# What is the standard enthalpy of formation of SO2(g)?

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A scientist measures the standard enthalpy change for the following reaction to be -171.2 kJ :

#2SO_2(g) + O_2(g) rightleftharpoons 2SO_3(g)#

Based on this value and the standard enthalpies of formation for the other substances, the standard enthalpy of formation of SO2(g) is ___

kJ/mol.

A scientist measures the standard enthalpy change for the following reaction to be -171.2 kJ :

#2SO_2(g) + O_2(g) rightleftharpoons 2SO_3(g)#

Based on this value and the standard enthalpies of formation for the other substances, the standard enthalpy of formation of SO2(g) is ___

kJ/mol.

##### 1 Answer

Well, we can always check our answer. To check, it should be

I actually got

Supposedly, the

#2"SO"_2(g) + "O"_2(g) rightleftharpoons 2"SO"_3(g)#

You have to look up

Now,

#DeltaH_(rxn)^@ = sum_P n_P DeltaH_(f,P)^@ - sum_R n_R DeltaH_(f,R)^@# where

#P# stands for products,#R# for reactants,#n# for the mols of stuff, and#DeltaH_f^@# is the enthalpy of forming 1 mol of the substance from its elements in their elemental states at#25^@ "C"# and#"1 bar"# .

We should know that

#-"171.2 kJ" = ["2 mol" xx -"395.77 kJ/mol"] - ["2 mol" xx DeltaH_(f,SO_2(g))^@ + "1 mol" xx "0 kJ/mol"]#

Resolving the units, we obtain:

#-"171.2 kJ" = -"791.54 kJ" - 2DeltaH_(f,SO_2(g))^@ "kJ"#

And so:

#color(blue)(DeltaH_(f,SO_2(g))^@ = -"310.17 kJ/mol")#

Apparently, this is rather off from what NIST has (