Laws of Electrolysis — Revision Notes

⚡ 30-Second Revision

Faraday's First Law: — $m = ZIt$ or $m = \frac{E}{F}It$
Faraday's Second Law: — $rac{m_1}{m_2} = \frac{E_1}{E_2}$ (for cells in series)
Electrochemical Equivalent (Z): — Mass deposited by $1,\text{C}$ of charge ($Z = \frac{E}{F}$)
Equivalent Weight (E): — Molar Mass ($M$) / Valency Factor ($n$) ($E = \frac{M}{n}$)
Faraday Constant (F): — Charge of $1,\text{mol}$ of electrons $approx 96500,\text{C/mol}$
Units: — Current in Amperes (A), Time in seconds (s), Mass in grams (g), Charge in Coulombs (C).
Gas Liberation: — $1,\text{mol}$ of gas at STP = $22.4,\text{L}$.

2-Minute Revision

Faraday's Laws of Electrolysis quantitatively describe the amount of chemical change occurring during electrolysis. The First Law states that the mass ($m$) of a substance deposited or liberated is directly proportional to the total quantity of electricity ($Q$) passed.

This is expressed as $m = ZIt$, where $Z$ is the electrochemical equivalent (mass per Coulomb), $I$ is current, and $t$ is time. Remember to convert time to seconds. The Second Law applies when the same quantity of electricity passes through different electrolytes (e.

g., cells in series). It states that the masses deposited are directly proportional to their chemical equivalent weights ($m_1/m_2 = E_1/E_2$). The equivalent weight ($E$) is crucial and calculated as Molar Mass divided by the valency factor ($n$), which is the number of electrons involved in the electrode reaction.

The electrochemical equivalent ($Z$) is related to equivalent weight by $Z = E/F$, where $F$ is the Faraday constant ($96500,\text{C/mol}$). For NEET, focus on applying these formulas, correctly identifying the valency factor, and performing unit conversions, especially for problems involving series connections or gas liberation at STP.

5-Minute Revision

A solid grasp of Faraday's Laws is indispensable for NEET. Start with the First Law: $m = ZIt$. This means the mass ($m$) of a substance deposited or liberated at an electrode is directly proportional to the current ($I$) and time ($t$) for which it flows.

The constant $Z$ is the electrochemical equivalent, specific to each substance. For instance, if $2,\text{A}$ flows for $30,\text{min}$ ($1800,\text{s}$) and $Z = 0.0003,\text{g/C}$, then $m = 0.0003 imes 2 imes 1800 = 1.

08, ext{g}$. Always ensure time is in seconds. Next, the **Second Law** is for comparing different substances. If two cells, say$AgNO_3$and$CuSO_4$, are in series, the same charge passes through both.

The masses deposited will be in the ratio of their equivalent weights: $m_{Ag}/m_{Cu} = E_{Ag}/E_{Cu}$. The equivalent weight ($E$) is key: $E = \text{Molar Mass} / n$, where $n$ is the valency factor (number of electrons in the half-reaction).

For $Ag^+ \to Ag$, $n=1$; for $Cu^{2+} \to Cu$, $n=2$. The Faraday constant ($F = 96500,\text{C/mol}$) links $Z$ and $E$: $Z = E/F$. This allows the general formula $m = (E/F)It$. Practice problems where you calculate $E$ for various ions (e.

g., $Al^{3+}$, $Fe^{2+}$, $Cl^-$) and apply it to both laws. Also, be ready for questions involving gas liberation, where you'll use the molar volume at STP ($22.4,\text{L/mol}$) after calculating moles of gas from Faradays.

For example, $2Cl^- \to Cl_2 + 2e^-$ means 2 Faradays produce $22.4,\text{L}$ of $Cl_2$ at STP.

Prelims Revision Notes

1
Faraday's First Law: — $m propto Q implies m = ZIt$. $Q$ is charge in Coulombs ($C$), $I$ is current in Amperes ($A$), $t$ is time in seconds ($s$). $m$ is mass in grams ($g$).
2
Electrochemical Equivalent (Z): — Mass deposited by $1,\text{C}$ of charge. Unit: $g/C$.
3
Faraday's Second Law: — For same $Q$ through different electrolytes, $rac{m_1}{m_2} = \frac{E_1}{E_2}$.
4
Equivalent Weight (E): — $E = \frac{\text{Molar Mass (M)}}{\text{Valency Factor (n)}}$. Valency factor ($n$) is the number of electrons involved per mole of substance in the electrode reaction (e.g., for $Cu^{2+} \to Cu$, $n=2$; for $Al^{3+} \to Al$, $n=3$; for $Cl_2$ from $Cl^-$, $n=2$ per mole of $Cl_2$).
5
Relationship between Z, E, and F: — $Z = \frac{E}{F}$.
6
Faraday Constant (F): — Charge of one mole of electrons. $F = 96485,\text{C/mol} approx 96500,\text{C/mol}$ (for NEET calculations).
7
Combined Formula: — $m = \frac{E}{F}It$.
8
Moles of electrons: — $Q = \text{moles of electrons} \times F$. So, $ext{moles of electrons} = Q/F$.
9
Gas Liberation: — For gases liberated at STP, $1,\text{mol}$ of gas occupies $22.4,\text{L}$. Use stoichiometry of half-reactions to relate moles of electrons to moles of gas.
10
Key Steps for Numericals:

* Convert time to seconds. * Identify the electrode reaction to find the valency factor ($n$). * Calculate equivalent weight ($E$). * Choose the appropriate Faraday's Law formula. * Substitute values and calculate carefully.

1
Common Traps: — Incorrect valency factor, wrong unit conversions (especially time), confusing $I$ with $Q$, or misapplying the laws (e.g., using Second Law for a single cell).

Vyyuha Quick Recall

For All Reactions, Amount Deposited Always Yields Same Equivalent Weight per Faraday. (FARADAYS EW/F) - This helps remember $m propto Q$, and $Z = E/F$ and $m = (E/F)It$ and $m_1/m_2 = E_1/E_2$ (same equivalent weight per Faraday for all substances).