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In the lecture today, we did an example about bond enthalpies. CH2=CH2 + H-Br → CH3-CH2Br. The change in enthalpy of the reaction was -58 KJ. Why is this an exothermic reaction?
It is exothermic because new bonds are being formed, so energy is being released.
When we calculated the individual bond enthalpies for the bonds that we broken in the reaction compared to the bonds that were formed, we found that the bond enthalpies of the bonds being formed were larger than the bonds broken. Since the bonds being formed are given negative enthalpies and the bonds being broken are given positive enthalpies, the resulting net enthalpy for the reaction is negative.
When bonds are formed, energy gets released. This results in a negative enthalpy, deltaH. If it is negative, then it is exothermic.
Whenever the enthalpy is negative it is an exothermic reaction as it means that it RELEASES that amount of energy rather than absorbing it.
Bonds being broken on the reactants side require energy, which is reported as a positive number. In this case, it was 978 kJ. On the products side, bonds are being formed, which release energy, reported as a negative number (-1086kJ). As a result, the difference between the two products was -59kJ, as more energy was released than was required. Since there is a net energy released, the reaction is exothermic.
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