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Name:  Jason
Status: student
Grade: 6-8
Country: N/A
Date: 11/17/2005

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 reciprocal of 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 KCl.

Vince Calder

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