Question

Part A: For which of the following reactions is ΔH∘rxn equal to ΔH∘f of the product(s)? You do not need to look up any values to answer this question.

2Li(s)+F2(g)→2LiF(s)

S(s)+O2(g)→SO2(g)

Li(s)+12F2(g)→LiF(s)

SO(g)+12O2(g)→SO2(g)

SO3(g)→12O2(g)+SO2(g)

Li(s)+12F2(l)→LiF(s)

Part B: The combustion of propane, C3H8, occurs via the reaction

C3H8(g)+5O2(g)→3CO2(g)+4H2O(g)

with heat of formation values given by the following table:

Substance	ΔH∘f (kJ/mol)
C3H8 (g)	-104.7
CO2(g)	−393.5
H2O(g)	−241.8

Calculate the enthalpy for the combustion of 1 mole of propane.

Answer 1

Part A: For which of the following reactions is ΔH∘rxn equal to ΔH∘f of the product(s)? You do not need to look up any values to answer this question.

In order for a reaction; to be the heat of formation of a sample, we must have:

- Pure elements

- 1 mol of product formed

- STP condition

- The most abundant phase

Then, knowing this:

2Li(s)+F2(g)→2LiF(s) --> will not work, since there are 2 mol of LiF formed

S(s)+O2(g)→SO2(g) --> correct, since elemental states

Li(s)+1/2F2(g)→LiF(s) -- > correct, Li is solid and F will be bonded as F2; the products is 1 mol of LiF solid salt

SO(g)+12O2(g)→SO2(g) --> false, since SO is not pure element

SO3(g)→12O2(g)+SO2(g) --> false, since SO3 is not pure

Li(s)+12F2(l)→LiF(s) --> false, since F2 will no tbe present as liquid

Part B: The combustion of propane, C3H8, occurs via the reaction

C3H8(g)+5O2(g)→3CO2(g)+4H2O(g)

HRxn = Hproducts - HReactants

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

HRxn = 3*-393.5+ 4*-241.8 - (-104.7 + 5*0)

HRxn = -2043 kJ/mol