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 calorimeter to minimize heat loss to the environment. This will provide better insulation and more accurate temperature readings.
2. Ensure that the spirit burner is positioned directly under the beaker to reduce any angle of heat loss and maximize the heat transfer to the water.

To calculate the molar heat of combustion, we first need to determine the heat absorbed by the water:

1. Calculate the mass of the water: 250.0 g

2. Calculate the temperature change: Final temperature - Initial temperature = 59.0 °C - 19.0 °C = 40.0 °C

3. Calculate the heat absorbed using the formula:

   $q = m imes c imes riangle T$

   Where:

   • $q$ = heat absorbed by water
   • $m$ = mass of water = 250.0 g
   • $c$ = specific heat capacity of water = 4.18 J/g°C
   • $riangle T$ = temperature change = 40.0 °C

   Substituting in:

   $q = 250.0 imes 4.18 imes 40.0 = 41800 ext{ J}$

4. Now, calculate the molar heat of combustion:

   Mass of ethanol burned = Initial mass - Final mass = 221.4 g - 219.1 g = 2.3 g.

   Moles of ethanol burned = $ext{Molar mass of ethanol} (C_2H_5OH) = 46.07 ext{ g/mol}$

   Moles of ethanol = rac{2.3}{46.07} ext{ mol} \\ ext{Moles} \\ = 0.0498 ext{ mol}

5. Finally, the molar heat of combustion ($ext{ΔH}_{comb}$) is calculated as:

   ext{ΔH}_{comb} = rac{q}{ ext{moles of ethanol}} = rac{41800 ext{ J}}{0.0498 ext{ mol}} \\ ext{ΔH}_{comb} \\ = 839000 ext{ J/mol} \\ = 839 ext{ kJ/mol}