Calculating Standard Enthalpy Change

May 2013
Hello! :)

I have honestly no idea on how to approach this seemingly easy question, can somebody please provide me with some assistance? I'm adding the enthalpy values up and then entering that as the answer (along with numerous other attempts that have been unsuccessful) and I just don't know where to begin or what to do!!

Use Hess's Law and the thermochemical equations labelled 1 and 2 below, and their standard enthalpy changes of reaction, to calculate the standard enthalpy change of reaction for oxidation of sulphur dioxide to sulphur trioxide, described by equation 3:

1) S8(s) + 8 O2(g) --> 8 SO2(g) -2375 kJ
2) S8(s) + 12 O2(g) 8 SO3(g) -3166 kJ
3) 2 SO2(g) + O2(g) 2 SO3(g) ??? kJ

How do I find the change in enthalpy for reaction 3 by using the above data? If somebody can please show me that would be amazing! Thanks!!


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May 2010
You need to draw the Hess' cycle to help you; start with inverting the first reaction so that you start with SO2 and end with S8 and O2.
May 2010
Hello Smokey,
Express heat of formation in eq 1 per mol of SO2formed
Express same in eq2 per mol of SO3 formed
Insert values in eq3 and solve for heat released