Explore BrainMass

bond energy and heat of formation

1. 0.412 mol of a solid was dissolved in 200 mL of water at 21.2 oC. After the solid had fully dissolved, the final temperature of the solution was 17.9 oC. What is the molar heat of solution of the substance?

2. Use Appendix B and the fact that the heats of formation of OF2 and OCl2 are 24.7 and 80.37 kJ/mol, respectively to determine the O-F and O-Cl bond energies.
DO-F = kJ/mol
DO-Cl = kJ/mol

3. Use Appendix B to determine the atomization energy of PCl3 and the average P-Cl bond energy in PCl3.
DP-Cl = kJ/mol
Use the P-Cl bond energy and the atomization energy of POCl3 to determine the P-O bond energy in POCl3.
DP-O = kJ/mol

4. Use Bond energies (Table 3.3) to estimate the heats of formation of the following gases:
Compound Hf (kJ/mol)

NF3 =

H2N-NH2 =

F2C=CH2* =

CH3Cl* =

*Hint: The standard state of carbon is solid graphite, but bond energies are for reactions in which all reactants and products are in the gas phase. You must use the heat of formation of carbon gas and bond energies to determine this energy.

5. C18H12(s)+ O2(g) CO2(g) + H2O(l)
ng = mol
What is Hcomb of chrysene if Ecomb = -8942.6 kJ/mol?
Hcomb = kJ/mol
What is the heat of formation of chrysene? Appendix B
Hf = kJ/mol

6. The combustion of 2.350 mmol of X caused the temperature of a bomb calorimter to rise from 23.900 oC to 26.100 oC. Assume T = 25.000 oC, Hcomb = -2896 kJ/mol, and ng = -2 mol gas/mol X for the combustion and determine the heat capacity of the calorimeter.
C = kJ/oC

7. The heat of combustion of liquid toluene (C7H8) is -3910.3 kJ/mol at 1 atm and 298 K.
a) What is the energy of combustion at 1 atm and 298 K?
Ecomb = ____________kJ/mol


Solution Preview

For you first question, calculate the amount of heat that was lost by the 20 mL of water and then divide that amount of heat by the number of moles of your mystery compound.

To calculate heat in this solution (since there are no phase changes) it will be a simple

q = mCp/T calculation

Mass is the mass of water (200 ml = 200 g)

Cp of water is usually 4.184 J/degrees C g

q = 200 g x 4.184 x 21.2-17.9

heat was lost so the answer should be positive.

For you second question, you must use the special case with respect to /Hatomization reactions...

/Hatom = total bond energy of starting material if all gases....

/Hatom = /Hf(products) - /Hf(reactant)

design yourself some atomization reactions that start with these compounds in gas form...

OF2 --> O + 2F for example... LABEL reaction A

Now we run the /H = /Hf(products) - /Hf(reactant) type calculations...

From the ...

Solution Summary

Bond energy and heat of formation are investigated. The solution is detailed and well presented.