Spontaneous reactions require activation energy?

[ldv1452]

"Spontaneous" reactions require activation energy?

To my understanding reactions are considered "spontaenous" if they require no additional energy for the reaction to occur. However, they require activation energy. Why is activation energy not considered "additional energy?" The term "spontaneous" doesn't seem to hold its intuitive meaning under this explanation.

 

[DaveC426913]

To my understanding reactions are considered "spontaenous" if they require no additional energy for the reaction to occur. However, they require activation energy. Why is activation energy not considered "additional energy?" The term "spontaneous" doesn't seem to hold its intuitive meaning under this explanation.

Spontaneous - at room temperature?

 

[ldv1452]

Spontaneous - at room temperature?

Well let's say we take a reaction at room temperature for example. If it is considered "spontaneous" at room temperature doesn't it still require activation energy?

 

[Borek]

Yes, it requires activation energy - but so low, that thermal motion of the molecules at room temperature is enough.

 

[DaveC426913]

This is what I was hinting at. A little too oblique I guess.

 

[ldv1452]

Yes, it requires activation energy - but so low, that thermal motion of the molecules at room temperature is enough.

So would it be correct to say that a "spontaenous" reaction is one where the activation energy is already available in the system as heat (thermal) energy?

 

[Borek]

Something like that.

 

[Ygggdrasil]

It really depends on how you define a "spontaneous" reaction. Most chemistry textbooks will define a "spontaneous" reaction as what I will call a thermodynamically spontaneous reaction. In these reactions, the sole requirement is that ΔG < 0. For these reactions, the reactions occur without a NET addition of energy to the system. A good example here would be burning gasoline. You need to add energy in the form of a spark, but the energy you get out is much greater than the activation energy you provide.

This definition, however, does not match the colloquial definition of "spontaneous." For example, the conversion of diamond to carbon dioxide is thermodynamically spontaneous. However, because the activation energy is so high, the reaction almost never happens at room temperature (hence, diamonds are forever and we don't need to worry about them "spontaneously" evaporating).

 

[ldv1452]

It really depends on how you define a "spontaneous" reaction. Most chemistry textbooks will define a "spontaneous" reaction as what I will call a thermodynamically spontaneous reaction. In these reactions, the sole requirement is that ΔG < 0. For these reactions, the reactions occur without a NET addition of energy to the system. A good example here would be burning gasoline. You need to add energy in the form of a spark, but the energy you get out is much greater than the activation energy you provide.

This definition, however, does not match the colloquial definition of "spontaneous." For example, the conversion of diamond to carbon dioxide is thermodynamically spontaneous. However, because the activation energy is so high, the reaction almost never happens at room temperature (hence, diamonds are forever and we don't need to worry about them "spontaneously" evaporating).

Good explanation. This makes more sense now. It would make life easier if words were used that didnt have such strongly ingrained colloquial definitions.