What are the products of electrolysis?

When compounds are electrolysed, new substances are produced at the electrodes. For example, when electricity is passed through molten sodium chloride, pale green chlorine gas comes off at the anode, and shiny molten sodium forms at the cathode.


Apparatus that helps to investigate the products at the electrodes when aqueous solutions are electrolysed

The apparatus in Figure 1 helps to investigate the products at the electrodes when aqueous solutions are electrolysed. When copper sulfate solution is electrolysed using this apparatus, a pink deposit of copper appears on the cathode. Bubbles of a colourless gas stream off at the anode and collect in the inverted test tube. This gas relights a glowing splint, showing that it is oxygen.

figure 1: apparatus that helps to investigate the products at the electrodes when aqueous solutions are electrolysed.

Products formed at the electrodes when various liquids and aqueous solutions are electrolysed

Table 1 also lists the products formed at the electrodes when various other liquids and aqueous solutions are electrolysed. Note that the products may come from the water when aqueous solutions are electrolysed.

Substance electrolysedProduct at anodeProduct at cathode
molten sodium chloridepale green chlorine gassodium
molten lead bromideorange-brown bromine gaslead
aqueous potassium iodideiodine which colours the
solution brown
hydrogen
aqueous copper sulfateoxygencopper (deposited on the
cathode)
dilute hydrochloric acidchlorinehydrogen
dilute sulfuric acidoxygenhydrogen
aqueous zinc bromidebromine which colours the
solution brown
zinc (deposited on the
cathode)
aqueous sodium chloridechlorinehydrogen
Table 1 The products formed at the electrodes when some liquids and aqueous solutions
are electrolysed

Which types of elements are produced at the anode? Which types of elements are produced at the cathode?

When acids and ionic (metal/non-metal) compounds conduct electricity, new substances are formed as follows.
– a metal or hydrogen is formed at the cathode.
– a non-metal (except hydrogen) is formed at the anode.

When acids and ionic (metal/non-metal) compounds are electrolytes, the compounds are decomposed by electrical energy. An element is produced at each electrode. This is very different from metals, which are not decomposed when they conduct electricity.

The first two electrolysis in Table 1 can be summarised in word equations as:

What are the products of electrolysis?
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