MadSci Network: Chemistry

Re: How does the molarity of acid affect the rate of electrolysis of HCl..?

Date: Thu Mar 12 14:01:53 1998
Posted By: John Letourneau, Lab Technician, Canadian Forestry Service
Area of science: Chemistry
ID: 889475052.Ch


To understand what is happening in your experiment, it may be necessary to 
define some of the parameters involved.

For chemists, it is necessary to have a unit of reference when dealing with 
quantities of chemicals.  As this reference, Avogadro's constant was chosen 
and is defined as 6.022 x 10e23 molecules per mol.  This means that a mol 
of any substance always contains the same number of molecules.  This is 
particularly important when we are comparing two different substances that 
don't have the same molecular weight (also known as molar mass).  As an 
example, if we were to compare 1 gram of NaCl (table salt) and 1 gram of 
water it would be difficult to tell if there are more salt molecules or 
water molecules without this frame of reference.  For this example, water 
has a molar mass of 18.01 g/mol and the salt has a molar mass of 58.44 

n = mass of substance / molar mass

Therefore, in our example there would be 0.05552 mol of water and 0.01711 
mol of the salt.

Molarity of solutions is related to this in that the molarity of a solution 
indicates the number of moles of solute for every liter of solution.  In 
turn, this can be related to the number of molecules in solution.

Now that the basics have been covered, we must now dive into the thick of 
things and answer the question of molarity effects on rate of electrolysis 
of HCl.  You are correct in saying that the rate of reaction in your case 
is dependant upon the concentration of the reactant in solution.  Every ion 
in your solution (H+ and CL- in your case) is surrounded by a much greater 
number of water molecules.  When a potential is applied to the electrodes 
the ions become attracted to the oppositely charged electrode and are drawn 
through the solution.  The ions continue to be drawn until they come in 
contact with the electrode (this is similar to a person attempting to move 
toward a door through a crowded room).  When contact is made the ion is 
converted either by gaining an electron or losing an electron.  The 
important part of this is how often an ion comes into contact with an 
electrode.  If there are more ions floating around in solution (in a 
solution of higher molarity), contact between an ion and an electrode will 
occur more frequently than in a more dilute solution.  This will appear as 
an increase in the rate of product formation (i.e.: an increase in the rate 
of electrolysis).

I hope that this is what you are looking for.


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