Top fuel dragsters (sometimes called funny cars) burn nitromethane. This combustion is represented by the equation below.
4 CH3NO2(l) + 3 O2(g)→ 4 CO2(g) + 6H2O(g) + 2N2(g)
?cH = -1418.4 kJ
16. Calculate the molar enthalpy of combustion for nitromethane.
17. A 5.00 g sample of nitromethane was burned in a calorimeter that contained 250 g of water at 20.5 degrees celcius. Calculate the final temperature of the water if all 5.00 g of nitromethane is combusted.
Hint – you will need to use the molar enthalpy of combustion that you calculated in Question 16.
6 years ago
Answered By Leonardo F
16) The molar enthalpy of combustion can be calculated by dividing the total enthalpy of reaction by the number of moles of nitromethane:
Molar enthalpy = -1418.4 kJ / 4 moles
Molar enthalpy = - 354.6 kJ/mol of nitromethane
17) We have first to calculate the number of moles in 5.00 g of nitromethane. The molar mass is:
M = 12.01 + 3(1.01) + 14.01 + 2(16)
M = 61.05 g/mol
n = m/M = 5.00 g / 61.05 g/mol
n = 0.0819 mol
Multiplying this value by the molar enthalpy, we can calculate the energy released by the combustion of 5.00 g of nitromethane:
Energy released = (0.0819 mol)(354.6 kJ/mol)
Energy released = 29.042 kJ
This energy is absorbed by the water in the form of heat. Hence:
Energy released = m.Cp.(T2 - T1)
The mass of the water is 250 g or 0.250 kg:
29.042 kJ = (0.250 kg)(4.1868 kJ/kg.K)(T2 - 20.5 °C)
T2 = 48.2°C
The final temperature of the water is approximately 48.2°C.