How to use Hess's Law to calculate enthalpy for the reaction?

Use bond enthalpies to estimate the enthalpy change for the reaction of hydrogen with ethylene. H2(g) + C2?

• (a) Use bond enthalpies to estimate the enthalpy change for the reaction of hydrogen with ethylene. H2(g) + C2H4(g) → C2H6(g) kJ (b) Calculate the standard enthalpy change for this reaction, using heats of formation. kJ Why does this value differ from that calculated in (a)? The heats of formation in the table have fewer significant figures and so are less precise. The heats of formation are theoretical values; whereas the bond enthalpies are experimental. The heats of formation are average values and are not as accurate. The bond enthalpies do not account for the fact that all these molecules are gases. The bond enthalpies are average values and are not as accurate.

• Answer:

  Some discussion and examples: http://www.chemteam.info/Thermochem/HessLawIntro3.html reactants ---> one H-H bond, one C-C bond, four C-H bonds products ---> one C-C bond, six C-H bonds You can eliminate like items: reactants ---> one H-H bond products ---> two C-H bonds Do the Hess' Law thing on the set just above with the one H-H and the two C-H part b: The bond enthalpies are average values and are not as accurate.