Q.Given the standard electrode potentials:
$K^+/K = -2.93\ V$, $Ag^+/Ag = 0.80\ V$, $Hg^{2+}/Hg = 0.79\ V$,
$Mg^{2+}/Mg = -2.37\ V$, $Cr^{3+}/Cr = -0.74\ V$
Arrange these metals in their increasing order of reducing power.

Concept: Standard Electrode Potentials — A more negative reduction potential means the metal is a stronger reducing agent (it loses electrons more readily).

Reasoning:

1. Reducing power is the ability to donate electrons. The more negative the standard reduction potential ($E^\circ$), the stronger the reducing agent.
2. Arrange the given $E^\circ$ values from most negative to most positive: $K^+/K = -2.93\ V$ (most negative → strongest reducing agent) $Mg^{2+}/Mg = -2.37\ V$ $Cr^{3+}/Cr = -0.74\ V$ $Hg^{2+}/Hg = 0.79\ V$ $Ag^+/Ag = 0.80\ V$ (most positive → weakest reducing agent)
3. The increasing order of reducing power is the reverse of this list: weakest to strongest.

The reducing power of a metal is its ability to lose electrons and get oxidised. In electrochemistry, this is directly linked to the standard electrode potential ($E^\circ$) of the metal/metal-ion half-cell.

A more negative $E^\circ$ means the metal is more easily oxidised — it is a stronger reducing agent. A more positive $E^\circ$ means the metal is harder to oxidise — it is a weaker reducing agent (and its ions are stronger oxidising agents).

So, to arrange metals in increasing order of reducing power, we need to go from the most positive $E^\circ$ (weakest reducing agent) to the most negative $E^\circ$ (strongest reducing agent).

Let's list the given potentials:

1. Identify the weakest reducing agent. The most positive potential is $Ag^+/Ag$ at $+0.80\ V$. Silver is the least willing to lose electrons, so it has the least reducing power. It comes first.

2. Next comes mercury. $Hg^{2+}/Hg$ is $+0.79\ V$, very close to silver but slightly less positive. So Hg is a slightly stronger reducing agent than Ag, but still much weaker than the others. It comes second.

3. Now we cross into negative potentials. Chromium has $E^\circ = -0.74\ V$. This is significantly more negative than Ag or Hg, meaning Cr is a much stronger reducing agent. It comes third.

4. Magnesium follows. $Mg^{2+}/Mg$ at $-2.37\ V$ is more negative than Cr, so Mg is a stronger reducing agent than Cr. It comes fourth.

5. Potassium is the strongest. $K^+/K$ at $-2.93\ V$ is the most negative potential given. Potassium is the most powerful reducing agent in this list. It comes last.

Method: Using Standard Electrode Potentials to Compare Reducing Power

Method Name: The More Negative $E^\circ$, the Stronger the Reducing Agent

Concept (Why this works)

Reducing power means the ability to lose electrons (get oxidized).

A more negative standard reduction potential ($E^\circ$) means the metal is more difficult to reduce — which means it is easier to oxidize (i.e., it is a stronger reducing agent).

Steps

1. List the given $E^\circ$ values (all are reduction potentials):

   • $K^+/K$: $-2.93\ V$
   • $Mg^{2+}/Mg$: $-2.37\ V$
   • $Cr^{3+}/Cr$: $-0.74\ V$
   • $Hg^{2+}/Hg$: $+0.79\ V$
   • $Ag^+/Ag$: $+0.80\ V$

2. Arrange in increasing order of $E^\circ$ (most negative → most positive):

   • $-2.93\ V$ (K)
   • $-2.37\ V$ (Mg)
   • $-0.74\ V$ (Cr)
   • $+0.79\ V$ (Hg)
   • $+0.80\ V$ (Ag)

3. Apply the rule:

   More negative $E^\circ$ = stronger reducing agent.

   So increasing reducing power means going from weakest (most positive) to strongest (most negative).

4. Final order (increasing reducing power):

   Ag < Hg < Cr < Mg < K

Key Result

Increasing reducing power:

$\boxed{Ag < Hg < Cr < Mg < K}$

Quick Check

• Ag ($+0.80\ V$) is the weakest reducing agent — it prefers to stay as $Ag^+$ rather than lose electrons.
• K ($-2.93\ V$) is the strongest — it readily loses electrons to become $K^+$.

Mistake 1: Confusing Reducing Power with Oxidising Power

The error:

Students often think: "Higher (more positive) electrode potential = stronger reducing agent."

This is wrong. Reducing power is the ability to lose electrons (get oxidised). A more negative $E^\circ$ means the metal is more eager to give away electrons.

How to avoid:

Remember the mnemonic:

More negative = more reactive (as a reducing agent).

• $K^+/K = -2.93\ V$ -> very strong reducing agent
• $Ag^+/Ag = +0.80\ V$ -> very weak reducing agent

Correct order (increasing reducing power):

$Ag < Hg < Cr < Mg < K$

Mistake 2: Forgetting the Sign Convention

The error:

Some students rank by absolute value (ignoring the negative sign), e.g., putting $K$ and $Mg$ in the middle.

How to avoid:

Always compare the actual signed values on a number line:

-3.0 V        -2.0 V        -1.0 V         0 V        +1.0 V
  K            Mg            Cr                     Hg   Ag
(most negative = strongest reducing agent)

Increasing reducing power = from most positive to most negative $E^\circ$.

Mistake 3: Mixing Up the Order (Ascending vs Descending)

The error:

The question asks for increasing order of reducing power. Students often write the decreasing order (strongest first).

How to avoid:

• Increasing = weakest first -> strongest last
• Decreasing = strongest first -> weakest last

For this data:

• Increasing reducing power: $Ag < Hg < Cr < Mg < K$
• Decreasing reducing power: $K > Mg > Cr > Hg > Ag$

Mistake 4: Not Writing the Metal Symbols Correctly

The error:

Writing $K^+$, $Ag^+$ etc. instead of the metal ($K$, $Ag$). The question asks for metals, not ions.

How to avoid:

The reducing agent is the metal in its elemental form ($M$), not the ion ($M^{n+}$). Always list the neutral metal.

Final Correct Answer

Increasing order of reducing power:

Reasoning:

• $Ag^+/Ag = +0.80\ V$ -> weakest reducing agent
• $K^+/K = -2.93\ V$ -> strongest reducing agent

Quick Checklist to Avoid These Mistakes