RP2: Enthalpy change measurement

• Assumption made in calorimetry:

  • Energy transferred to the water from reacting chemicals is equal to the energy released by the reaction
• Sources of error:

  • Heat loss
  • Incomplete combustion of fuel
  • Heat energy transferred to the metal calorimeter
  • Some fuel evaporates
• Improvements to minimise sources of error:

  • Add a lid
  • Insulate sides of calorimeter
  • Reduce distance between flame
  • Put sleeve around flame to protect it from draughts
• Steps to measure an enthalpy change using cooling curve:

  1. Record the temperature for a suitable time (3 mins) before adding reactants together
  2. Establish an accurate initial temperature
  3. Mix reactants then record temperature every minute until trend is seen
  4. Plot a graph of temperature against time
  5. Extrapolate the cooling curve back to the point of addition
  6. Establish a theoretical temperature change accounting for heat loss
• Measuring an enthalpy of solution:

  1. Measure water with apparatus
  2. Use suitable mass
  3. Insulated container and add known mass
  4. Use before and after weighing method
  5. Record initial temperature
  6. Record temperature at regular timed intervals
  7. Plot temperature v time
  8. Extrapolate lines to point of addition and use Q=mcT
• Types of calorimetry experiments:

  • Burning a fuel
  • Adding a solid to a solution
  • Adding two solutions together
• Reducing uncertainty using the same thermometer:

  • Use larger mass of fuel
• Reducing the percentage uncertainty when using solutions:

  • Increase the concentration of the solutions
• Describe how to find the enthalpy of solution:

  • Measure water with measuring cylinder
  • Mass of 100cm3
  • Into a polystyrene cup with a lid
  • Add known mass of solid
  • Use before and after weighing method
  • Record initial temperature three times
  • Record temperature at regular timed intervals for 5 or more mins
  • Plot temperature against time
  • Extrapolate back to the point of addition to find the final temperature and find temperature change
• Reducing the percentage uncertainty of calorimetry:

  • Larger mass of solid used
  • Change the concentration of reagent or solution