The standard enthalpy of formation of n-octane is -249.95 kJ/mol. Compute the amount of heat liberated when 5.24 g of n-octane is burned completely with excess oxygen to form carbon dioxide and liquid water.

Answer 1

Hint:
Write the balanced combustion reaction. You can find the standard enthalpies of formation for O2, CO2, and H2O from many sources, including the CRC Handbook of Chemistry and Physics.
Solution:
Balanced combustion reaction: C8H18 + 12.5 O2 --> 8 CO2 + 9 H2O
From the CRC Handbook of Chemistry & PhysicsStandard enthalpies of formation (25C):
n-octane -249.95, oxygen 0.00, CO2 -393.5, H2O(l) -285.8 kJ/mol
ÄHrxn = -5470.25 kJ/mol
moles of octane burned = (5.24 g)/(114.231 g/mol) = X mol
Total heat released = 5470.25 kJ/mol * X mol = Answer kJ

Answer 2

alchol releases 29.7kJ/g when it burns. Convert this value to the number of calories per gram