Enthalpy Changes

• Enthalpy


  The heat stored in a chemical system
• Exothermic (-ΔH)


  A product releases heat into the surroundings, therefore feels hotter
• Exothermic reaction profile

  
• Endothermic (+ΔH)


  A product absorbs heat from its surroundings, therefore feels cooler)
• Endothermic reaction profile

  
• Activation energy

  The energy required to start a chemical reaction
• Standard conditions

  298K and 100kPa
• Standard states

  Physical states under standard conditions
• Enthalpy change of reaction

  The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation under standard conditions
• Enthalpy change of formation

  The energy required to form one mole of a gaseous compound is formed from its elements under standard conditions
• Enthalpy change of combustion


  The energy required to completely burn one mole of a gaseous compound under standard conditions
• Enthalpy change of neutralisation

  The energy change of a reaction of an acid and a base associated with the formation of 1 mole of water, it is measured per mole of water produced
• Equation to measure enthalpy change

  Equation to measure enthalpy change

  Q=MCΔT where

  Q= Heat energy (J)

  M= Mass of solution (g)

  c = Specific heat capacity

  ΔT = Change in temp (K)
• Average bond enthalpy

  The breaking of 1 mole of bonds in gaseous molecules
• Making bonds is

  Exothermic
• Breaking bonds is

  Endothermic
• Hess cycle


  -An alternative route is used

  -Products and reactants are the same

  -Energy of initial reaction is equal to energy of alternative route reaction
• Why can't enthalpy of formation be calculated directly?

  The reaction may produce multiple compounds
• Which direction are the arrows pointing in a hess cycle of formation?

  Up (forming the products)
• Which direction are the arrows pointing in a hess cycle of combustion?

  Down