PYQ + Trends
1. TITLE
NEET 2019 Q1: Reducing Power Trends: The Negative Eā Rule — Full solution + Why they asked this
2. THE ACTUAL NEET QUESTION
NEET 2019 (Odisha) — Question
The standard electrode potential ($E^{\circ}$) values of $Al^{3+}/Al$, $Ag^{+}/Ag$, $K^{+}/K$ and $Cr^{3+}/Cr$ are -1.66 V, 0.80 V, -2.93 V and -0.74 V, respectively. The correct decreasing order of reducing power of the metal is:
A) $Ag>Cr>Al>K$
B) $K>Al>Cr>Ag$
C) $K>Al>Ag>Cr$
D) $Al>K>Ag>Cr$
3. OFFICIAL ANSWER
NEET Answer Key: (B)
4. COMPLETE SOLUTION
Concept Explanation:
* By IUPAC convention, standard electrode potentials are given as standard reduction potentials.
* The Negative $E^{\circ}$ Rule: A negative $E^{\circ}$ means that the redox couple is a stronger reducing agent than the standard $H^{+}/H_{2}$ couple. In simpler terms, the more negative the value of the reduction potential, the stronger the reducing agent.
* Conversely, a more positive $E^{\circ}$ value indicates a greater tendency to get reduced, making it a stronger oxidising agent.
Step-by-step arrangement:
To find the decreasing order of reducing power, we must arrange the metals from the most negative $E^{\circ}$ to the most positive $E^{\circ}$:
1. Potassium (K): $E^{\circ} = -2.93\text{ V}$ (Most negative = Strongest reducing agent)
2. Aluminium (Al): $E^{\circ} = -1.66\text{ V}$
3. Chromium (Cr): $E^{\circ} = -0.74\text{ V}$
4. Silver (Ag): $E^{\circ} = +0.80\text{ V}$ (Most positive = Weakest reducing agent)
Therefore, the correct decreasing order of reducing power is $K > Al > Cr > Ag$.
Why other options are wrong:
* Option A represents the exact reverse: the decreasing order of oxidising power.
* Options C and D feature an incorrect sequence that does not correlate mathematically with the provided $E^{\circ}$ values.
5. WHY NEET ASKED THIS
Explain the examiner's intent:
* What concept were they testing? The examiner wanted to test the direct, inverse relationship between standard reduction potential values and reducing capability.
* What's the common mistake they expected students to make? Students frequently confuse "reducing power" with "reduction potential." A common error is assuming that a higher reduction potential equals a higher reducing power.
* Which option was the "trap"? Option A was the ultimate trap. If a student mistakenly sorted the values in decreasing order numerically ($+0.80 > -0.74 > -1.66 > -2.93$), they would select Option A, which actually represents the strength of the oxidising agents.
6. TREND ANALYSIS
PATTERN ALERT:
| Year | How they asked | Twist |
|---|---|---|
| 2019 | Provided actual metals (Al, Ag, K, Cr) and their specific $E^{\circ}$ values. | Straightforward application of the Negative $E^{\circ}$ rule to test ranking ability. |
| 2012 (Mains) | Provided $E^{\circ}$ values for halogens ($F_2$, $Cl_2$, $Br_2$, $I_2$) and asked for both the strongest oxidising and reducing agents. | Combined both ends of the concept (highest positive vs. lowest positive) in a single question. |
| 2011 | Used hypothetical metals X, Y, and Z with $E^{\circ}$ values -1.2 V, +0.5 V, and -3.0 V. | Prevented students from relying on a memorised reactivity series by forcing them to use the raw $E^{\circ}$ data. |
Prediction for NEET 2026:
Expect an integrated question where you must use $E^{\circ}$ values to not only determine reducing power but also to predict if a specific metal can displace hydrogen gas from dilute acids (e.g., metals with negative $E^{\circ}$ can reduce $H^{+}$).
7. SIMILAR QUESTIONS TO PRACTICE
"If you understood this, you can solve:"
* NEET 2011 Q4: Standard electrode potentials of three metals X, Y and Z are -1.2 V, +0.5 V and -3.0 V respectively. Determine the reducing power.
* NEET 1994 Q14: Standard reduction potentials of $Li^{+}/Li$, $Ba^{2+}/Ba$, $Na^{+}/Na$ and $Mg^{2+}/Mg$ are given. Identify the strongest oxidising agent.
* NEET Mains 2012 Q3: Given standard reduction potentials for $F_2$, $Cl_2$, $Br_2$, and $I_2$, identify the strongest oxidising and reducing agents.
8. QUICK REVISION FOR THIS TYPE
For Standard Electrode Potential questions, always:
- First check if the values provided are explicitly stated as standard reduction potentials (usually the default by IUPAC convention).
- Then apply the golden rule: More Negative $E^{\circ}$ = Stronger Reductant (better at getting oxidised). Conversely, More Positive $E^{\circ}$ = Stronger Oxidant (better at getting reduced).
- Watch out for the wording! "Reducing agent" or "reducing power" refers to the species losing electrons. "Reduced form" or "reduction potential" relates to the species gaining electrons.
9. NCERT BASE
This question tests: NCERT Class 12, Chapter 2 (Electrochemistry), Page 37 (Table 2.1: Standard Electrode Potentials). It is also fundamentally linked to Class 11, Chapter 7 (Redox Reactions).
10. CTA
All PYQs from this chapter → Redox Reactions & Electrochemistry Guide
Practice similar questions → Mock exams