[Peng Zhong]

Hi all,

A system containing HCl(g), H2(g), and Cl(g) at T=273K and P=1atm. Initially it has a lot more HCl(g) than H2(g) and Cl2(g) so that the reaction quotion [HCl(g)]/(H2(g)]*[Cl2(gP]) for the reaction H2(g)+Cl2(g)=HCl(g) is a lot higher than the equilibrium K value at T=273 and P=1atm. This will favor the reverse reaction according to the laws of equilibrium.

I know that if the system favors the forward reaction, a spark of ultraviolet light must be used to initiate a fast forward reaction. In the case described above, the reverse reaction is favored, but what mechanism can trigger a fast reverse reaction?

Thank you very much.

Peng zhong

[Borek]

Technically there is no such thing as forward and reverse reactions - it all depends on how you write the reaction equation, and you can write it both ways.

And in general all reactions require activation energy.

[Peng Zhong]

Borek, sorry but you didn't really answer my question and I think you are wrong because there is forward and reverse reactions in all chemical reactions. Someone please help.

[Borek]

there is forward and reverse reactions in all chemical reactions

Yes, but they are defined in reference to the reaction equation, and reaction equation - especially for reversible processes - can be written any way you like, making the distinction irrelevant.

What I am aiming at is that if the process can run both ways the same rules apply to both forward and reverse process, as which is called which is a matter of convention:

CH₃COOH CH₃COO^- + H⁺

Forward process - dissociation, reverse process - protonation.

CH₃COO^- + H⁺ CH₃COOH

Forward process - protonation, reverse process - dissociation.

Neither description is better than the other, they are perfectly equivalent.