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An endothermic reaction can only occur when there is excess free energy in the environment, while an exothermic reaction can occur in the absence of this energy. Therefore, the larger the deltaH value, the less spontaneously the reaction will occur.
If the reaction has a positive enthalpy, then it is endothermic and is non -spontaneous as it requires heat in order to occur. If the reaction has a negative enthalpy, then it is exothermic and is spontaneous.
If a reaction is spontaneous, it means that no additional energy is needed for the reaction to occur. It will just happen naturally (although this doesn’t indicate how fast a reaction will occur). You can determine spontaneity using Gibbs free energy or delta G — if the value is negative in a reaction, it will be spontaneous. Since delta G is determined in part by enthalpy, the enthalpy can affect is delta G is positive or negative.
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