In electrolysis, the equivalent mass of a substance is the mass of that substance deposited or liberated at an electrode when one mole of electrons is passed through the electrolytic cell. It's directly related to the number of electrons transferred during the electrochemical reaction.
Understanding Equivalent Mass
The concept of equivalent mass in electrolysis hinges on Faraday's laws of electrolysis. These laws state that the amount of substance deposited or liberated at an electrode is directly proportional to the quantity of electricity passed through the cell. One Faraday (F), which is approximately 96485 coulombs (C), represents the charge of one mole of electrons.
Therefore, the equivalent mass can be defined as:
- The mass of a substance deposited or liberated by the passage of one Faraday (one mole of electrons) of electricity.
Alternatively, we can also define it as:
- The mass of substance that reacts with or releases one mole of electrons.
The equivalent mass is calculated differently depending on the substance involved.
Calculating Equivalent Mass
To calculate the equivalent mass, you need to consider the substance's molar mass (M) and its valence (n), which represents the number of electrons gained or lost per atom or ion during the reaction. The formula is:
Equivalent Mass = Molar Mass / Valence (n)
Example:
Let's consider the electrolysis of copper(II) sulfate (CuSO₄). Copper(II) ions (Cu²⁺) have a valence of 2 because each ion gains 2 electrons to become metallic copper. If the molar mass of copper is 63.55 g/mol, the equivalent mass of copper in this reaction would be:
Equivalent Mass (Cu) = 63.55 g/mol / 2 = 31.775 g/mol
Equivalent Mass vs. Electrochemical Equivalent
It's important to distinguish between equivalent mass and the electrochemical equivalent (Z). While both relate to the mass deposited during electrolysis, they differ in their units:
- Equivalent Mass: Expressed in grams per mole (g/mol). Focuses on one mole of electrons.
- Electrochemical Equivalent (Z): Expressed in grams per coulomb (g/C). Focuses on the mass deposited per coulomb of charge.
The relationship between these two is given by:
Z = Equivalent Mass / Faraday's Constant (F)
Practical Applications
Determining equivalent mass via electrolysis helps in:
- Determining the molar mass of unknown metals: By measuring the mass of metal deposited and the charge passed, one can calculate the equivalent mass and, subsequently, the molar mass.
- Refining metals: Electrolysis is used to purify metals, like copper, by depositing pure metal at the cathode. The equivalent mass aids in process control.
- Understanding electrochemical reactions: The equivalent mass gives insight into the stoichiometry of the reaction involving electron transfer.
