Changes in Internal Energy, Af

Use the bond energies listed in Table 15-2 to estimate the heat of reaction at 298 K for the following reaction. 

Two moles of C — C bonds and seven moles of C — H bonds are the same before and after reaction, so we do not need to include them in the bond energy calculation. The only reactant bonds that are broken are one mole of C — H bonds and one mole of Cl — Cl bonds. On the product side, the only new bonds formed are one mole of C — Cl bonds and one mole of H — Cl bonds. We need to take into account only the bonds that are different on the two sides of the equation. As before, we add and subtract the appropriate bond energies, using values from Table 15-2. 

We would get the same value for A/fJxn if we used the full bond energy form of Hess s Law and assumed that all bonds in reactants were broken and then all bonds in products were formed. In such a calculation the bond energies for the unchanged bonds would cancel. Why Try it  

The terms q and w represent heat and work, respectively. These are two ways in which energy can flow into or out of a S) tem. Work involves a change of energy in which a body is moved through a distance, d, against some force, that is, w = fd. 

A = (amount of heat absorbed by system) + (amount of work done on s) tem) 

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Hess s Law 15-9 Bond Energies 15-10 Changes in Internal Energy, Af 15-11 Relationship Between AH and Af... 

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