Factors that affect electrolysis

There are various factors that affect the electrolysis of a compound. These factors are discussed as follows:

• The electrode’s nature

The extent of electrolysis depends majorly on the electrodes dipped in the electrolytic solution. If the electrode contains metals that are more reactive or have a high reduction tendency, then it will readily react with the ions of the electrolyte and start decreasing in size. But if some non-reactive or inert electrodes such as that of platinum or graphite are used, then they possess no change in them. 

For example: In the electrolysis of a solution of sulfuric acid in water, if platinum electrodes are used, then there will be no reduction by platinum at anode, and the concentration of sulphate ion starts increasing in the solution, but if electrodes of copper are used at anode then it will reduce the sulphate ion to maintain its concentration.

• Nature of electrolytes

Generally, current needs a carrier such as electrons to flow through a material. Hence, if the electrolyte is non-conducting or contains no ions, then it will stop the process of electrolysis. In general, electrolysis involves the movement of charged particles such as ions (cations and anions) towards the oppositely charged electrodes. Hence, the electrolyte must contain free mobile ions in order to complete the process of electrolysis. If the concentration of salt or acid or base in the electrolytic solution is altered, then it also brings change to the major product formed in the reaction.

•  Electrode potential of ions 

The ions in the electrolyte have a major influence over the end product formed. The reduction of potential variation of ions leads to the generation of multiple ionic products sometimes. Not only that, in the aqueous phase of the salt, water also participates in the electrolysis and alters the end product.

•  Overvoltage 

Generally, electrolysis reactions prefer thermodynamically stable products, but if there is an oversupply of voltage during the reaction, it alters the end product. Sometimes kinetically stable products are formed due to this. This usually depends on the reduction potential of ions, which is highly affected by the voltage of the current supplied.

Electrolysis of aqueous sodium chloride

• Sodium Chloride is an ionic salt that readily dissociates in water as sodium cation and chloride anion
• Generally, the electrolysis of NaCl, when dissolved in water, becomes a lot easier than the electrolysis of NaCl in the molten state
• Also, in the case of an aqueous solution, water also gets dissociated into H+  ion, and OH- ions, and they also participate in the electrolysis reaction

• Hence, the end product of the electrolysis of NaCl produces H2 gas at cathode and Cl2 gas at anode. Also, the Na+ ion and OH- ion in the remaining solution react together to form NaOH
• This process is also known as the Chlor-Alkali process because the products constituted are chlorine (chlor) and NaOH (alkali). The general reaction of this electrolysis process is:

2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)

At Cathode: 

2H2O(l) + 2e– → H2(g) + 2OH–       E° = -1.0 V

Na+(l) + e– → Na(l)            E° = -2.71V

At Anode: 

2H2O → O2(g) + 4H+ + 4e–          E° =-1.42 V

2Cl– → Cl2 + 2e–              E° =- 1.36V

Hence, the major product formed from the electrolysis of aqueous NaCl. 

1. i) At cathode, the product will be sodium metal or hydrogen gas.
2. ii) At anode, the product will be chlorine gas or oxygen gas.

A side product of NaOH is also formed depending on the concentration of NaCl in water.

Effect of concentration of NaCl in electrolyte on the product of electrolysis

Case 1)  When the concentration of NaCl is low (highly diluted solution): At a higher dilution of NaCl, water dominates the electrolyte. Also, we know pure water is not a good conductor; hence the product depends on the dissociation of ions caused by the concentration of NaCl. Thus, at cathode H2 gas is liberated, while at anode, O2 gas is liberated. This is also because of low concentrations of NaCl.

Case 2)  When the concentration of NaCl is high (Low diluted solution): At higher concentrations of NaCl in electrolyte, it dominates the end product of the electrolysis. H2 gas is obtained at cathode, and Cl2 gas is liberated at anode. Generally, electrolysis of brine (a solution of NaCl in water with a ratio of 9:1 of both respectively) produces NaOH as a product. This process is also known as the chlor-alkali process.

Case 3) When the concentration of NaCl is intermediate (adequate dilution): At intermediate concentrations of NaCl in water, all the expected products are in major as the quantity of both water and NaCl is equal. At anode, Cl2 and O2 gases are liberated, and at cathode, Na metal and H2  gas are liberated. And the reaction of remaining OH- and Na+ in the solution reacts to form NaOH.

Conclusion

So, at last, it is concluded that electrolysis is a process that uses electricity to do a chemical change. In the electrolysis of aqueous solution of NaCl typically, the major products at anode are Cl2 gas and O2 gas while those on cathode are H2 gas and Na metal. NaOH is also formed as a side product, which totally depends upon the concentration of NaCl in water.

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