Electrolysis & Faraday's Laws
Force non-spontaneous reactions to occur. Master preferential discharge theory, conquer the aqueous overvoltage traps, and execute rapid Faraday mass calculations.
Module Focus: Electrical to Chemical Energy
An Electrolytic Cell is the exact opposite of a Galvanic Cell. Here, we use an external voltage source (a battery) to drive a non-spontaneous chemical reaction ($\Delta G > 0$). In NEET, you must calculate exactly how much product is formed (Faraday's Laws) and theoretically predict what product will form when multiple ions are competing at the electrodes (Preferential Discharge).
1. Faraday's Laws of Electrolysis
Michael Faraday established the quantitative relationship between the amount of electricity passed through an electrolyte and the amount of substance deposited or liberated at the electrodes.
The mass ($w$) of a substance deposited is directly proportional to the quantity of electricity ($Q$) passed.
- $I$ = Current in Amperes (A)
- $t$ = Time in seconds (s)
- $Z$ = Electrochemical equivalent ($Z = \frac{E}{96500}$)
1 Faraday is the total electrical charge carried by exactly 1 mole of electrons.
NEET Shortcut Rule:
Passing 1 Faraday of charge deposits exactly 1 gram-equivalent of any substance.
When the exact same quantity of electricity is passed through different electrolytes in series, the masses deposited are proportional to their Equivalent Weights ($E$).
Because they are in series, the same number of moles of electrons (Faradays) flows through both. Therefore, the Moles of Equivalents deposited must be strictly equal.
Reminder: Equivalent Weight ($E$) = $\frac{\text{Molar Mass}}{\text{n-factor}}$
2. Preferential Discharge Theory
If an electrolyte is aqueous, water is present. Water can be oxidized at the anode to produce $O_2$, or reduced at the cathode to produce $H_2$. Therefore, ions must compete against water.
The species with the Higher Standard Reduction Potential (SRP) gets reduced first.
- Active Metals vs Water: Elements above Hydrogen in the reactivity series ($Li, K, Na, Mg, Al$) have highly negative SRPs. Therefore, $H_2O$ is reduced to $H_2$ gas instead of the metal depositing.
- Transition Metals vs Water: Elements below Hydrogen ($Cu, Ag, Au$) have positive SRPs. The metal deposits instead of $H_2$ gas.
The species with the Lower SRP (Higher Oxidation Potential) gets oxidized first.
- Polyatomic vs Water: Complex ions like $SO_4^{2-}$, $NO_3^-$, $CO_3^{2-}$ are very hard to oxidize. Therefore, $H_2O$ is oxidized to $O_2$ gas instead.
- Halides vs Water: $I^-$ and $Br^-$ are easily oxidized to $I_2$ and $Br_2$. $F^-$ is never oxidized in aqueous solution (water yields $O_2$ instead).
Thermodynamically, water should be oxidized before $Cl^-$. Why does $Cl_2$ gas form instead of $O_2$?
Thermodynamically, water is easier to oxidize than chloride. However, the oxidation of water to form oxygen gas ($O_2$) is a kinetically slow process that requires a high activation energy.
To make it happen at a reasonable rate, we must supply extra voltage (called Overpotential or Overvoltage). Because of this extra barrier, the oxidation of $Cl^-$ becomes kinetically favorable, and $Cl_2$ gas is produced at the anode.
$Na^+$ and $OH^-$ are left behind. The solution becomes aqueous $NaOH$, and the pH increases strongly.
3. Active vs. Inert Electrodes
The nature of the electrode itself can change the products of electrolysis. Inert electrodes (like Platinum or Graphite) do not participate in the reaction. Active electrodes (like Copper or Silver) will participate if their oxidation is easier than the ions in solution.
- Cathode (-): $Cu^{2+}$ is reduced $\rightarrow$ $Cu$ metal deposits.
- Anode (+): Pt is inert. Water is oxidized $\rightarrow$ $O_2$ gas is evolved.
- Result: Solution becomes $H_2SO_4$ (pH drops).
- Cathode (-): $Cu^{2+}$ is reduced $\rightarrow$ $Cu$ metal deposits.
- Anode (+): Copper is an active metal. It is easier to oxidize Cu metal than water. $\rightarrow$ The Cu Anode dissolves ($Cu \rightarrow Cu^{2+} + 2e^-$).
- Result: Concentration of $CuSO_4$ remains constant (used for Cu refining).
NEET Grand Test: Electrolysis
15 High-Yield Questions testing Faraday calculations, the overvoltage exception, and electrode specific products.
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