Question

When a 3.80 g sample of C8H18(l) is burned in a bomb calorimeter, the temperature of the calorimeter rises by 27.3 oC. The heat capacity of the calorimeter, measured in a separate experiment, is 6.18 kJ/oC. Determine the ΔE for C8H18(l) in units of kJ/ mole C8H18(l).

Answer 1

Answer: The enthalpy of the reaction is -5112.5 kJ/mol

Explanation:

To calculate the heat absorbed by the calorimeter, we use the equation:

$q=c\Delta T$

where,

q = heat absorbed

c = heat capacity of calorimeter = 6.18 kJ/°C

$\Delta T$ = change in temperature = 27.3°C

Putting values in above equation, we get:

$q=6.18kJ/^oC\times 27.3^oC=168.714kJ$

Heat absorbed by the calorimeter will be equal to the heat released by the reaction.

Sign convention of heat:

When heat is absorbed, the sign of heat is taken to be positive and when heat is released, the sign of heat is taken to be negative.

To calculate the number of moles, we use the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$

Given mass of octane = 3.80 g

Molar mass of octane = 114 g/mol

Putting values in above equation, we get:

$\text{Moles of octane}=\frac{3.80g}{114g/mol}=0.033mol$

To calculate the enthalpy change of the reaction, we use the equation:

$\Delta E=\frac{q}{n}$

where,

q = amount of heat released = -168.714 kJ

n = number of moles = 0.033 moles

$\Delta E$ = enthalpy change of the reaction

Putting values in above equation, we get:

$\Delta E=\frac{-168.714kJ}{0.033mol}=-5112.5kJ/mol$

Hence, the enthalpy of the reaction is -5112.5 kJ/mol