3.1.4 - Energetics

• 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.