I think I'm getting the wrong answer to this problem I got -991.61

Question

The combustion of propane by the following unbalanced chemical reaction:

a C3H8(g) + b O2(g) --> c CO2(g) + d H2O(g)

How many kilojoules of Gibb's Free Energy are liberated by burning 21.086g of propane in excess oxygen?

Substance ΔG°f(kJ/mol)

C3H8(g) -23.42

O2(g 0

CO2(g) -394.36

H2O(g) -228.59

Answer 1

I get pretty much the same answer that you did. Here are my calculations.

First, let's write the correctly balanced equation

C₃H₈(g) + 5O²(g) ==> 3CO₂(g) + 4H₂O(g) ... BALANCED EQUATION

Next, using the ∆Gº values, we find the heat energy from burning 1 mol of C3H8 according to the balanced equation and the provided values for ∆Gº_f

C₃H₈(g) + 5O₂(g) ==> 3CO₂(g) + 4H₂O(g)

-23.42...........0................3x-394.36....4x-228.59

∑reactants = -23.42 kJ

∑products = -1183.08 + (-914.36) = -2097.44 kJ

∆G for 1 mole C3H8 = ∑products - ∑reactants = -2097.44 kJ + 23.42 kJ = -2074.02 kJ / mole

Since we have 21.086 g of C3H8, we can now find the heat from this as follows:

21.086 g C3H8 x 1 mol C3H8 / 44.096 g = 0.47818 moles

0.47818 mols x -2074.02 kJ/mol = -991.76 kJ

ANSWER: 991.76 kJ liberated from the combustion reaction