# Are all substitution reactions reversible?

Akash47
Consider a reaction:
H2+CuCl2= Cu+2HCl
This is a substitution reaction.But is this may not be a reversible reaction since Cu is less active than .So Cu can't substitute H from HCl and make a backward reaction.Is my thinking right?

## Answers and Replies

trurle
Consider a reaction:
H2+CuCl2= Cu+2HCl
This is a substitution reaction.But is this may not be a reversible reaction since Cu is less active than .So Cu can't substitute H from HCl and make a backward reaction.Is my thinking right?
See Gibbs energy relation to the rate of direct and reverse reaction.
You need to sum entropic and enthalpic parts of Gibbs energy on left and right part of reaction equation to find the rates of both reverse and direct reaction.

Akash47
See Gibbs energy relation to the rate of direct and reverse reaction.
You need to sum entropic and enthalpic parts of Gibbs energy on left and right part of reaction equation to find the rates of both reverse and direct reaction.
But isn't my opinion right what I've said in my first post?

trurle
Consider a reaction:
H2+CuCl2= Cu+2HCl
This is a substitution reaction.But is this may not be a reversible reaction since Cu is less active than .So Cu can't substitute H from HCl and make a backward reaction.Is my thinking right?
At room temperature, Gibbs energy:
Of HCl: -2*131170+2*51(T-T0)
Of CuCl2: -1*103000+2*91(T-T0)
You can see the HCl Gibbs energy of HCl is much more negative, and difference of entropies is small so heating do not help much.
According to equation,
Keq=5*10^27 for reaction in question at room temperature, this mean reaction is very difficult to reverse to the formation of copper chloride.