Calorimetry

Calorimetry

1. method for measuring energy released by chemical reactions

2. Q=mcdeltaT

   • Q=energy in joules
   • m=mass of water in grams
   • c=specific heat capacity of water(4.2 J/g degrees celsius)
   • delta T=change in temp

3. errors

   • lot of heat loss to the surroundings
   • not all of the food/fuel burns
   • water should be stirred
   • some energy heat the container

4. Energy level diagrams

   • supply energy to break bonds
   • energy released when making bonds
   • exothermic - energy given out (high energy level to low level energy
     • energy released from forming new bonds is greater than energy needed to break bonds
     • eg combustion/neutralisation
     • deltaH is negative
   • activation energy
     • catalyst can provide a different pathway that has a lower activation energy
     • the minimum amount of energy that colliding reactant particles must have for a reaction to occur
   • endothermic-energy taken in (low energy level to high energy level)
     • energy needed to break bonds is greater than the energy released from forming new bonds
     • deltaH is positive
     • thermal decomposition
   • Bond energies
     • the amount of energy needed to break a particular chemical bond
     • measured in KJ/mole
     • 1) calculate total amount of energy needed to break all the bonds in reactants
       • 2) calculate total amount of energy released in making all the bonds in products
         • 3) overall energy change = energy in - energy out
     • exothermic reaction, the energy needed to break existing bonds is less than the energy released from forming new bonds
     • endothermic reactions , the energy needed to break existing bonds is more than the energy released from forming new bonds
     • Hydrogen power
       • when hydrogen burns in air the only product is water
         • does not contain carbon - so hydrogen burns with an invisible flame
       • alternative fuel because of the concern that burning fossil fuels produce greenhouse gases and is accererating the climate change
         • no emissions of smoke or carbon dioxide when burning hydrogen
       • drawback - must be liquified or pressurised so it can occupy a smaller volume
         • in terms of energy density / gram --- hydrogen releases far more energy than other fuels
           • but at room temp it is a gas so energy density is far lower than that of petrol an other liquid fuels
             • hydrogen fuelled vechiles would need huge tanks if they stored hydrogen as a gas at atmospheric pressure
       • fuel cells produce electricity by the reaction of hydrogen with oxygen
         • water only waste product
         • hydrogen and oxygen do not react together directly in a fuel cell
           • kept apart by partially permeable membrane that only allows hydrogen ions
         • fuel cells very efficient in producing electricity as long as they are supplied with fuel and oxygen
         • hydrogen not truly clean fuel as it is made from fossil fuels directly or indirectly using electricitty......so as a fuel it uses non-renewable resources and produces CO2 emissions
           • investigation of modyifying photosynthesis so that algae produce hydrogen