Re: How is the rate of electrolysis affected by temperature

Hello J.--

The easy answer to the question: "Does ___________ affect the rate of the reaction?" is "Yes!".

Everything from temperature and concentration to the shape of a container can affect kinetics.  This is sort of a joke for many chemists and they go so far as to blame the color of their socks for unusual kinetics data!  The more difficult question, of course, is how much effect does it have...

(The quick answer is that increasing the concentration of the copper solution will always increase the rate, but too much could cause copper sulfate to precipitate during the plating.  Temperature is more complicated but increasing it will generally increase the rate.)

Essentially, the rate of the deposition of copper is controlled by how quickly Cu ions are reduced at the electrode surface.  This, in turn, is mostly controlled by two different things:  (1) the current density and (2) availability of the metal cations.

Current density is estimated by the Butler-Volmer equation.  The important things for us to know is that it decreases exponentially with temperature,  increases exponentially with potential, is a function of the metals involved, and is a function of electrode shape.  Copper has a current exchange density between that of platinum and nickel and the typical plating times reflect this.  Corners and edges of electrodes tend to have a higher current density so electrode shape is important.

The availability of the metal cations is controlled by diffusion and by the concentration.  As cations are reduced and plated they must be replaced by other cations in order for the reaction to continue.  The ability of cations to diffuse depends upon many things including temperature, physical agitation, and the solvent.  Diffusion will be faster with higher temperatures and more stirring.  In fact, Mohler (see references) writes that the rate of this very reaction can be increased easily by ten times simply by vigorous stirring!

So you can see clearly that higher concentration will result in a faster reaction rate.  Some typical values for a clean and speedy deposition in acid copper plating are as follows:
 
 

The sulfuric acid will slowly have to be replaced during the reaction.  It limits the solubility of copper sulfate and that is why one uses a lower initial value and then replenishes it.

Temperature has a more complicated effect.  As the temperature is increased, the current density decreases, diffusion increases, and solubility increases.  So one must discover which is more important--ideally with experiment.  However, at low concentrations the rate of deposition will be controlled by diffusion--so higher temperature would be better.  At high concentrations, the rate will be controlled by current density so lower temperature would be better.   (You could take advantage of this situation with a procedure like the second one below.)

So the simpler way would be to do a high concentration solution at room temperature.

However, it might be interesting to start at room temperature with 250 g/l copper sulfate and 60 g/l sulfuric acid with much stirring.  Then as the reaction proceeds, increase the temperature slowly until it is nearly boiling at the end of the reaction.  Another addition of sulfuric acid half-way might also be good.  (And, of course, be careful with acid...)

Cheers!
Jason

Here are some links that may be useful:

References:

Mohler, James B., Electroplating and Related Processes, Chemical Publishing Company, New York 1969
Atkins, Peter, Physical Chemistry, Freeman and Company, New York 1994