9.2 - Measuring Enthalpy Change

Question 1

What is the Kelvin scale?

Answer 1

degrees C +273 = K

Question 2

What is the formula for finding energy?

Answer 2

q=mc(delta)T

q - heat energy in J
m - mass in g
delta T - change in temperature
c - specific heat capacity

Question 3

Give the method for the experimental determination of enthalpy change of combustion.

Answer 3

1. Measure 150 cm3 of water and record the initial temperature
2. Add methanol to a spirit burner and weigh together
3. Light the burner as it heats the water
4. After 3 minutes record the water temperature as fire is extinguished
5. Re-weigh spirit burner
   Use q=mc(delta)T
   Then, do q/(mol of substance) = enthalpy change of combustion in kJ/mol

Question 4

Why are there inaccuracies in the experimental value of enthalpy change of combustion?

Answer 4

• Heat loss to surrounding rather than water
• Incomplete methanol combustion
• Methanol may evaporate from wick, changing mass reading
• Non-standard conditions