The enthalpy of combustion of heptane is -4817 kJ/mol. This means that when one mole of heptane undergoes complete combustion, 4817 kJ of heat is released.

To calculate the amount of heat released when 250 kg of heptane undergoes complete combustion, we need to first convert the mass of heptane to moles. The molar mass of heptane is 100.2 g/mol, so 250 kg of heptane is equal to:

$$250\times1000\ g\times\frac{1\ mol}{100.2\ g}=2494.7\ mol$$

Now we can use the enthalpy of combustion to calculate the amount of heat released:

$$2494.7\ mol\times(-4817\ kJ/mol)=-12030\times10^3\ kJ$$

Therefore, when 250 kg of heptane undergoes complete combustion, 12030 MJ of heat is released.