Redox Reactions in Titrimetry — Revision Notes

⚡ 30-Second Revision

Redox Titrimetry: — Volumetric analysis based on electron transfer.
Oxidation: — Loss of electrons, increase in oxidation number.
Reduction: — Gain of electrons, decrease in oxidation number.
n-factor: — Electrons transferred per mole.

- $\text{KMnO}_4$: Acidic ($n=5$), Neutral ($n=3$), Strongly Alkaline ($n=1$). - $\text{K}_2\text{Cr}_2\text{O}_7$: ($n=6$). - $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+}$: ($n=1$). - $\text{C}_2\text{O}_4^{2-} \rightarrow 2\text{CO}_2$: ($n=2$). - $\text{H}_2\text{O}_2 \rightarrow \text{O}_2$: ($n=2$). - $\text{S}_2\text{O}_3^{2-} \rightarrow \text{S}_4\text{O}_6^{2-}$: ($n=1$).

Equivalence Principle: — $N_1V_1 = N_2V_2$ or $M_1V_1n_1 = M_2V_2n_2$.
Indicators: — Redox indicators (e.g., diphenylamine), Starch (for $\text{I}_2$), $\text{KMnO}_4$ (self-indicator).
Equivalence Point: — Stoichiometric completion.
Endpoint: — Observed color change.

2-Minute Revision

Redox titrimetry is a quantitative analytical method that uses redox reactions to determine unknown concentrations. The fundamental idea is electron transfer: one substance is oxidized (loses electrons), and another is reduced (gains electrons).

The 'n-factor' is crucial, representing the number of electrons exchanged per mole of a substance in a specific reaction. For instance, $\text{KMnO}_4$ has an n-factor of 5 in acidic medium, 3 in neutral, and 1 in strongly alkaline medium.

$\text{K}_2\text{Cr}_2\text{O}_7$ always has an n-factor of 6. At the 'equivalence point,' the equivalents of the titrant and analyte are equal, expressed by $N_1V_1 = N_2V_2$ or $M_1V_1n_1 = M_2V_2n_2$.

The 'endpoint,' signaled by an indicator's color change (or self-indication by $\text{KMnO}_4$), marks the experimental completion. Common titrations involve permanganometry, dichrometry, and iodometry/iodimetry.

Always correctly identify n-factors and apply the equivalence formula for numerical problems.

5-Minute Revision

Redox titrimetry is a precise volumetric technique for determining the concentration of an unknown solution by reacting it with a standard solution in a redox reaction. The core principle is the transfer of electrons, where one species is oxidized (loses electrons, oxidation number increases) and the other is reduced (gains electrons, oxidation number decreases).

The 'n-factor' is key: it's the number of electrons exchanged per mole of a reactant in a specific redox process. For example, in acidic medium, $\text{KMnO}_4$ (Mn from +7 to +2) has an n-factor of 5.

Oxalic acid ($\text{H}_2\text{C}_2\text{O}_4$, C from +3 to +4) has an n-factor of 2. Sodium thiosulfate ($\text{Na}_2\text{S}_2\text{O}_3$, S from +2 to +2.5) has an n-factor of 1.

At the 'equivalence point,' the moles of electrons lost by the reducing agent exactly equal the moles of electrons gained by the oxidizing agent. This is mathematically represented by the equivalence principle: $N_1V_1 = N_2V_2$, where $N$ is normality ($N = M \times n$). Alternatively, using molarity, $M_1V_1n_1 = M_2V_2n_2$.

To detect this point, 'redox indicators' are used, which change color at a specific redox potential. For instance, starch is used for iodine titrations (blue with $\text{I}_2$, colorless with $\text{I}^-$). $\text{KMnO}_4$ is unique as a 'self-indicator' in acidic medium, its intense purple color disappearing until the first excess drop gives a persistent pink. The 'endpoint' is the observed color change, ideally very close to the theoretical equivalence point.

Worked Example: If 20 mL of 0.1 M $\text{KMnO}_4$ (acidic) reacts with 25 mL of $\text{H}_2\text{O}_2$ solution, find the molarity of $\text{H}_2\text{O}_2$.

1
n-factors: — $\text{KMnO}_4$ (acidic) $n_1=5$. $\text{H}_2\text{O}_2$ (O from -1 to 0) $n_2=2$.
2
Formula: — $M_1V_1n_1 = M_2V_2n_2$.
3
Substitute: — $0.1,\text{M} \times 20,\text{mL} \times 5 = M_{\text{H}_2\text{O}_2} \times 25,\text{mL} \times 2$.
4
Solve: — $10 = M_{\text{H}_2\text{O}_2} \times 50 \Rightarrow M_{\text{H}_2\text{O}_2} = 10/50 = 0.2,\text{M}$.

Remember to always balance the half-reactions mentally or on paper to confirm n-factors and understand the reaction's stoichiometry.

Prelims Revision Notes

Redox Titrimetry: NEET Quick Revision

1. Fundamentals:

Redox Reaction: — Electron transfer. Oxidation (loss of e-, increase in O.N.), Reduction (gain of e-, decrease in O.N.).
Oxidizing Agent: — Gets reduced, causes oxidation.
Reducing Agent: — Gets oxidized, causes reduction.
Titrant: — Known concentration, in burette.
Analyte: — Unknown concentration, in conical flask.

2. Key Concepts & Formulas:

n-factor (Equivalence Factor): — Number of electrons gained/lost per mole of substance in a specific reaction.

* **KMnO$_4$:** * Acidic medium ($\text{MnO}_4^- \rightarrow \text{Mn}^{2+}$): $n=5$ * Neutral/Weakly alkaline ($\text{MnO}_4^- \rightarrow \text{MnO}_2$): $n=3$ * Strongly alkaline ($\text{MnO}_4^- \rightarrow \text{MnO}_4^{2-}$): $n=1$ * **K$_2$Cr$_2$O$_7$:** ($\text{Cr}_2\text{O}_7^{2-} \rightarrow 2\text{Cr}^{3+}$): $n=6$ * **FeSO$_4$:** ($\text{Fe}^{2+} \rightarrow \text{Fe}^{3+}$): $n=1$ * **Oxalic Acid (H$_2$C$_2$O$_4$):** ($2\text{C}^{3+} \rightarrow 2\text{C}^{4+}$): $n=2$ * **H$_2$O$_2$ (as reducing agent):** ($2\text{O}^{-1} \rightarrow \text{O}_2^0$): $n=2$ * **Na$_2$S$_2$O$_3$:** ($2\text{S}_2\text{O}_3^{2-} \rightarrow \text{S}_4\text{O}_6^{2-}$): $n=1$

Normality (N): — $N = M \times n$ (M = Molarity).
Equivalence Principle: — At equivalence point, equivalents of titrant = equivalents of analyte.

* $N_1V_1 = N_2V_2$ * $M_1V_1n_1 = M_2V_2n_2$

3. Indicators:

Redox Indicators: — Change color based on redox potential. E.g., Diphenylamine (for dichromate titrations).
Starch: — Specific for iodine titrations (blue with $\text{I}_2$, colorless with $\text{I}^-$).
Self-indicator: — $\text{KMnO}_4$ (purple $\rightarrow$ colorless $\text{Mn}^{2+}$, first excess drop gives pink).

4. Key Terms:

Equivalence Point: — Theoretical point of stoichiometric completion.
Endpoint: — Experimental point of indicator color change.
Standard Solution: — Solution of accurately known concentration.

* Primary Standard: High purity, stable, directly weighable (e.g., oxalic acid, $\text{K}_2\text{Cr}_2\text{O}_7$). * Secondary Standard: Needs standardization against a primary standard (e.g., $\text{KMnO}_4$, $\text{NaOH}$).

Vyyuha Quick Recall

N-Factor for KMnO4: All Neutrals Strongly Alkaline -> 5, 3, 1 (Acidic, Neutral, Strongly Alkaline)