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Electrolysing neutral aqueous sodium chloride reduces water rather than hydrogen ion because the hydrogen ion concentration is:
AExactly the standard 1 M
BImpossible to measure
CFar below the standard 1 M
DFar above the standard 1 M
Answer & Solution
Correct answer: C. Far below the standard 1 M
1. Standard electrode potentials assume every dissolved species sits at 1 molar.
2. A neutral solution does not meet that assumption for hydrogen ion.
3. In a neutral aqueous sodium chloride solution the concentration of hydrogen ion is far below the standard state value of 1 molar, approximately ten to the minus seven molar.
4. So the observed cathode reaction is actually reduction of water.
5. This is a standard example of why tabulated potentials give only estimates.
_Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 17 'Electrochemistry', sections 17.1-17.7_
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