I am wondering what would happen if a metal strip of copper and a metal strip of zinc were immersed in solution of $\ce{HCl}$ in standard electrochemical conditions. I have calculated the standard change in Gibb's free energy $\Delta_\mathrm rG^0$ using the relation $\Delta_\mathrm rG^0=-zF\Delta E^0$ where $z$ is the number of electrons exchanged, $F$ is faraday's constant, and $\Delta E^0$ is the standard change in electrode potential. I have found the following values:

$$\Delta_\mathrm rG^0_{\ce{Zn}}=-147\ \mathrm{kJ/mol}, \Delta_\mathrm rG^0_{\ce{Cu}}=-66\ \mathrm{kJ/mol}$$

It appears that the reaction between $\ce{Zn}$ and $\ce{H+}$ is more spontaneous. Does this mean that the reaction with $\ce{Cu}$ will not take place at all?

  • $\begingroup$ Enthalpy or free energy? These are two different things. Make up your mind. $\endgroup$ – Ivan Neretin Jun 8 '16 at 17:46

Thermodynamics spontaneity and kinetics of reactions are two different things. Just because something has more driving force, doesn't mean it is going to go faster. In fact, there are cases where higher driving force actually causes the reaction to go slower. For more, I would recommend the reading the Nobel lecture of Rudolph A. Marcus who developed theories explaining what is now known as the "Marcus Inverted Region".

Kinetics of reactions depend on more complex issues that are harder to control and predict.


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