Electrochemistry :Faraday's Law &Product of electrolysis.Class -12
Faraday’s First Law of Electrolysis
Faraday’s first law of electrolysis states that the mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electric charge transferred at that electrode. As charge is the product of strength of current and time, the mass (m) of the substance altered at an electrode is directly proportional to the strength of the current (i) and the time (t) for which the current passes, i.e.,
m ∝ it.
Or
m = Zit, ------------------(2)
where Z is the proportionality constant, and is characteristic of the substance or the element of which the electrode is made. Z is called electro-chemical equivalent (ECE) of the substance. From equation (2), we have
Z = m /( it) ----------------(3).
Electrochemical Equivalent (ECE)
The electrochemical equivalent (ECE) of an element is defined as the mass of its ions liberated at the electrode when one coulomb of electricity is passed through the electrolyte. We know that the quantity of electricity, i.e., the amount of charge q = it. Therefore, equation (3) becomes
Z = m / q ----------------(4).
Since m is in gram and q in coulomb, the unit for Z is gram per coulomb. The electrochemical equivalents of some common elements are given in the table here.
Faraday’s first law of electrolysis states that the mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electric charge transferred at that electrode. As charge is the product of strength of current and time, the mass (m) of the substance altered at an electrode is directly proportional to the strength of the current (i) and the time (t) for which the current passes, i.e.,
m ∝ it.
Or
m = Zit, ------------------(2)
where Z is the proportionality constant, and is characteristic of the substance or the element of which the electrode is made. Z is called electro-chemical equivalent (ECE) of the substance. From equation (2), we have
Z = m /( it) ----------------(3).
Electrochemical Equivalent (ECE)
The electrochemical equivalent (ECE) of an element is defined as the mass of its ions liberated at the electrode when one coulomb of electricity is passed through the electrolyte. We know that the quantity of electricity, i.e., the amount of charge q = it. Therefore, equation (3) becomes
Z = m / q ----------------(4).
Since m is in gram and q in coulomb, the unit for Z is gram per coulomb. The electrochemical equivalents of some common elements are given in the table here.
Faraday’s Second Law of Electrolysis
Faraday’s second law of electrolysis states that when the same quantity of electricity passes through different electrolytes, the mass of an elemental material altered at an electrode is directly proportional to the element's chemical equivalent. Chemical equivalent is also referred to as equivalent weight. Chemical equivalent is the ratio of the atomic weight (A) of an element to its valency (V).
Consider three electrolytes, say copper sulphate, silver nitrate and zinc sulphate, in three different voltameters connected in series with the respective electrodes in them, as shown here.
Let E1, E2, E3 be the chemical equivalents of copper, silver and zinc, respectively. Also, let Z1, Z2 and Z3 be their respective electro chemical equivalents, and m1, m2, m3 be their respective masses liberated when the same quantity (q) of electricity is passed through their respective electrolytes. According to Faraday’s second law,
(m1 / E1) = (m2 / E2) = (m3 / E3) ----------------(5).
Or
m1 : m2 : m3 = E1 : E2 : E3, ----------------(6)
where charge q is the same for all. This is one form of Faraday’s second law of electrolysis. Applying the first law to these electrolytes, we have
m1 = Z1q, m2 = Z2q and m3 = Z3q ----------------(7).
In other words,
m1 : m2 : m3 = Z1 : Z2 : Z3, ----------------(8)
where charge, q is the same for all. From equations (6) and (8),
Z1 : Z2 : Z3 = E1 : E2 : E3.----------------(9)
Equation (9) represents another form of Faraday’s second law of electrolysis.It shows that the ratio of the electrochemical equivalents of elements is equal to the ratio of their chemical equivalents.
Activity to Verify Faraday’s First Law of Electrolysis
Repeat the electrolysis with the copper sulphate electrolyte. Pass a current i1 for a time t1. Let the mass of copper deposited on the cathode after t1 second be m1. Then
Z = m1/( i1 ×t1).
Change the position of the rheostat again, and allow a different current i2 to pass through the electrolyte for a time t2. Determine the mass m2 deposited at the cathode C. Then
Z = m2 / (i2 × t2).
We will find that the value of Z is the same. This verifies Faraday’ first law of electrolysis.
Verification of Faraday’s Second law
In addition to the copper voltameter containing copper sulphate electrolyte, take two more voltameters, one a silver voltameter containing silver nitrate electrolyte, and the other a zinc voltameter containing zinc sulphate electrolyte. Connect the three voltameters in series. In each of the three voltameters, the electrodes A and C are made of the metals of the respective solutions. That is, the electrodes in the copper sulphate solution are made of copper, in the silver nitrate solution of silver, and in the zinc sulphate solution of zinc.
Let the initial masses of the three cathodes in the three voltameters be m1i, m2i and m3i, respectively. Adjust the rheostat such that current i is read in the ammeter. Since the three voltameters are in series, the same current passes through all the electrolytes. Allow the current to pass for about half an hour. At the end of half an hour, let the mass of the three cathodes be m1f, m2f and m3f, respectively. The differences between the corresponding final and initial masses give the masses of copper, silver and zinc deposited on the respective cathodes. That is, m1 = m1f - m1i, m2 = m2f - m2i, and m3 = m3f - m3i. When the ratios m1 : E1, m2 : E2 and m3 : E3 are calculated, it is found that they are equal, where charge q is the same for all. Hence
m1 : E1, m2 : E2 and m3 : E3.
This verifies Faraday’s second law of electrolysis. The Faraday constant is the quantity of charge that must be passed through an electrolyte to deposit one gram equivalent (equivalent weight expressed in grams) of the substance at an electrode. The Faraday constant has a fixed value of 96,500 C which means that when a charge of 96,500 C is passed through an electrolyte (say copper sulphate), 1 gram equivalent of copper (31.77 g) is deposited at the cathode. Therefore, the chemical equivalent (E) of copper is equal to the relative atomic mass of copper divided by its valency, which is equal to 31.77.
Let us find the ECE, or (Z), of copper in the copper sulphate electrolysis activity that we just carried out. Let W1 grams be the initial mass of the cathode (C). Repeat the electrolysis activity for a duration of t second, and record the current i ampere. At the end of t seconds, the weight of the cathode will be W2 gram. The difference (W2 - W1) gives the mass m of the copper deposited on the cathode. From Faraday’s first law, m = Zit, the ECE of copper can be calculated, by rearranging the formula as Z = m/(it) gram per coulomb.
Applications of Electrolysis
- In metallurgy, it is used for refining and extracting metals.
- Certain metals like copper, tin, lead, gold, zinc, chromium and nickel are purified and extracted by electrolysis.
- Electroplating is the process of coating a thin film of costlier or less corrodible metal on a base metal by electrolysis. For example, a silver film can be deposited on a copper base, or a plating of gold can be made on ornaments with a copper base. Here, the article to be electroplated forms the cathode, and the metal to be coated forms the anode. The electrolyte is a solution containing the salt of the anode material.
- Electrotyping is a method of obtaining an exact copy of an engraved block containing letters or figures by electrolysis.
