Electrolysis is a process that uses an electrical current to separate ionic compounds into their constituent elements.
When a direct current is passed through an ionic substance, the ions migrate to the oppositely charged electrode and drive a non-spontaneous oxidation-reduction reaction.
If you’re studying GCSE chemistry, you may have already used electrolysis to separate oxygen and hydrogen from water. However, the technique has various industrial applications too, including electroplating and metal extraction. Fuel cell electric vehicles are also powered by hydrogen that’s produced through electrolysis.
In this post:
Introduction to electrolysis: what is electrolysis?
When connected to a direct current, the free-moving ions in an electrolyte are attracted to the oppositely charged electrode. This produces a chemical change called a non-spontaneous oxidation-reduction reaction.
Electrolysis requires an input of electrical energy to force a reduction reaction in the cathode (negative electrode) and an oxidation reaction in the anode (positive electrode). The process is carried out in an electrolytic cell (more on this below).
Oxidation and reduction reactions occur in tandem during electrolysis. As electrons move away from the anode, they’re received by the cathode. In the electrolysis of water, oxygen is produced at the anode while hydrogen is produced at the cathode.
Gases aren’t the only products that can be formed during electrolysis. As shown in the balanced chemical equation below, if sodium chloride (table salt) is added as an electrolyte, sodium hydroxide and chlorine are also produced.
2 NaCl + 2 H2O = 2 NaOH + Cl2 + H2
Electrolytes: definition and examples
Electrolytes are any ionic compounds that are dissolved in water. The most common example of an electrolyte is sodium chloride, which is commonly found in the blood.
Other electrolytes include ions of magnesium, phosphorus and potassium. These play crucial roles in helping to keep cells and organ systems functioning normally. For example, electrolytes carry electrical signals from the brain to move skeletal muscles.
Set-up of an electrolytic cell: components and their functions
An electrolytic cell has three main components – the electrodes (an anode and a cathode), the electrolyte, and the battery or power source.
The electrodes are where the oxidation-reduction reactions occur in an electrolytic cell. They can be made from any type of metal, provided they’re either inert or relatively stable and don’t react with the electrolyte. During electrolysis, positively-charged ions are attracted to the negative electrode (cathode), while negatively-charged ions are attracted to the positively-charged electrode (anode).
The electrolyte is typically an aqueous solution that contains dissolved ions. Molten substances including salts like sodium chloride can also act as electrolytes. The electrolytes involved in electrolysis aren’t as reactive as the acids that are typically used in galvanic cells.
Unlike a galvanic cell, which converts energy from a spontaneous redox reaction into electrical energy, the oxidation-reduction reaction in an electrolytic cell is non-spontaneous. An electrolytic cell therefore requires an external electricity source such as a battery or AC power supply to drive the reaction.
Electrolysis solutions: aqueous vs. molten
Electrolysis can only be performed on a fluid (this can either be an aqueous solution or an ionic molten substance). It’s also worth noting that the process won’t work if the substance doesn’t conduct electricity. That’s because an electrical current needs to pass through the substance to induce the oxidation-reduction reaction.
Aqueous electrolysis involves electrically-conductive substances dissolved in water. When salts and soluble minerals are subjected to electrolysis, they will always separate water into hydrogen and oxygen. Simultaneously, the ionic components are also split and, in some cases, may react with water. An example of this is the reaction between sodium ions and hydroxide ions to form sodium hydroxide.
Molten electrolysis, meanwhile, involves ionic substances that are melted to become liquid. For example, lead bromide can be subjected to molten electrolysis when it’s heated to its melting point of 373 °C. Electricity is then passed through the molten substance to split it into its constituent elements. The electrodes used in molten electrolysis are generally inert and have a high resistance to melting.
Molten electrolysis may be necessary if you want to derive a higher concentration of the constituent elements. It also helps to prevent the constituent components from reacting with water.
Factors affecting electrolysis
Various factors can affect the rate of reaction during electrolysis. These include the concentration of the electrolyte, the temperature, and the voltage.
- Concentration – this refers to the amount of electrolyte that’s dissolved in the water. A greater amount of substance means there are more ions that can be separated from the compounds.
- Temperature – the temperature is crucial to both aqueous electrolysis and molten electrolysis. A higher temperature enables the molecules and ions to move faster, which then facilitates faster reactions.
- Voltage – increasing the voltage of the electricity also increases the rate of reactions during electrolysis. Voltage is the “pressure” that pushes electrons.
Applications of electrolysis
Electrolysis has a wide range of industrial applications, including the extraction of metals and electroplating. The technique is usually performed as part of the wider manufacturing process. Some of the common uses of electrolysis are summarised below.
Electroplating
Many of the metals used in the automotive and shipbuilding industries require multiple layers of protection. Electroplating – the process of plating one metal onto another using electrolysis – enables manufacturers to protect the base metal from corrosion. In cases such as chrome plating, the technique can also be used to enhance the aesthetic appeal and finish.
Extraction of metals
Electrolysis is often used to extract metals from molten ores or molten ionic compounds. Aluminium, for example, can be extracted from its ore (known as bauxite) using this process.
The bauxite is firstly converted into aluminium oxide and then electrolysed in a solution of molten cryolite (another aluminium compound). Electrolysis is particularly useful for extracting highly-reactive metals.
Electrorefining of metals
Metals that have been extracted from ores through the heating process can be further refined using electrolysis. Materials like copper and silver are often purified in this way. The impure metal serves as the anode while a very pure sample of the metal acts as the cathode.
During electrolysis, the impure components are removed from the metal and deposited on the cathode.
Electrolytic reduction of metals from their compounds
Reduction reactions can be used to extract metals from ionic compounds. Metals may either precipitate in an aqueous solution or pure metals can be extracted from molten compounds.
Safety in electrolysis
Whether you’re performing electrolysis in the school laboratory or an industrial setting, there will always be some risks involved. The electrolyte may be corrosive or poisonous, for example, or the by-products could be explosive, such as in the case of water electrolysis. There’s also a risk of electrocution, particularly if you’re using a high-voltage electrical current.
Handling of electrolytes
It’s important to take care when handling the electrolyte solution to minimise the risk of injury. Always wear protective equipment such as goggles, gloves, and a lab gown.
Electrical precautions
Large-scale electrolysis, such as metal electroplating, can be dangerous because of the high voltage involved. Never touch the electrodes or electrolytes when the process is underway and always wear the correct protective equipment. You should also make sure you receive proper training before operating the equipment.
Summary
Electrolysis works by producing a non-spontaneous oxidation-reduction reaction. The movement of electrons split the constituent ions of the compound into their elements. As part of your GCSE chemistry studies, you will probably use electrolysis to separate oxygen and hydrogen from water. Electrolysis also has several industrial applications such as electroplating and metal extraction from compounds.
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