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So the example at the end of class was using bond enthalpies to calculate the rxn enthalpy. The answer came out to -58kj which is exothermic. I was thinking about this ... in order for this reaction to occur, you would have to take out 58kj from the system. So would you ever naturally find the reactants if they are more stable on the product side?
Interesting question! I can actually pull in information I've learned in LS7A to explain this: as others mentioned, negative enthalpy or exothermic reactions don't necessarily indicate that the reaction is spontaneous, it is actually the sign of the change of Gibbs free energy that does (a negative value of change means the reaction is spontaneous, a positive value of change means it is not spontaneous). The equation for Gibbs free energy is ΔG = ΔH - TΔS, so you can see that the value of ΔG is affected also by temperature and change in entropy and that even negative values of ΔH can result in positive ΔG.
The method explained in class is the most common and easiest way to determine if a reaction is exo- or endo- thermic because it uses the definition. If the reaction produces/releases heat to the surroundings it is exothermic, and if it absorbs heat it is endothermic.
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