Chemistry Question, I need some help please

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

 

Justis,

 

Hope this still helps!

 

For part (a), we need to know deltaH^o_f (at 298K) for each of the products and reactants.  Remember that the heat of formation of "any" element in its thermodynamic standard state is defined as zero.

 

deltaH^o_f  HCl_(g) = -92.307 kJ/mole

deltaH^o_f  O_2(g)  =    0 kJ/mole

deltaH^o_f  Cl_2(g) =    0 kJ/mole

deltaH^o_f  H₂O_(g) = -241.818 kJ/mole

 

deltaH^o_f rxn = summation(deltaH^o_f products) - summation(deltaH^o_f reactants)

 

so

deltaH^o_f rxn = [(2)(deltaH^o_f H₂O_(g)) + 2(deltaH^o_f Cl_2(g) )] - [4(deltaH^o_f  HCl_(g)) + 1(deltaH^o_f O_2(g))]

 

deltaH^o_f rxn = [2(-241.818) + 2(0)] - [4(-92.307) + 1(0)]

                  = -483.636 + 369.228

or

deltaH^o_f rxn = -114.408 kJ/mole

 

 

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For part (b), we need to know the bond dissociation energies (BDE) at 298 K for each of the bonds in the equation.

 

BDE O-H = 463 kJ/mole

BDE Cl-Cl = 243 kJ/mole

BDE O=O = 485 kJ/mole

BDE H-Cl = 432 kJ/mole

 

Energy needed to break bonds = 4(energy to break H-Cl bond) + 1(energy to break O=O bond)

 

Energy to break bonds = 4(432) + 1(485)   (positive since energy required)

 

Energy to break bonds = 1728 + 485 = 2213 kJ/mole

 

Do same for bond formation.

 

Energy to form bonds = 4(energy to form O-H bond) + 2(energy to form Cl-Cl bond)
 
Energy to form bonds = 4(-463) + 2(-243)   (negative since energy released)
 
Energy to form bonds = -1852 + -486 = -2338 kJ/mole

 

 

deltaH^o_f rxn = energy to break bonds + energy released by bond formation

 

deltaH^o_f rxn = 2213 + (-2338) kJ/mole

 

deltaH^o_f rxn = -125 kJ/mole

 

 

 

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For question (c):

 

Most useful bond energies are determined by averaging the values determined from entire classes of compounds, not just individual compounds.  For example, the C-H and C-C bond energies are obtained by looking at hydrocarbons in general, not just from methane or ethane.  As a result of the averaging, differences will be seen between both methods.