Preparation and Properties of K2Cr2O7 and KMnO4 — Revision Notes

⚡ 30-Second Revision

$K_2Cr_2O_7$ (Potassium Dichromate)

- Orange solid, orange solution. - Cr oxidation state: +6. - Oxidizing agent in acidic medium: $Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} (green) + 7H_2O$. - n-factor (acidic): 6. - Equilibrium: $2CrO_4^{2-} (yellow) + 2H^+ \rightleftharpoons Cr_2O_7^{2-} (orange) + H_2O$. - Primary standard. - Chromyl Chloride Test: specific for $Cl^-$, forms red $CrO_2Cl_2$.

$KMnO_4$ (Potassium Permanganate)

- Dark purple solid, deep purple solution. - Mn oxidation state: +7. - Oxidizing agent, pH-dependent: - Acidic: $MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} (colorless) + 4H_2O$. n-factor: 5. - Neutral/Weakly Alkaline: $MnO_4^- + 2H_2O + 3e^- \rightarrow MnO_2 (brown) + 4OH^-$. n-factor: 3. - Strongly Alkaline: $MnO_4^- + e^- \rightarrow MnO_4^{2-} (green)$. n-factor: 1. - Not a primary standard (decomposes, reacts with impurities).

2-Minute Revision

Potassium dichromate ($K_2Cr_2O_7$) and potassium permanganate ($KMnO_4$) are key oxidizing agents. $K_2Cr_2O_7$ is orange, with chromium in the +6 oxidation state. It's prepared from chromite ore ($FeCr_2O_4$) by roasting, acidification, and $KCl$ treatment.

In acidic solutions, it's a strong oxidizer, reducing to green $Cr^{3+}$ ions (n-factor = 6). The chromate ($CrO_4^{2-}$, yellow) and dichromate ($Cr_2O_7^{2-}$, orange) ions interconvert based on pH: acid favors dichromate, base favors chromate.

It's a primary standard and used in the chromyl chloride test for chlorides.

$KMnO_4$ is dark purple, with manganese in the +7 oxidation state. It's prepared from pyrolusite ore ($MnO_2$) by fusion with $KOH$ to form green manganate ($K_2MnO_4$), followed by oxidation to permanganate.

$KMnO_4$ is a powerful and versatile oxidizer, with its reduction products and n-factor varying with pH: in acidic media, it forms colorless $Mn^{2+}$ (n=5); in neutral/weakly alkaline, brown $MnO_2$ (n=3); and in strongly alkaline, green $MnO_4^{2-}$ (n=1).

Unlike $K_2Cr_2O_7$, $KMnO_4$ is not a primary standard due to its instability and reactivity. Remember the distinct color changes for each compound and medium.

5-Minute Revision

Let's quickly review the essentials of potassium dichromate ($K_2Cr_2O_7$) and potassium permanganate ($KMnO_4$), focusing on their preparation, properties, and key differences for NEET.

**Potassium Dichromate ($K_2Cr_2O_7$)**:

Preparation — Starts from chromite ore ($FeCr_2O_4$).

1. Roasting: $FeCr_2O_4$ is roasted with $Na_2CO_3$ and $O_2$ to form yellow sodium chromate ($Na_2CrO_4$). ($Cr^{3+} \rightarrow Cr^{6+}$) 2. Acidification: $Na_2CrO_4$ solution is acidified (e.g., with $H_2SO_4$) to convert chromate to orange sodium dichromate ($Na_2Cr_2O_7$). 3. Conversion: $Na_2Cr_2O_7$ is treated with $KCl$ to precipitate less soluble $K_2Cr_2O_7$.

Properties — Orange crystalline solid. Strong oxidizing agent in acidic medium.

* Redox: $Cr_2O_7^{2-} (orange) + 14H^+ + 6e^- \rightarrow 2Cr^{3+} (green) + 7H_2O$. (n-factor = 6) * Equilibrium: $2CrO_4^{2-} (yellow) + 2H^+ \rightleftharpoons Cr_2O_7^{2-} (orange) + H_2O$. (Acid shifts right, base shifts left). * Tests: Chromyl chloride test for $Cl^-$ (red $CrO_2Cl_2$ vapors). * Standard: A primary standard.

**Potassium Permanganate ($KMnO_4$)**:

Preparation — Starts from pyrolusite ore ($MnO_2$).

1. Fusion: $MnO_2$ is fused with $KOH$ and an oxidizer ($O_2$ or $KNO_3$) to form green potassium manganate ($K_2MnO_4$). ($Mn^{4+} \rightarrow Mn^{6+}$) 2. Oxidation: $K_2MnO_4$ is oxidized to purple $KMnO_4$ either chemically (e.g., by $Cl_2$, $CO_2$) or electrolytically. ($Mn^{6+} \rightarrow Mn^{7+}$)

Properties — Dark purple crystalline solid. Extremely strong and versatile oxidizing agent, highly pH-dependent.

* Acidic Medium: $MnO_4^- (purple) + 8H^+ + 5e^- \rightarrow Mn^{2+} (colorless) + 4H_2O$. (n-factor = 5) * Neutral/Weakly Alkaline Medium: $MnO_4^- (purple) + 2H_2O + 3e^- \rightarrow MnO_2 (brown ppt) + 4OH^-$. (n-factor = 3) * Strongly Alkaline Medium: $MnO_4^- (purple) + e^- \rightarrow MnO_4^{2-} (green)$. (n-factor = 1) * Standard: Not a primary standard (decomposes, reacts with impurities).

Key Differences to Remember: Colors, oxidation states, n-factors in different media, and primary standard status. Practice balancing redox equations and calculating equivalent weights for both compounds.

Prelims Revision Notes

Potassium Dichromate ($K_2Cr_2O_7$)

Formula — $K_2Cr_2O_7$
Color — Orange-red solid, orange solution.
Chromium Oxidation State — +6 in $Cr_2O_7^{2-}$.
Preparation — From chromite ore ($FeCr_2O_4$).

1. Roasting with $Na_2CO_3$ and $O_2$: $FeCr_2O_4 \rightarrow Na_2CrO_4$ (yellow). 2. Acidification: $Na_2CrO_4 \rightarrow Na_2Cr_2O_7$ (orange). 3. Precipitation: $Na_2Cr_2O_7 + KCl \rightarrow K_2Cr_2O_7 (s)$.

Oxidizing Nature (Acidic Medium)

* Powerful oxidizing agent. * Reduced to $Cr^{3+}$ (green). * Half-reaction: $Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O$. * n-factor = 6. * Equivalent weight = Molar Mass / 6.

Chromate-Dichromate Equilibrium — $2CrO_4^{2-} (yellow) + 2H^+ \rightleftharpoons Cr_2O_7^{2-} (orange) + H_2O$.

* Acid favors dichromate (orange). * Alkali favors chromate (yellow).

Chromyl Chloride Test — Specific for $Cl^-$. Forms red-brown $CrO_2Cl_2$ vapors.
Primary Standard — Yes, due to high purity and stability.

Potassium Permanganate ($KMnO_4$)

Formula — $KMnO_4$
Color — Dark purple solid, deep purple solution.
Manganese Oxidation State — +7 in $MnO_4^-$.
Preparation — From pyrolusite ore ($MnO_2$).

1. Fusion with $KOH$ and $O_2$: $MnO_2 \rightarrow K_2MnO_4$ (green manganate). 2. Oxidation of $K_2MnO_4$ to $KMnO_4$: Chemical (e.g., $Cl_2$, $CO_2$) or Electrolytic.

Oxidizing Nature (pH-Dependent)

* Acidic Medium: * Reduced to $Mn^{2+}$ (colorless). * Half-reaction: $MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O$. * n-factor = 5. * Equivalent weight = Molar Mass / 5. * Neutral or Weakly Alkaline Medium: * Reduced to $MnO_2$ (brown precipitate).

* Half-reaction: $MnO_4^- + 2H_2O + 3e^- \rightarrow MnO_2 + 4OH^-$. * n-factor = 3. * Equivalent weight = Molar Mass / 3. * Strongly Alkaline Medium: * Reduced to $MnO_4^{2-}$ (green manganate).

* Half-reaction: $MnO_4^- + e^- \rightarrow MnO_4^{2-}$. * n-factor = 1. * Equivalent weight = Molar Mass / 1.

Primary Standard — No, due to decomposition, reactivity, and impurities.
Uses — Disinfectant, volumetric analysis, organic synthesis.

Vyyuha Quick Recall

For $KMnO_4$ reduction products and n-factors in different media, remember 'A-5, N-3, S-1':

Acidic: 5 electrons ($Mn^{2+}$)
Neutral/Weakly Alkaline: 3 electrons ($MnO_2$)
Strongly Alkaline: 1 electron ($MnO_4^{2-}$)

For colors of chromium species: 'C-Y-D-O-R-G'

Chromate: Yellow
Dichromate: Orange
Reduced $Cr^{3+}$: Green