Calculating enthalpy change of the combustion of alcohols

• what sort of reaction is combustion?

  an exothermic reaction, so during the reaction the system releases heat energy into the surroundings.
• what does the 1st method investigate?

  enthalpy changes that occur during the combustion of alcohols. The heat energy released is used to heat up a known volume of water.
• what is step 1?

  use a measuring cylinder to transfer 100cm3 of water into a glass beaker or metal can (the calorimeter).
• what is step 2?

  use a clamp stand and clamp to position the calorimeter at a height suitable for the spirit burner to be placed under it.
• what is step 3?

  measure and record the mass of the spirit burner, lid and contents before placing it below the calorimeter.
• what is step 4?

  set up draught shields around the clamp stand and spirit burner. This is to minimise heat loss to surroundings
• what is step 5?

  use a thermometer to record the initial temperature of the water in the calorimeter.
• what is step 6?

  remove the lid from the spirit burner and ignite the wick.
• what is step 7?

  stir the water with the thermometer and continue to heat until the temperature has risen 20-25'C.
• what is step 8?

  extinguish the spirit burner by replacing lid over wick.
• what is step 9?

  continue to stir the water and record the highest temperature reached.
• what is step 10?

  allow the spirit burner to cool, then measure and record the final mass of the spirit burner, lid and contents.
• what is step 11?

  calculate the max temperature change (difference between the initial temp and final temp)
• what assumption do we make about the experiment results?

  we assume that all the heat energy is transferred from the alcohol to the water, BUT THIS ISNT THE TRUTH!!
• are draught shields 100% effective?

  no, they can be used to minimise heat loss but A LOT of heat energy is lost to other surroundings (air, beaker and equipment surrounding the flame). So, the enthalpy change will be much lower than expected as not as much heat energy is transferred to water as should.
• how can we reduce amount of heat loss (not draught shields)?

  using insulating material around the calorimeter and using a lid on the calorimeter.
• what are other potential sources of error?

  incomplete combustion and fuel evaporation from wick.
• how can we try and avoid fuel evaporation?

  try keeping the wick as short as possible.
• how are accepted data book values obtained?

  using a bomb calorimeter.
• what does the bomb calorimeter do?

  provides accurate values for the enthalpy of combustion of fuels under standard conditions through the following features:

  • use of a lid and air jacket to provide insulation which reduces heat loss to the surroundings during combustion.
  • fuel is ignited through an electrically heated wire to avoid fuel evaporation from the wick.
  • the fuel is combusted in excess oxygen to prevent incomplete combustion.
  • a stirrer is used to make sure the heat is evenly distributed throughout the water.