Enthalpy changes

• System


  The atoms and bonds involved in the chemical reaction
• Law of conservation


  The amount of energy in an isolated system remains the same. Energy cannot be destroyed or created, It can only be transferred from one form to another
• Breaking bonds


  Energy is taken in → endothermic reaction
• Making bonds


  Energy is released → exothermic reaction
• Endothermic reaction


  A reaction with an overall positive enthalpy change (+ΔH) → enthalpy of products > enthalpy of reactants

  
• Exothermic reaction


  A reaction with an overall negative enthalpy change (-ΔH) → enthalpy of products < enthalpy of reactants

  
• Enthalpy change diagram


  • Endothermic and exothermic

  
• Activation energy


  The minimum energy required for a reaction to take place
• The arrow for activation energy always points upwards on an enthalpy profile diagram
• Standard conditions


  100 kPa, 298 K
• In standard state


  The state an element / compound exists at in standard conditions (100 kPa, 298 K)
• Enthalpy change of formation

  The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions
• Equation representing standard enthalpy of formation

  • H2 (g) + ½ O2 (g) → H2O (l)
• Enthalpy change of combustion

  The energy change that takes place when 1 mole of a substance is completely combusted
• Equation representing standard enthalpy of combustion


  • C (s) + O2 (g) → CO2 (g)
• Enthalpy change of neutralisation


  The energy change that takes place when 1 mole of water is formed from a neutralisation reaction
• Enthalpy change of reaction


  The energy change associated with a given reaction
• Calculating enthalpy change from experimental data

  Use the equation Q = mcΔT, where m is the mass of the substance being heated (usually water), c is the specific heat capacity of that substance (water's SHC = 4.18gJ-1K-1) and ΔT is the change in temperature

  
• Simple calorimeter


  • Diagram
• Advantages of using a bomb calorimeter

  • Minimises heat loss
  • Pure oxygen used → ensures complete combustion
• Experimental methods for enthalpy determination may not be accurate due to heat loss to surroundings, not being in standard conditions, and the reaction may not go to completion
• Average bond enthalpy


  The mean energy required to break 1 mole of bonds in gaseous molecules
• Using bond enthalpies will not be as accurate as using standard enthalpy of combustion/formation because bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion and formation apply just to that molecule
• Calculating enthalpy change of reaction using average bond enthalpies

  ΔH = Σ (bond enthalpies of reaction) - Σ (bond enthalpies of products)