Enthalpy Changes

• Exothermic 


  If the enthalpy value of reactants > products then energy must have been lost to the surroundings

  E.g. Combustion of fuels, Respiration
• Endothermic


  If energy of reactants < products then energy must have been gained from the surroundings

  E.g. Thermal decomposition of carbonates
• Enthalpy Change diagrams

  
• Standard Conditions


  Pressure 100kPa

  Temp 298 K

  Conc 1.0 mol dm⁻³ (for reactions with aq solutions)
• Heat energy


  Q = mcΔT
• Enthapy change


  ΔH = q ÷ n
• Exothermic Reaction


  ΔT ↑

  ΔH ↓
• Endothermic


  ΔT↓

  ΔH↑
• Standard enthalpy change of Formation ΔբH


  The enthalpy change when one mole of a substance is formed from its elements

  As this involves making bonds, it is usually exothermic
• Standard enthalpy change of Combustion Δ꜀H


  The enthalpy change that takes place during complete combustion of 1 mol of a substance
• Enthalpy Change if Combustion using Calorimetry


  Why experimental value may be less exothermic than book value:

  • Non-standard conditions
  • Incomplete combustion
  • Some beat is lost to surroundings, so smaller energy is transferred to water & smaller temp rise- put on lid or shield round beaker

  
• Standard Enthalpy Change of Neutralisation using Calorimeter

  1. Known vol acid of known conc placed into insulated plastic cup & temp of acid is measured
  2. Known vol alkali of known conc (stoichiometric equivalent or excess) placed into beaker & temp measured
  3. Average temp (Initial temp) of acid + alkali determined
  4. Acid + Alkali mixed in polystyrene cup & highest temp reached measured (final temp)
  5. From this, temp change is determined

  
• Scale for Stoichiometry for acid as limiting reagent

  
• Enthalpy Change of Neutralisation using Calorimetry

  
• Enthalpy Change of Combustion

  
• Number line diagrams to find the Change in value


  Δ= (end value) - (start value)

  
• Hess' Law


  The enthalpy change of a reaction only depends on the initial and final states, and is independent of the route taken
• Enthalpy cycle


  One direction - other direction

  
• Average Bond Enthalpy 


  The average enthalpy change that takes place when breaking, by homolytic fission, 1 mol of a given type of bond in the molecules of a gaseous species
• ΔH = bonds broken - bonds formed

  
• Standard enthalpy change of combustion

  
• Standard Enthalpy Change of Formation