Enthalpy changes

• Breaking bonds

  Energy is taken in to break bonds → endothermic 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
• Making bonds

  Energy is released to make bonds → exothermic reaction
• System in a chemical reaction

  The atoms and bonds involved in the chemical reaction
• Endothermic reaction

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

  The minimum energy required for a reaction to take place
• Activation energy arrow on enthalpy profile diagram

  Always points upwards
• Exothermic reaction

  A reaction with an overall negative enthalpy change (-ΔH) → enthalpy of products < enthalpy of reactants
• Explain the law of conservation
• What energy change is making bonds associated with?
• What energy change is breaking bonds associated with?
• What is an exothermic reaction?
• What does system mean in a chemical reaction?
• Which way does the arrow for activation energy point on an enthalpy profile diagram?
• What is an endothermic reaction?
• What does activation energy mean?
• What are the standard conditions?
• What does “in standard state” mean?
• Define enthalpy change of formation
• Give an example of an equation which represents standard enthalpy of formation
• Define enthalpy change of neutralisation
• Define enthalpy change of combustion
• Give an example of an equation which represents standard enthalpy of combustion
• How can you calculate enthalpy change from experimental data?
• What does enthalpy change of reaction mean?
• The enthalpy change of formation is 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
• There are many examples, e.g., H2 (g) + ½ O2 (g) → H2O (l)
• The enthalpy change of neutralisation is the energy change that takes place when 1 mole of water is formed from a neutralisation reaction
• The arrow for activation energy always points upwards
• Standard conditions

  • 100 kPa
  • 298 K
• “In standard state” means the state an element/compound exists at in standard conditions (100 kPa, 298 K)
• For example, C (s) + O2 (g) → CO2 (g)
• The enthalpy change of reaction means the energy change associated with a given reaction
• The enthalpy change of combustion is the energy change that takes place when 1 mole of a substance is completely combusted
• What does average bond enthalpy mean?
• Advantages of using a bomb calorimeter

  • Minimises heat loss
  • Pure oxygen used → ensures complete combustion
• How to calculate enthalpy change of reaction using average bond enthalpies

  ΔH = Σ (bond enthalpies of reaction) - Σ (bond enthalpies of products)
• Why might experimental methods for enthalpy determination not be accurate?
• How to calculate 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
• Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?