3.1.4 - Energetics

Cards (20)

  • Enthalpy change: heat energy change at a constant pressure
  • Examples of exothermic processes:
    • combustion of fuels
    • oxidations of carbohydrates
  • Examples of endothermic processes:
    • thermal decomposition of calcium carbonate
    • melting/boiling
  • Standard conditions:
    •  100kPa pressure
    • 298K temperature
    • Solutions at 1 mol/dm^3
    • All substances at their standard state
  • Calculating the energy change in a reaction:
    q = mcΔt

    energy change = mass of solution x heat capacity x temperature change
  • Hess's law: total enthalpy change for a reaction is independent of the route
  • Mean bond enthalpy: The enthalpy needed to break a covalent bond, averaged over different molecules
  • Mean bond enthalpy only applies when the substances are in the gaseous state
  • Enthalpy change: Heat energy change at constant pressure
  • Standard enthalpy of formation:
    • The enthalpy change at constant pressure when 1 mole of a compound
    • Is formed from its constituent elements
    • With the reactants and products in standard states
  • Energy is taken in to break bonds → endothermic reaction
  • Energy is released to make bonds → exothermic reaction
  • What is an endothermic reaction?
    • One with an overall positive enthalpy change (+ΔH)
    • → energy in breaking bonds > energy out making bonds
  • What is an exothermic reaction?
    • One with an overall negative enthalpy change (-ΔH)
    • → energy in breaking bonds < energy out making bonds
  • If a reversible reaction is endothermic one way, what type of reaction is the other way? exothermic
  • Label these, exothermic or endothermic?
    A) Exothermic
    B) Endothermic
  • What is the difference between heat and temperature?

    Heat is the sum of all particles’ energy, therefore it is affected by the amount of substance; temperature is related to the mean kinetic energy of the particles in a system, so is independent of the number of particles present.
  • Draw a simple calorimeter
    A) Heatproof mat
    B) Spirit Burner
    C) Tripod
    D) Gauze
    E) Thermometer
  • How could this calorimeter be made more accurate?
    Add draught screens at the sides, add a lid on top of the beaker, add mineral wool around the beaker → all to insulate and reduce heat lost to the surroundings
  • What is a flame calorimeter; how does it differ to a simple calorimeter?
    • Reduces heat lost to the surrounding to give more accurate results:
    • has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in pure oxygen, not air.