Salt and Electrolysis
How does salt (NaCl) effect the process of hydrolysis of
water by electrolysis?
Usually chemists reserves the term "hydrolysis" to mean the reaction
of water with some OTHER substance, and the term "electrolysis" to mean
the decomposition of water into its constituent elements (H2 and O2) by
passing an electric current (a direct current DC, not an alternating
current AC) through the water.
The major effect of adding salt to water is to icrease its
conductivity, or the same thing reduce its resistiviy. This is so more
electric current can be passed through the solution. I will give you some
comparisons below, but first, the units of measure
need to be clarified. By Ohm's Law:
E = I x R where E is the voltage in volts, I is the current in amperes, and
R is the resistance in ohms. In terms of fundamental units the resistance R
has crazy units: (meters)^2 x (kilogram) x (seconds)^-3 x (ampere)^-2 -- a
bit weird but that is how it turns out in SI units, and is the reason we
state resistance in "ohms" in laboratory work.
The conductivity, which has the name "Siemens", is just the
the resistance, that is: 1/R. The thing to remember is: Large values of the
resistance (ohms) means a poor conductor; low values of the resistance
(ohms) means a good conductor. Conversely, large values of the conductivity
(Siemens) means a good conductor; low values of the conductivity (Siemens)
means a poor conductor.
Extremely pure water (very difficult to make in the lab, but possible)
is a very poor conductor of electricity. And only a very low concentration of
a dissolved IONIC substance is necessary to cause a huge increase in the
conductivity of the solution (or the other way around huge decrease in the
resistivity of the solution). Here are some comparisons expressed as ohms.
(The reason for using ohms is people are more familiar with resistors --
those little black things on a circuit board with the colored stripes which
codes the value of the resistor.):
"pure water" (2 x 10^5 ohms); water saturated with atmospheric CO2 (1 ohm);
1 molar KCl [similar to NaCl]
(1 x 10 ^-5).
This value for atmospheric CO2 is a bit variable because the amount of
CO2 in air can vary somewhat. The reaction is: CO2 + H2O ---> H2CO3 == H[+1]
+ HCO3[-1]. But you can see that the ability of an ionic solution to conduct
electricity is VERY sensitive to the concentration of ionic species
present -- decreasing by 10 orders of magnitude for pure water to 1 molar
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Update: June 2012