Spontaneous Reactions

[narek kurkjian 2I]

How can an endothermic reaction occur spontaneously?

[Angela Ke]

I don't think endothermic reactions can occur spontaneously because the definition of a spontaneous reaction is one that occurs without additional energy input. Endothermic reactions require additional heat to proceed.

[Abeni Liu 2B]

While endothermic and exothermic reactions are determined using delta H (enthalpy), spontaneity is determined using delta S (entropy). Since these two values are different, endothermic reactions can be spontaneous. This would mean a positive delta H and a positive delta S.

[Ankit Budhiraja 3D]

The occurrence of endothermic reactions taking place spontaneously is determinant on the composition of deltaG. Specifically, deltaG being a negative value implies that a reaction is exergonic and is occurring spontaneously. When deciding if an endergonic reaction can occur spontaneously we must consider when deltaG can be negative considering the presence of a positive deltaH value. Consider the equation deltaG = deltaH - (T(deltaS)). This equation signifies that for deltaG to be negative we must know the deltaS value's size in comparison to deltaH. Also deltaS must be positive so that it is being subtracted from deltaH, thus influencing the possibility for deltaG to be negative and the reaction occurring to be spontaneous despite being endergonic. Basically, deltaS must be positive and the value of (T(deltaS)) must be greater than deltaH to yield this result.

[Adam Jameel 2A]

In addition to what the earlier responses emphasized, the idea of spontaneity results from the idea that a reaction does not need any energy to start - unlike in an endergonic reaction which requires energy to break bonds (thus increasing delta H and delta G overall in a reaction). Spontaneous reactions are thoroughly only exergonic.

[Catherine Grigorian 3L]

In lecture, spontaneous reactions were defined as favorable, meaning it occurs by itself, without the need for an energy input. For an exothermic reaction, we know that the delta U or delta H are negative values. Therefore, it would be spontaneous for an exothermic reaction because it goes to a lower enthalpy. In addition to this information, you must also know about the entropy. If the entropy increases it facilitates a process to occur by itself, therefore it is favorable and spontaneous, and vice versa. Therefore, for an endothermic reaction, the entropy decreases, therefore I think it would not be favorable, and not be spontaneous.

[Audrey2B]

Is it true that whether or not a reaction is spontaneous does not predict the rate of reaction?

[Janvi Bharucha 3D]

For a reaction to be spontaneous, the total entropy of the system and its surroundings must increase. This can occur even if the reaction is endothermic and absorbs heat from the surroundings, as long as the entropy change of the system is greater than the entropy change of the surroundings.

The increase in the entropy of the system can be greater than the decrease in the entropy of the surroundings, thereby resulting in a net increase in the total entropy of the system and its surroundings. This makes the reaction spontaneous.