The molar heat of combustion of ethanol is 1367 kJ mol⁻¹ - HSC - SSCE Chemistry - Question 10 - 2008 - Paper 1

To calculate the amount of ethanol required, we first need to find the heat energy required to raise the temperature of the water.

1. Calculate the heat energy needed (Q): The required temperature change for water is:

   100 °C - 50 °C = 50 °C

   The heat energy needed can be calculated using the formula:

   $Q = mc heta$

   Where:

   • m = mass of the water = 1.0 kg
   • c = specific heat capacity of water = 4.18 kJ/(kg·°C)
   • θ = change in temperature = 50 °C

   Inserting values:

   $Q = 1.0 ext{ kg} imes 4.18 ext{ kJ/(kg·°C)} imes 50 ext{ °C} = 209 ext{ kJ}$

2. Determine moles of ethanol required: With the molar heat of combustion of ethanol being 1367 kJ/mol, the number of moles needed to produce 209 kJ is:

   ext{moles} = rac{Q}{ ext{molar heat}} = rac{209 ext{ kJ}}{1367 ext{ kJ/mol}} ext{ moles} ≈ 0.153 ext{ mol}

3. Calculate mass of ethanol required: The molar mass of ethanol (C₂H₅OH) is approximately 46 g/mol. Therefore, the mass of ethanol required is:

   $ext{mass} = ext{moles} imes ext{molar mass} = 0.153 ext{ mol} imes 46 ext{ g/mol} ≈ 7.0 ext{ g}$

Thus, the quantity of ethanol required is approximately 7.0 g.