2.1 Thermochemistry

• Enthalpy change


  The overall energy change of a reaction, depending on how much energy is transferred when bonds are broken and formed
• Positive enthalpy change


  Energy is taken in from the surroundings, the reaction is endothermic
• Negative enthalpy change


  Energy is released, the reaction is exothermic
• Enthalpy of formation (∆​f​H​o​)


  The enthalpy change when one mole of a substance is produced from its elements under standard conditions
• Enthalpy of combustion (∆​c​H​o​)

  The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
• Energy in a reaction system must be conserved, as it cannot be created or destroyed
• Hess's Law


  The overall enthalpy change for a reaction is the same, regardless of the route taken
• Triangular cycle method


  1. Used to determine enthalpy changes for reactions that cannot be found directly using an experimental method

  2. Involves an intermediate product

  3. The direction of the arrows indicates whether the values should be added or taken away
• Enthalpies of formation


  When setting up the triangular diagram, the arrows point out from the central product as both reactants are formed from the elements
• Enthalpies of combustion


  When setting up the triangular diagram, the arrows point towards the central product (H2O and CO2) as both reactants burn to form the products
• Average bond enthalpies


  The energy required to break one mole of the stated bond in a gaseous state, under standard conditions
• Average bond enthalpies are an approximation as they are the average bond enthalpy for a bond from a range of different compounds
• Calorimetry


  1. An experimental method for finding enthalpy change by measuring temperature change over time

  2. A reaction is carried out in a sealed, insulated container and the temperature noted at regular intervals

  3. The measured change in temperature is proportional to the energy change
• Specific heat capacity


  The energy required to raise 1g of a substance by 1K without a change of state
• Enthalpy change values found using calorimetry are never completely accurate as energy is easily lost from the system through conduction or convection and inaccuracies in measuring temperatures
• Standard conditions are 298K, 100kPa
• Enthalpies are measured at constant pressure (standard conditions)
• The enthalpy change is the energy transferred as heat during a reaction
• Exothermic reactions release energy into surroundings
• Endothermic reactions absorb energy from surroundings
• ΔH°f = Enthalpy change when one mole of a compound forms from its elements under standard conditions
• ΔH°r = Enthalpy change when one mole of reactants form products under standard conditions
• ΔH°c = Enthalpy change when one mole of a compound sublimes under standard conditions