Paper 3: Required practical 2 - Measuring enthalpy change

Question 1

How can you reduce the uncertainty in the mass measurement?

Answer 1

• Use a balance with a greater resolution
• Use a larger mass

Question 2

How do you calculate enthalpy change of reaction experimentally?

Answer 2

• q=mcΔT
• Convert to J, divide by the number of moles of the limiting reactant
• Add a sign to show whether it is exothermic or endothermic

Question 3

Why may an experimental value for enthalpy change be different to the theoretical value?

Answer 3

1. Heat loss to apparatus/ surroundings
2. Incomplete combustion
3. Non-standard conditions
4. Evaporation of alcohol/water

Question 4

How do you prevent heat loss to surroundings/ apparatus?

Answer 4

• Insulate the beaker by placing it in a polystyrene cup with a lid
• Avoid large temperature differences between surroundings and calorimeter
• Use a bomb calorimeter

Question 5

Other than preventing heat loss, how can the accuracy of this experiment be improved?

Answer 5

• Read thethermometer at eye level to avoid parallax errors
• Stir the solution so the temperature is evenly distributed
• Use a digital thermometer for more accurate and faster readings
• Use greater concentrations and masses, leading to a greater temperature change and thus smaller uncertainty