Thermochemistry

Question

C3H8 + 5O2 ---> 3CO2 + 4H20

Delta H = -2043.9 KJ/Mol

If 25.0g of C3H3 reacts with 25.0g of O2, how much energy will be released?

Answer 1

C₃H₈ + 5O₂ ==> 3CO₂ + 4H₂O ... balanced equation for combustion of propane

∆H rxn = -2043.9 kJ/mol

25.0 g C3H8 x 1 mol C3H8 / 44 g = 0.568 moles C3H8

25.0 g O2 x 1 mol O2 / 32 g = 0.781 moles O2

Which is limiting? O2 is limiting since it takes 5x as much O2 as C3H8 and there isn't that much O2.

Now that we know O2 is limiting, we can use that fact to calculate the energy released.

0.781 moles O2 x 1 mol C3H8 / 5 moles O2 = 0.156 moles C3H8 will be combusted

0.156 moles C3H8 x -2043.9 kJ/mol = -319 kJ of energy released

Or you could have done it this way...

0.781 moles O2 x -2044 kJ/5 mol O2 = -319 kJ of energy released