Required Practical 2: Enthalpy change

Question 1

Explain how you could improve the standard method of measuring enthalpy change and explain how you could measure the temperature change (6 marks)

Answer 1

Apparatus:

• Use a burette/pipette (instead of a measuring cylinder)
• Use a polystyrene cup (instead of a beaker) / insulate beaker
• Reweigh the watch glass after adding the solid
• Use powdered solid

Temperature Measurements:

• Measure and record the initial temperature of the solution for a few minutes before addition
• Measure and record the temperature after the addition at regular intervals (eg each minute) for 8+ minutes/until a trend is observed

Temperature Determination:

• Plot a graph of temperature against time
• Extrapolate to the point of addition
• Determine ΔT at the point of addition

Question 2

Explain how you could reduce percentage uncertainty of using the thermometer (2 marks)

Answer 2

• Increase the magnitude of temperature change

- By increasing the concentration of the acid/alkali added

Question 3

Outline a simple practical experiment including the measurements to be taken, in order to determine the enthalpy of combustion for an unknown alcohols (5 marks)

Answer 3

• Weigh the spirit burner (alcohol) before and after combustion
• Water in a calorimeter / beaker
• Measure volume of water (or mass)
• Burn the alcohol to heat the water
• Measure temperature rise in water

Question 4

Suggest one improvement that would reduce errors due to heat loss in the student’s experiment (1 mark)

Answer 4

• Reduce the distance between the flame and the beaker
• Put a sleeve around the flame to protect from drafts
• Add a lid