Students were asked to perform a first-hand investigation to determine the molar heat of combustion of ethanol - HSC - SSCE Chemistry - Question 17 - 2001 - Paper 1

1. Use a more effective insulating material for the calorimeter to minimize heat loss to the environment.
2. Ensure a more precise placement of the burner under the beaker to maximize heat transfer and reduce any heat loss.

To calculate the molar heat of combustion, we need to find the temperature change and use the formula:

1. Calculate the temperature change:

   $\Delta T = T_{final} - T_{initial} = 59.0 °C - 19.0 °C = 40.0 °C$

2. Calculate the amount of heat absorbed by the water:

   $q = m \cdot c \cdot \Delta T$ Where:

   • $m$ = mass of water = 250.0 g
   • $c$ = specific heat capacity of water = 4.18 J/g°C
   • $\Delta T$ = temperature change = 40.0 °C

   $q = 250.0 \, \text{g} \cdot 4.18 \, \text{J/g°C} \cdot 40.0 \, \text{°C} = 41800 \, \text{J}$

3. Calculate the moles of ethanol burned:

   $\text{Moles of ethanol} = \frac{(Mass \, of \, burner \, initial - Mass \, of \, burner \, final)}{Molar \, mass \, of \, ethanol} = \frac{(221.4 - 219.1) \, g}{46.07 \, g/mol} = 0.0497 \, mol$

4. Finally, calculate the molar heat of combustion:

   $Molar \, heat \, of \, combustion = \frac{q}{\text{moles}} = \frac{41800 \, J}{0.0497 \, mol} \approx 840800 \, J/mol$

   Therefore, the molar heat of combustion of ethanol is approximately 840.8 kJ/mol.