A scientist measures the standard enthalpy change for the following reaction to be -172.4 kJ :
2SO2(g) + O2(g) 2SO3(g)
Based on this value and the standard enthalpies of formation for the other substances, the standard enthalpy of formation of SO2(g) is what in kJ/mol?
To determine the standard enthalpy of formation of SO2(g) given the standard enthalpy change and the standard enthalpies of formation for the other substances, we can use Hess's Law.
Hess's Law states that if a reaction can be written as a series of steps, the overall enthalpy change is the sum of the enthalpy changes of each step. In this case, we can write the chemical equation for the formation of SO2(g) as:
SO2(g) S(s) + O2(g)
The standard enthalpy change for the formation of SO2(g) can be calculated by subtracting the standard enthalpy changes of the formation of the other substances from the overall enthalpy change of the reaction. Let's denote the standard enthalpies of formation for S(s) and O2(g) as ΔHf1 and ΔHf2, respectively.
The given reaction is:
2SO2(g) + O2(g) 2SO3(g) ΔH = -172.4 kJ
We can rewrite the equation as follows:
2(S(s) + O2(g)) + O2(g) 2SO3(g)
Now, let's consider the standard enthalpy changes for the individual steps involved:
Step 1:
2(SO2(g)) 2(S(s) + O2(g)) ΔH1'
Step 2:
2(S(s) + O2(g)) + O2(g) 2SO3(g) ΔH2'
By applying Hess's Law, we can calculate the standard enthalpy change for each step:
ΔH1' = 2 × (ΔHf1 + ΔHf2)
ΔH2' = ΔH
We know that ΔH = -172.4 kJ. Let's substitute the values:
ΔH1' = 2 × (ΔHf1 + ΔHf2)
ΔH2' = -172.4 kJ
Now, rearranging the equation for ΔH1':
ΔH1' = 2 × (ΔH2' - ΔHf2)
We can now calculate the ΔH1' using the given values. Once we have ΔH1', we can determine the value of ΔHf1 (the standard enthalpy of formation of SO2(g)):
ΔH1' = 2 × (ΔH2' - ΔHf2)
ΔHf1 = ΔH1' / 2
By substituting the known values, you can calculate the answer in kJ/mol.
dHrxn = (n*dHf products) - (n*dHf reactants)
You know dHrxn and you can look up dHf O2 (it is zero) and SO3, calculate the only unknown of dHf SO2.