heat of combustion

Heat of Combustion Questions

1. When bonds are broken, energy is required. When bond are formed, energy is released. All combustion reactions are exothermic. Suggest a reason for this in terms of the bonds broken and bonds formed. The energy required to break the bonds is greater than the energy required to form.

2. Explain why the heat of combustion of alkanols increases as the length of the carbon chain increases. The heat of combustion of alkanol increases as the length of carbon chain increases due to greater dispersion forces resulting in more energy required to break the bond.

3. An experiment was carried out to measure the heat of combustion of a fuel, 1-propanol. The student found that when 1.0g of 1-propanol had been used up, the temperature of the water had risen by 15oC. He knew the specific heat capacity of water and was able to calculate the amount of heat absorbed by 200g of water.

Calculate the amount of heat released by the combustion of 1-propanol.

4. Calculate the heat of combustion per mole.

5. Calculate the heat released per gram for the combustion of each of the following fuels. The heats of combustion for each fuel (in kJmol-1) is shown.
Fuel
Molecular Formula
Molar heat of combustion (kJmol-1)
Heat released per gram (kJg-1)
Hydrogen
H2
285

Ethanol
C2H5OH
1367

Ethyne
C2H2
1300

Propane
C3H8
2220

Sucrose
C12H22O11
5650

Octane
C8H18
5460

6. A student wished to determine the heat of combustion of ethanol. He used a spirit burner containing ethanol to heat 250g of water in a beaker. The temperature rose from 15oC to 31oC. During combustion, the burner lost 0.90g in mass, due to the ethanol combusting. Calculate the heat of combustion in ethanol.

7. The heat of combustion of ethanol is 1367 kJmol-1. Calculate the theoretical mass of ethanol required to heat 50mL of water from 21.0oC to 30.0oC.