Measurement of enthalpy change

• How can you reduce the uncertainty in the mass measurement?

  • Use a larger mass
  • Use a balance with a higher resolution
• Why may an experimental value for enthalpy change be different to a theoretical value

  • Heat loss to surroundings or apparatus
  • Incomplete combustion
  • Non-standard conditions
  • Evaporation of water
• What are the methods to prevent heat loss in a calorimeter?

  • Insulate the beaker with a polystyrene cup
  • Avoid large temperature differences with surroundings
  • Use a bomb calorimeter
• Why is stirring the solution important in an experiment?

  It ensures even temperature distribution
• What are the methods to improve the accuracy of an experiment?

  • Read thermometer at eye level
  • Stir the solution for even temperature
  • Use a digital thermometer
  • Use greater concentrations and masses
• Diagram of determining enthalpy change

  Use would use a polystyrene cup instead of a beaker

  Thermometer bulb needs to be fully immersed into solution

  Add a lid

  
• Graph for enthalpy change would look different for exothermic ( on left )and endothermic ( on right )reactions

  
• Describe how you would carry out an experiment to determine the enthalpy of solution of anhydrous MgCl2

  1. Set up clamp, Bunsen burner ,heat proof mat and clamp stand
  2. Pour 50 cm³ of distilled water in a polystyrene cup using a measuring cylinder
  3. Place a lid with a hole for a thermometer and glass rod to stir
  4. Quickly add the mass of anhydrous MgCl2 into polystyrene cup and record initial temperature
• Describe how you would carry out an experiment to determine the enthalpy of solution of anhydrous MgCl2 pt2

  1. Then you would heat the cup
  2. Every 30 seconds, measure and record temperature for 5 minutes
  3. Then draw a time Vs temperature graph and extrapolate lines to find ∆T
  4. Using ∆T find q using q = mc∆T
  5. Then finally do -q÷n to find enthalpy change
• Calorimetry is the process of measuring the amount of heat released or absorbed during a chemical reaction