Chemistry, find standard heat of reaction of this reaction. Please help me understand how to do these problems

Question

This question is about the reaction
4HCl(g)+O2(g)-->2H20(g)+2Cl2(g)
A) Using the values of ΔfH°, find the standard heat of the reaction of this reaction.
B) Using average bond energies, estimate the standard heat of the reaction of this reaction.
C) Explain why the results in the two parts above are different

Answer 1

A) Heat of formation of a molecule is the energy to form that molecule which is the opposite of the energy required to break it apart (so if one is positive the other is negative).  So the heat of reaction is the sum of the energy of formations of the products minus sum of the energy of formation of the reactants.  This is taking the energy required to make the products and subtracting the energy required to form the reactants because it takes the same amount of energy to break them but negative).

ΔfH° Cl₂= 0kJ/mol (elemental forms are 0kJ/mol)

ΔfH° H₂0= -241.8kJ/mol

ΔfH° O₂= 0kJ/mol

ΔfH° HCl= -92.3kJ/mol

 

Heat of reaction= (2(0kJ/mol) + 2(-241.8kJ/mol)) - (0kJ/mol + 4(-92.3kJ/mol))= 2(-241.8kJ/mol) +4(92.3kJ/mol)= -114.4kJ/mol