bonds
• Calorimetry


  Measuring enthalpy changes
• Standard enthalpy change of combustion (ΔcH⦵)


  When one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states
• Standard enthalpy change of formation (ΔfH⦵)


  When one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
• Standard enthalpy change of neutralisation (ΔneutH⦵)


  Accompanies the reaction of an acid and base to form 1 mole of H2O(L) under standard conditions, with all reactants and products in their standard states
• Standard conditions

  • Temperature - 298K (25°C)
• Hess's law


  If a reaction takes place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each one
• Average bond enthalpies


  • Average enthalpy change that takes place when breaking, by homolytic fission, 1 mole of a given type of bond in the molecules of a gaseous species
  • +ve as it requires energy to break bonds
  • Note: GASEOUS so it will not be the same as with standard states
• Calorimetry


  Measurement of enthalpy changes in chemical reactions
• Calorimeter

  • Made from a polystyrene cup, vacuum flask or metal can
  • Carry out reaction, with a solution of deionised water included to help measure enthalpy change
  • Remember to measure volume of solution to find m
  • Find temperature change caused by reaction
  • Use volume and concentration or mass and mr to find moles of substance used in reaction
  • Calculate energy transferred using q=mcΔT
  • Convert q from J in kJ and divide by number of moles to find ΔH - enthalpy change of reaction
• Standard conditions

  • Concentration – 1moldm-3
• Standard Conditions

  • pressure - 100kPa