1.4 Energetics

• Define enthalpy

  Thermal energy stored in a chemical system
• Define enthalpy change.

  Heat energy change under constant pressure
• What is the standard enthalpy change?

  Enthalpy change under standard conditions (298K and 100kPa)
• What is the standard enthalpy of formation?

  Enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions
• What is the standard enthalpy of combustion?

  Enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in their standard states under standard conditions
• What is an exothermic reaction?

  Heat energy is released to surroundings therefore ΔH is -ve
• What is an endothermic reaction?

  Heat energy is absorbed from surroundings therefore ΔH is +ve
• When is a reaction exothermic?

  When energy released from bond making is greater than energy taken in when bonds are being broken

  Temperature increases
• When is a reaction endothermic?

  Energy required to break bonds is greater than energy released from making bonds

  Temperature decreases
• How do you calculate ΔH?

  ΔH = ΔHproducts - ΔHreactants
• What is activation energy?

  Minimum energy required for a reaction to occur
• What is calorimetry?


  the process of measuring the amount of heat released or absorbed during a chemical reaction
• Why do experimental values in calorimetry often differ from data book values?

  - reaction may be incomplete

  - incomplete combustion

  - heat capacities and densities of the solutions are only approximates

  - heat may be lost to surroundings

  - experimental conditions may not be standard conditions

  - fuel or water may evaporate before reweighing
• What equation measures heat change?

  q=mcΔT

  
  

  Q= heat change (J)

  M= mass of substance (g)

  C = specific heat capacity (4.18 Jg-1K-1)

  ΔT = temperature change (K)
• What are bond enthalpies?

  Energy needed to break a bond
• What is the mean bond enthalpy?

  Enthalpy change when one mole of covalent bonds is broken to give the free atoms, averaged over a range of compounds
• Is bond breaking an exothermic or endothermic process?

  Endothermic as requires energy therefore +ve
• Is bond making an exothermic or endothermic process?

  Exothermic as energy is released therefore -ve
• What is Hess's Law?

  The ΔH for a reaction is independent of a route taken