3.1.4 Energetics

• In an open system energy and matter can be exchanged between the system and surroundings.
• In a closed system, only energy can be exchanged between the system and surroundings.
• In an isolated system, neither energy nor matter can be exchanged between system and surroundings.
• An exothermic reaction is a reaction that releases energy to the surroundings, examples include combustion and neutralisation.
• An endothermic reaction is a reaction that takes in energy from the surroundings, examples include thermal decomposition and photosynthesis.
• Enthalpy change (ΔH) is the heat energy change measured under conditions of constant pressure.
• Standard conditions include a temperature of 298K and a pressure of 100kPa.
• The enthalpy change of formation (ΔfH) is the enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions, with all reactants and products in their standard states.
• The enthalpy change of combustion (ΔHc) is the enthalpy change when 1 mole of a substance is burnt completely in oxygen when all products and reactants are in their standard states, under standard conditions.
• Mean bond enthalpy is the enthalpy change needed to break a covalent bond. It is an average over different compounds as it varies from compound to compound.
• Hess' law states that the enthalpy change of a reaction is independent of the route taken
• Bond breaking is endothermic, while bond making is exothermic.
• enthalpy change = bonds broken - bonds made
• Calorimetry is used to work out the enthalpy change of combustion.
• Calorimetry experiment

  • weigh the fuel before
  • fuel is burned to raise the temperature of the water by a specific amount
  • lid is placed on top of beaker with water to prevent heat loss and wind shields placed by flame to prevent a draught moving the flame
  • fuel weighed after to calculate mass of fuel burned
• The equation for calorimetry is q=mcΔT where m is the mass of substance (g), c is its specific heat capacity (g/K) and ΔT is the change in temperature (T).
• Enthalpy = q / moles