PYQ + Trends
1. TITLE
NEET 2023 Q: Balancing Redox: The Acidic Dichromate Method — Full solution + Why they asked this
2. THE ACTUAL NEET QUESTION
NEET 2023 — Question
On balancing the given redox reaction,
$aCr_{2}{O_{7}}^{2-} + bSO_{3}^{2-}(aq) + cH^{+}(aq) \rightarrow 2aCr^{3+}(aq) + bSO_{4}^{2-}(aq) + \frac{c}{2}H_{2}O(l)$
the coefficients a, b and c are found to be, respectively-
A) 3, 8, 1
B) 1, 8, 3
C) 8, 1, 3
D) 1, 3, 8
3. OFFICIAL ANSWER
NEET Answer Key: (D)
4. COMPLETE SOLUTION
Step-by-step calculation using the Ion-Electron Method:
This method explicitly shows the electrons transferred to balance the equation.
- Step 1: Split into Half-Reactions
- Reduction Half: $Cr_2O_7^{2-} + 6e^- \rightarrow 2Cr^{3+}$
- Oxidation Half: $SO_3^{2-} \rightarrow SO_4^{2-} + 2e^-$
- Step 2: Equalise Electrons
Multiply the oxidation half by 3 so the number of electrons lost equals the number of electrons gained ($6e^-$).- $3 \times (SO_3^{2-} \rightarrow SO_4^{2-} + 2e^-)$
- Step 3: Combine and Balance Oxygen & Hydrogen
- Overall skeletal reaction: $Cr_2O_7^{2-} + 3SO_3^{2-} \rightarrow 2Cr^{3+} + 3SO_4^{2-}$
- To balance 'O' atoms, add $H_2O$ on the side deficient in oxygen (RHS):
$Cr_2O_7^{2-} + 3SO_3^{2-} \rightarrow 2Cr^{3+} + 3SO_4^{2-} + 4H_2O$ - To balance 'H' atoms in an acidic medium, add $H^+$ on the left side:
$Cr_2O_7^{2-} + 3SO_3^{2-} + 8H^+ \rightarrow 2Cr^{3+} + 3SO_4^{2-} + 4H_2O$
- Final Coefficients:
Comparing this to the given equation $aCr_{2}{O_{7}}^{2-} + bSO_{3}^{2-} + cH^{+} \rightarrow ...$ we get a = 1, b = 3, and c = 8.
5. WHY NEET ASKED THIS
Explain the examiner's intent:
* What concept were they testing? The examiner wanted to test your mastery of the Half-Reaction (Ion-Electron) Method for balancing redox equations in an acidic medium, specifically checking if you know how to equalise electron transfer.
* What's the common mistake they expected students to make? Students often forget to multiply the entire oxidation half-reaction by 3 to balance the 6 electrons required by the dichromate ion, leading to incorrect stoichiometry for the sulphite/sulphate pair.
* Which option was the "trap"? Option (B) 1, 8, 3 is a classic trap. It correctly identifies $a=1$ but deliberately swaps the coefficients for $H^+$ ($c$) and $SO_3^{2-}$ ($b$) to catch students who memorised numbers without aligning them to the specific variables requested.
6. COMPARISION TABLE
PATTERN ALERT: Redox balancing and coefficient identification is a highly recurring pattern!
| Year | How they asked | Twist |
| :--- | :--- | :--- |
| 2024 (Re) | Identifying which oxidation states are not shown by Mn in a disproportionation reaction ($3MnO_4^{2-} + 4H^+ \rightarrow 2MnO_4^- + MnO_2 + 2H_2O$). | Shifted from purely balancing to analyzing the oxidation states within an already balanced acidic equation. |
| 2023 | Balancing acidic dichromate + sulphite ($a, b, c$ coefficients). | Standard step-by-step balancing to find multiple variables. |
| 2018 | Balancing acidic permanganate + oxalate ($MnO_4^- + C_2O_4^{2-} + H^+$). | Same pattern, required matching the exact coefficients (2, 5, 16) for a very famous NCERT reaction. |
Prediction for NEET 2026:
Expect a basic medium balancing question. NEET has heavily tested acidic mediums recently. A prime candidate is the balancing of Permanganate ($MnO_4^-$) with Iodide ($I^-$) or Bromide ($Br^-$) to yield $MnO_2$ and $IO_3^- / BrO_3^-$.
7. SIMILAR QUESTIONS TO PRACTICE
"If you understood this, you can solve:"
* NEET 2018 Q: For the redox reaction $MnO_4^- + C_2O_4^{2-} + H^+ \rightarrow Mn^{2+} + CO_2 + H_2O$, find the correct coefficients.
* NEET 2024 Re-Exam Q: Find the oxidation states not shown by Mn in the given balanced disproportionation reaction.
* Basic Medium Variation: Balance $MnO_4^- + Br^- \rightarrow MnO_2 + BrO_3^-$ in basic medium (NCERT Problem 7.9).
8. QUICK REVISION FOR THIS TYPE
For Balancing Redox questions, always:
- First check: The medium specified (Acidic vs. Basic). This determines whether you balance charges using $H^+$ or $OH^-$.
- Then apply: Split into two half-reactions. Balance atoms (other than O and H), then balance O using $H_2O$, and H using $H^+$. Finally, equalise the number of electrons lost and gained by multiplying the half-reactions.
- Watch out for: The final sequence of requested variables. Examiners often ask for $a, b, c$ in an order that doesn't match the left-to-right reading of the equation to set a trap.
9. NCERT BASE
This question tests: NCERT Class 11, Part 2, Chapter 7 (Redox Reactions), Page 247.
This is a direct adaptation of Problem 7.8 in the NCERT textbook: "Write the net ionic equation for the reaction of potassium dichromate(VI), $K_2Cr_2O_7$ with sodium sulphite, $Na_2SO_3$, in an acid solution...".
10. CTA
All PYQs from this chapter → Redox Reactions & Electrochemistry Guide
Practice similar questions → Mock exams