Understanding Electrolysis

Understanding Electrolysis
Posted in: Science Blogs

Electrolysis is a chemical process that involves the use of an electric current to drive a non-spontaneous chemical reaction. It is commonly used to separate compounds into their constituent elements or to induce chemical changes in a substance. Examples in school include understanding the stoichemistry of water, and the plating of one metal to another. Understanding electrolysis requires knowledge of its basic principles, the components involved, and the underlying reactions. 

In electrolysis, a substance is either dissolved in a liquid or in a molten state, known as an electrolyte, which allows the flow of electric charge. The electrolyte contains ions that can either be positively charged (cations) or negatively charged (anions). Two electrodes: a cathode (negative electrode) and an anode (positive electrode), are submerged in the electrolyte. These electrodes are usually made of inert (unreactive) materials like carbon or platinum, (copper is used in electroplating experiments). 

  • Electrolysis means splitting with electricity 
  • Completing an electrical circuit using free ions in a liquid or solution 
  • Needs power source (DC) 
  • Needs electrodes; somewhere for the ions to go (be attracted to) 
  • Used to ‘plate’ metal in industry, to extract metal from its ore, manufacturing chemicals, and is also employed in pH probes 

Set up: 

  1. The electrolyte solution is added to a suitable container. 
  2. Electrodes and a power supply are added as in the diagram above. 
  3. The power supply is switched on and bubbles should appear at one of the electrodes. 
  4. Depending on the electrolyte and the electrodes used you may see; plating of a metal at one electrode, a diminishing electrode (as it is used in the reaction), gas produced at one electrode. 
  5. Adding a microammeter into the circuit, (in series) will indicate that current is flowing.  

The science: 

Electrolysis is a good way of proving the existence of ions in a molten state or a soluble state. 
Ionic compounds (typically a metal and non-metal) are able to be split into their constituent ions (charged particles) with the addition of power. 
These charged particles act to bridge the ‘gap’ in the circuit above. The negative ions will be attracted to the positive anode and at the same time, positive metal ions will be attracted to the cathode (negative electrode). 

Molten compound: 

In a molten ionic compound this is a straightforward split, e.g., molten lead bromide (PbBr2): 
PbBr2→ Pb2+ + 2Br- 
The bromine ions (Br-) will be attracted to the anode (+) and gain electrons here to become bromine gas (Br2), this will be given off at the anode.  
The lead ions (Pb2+) will move to the cathode (-) and lose electrons to it becoming lead solid (Pb) and molten lead drops would be formed here. 

*The reason that this ‘connects’ the circuit becomes clearer when you see that conventional current flow is described as moving from the positive terminal of the power source towards the negative terminal. The flow of electrons in a circuit, however, has been shown to be from the negative terminal to the positive, therefore the Pb2+ flows to the negative electrode and gains electrons coming the other way, to become solid lead. The Br- flows to the anode, losing electrons to the anode (which then continue the electron flow around the circuit) and becoming Br2 gas 


Here, there are also the ions in the water to be aware of: H+ and OH-  
Some of the most common solutions used in electrolysis (in school) are sodium chloride and copper sulfate, and sulfuric acid (used in the electrolysis of water, in the Hofmann voltameter) 


Sodium chloride solution, NaCl (aq):  
NaCl → Na+ + Cl- also, because this is a solution:  
H2O → H+ + OH- 
Chlorine ions and hydroxide (OH-) ions will head to the +ve anode 
Sodium ions and hydrogen ions will head towards the – ve cathode 
But how do you know what is given off/comes out of solution at each electrode? 

At the Cathode: 

For this you need to know how reactive the ions are with respect to each other. 



The more reactive an element is, the easier it forms ions, but it is also harder for it to return to the atomic state (i.e., to pick up electrons and return to neutral), in other words it prefers to stay as ions in solution. 
So, if hydrogen is the less reactive ion at the cathode, hydrogen gas will be produced, because the other (metal) ion is more reactive and wants to stay in solution. 
If hydrogen is the more reactive element at the cathode it will stay in solution, and the other ion will form solid metal instead. 
For the aqueous NaCl example above: 
Hydrogen is less reactive than sodium, so: 2H+ + 2e- → H2(g) at the cathode 
Sodium stays as Na+ in solution 
For aqueous silver chloride, the hydrogen ions stay in solution (as hydrogen is now the more reactive element), and silver metal is deposited at the cathode: Ag2+ +2e- → Ag(s) 

At the Anode: 

Here, the negative ions are collecting, these will be OH- (from the water) and another ion. 
If this other ion is a halide (chloride, bromide, iodide) then the halogen gas will be produced (chlorine, bromine or iodine). 
If it is anything else (sulfate, nitrate etc) then hydroxide ions will form water and oxygen which will be produced at the anode. 

So, for aqueous NaCl: 

2Cl- (aq) → Cl2(g) +2e- and the OH- stays in solution (this will make NaOH with the Na+ ions already in solution). You can test for chlorine gas by seeing if it bleaches some damp litmus paper. 
So, the full equation for this would be: 

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


Inert (unreactive) electrodes are often used in electrolysis set ups so that they act as a conducting element in the circuit but are not affected by and don’t take part in the reaction of the ions in solution. 
These are usually carbon or platinum. If you use non-inert electrodes, they can decompose in the electrolyte solution and add to the reaction. This can be useful if you want to purify a metal (such as copper). 

  • Using impure copper as the anode and a tiny piece of pure copper as the cathode, and copper sulfate solution as the electrolyte, copper ions will transfer from the anode to the cathode, decreasing the mass of the anode and increasing the amount of pure copper at the cathode. 

Electrolysis of water: the Hofmann voltameter 
Although the Hofmann apparatus is used with dilute sulfuric acid, it is actually to demonstrate the electrolysis of water, and the stoichemistry of water (2 molecules of hydrogen to one of oxygen). 


The aqueous solution contains sulfuric acid, H2SO4 and water, H2
At the cathode there will be H+ ions gathering. As there are no other positive ions here,
H+ will gain electrons and become hydrogen gas: 2H+ +2e- → H2(g) 

At the anode there will be OH- and SO42- gathering. Because there is no halide present, the sulfate ions will stay in solution and the OH- will discharge at the electrode and water and oxygen gas will be produced: 4OH- → 2H2O(aq) + O2(g) + 4e- and the ratio of hydrogen
gas: oxygen gas produced is 2:1. 

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