[twinklestar1012]

Hi everybody from this forum

I've got a problem in understanding the relationship between activation energy and spontaneity and feasibility of a chemical reaction.

I know that spontaneity of a reaction can be determined by:
delta G = delta H - T delta S
if delta G <0, reaction is spontaneous
if delta G <0, reaction is not spontaneous

I also know that -
If Ea is high,
--> reactants: kinetically stable
--> reaction is unlikely to occur
If Ea is low,
--> reactants: kinetically unstable
--> reaction is likely to occur

Besides, I know that the feasibility of a chemical reaction depends of S and delta H.

However, I feel confused after studying the concepts above:
is feasibility = spontaneity of a reaction?
and is Ea related to feasibility / spontaneity of a reaction?
I guess Ea is not related to feasibility, because once Ea is provided, the reaction is feasible actually.
But at the same time, I think because the magnitude of Ea determines whether the reaction is likely to occur, so it should be related to the feasibility of the reaction.
Can you correct me?
If Ea will control the feasibility/ spontaneity of a reaction, how come it is not included in the equation delta G = delta H - T delta S?

thanks a lot!

[Yggdrasil]

The ΔG of the reaction tells you whether the reaction is spontaneous or not (i.e. whether the reaction is actually possible).  The E_a tells you whether the reaction will occur in a reasonable timescale or not.  It is possible for a reaction to be spontaneous but not feasible.  For example, graphite has a lower free energy than diamond, so the transformation of diamond to graphite is spontaneous.  However, the activation energy from this process is extremely high, so the process essentially will never occur (hence the saying that diamonds are forever).

[twinklestar1012]

The ΔG of the reaction tells you whether the reaction is spontaneous or not (i.e. whether the reaction is actually possible).

so you mean feasibility is the same as spontaneity? but aren't they different?

However, the activation energy from this process is extremely high, so the process essentially will never occur (hence the saying that diamonds are forever).

so that means because the Ea is too high, so that the reaction is not "feasible"?
And am I correct to say that:
Feasibility (whether a reaction is possible) of a reaction is affected by 1.  :delta:H
                                                                                              2. E_a
                                                                                              3.  :delta:G
Spontaneity (whether a reaction can occur without the supply of external source of energy) of a reaction is affected by 1.  :delta:H
                           2. :delta:G

Thanks.

[Yggdrasil]

The ΔG of a reaction determines whether the reaction is possible in theory.  If ΔG > 0 then you cannot get the reaction to occur without performing work on the system using an external energy source (such as performing electrical work to get an electrolysis reaction to occur).

In order for a reaction to be possible in practice, the reaction must not violate the laws of thermodynamics (so ΔG must be negative) and the reaction must occur at a reasonable rate (so E_a should not be too big compared to the thermal energy available).

The change in enthalpy of a reaction (ΔH) can have some practical implications (i.e. whether you would need an input of heat/active cooling to maintain the reaction at a certain temperature, but I would say that it is not a major factor in determining whether a reaction is feasible.  It is not a factor in the spontaneity of a reaction as spontaneity depends only on the ΔG of the reaction.  For example, ice melts spontaneously at room temperature despite being an endothermic process (i.e. ΔH > 0).  The reaction requires an external source of energy (heat) but this external source is not doing work on the system.

A final note.  The word "spontaneity" has a specific meaning in thermodynamics, namely that the ΔG of a reaction is negative.  "Feasibility" does not have a specific meaning, so it could mean different things to different people.