# Bond Energies Problems and Concepts Carol Brown Saint Mary’s Hall.

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Bond Energies Problems and Concepts Carol Brown Saint Mary’s Hall

An Organic Story From the instant I laid eyes on O‚Chem I felt a bond between us, actually, more like a triple bond. It was an incredibly polarizing attraction. The feeling was out of this orbital. I mean before I met O‚Chem, I was merely a single electron. So it was a pretty radical experience. Then, however, the torsional twisting was taking a strain on our relationship. Sadly, our thermodynamics was uphill; we just didn't have the right chemistry. It was an unstable relationship, or maybe it was just a lack of equilibrium. We weren’t able to rearrange into a more stable configuration. We decided that we were equally at fault, so it ended in a homolytic split.

The Basics l Breaking a bond is always endothermic! l (Breaking up is hard to do.) l ∆H is positive l Making a bond is always exothermic! l ∆H is negative

The reaction itself will be either exothermic or endothermic depending upon the number and type of bonds broken and formed.

Steps for solving the problems l Draw correct Lewis structures for each substance. l Total the energy needed for breaking bonds. Remember it is a positive number. l Total the energy gained by forming bonds. Remember it is a negative number. l Add the two for the (approximate) ∆H rxn.

#1. Calculate the enthalpy of reaction for the process H 2 (g) + Cl 2 (g) --> 2 HCl(g)

#1. Calculate the enthalpy of reaction for the process H 2 (g) + Cl 2 (g) --> 2 HCl(g) -184.7 kJ

#2. Estimate the enthalpy change for the combustion of hydrogen gas. 2 H 2 (g) + O 2 (g) --> 2 H 2 O(g)

#2. Estimate the enthalpy change for the combustion of hydrogen gas. 2 H 2 (g) + O 2 (g) --> 2 H 2 O(g) -483.6 kJ

#3. Predict the enthalpy of reaction of the following reaction: 2 C 2 H 6 (g) + 7 O 2 (g) --> 4 CO 2 (g) + 6 H 2 O(g)

#3. Predict the enthalpy of reaction of the following reaction: 2 C 2 H 6 (g) + 7 O 2 (g) --> 4 CO 2 (g) + 6 H 2 O(g) -2759.1 kJ

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