Faraday’s Law of Electrolysis is a principle that describes the behavior of electrons and ions in an electrolytic solution. It is fundamental to every electrochemist since it plays a crucial role in many industrial processes and applications, such as the production of aluminum, the refining of metals, electroplating, the treatment of wastewater, and more. In this post, we will explore the principles of Faraday’s Law of Electrolysis, the factors that affect it, and why it is important in electrochemical reactions.
What is Faraday’s Law of Electrolysis?
Faraday’s Law of Electrolysis is a principle in electrochemistry that states that the amount of a substance that is dissolved or deposited during an electrolytic process is proportional to the electric charge passed through the solution. It was first discovered by Michael Faraday in the early 19th century and remains a fundamental concept in the study of electrochemistry.
It is divided into 2 interrelated laws.
Faraday’s First Law of Electrolysis
Faraday’s First Law of Electrolysis states that the mass deposited or dissolved in a solution during electrolysis is directly proportional to the electric charge that is passed through the system. In other words, the greater the electric charge, the greater the amount of substance that is deposited or dissolved.
It can be expressed as:
Where:
m is the mass deposited or dissolved,
Z is the proportionality constant, also known as electrochemical equivalent
Faraday’s Second Law of Electrolysis
Faraday’s Second Law of Electrolysis states that if the same amount of electric charge is passed through different electrolytes, the mass deposited or dissolved during electrolysis is proportional the their chemical equivalent or equivalent weight.
What is a chemical equivalent or equivalent weight?
Each ionic chemical compound has a molecular mass and a valency associated. To further understand this concept, let’s look at a couple of examples.
For simplicity, we will use two examples of metal deposition: sodium and aluminium.
The deposition reaction of sodium is as follows:
As we can see, if we combine 1 cation of sodium with one electron we obtain sodium metal.
But for the case of aluminium the reaction works differently:
Unlike sodium, aluminiium requires 3 electrons to transform a cation into a metal. That is, 3 times more electricity!
This, together with the fact that sodium and aluminium cations have different atomic weights, it means that, in order to be able to accurately estimate the mass deposited, we need to take into account both factors into consideration. This is done by calculating the chemical equivalent or equivalent weight.
The equivalent weight is defined as the ratio between the atomic mass of the chemical species and its valency.
Where:
E is the equivalent weight in g/mol
M is the atomic mass in g/mol
and n is the valency.
What is the electrochemical equivalent weight?
Now we know what an equivalent weight is and why it is important in Faraday’s Laws of Electrolysis. A derivation of this number is the so-called electrochemical equivalent weight, Z, that we introduced when explaining Faraday’s First Law of Electrolysis.
While the equivalent weight, E, expresses the grams per mol for a specific reaction, the electrochemical equivalent weight expresses tha same concept in grams per Coulomb. This is accomplished by dividing the equivalent weight by Faraday’s Constant as follows:
Where:
Z is the electrochemical equivalent weight in g/C,
E is the equivalent weight in g/mol,
F is Faraday’s Constant 96485 C/mol,
M is the atomic mass in g/mol
and n is the valency or number of electrons involved in the reaction.
Faraday’s Law of Electrolysis Equation
If we combine the concepts from both the First and Second Laws, we obtain a unified equation to describe the mass deposited or dissolved during an electrolysis experiment:
Where:
m is the mass deposited or dissolved during electrolysis in g,
M is the atomic mass of the chemical species being deposited or dissolved in g/mol
Q is the total electric charge passed during electrolysis in C,
F is Faraday’s Constant, 96485 C/mol,
and n is the number of electrons involved in the reaction.
Why is Faraday’s Law of Electrolysis important in electrochemical reactions?
Understanding Faraday’s Law of Electrolysis provides electrochemists with a framework that explains the behavior of electrons and chemical species in electrochemical reactions. With this information, it is possible to control the outcome of electrochemical reactions and optimize many industrial processes such as anodization and electroplating.
One particular use of Faraday’s Law of Electrolysis is to estimate the amount of energy required to dissolve or deposit a substance using electrolysis. However, in the real world it is likely that the actual amount of energy used is larger due to losses. Doing so, provides industries with an estimation of how much electricity they should be spending in their processes and identify potential sources of these energy losses, enabling them to optimize them and become more energy-efficient.
As we have seen, Faraday’s Law of Electrolysis is one of the key concepts in electrochemistry. Its principle can be applied to a broad range of industries, from the production of chemicals and metals, to energy storage devices like batteries. If you would like to know more about electrochemistry, check out our blog, where we post regularaly about electrochemistry and its applications.
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