Assertion: Current stops flowing when ECell = 0. Reason: Equilibrium of the cell reaction is attained. (i) Bot

NCERT Exemplar · Q66

E_{Cell} = 0

(i) Both assertion and reason are true and the reason is the correct explanation of assertion.

(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.

(iii) Assertion is true but the reason is false.

(iv) Both assertion and reason are false.

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Concept understanding — Cell Representation Nernst Equation

Cell Representation and the Nernst Equation: From Intuition to Precision

Imagine you have a Daniell cell — a zinc rod in zinc sulphate solution connected by a salt bridge to a copper rod in copper sulphate solution. You know it produces a voltage. But what happens if you dilute the copper sulphate solution? Or if you change the temperature? The voltage changes. The Nernst equation is the tool that tells you exactly how much it changes.

The Intuition First

A battery works because the two half-cells "want" to react — zinc wants to lose electrons, copper ions want to gain them. This "want" is measured as a tendency, or potential. But the strength of that tendency depends on how crowded the ions are.

Think of it like this: If you have a room full of people who all want to leave (like zinc ions wanting to form), the push to get out is stronger when the room is packed. If the room is nearly empty, the push is weaker. Similarly, for copper ions wanting to enter the metal (gain electrons), the pull is stronger when there are many copper ions around, and weaker when there are few.

The Nernst equation quantifies this: the actual cell potential depends on the concentrations (or activities) of the ions involved.

The Precise Statement

For a general cell reaction:

aA + bB \rightarrow cC + dD

The cell potential $E$ under non-standard conditions is given by:

E = E^\circ - \frac{RT}{nF} \ln Q

Where:

$E$ = cell potential under the given conditions (in volts)
$E^\circ$ = standard cell potential (when all reactants/products are at 1 M, 1 atm, 25°C)
$R$ = universal gas constant (8.314 J/mol·K)
$T$ = temperature in Kelvin
$n$ = number of moles of electrons transferred in the balanced half-reactions
$F$ = Faraday constant (96,485 C/mol)
$Q$ = reaction quotient = $\frac{[C]^c [D]^d}{[A]^a [B]^b}$ (using concentrations for dilute solutions)

At 25°C (298 K), the equation simplifies to a very practical form:

E = E^\circ - \frac{0.0591}{n} \log_{10} Q

The 0.0591 comes from $\frac{2.303RT}{F}$ at 298 K. Notice it uses $\log_{10}$ (common log), not natural log.

Cell Representation: How We Write It

In electrochemistry, we represent a cell with a shorthand notation. For the Daniell cell:

\text{Zn(s)} \mid \text{Zn}^{2+}(\text{aq}) \parallel \text{Cu}^{2+}(\text{aq}) \mid \text{Cu(s)}

The single vertical line $\mid$ represents a phase boundary (solid electrode | solution). The double line $\parallel$ represents the salt bridge.

The anode (oxidation) is written on the left, the cathode (reduction) on the right. Electrons flow from left to right in the external circuit.

Applying the Nernst Equation to a Cell Representation

For the Daniell cell, the half-reactions are:

Anode (oxidation): $\text{Zn(s)} \rightarrow \text{Zn}^{2+}(\text{aq}) + 2e^-$
Cathode (reduction): $\text{Cu}^{2+}(\text{aq}) + 2e^- \rightarrow \text{Cu(s)}$

Overall: $\text{Zn(s)} + \text{Cu}^{2+}(\text{aq}) \rightarrow \text{Zn}^{2+}(\text{aq}) + \text{Cu(s)}$

Here $n = 2$ (two electrons transferred). The reaction quotient is:

Q = \frac{[\text{Zn}^{2+}]}{[\text{Cu}^{2+}]}

So the Nernst equation becomes:

E = E^\circ - \frac{0.0591}{2} \log_{10} \frac{[\text{Zn}^{2+}]}{[\text{Cu}^{2+}]}

Note

Solids (Zn, Cu) do not appear in $Q$ because their concentrations are constant (activity = 1).

A Worked Example

Suppose you have a Daniell cell where $[\text{Zn}^{2+}] = 0.1$ M and $[\text{Cu}^{2+}] = 1.0$ M at 25°C. $E^\circ$ for the cell is 1.10 V.

$E = 1.10 - \frac{0.0591}{2} \log_{10} \frac{0.1}{1.0}$ …

Why this formula?

Cell Representation & the Nernst Equation: Why It Works

The Core Question

Why does a cell's voltage change when concentrations change? The Nernst equation answers this — but the reason lies in the link between chemical free energy and electrical work.

1. The Fundamental Link: Gibbs Free Energy & Cell Potential

A galvanic cell does electrical work. The maximum useful work a cell can do equals the change in Gibbs free energy ( $\Delta G$ ):

\Delta G = -nFE_{\text{cell}}

Where:

$n$ = moles of electrons transferred
$F$ = Faraday constant ( $96,485 \, \text{C mol}^{-1}$ )
$E_{\text{cell}}$ = cell potential (volts)

Why negative? A spontaneous reaction has $\Delta G < 0$ and $E_{\text{cell}} > 0$ — the negative sign makes this consistent.

2. The Chemical Side: $\Delta G$ Depends on Concentration

For a general redox reaction:

aA + bB \rightarrow cC + dD

The Gibbs free energy under non-standard conditions is:

\Delta G = \Delta G^\circ + RT \ln Q

Where $Q$ is the reaction quotient:

Q = \frac{[C]^c [D]^d}{[A]^a [B]^b}

Why this form? It comes from the relationship between chemical potential and concentration — the entropy of mixing drives concentration dependence.

3. Combining Both Sides: The Derivation

Set the electrical work equal to the chemical free energy change:

-nFE_{\text{cell}} = -nFE^\circ_{\text{cell}} + RT \ln Q

Divide both sides by $-nF$ :

E_{\text{cell}} = E^\circ_{\text{cell}} - \frac{RT}{nF} \ln Q

This is the Nernst equation.

4. The "Why" in Plain Terms

Concept	Physical Meaning
$E^\circ_{\text{cell}}$	Voltage when all species are at 1 M (standard state)
$-\frac{RT}{nF} \ln Q$	Correction factor — adjusts voltage for real concentrations
$Q$	Tells you how far the reaction is from equilibrium

Key insight: When $Q = K$ (equilibrium), $E_{\text{cell}} = 0$ — the battery is dead because no net reaction occurs.

5. The Common Form (log base 10)

At $25^\circ \text{C}$ ( $298 \, \text{K}$ ):

\frac{RT}{F} \ln 10 \approx 0.0592 \, \text{V}

So:

E_{\text{cell}} = E^\circ_{\text{cell}} - \frac{0.0592}{n} \log_{10} Q

Why convert to log? Exam convenience — most concentration values are powers of 10.

--- …

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The key idea is that a cell stops producing current when it reaches equilibrium, where $E_{Cell} = 0$ and the reaction quotient $Q$ equals the equilibrium constant $K$ . Both the assertion and reason are true, and the reason correctly explains the assertion — so option (i) is correct.

Let’s start with the core concept. A galvanic cell works because there’s a difference in electrode potentials, which drives electrons through the external circuit. This potential difference is the cell potential $E_{Cell}$ . As the cell discharges, the concentrations of reactants and products change, and $E_{Cell}$ gradually falls. When the cell reaction reaches equilibrium, the forward and reverse rates become equal — no net reaction occurs, and no current flows. At equilibrium, the Nernst equation gives $E_{Cell} = 0$ , because the reaction quotient $Q$ equals the equilibrium constant $K$ .

Now, let’s break it down step by step.

What does $E_{Cell} = 0$ mean physically?

The cell potential is the driving force for electron flow. When $E_{Cell} = 0$ , there is no net driving force — electrons don’t flow spontaneously in either direction. So current stops. This is exactly what the assertion says.
Why does $E_{Cell}$ become zero?

The Nernst equation for a cell reaction $aA + bB \rightarrow cC + dD$ is:

E_{Cell} = E^\circ_{Cell} - \frac{RT}{nF} \ln Q

where $Q = \frac{[C]^c [D]^d}{[A]^a [B]^b}$ . As the reaction proceeds, $Q$ changes. At equilibrium, $Q = K$ (the equilibrium constant). Also, at equilibrium, $\Delta G = 0$ . The relation $\Delta G = -nFE_{Cell}$ then forces $E_{Cell} = 0$ . So the Nernst equation becomes:

0 = E^\circ_{Cell} - \frac{RT}{nF} \ln K

which is consistent — it just gives the standard relation $E^\circ_{Cell} = \frac{RT}{nF} \ln K$ .

Is the reason correct? …

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Common Mistakes & How to Avoid Them

Mistake 1: Thinking current completely stops at equilibrium

The error: Students assume $E_{Cell} = 0$ means zero current flow in the external circuit.
Why it's wrong: At equilibrium, the net current is zero, but microscopic equal and opposite currents (forward and reverse reactions) continue. The cell is not "dead" — it's balanced.
How to avoid: Remember: equilibrium is dynamic, not static. Use the analogy of a market — prices stop changing, but buying and selling still happen at equal rates.

Mistake 2: Confusing $E_{Cell} = 0$ with a "dead" battery

The error: Students think a cell with $E_{Cell} = 0$ is like a discharged battery where no reaction is possible.
Why it's wrong: A discharged battery has $E_{Cell} > 0$ but reactants are depleted. At $E_{Cell} = 0$ , the cell is at chemical equilibrium — reactants and products coexist with no net change.
How to avoid: Distinguish between:
- Dead battery: Reactants exhausted, $E_{Cell} > 0$ possible but no reaction.
- Equilibrium cell: $E_{Cell} = 0$ , reaction still occurs in both directions equally.

Mistake 3: Misapplying the Nernst equation

The error: Students write $E_{Cell} = E^\circ_{Cell} - \frac{0.059}{n} \log Q$ and forget that at equilibrium $Q = K$ , so $E_{Cell} = 0$ .
Why it's wrong: They may think $E_{Cell} = 0$ only when $Q = 1$ , not when $Q = K$ .
How to avoid: Always write:

E_{Cell} = E^\circ_{Cell} - \frac{0.059}{n} \log Q

At equilibrium: $Q = K$ and $E_{Cell} = 0$ , so:

E^\circ_{Cell} = \frac{0.059}{n} \log K

This is a direct consequence of equilibrium.

Mistake 4: Thinking the Reason is false

The error: Students reject "Equilibrium of the cell reaction is attained" as the reason.
Why it's wrong: The reason is true — $E_{Cell} = 0$ is the definition of electrochemical equilibrium.
How to avoid: Memorise the key link: …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 13 on this concept.

CBSE 2026Set ANNUAL1 mark
$Q. What is the potential difference between the two electrodes of the galvanic cell called?$
›Reveal solutionSolution

The potential difference between the two electrodes of a galvanic cell (measured when no current is drawn) is called the electromotive force (EMF) or cell potential, $E_{\text{cell}}$ .

Concept. In a galvanic (voltaic) cell, the two half-cells are at different electrode potentials. The difference between the cathode and anode potentials is what pushes electrons through the external circuit:

$E_{\text{cell}} = E_{\text{cathode}} - E_{\text{anode}}$

…

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CBSE 2025Set ANNUAL1 mark
$Zn(s)+Cu^{2+}(aq) \rightarrow Zn^{2+}(aq)+Cu(s)$
›Reveal solutionSolution
An external potential exactly equal and opposite to the cell's own EMF brings the system to balance, so no net current flows in either direction — this is the basis of potentiometric EMF measurement.

The Daniell-type cell $Zn(s)|Zn^{2+}(aq)\|Cu^{2+}(aq)|Cu(s)$ spontaneously drives current in the galvanic direction (electrons flow from Zn anode to Cu cathode through the external circuit) with an EMF of 1.1 V under standard conditions.

If an external opposing potential is applied, it works against this spontaneous cell reaction:

If the external potential is less than 1.1 V, the cell's own EMF still dominates, and current continues to flow in the original (galvanic) direction, though at a reduced magnitude.

If the external potential is greater than 1.1 V, it overpowers the cell's own EMF, and current is forced to flow in the reverse direction (the cell now behaves as an electrolytic cell, being charged/driven backward). …
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CBSE 2025Set ANNUAL1 markMCQ
$Q. The correct statement in a cell of zinc and copper is$
(a) zinc acts as cathode and copper as anode
(b) zinc acts as anode and copper as cathode
(c) the standard reduction potential of zinc is more than that of copper
(d) the flow of electrons is from copper to zinc
›Reveal solutionSolution

Zinc has a lower (more negative) standard reduction potential than copper, so it is oxidized (anode) while copper is reduced (cathode).

In a Daniell-type zinc–copper cell, E°(Zn²⁺/Zn) = −0.76 V is lower than E°(Cu²⁺/Cu) = +0.34 V. The electrode with the lower (more negative) reduction potential is oxidized — zinc loses electrons and acts as the anode (Zn → Zn²⁺ + 2e⁻) — while the electrode with the higher reduction potential is reduced — copper gains electrons and acts as the cathode (Cu²⁺ + 2e⁻ → Cu). Electrons flow …

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CBSE 2024Set D1 markMCQ
$Q. The electromotive force of the cell Zn | ZnSO4 || CuSO4 | Cu is 1.1 volt. Its cathode is$
(a) Zn
(b) Cu
(c) ZnSO4
(d) CuSO4
›Reveal solutionSolution
Reduction happens at the cathode; Cu2+ is reduced to Cu, so Cu is the cathode.

In the Daniell cell Zn | ZnSO4 || CuSO4 | Cu:

Anode (oxidation, left): Zn -> Zn2+ + 2e-

Cathode (reduction, right): Cu2+ + 2e- -> Cu …
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CBSE 2024Set ANNUAL1 markMCQ
$Q. An electrochemical cell can behave like an electrolytic cell when _______.$
(a) Ecell = 0
(b) Ecell > Eext
(c) Eext > Ecell
(d) Ecell = Eext
›Reveal solutionSolution
A galvanic (electrochemical) cell starts behaving like an electrolytic cell when an external potential greater than the cell's own emf is applied against it, reversing the direction of current flow.

Consider a Daniell cell: Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s), which normally works as a galvanic cell producing a cell potential Ecell, with electrons flowing from Zn (anode) to Cu (cathode) through the external circuit.

If an external opposing emf (Eext) is applied to this cell:

When Eext < Ecell, the cell continues to work as a galvanic cell, but the current decreases.

When Eext = Ecell, no current flows through the cell (this is used to measure the cell's emf accurately, e.g. using a potentiometer). …
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CBSE 2024Set ANNUAL1 mark
$Q. Write True/False: A hydrogen bridge is used to maintain continuity of ion flow in a Daniell cell.$
›Reveal solutionSolution

This statement is FALSE. A Daniell cell uses a salt bridge (e.g. containing KCl or KNO3 in agar-agar gel), not any "hydrogen bridge", to complete the internal circuit.

A Daniell cell consists of a Zn electrode dipped in ZnSO4 solution (anode) and a Cu electrode dipped in CuSO4 solution (cathode), connected externally by a wire and internally by a salt bridge. The salt bridge allows ions to migrate between the two half-cells, maintaining electrical neutrality in each compartment as the cell reaction proceeds, and completes the internal circuit …

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CBSE 2023Set 56/1/11 markMCQ
$Zn + 2Ag^+ \rightarrow Zn^{2+} + 2Ag$
›Reveal solutionSolution
By convention the anode (oxidation) is written on the left and the cathode (reduction) on the right. Zinc is oxidised and silver ions are reduced, so the cell is $Zn \mid Zn^{2+} \parallel Ag^+ \mid Ag$ — option (D).

A cell diagram is written anode (left) $\parallel$ cathode (right), with each half-cell running from the electrode metal outward and the double bar $\parallel$ marking the salt bridge.

For the reaction
$Zn + 2Ag^+ \rightarrow Zn^{2+} + 2Ag$

Zinc loses electrons: $Zn \rightarrow Zn^{2+} + 2e^-$ (oxidation, anode, left).

Silver ions gain electrons: $Ag^+ + e^- \rightarrow Ag$ (reduction, cathode, right).

Writing the anode as metal $\mid$ ion and the cathode as ion $\mid$ metal gives
$Zn \mid Zn^{2+} \parallel Ag^+ \mid Ag$
Checking the options: …
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CBSE 2022Set E1 markMCQ
$Q. The standard electrode potentials for the following reactions are given ( At 25°C ): Ag+(aq) + e- -> Ag(s), E° Ag+/Ag = +0.80 V ; Sn2+(aq) + 2e -> Sn(s), E° Sn2+/Sn = -0.14 V. The electromotive force (EMF) of the given cell Sn | Sn2+ (1M) || Ag+ (1M) | Ag is$
(a) 0.66 V
(b) 0.80 V
(c) 1.08 V
(d) 0.94 V
›Reveal solutionSolution
For Sn | Sn2+ || Ag+ | Ag, EMF = E°(Ag+/Ag) - E°(Sn2+/Sn) = 0.80 - (-0.14) = 0.94 V.

In the cell notation the left electrode is the anode (oxidation) and the right is the cathode (reduction):

Cathode (reduction): Ag+ + e- -> Ag, E° = +0.80 V

Anode (oxidation): Sn -> Sn2+ + 2e-, E°(Sn2+/Sn) = -0.14 V

E°cell = E°cathode - E°anode = (+0.80) - (-0.14) = +0.94 V.

…
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CBSE 2022Set ANNUAL1 markMCQ
$Q. Which one of the following statements is incorrect for a voltaic cell ?$
(a) It converts chemical energy to electrical energy.
(b) It uses electrical energy to carry out chemical changes.
(c) It is based on a redox reaction.
(d) It has $-\Delta G$ .
›Reveal solutionSolution
A voltaic cell produces electricity from a spontaneous redox reaction — it does not consume electrical energy, so statement (b) describes an electrolytic cell instead.

Checking each option against what a voltaic (galvanic) cell actually does:

(a) True — a voltaic cell converts chemical energy into electrical energy.

(b) False — this describes an electrolytic cell, which uses externally supplied electrical energy to force a non-spontaneous chemical change. A voltaic cell does the opposite. …
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CBSE 2022Set ANNUAL1 markMCQ
$Mg\,|\,Mg^{2+}\,||\,Cu^{2+}\,|\,Cu$
(a) $Mg$ as cathode
(b) $Cu$ as cathode
(c) $Cu$ is oxidizing agent
(d) None of the above
›Reveal solutionSolution
By IUPAC convention the electrode written on the LEFT of a cell is the anode and the one on the RIGHT is the cathode. Here Mg is the anode and Cu is the cathode. Option (B).

The cell is written as $Mg\,|\,Mg^{2+}\,||\,Cu^{2+}\,|\,Cu$ .

Convention: anode (negative, oxidation) is written on the left; cathode (positive, reduction) is written on the right.

The electrode reactions are:

Anode (Mg, oxidation): $Mg \rightarrow Mg^{2+} + 2e^-$

Cathode (Cu, reduction): $Cu^{2+} + 2e^- \rightarrow Cu$ …
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CBSE 2021Set A1 markMCQ
$Q. Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s) is$
(a) Weston cell
(b) Daniel cell
(c) Calomel cell
(d) None of these
›Reveal solutionSolution
A zinc-copper galvanic cell with this notation is the Daniell cell.

The cell Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s) is the Daniell cell, a galvanic (voltaic) cell.

At the anode (LHS): Zn(s) → Zn2+ + 2e- (oxidation).

At the cathode (RHS): Cu2+ + 2e- → Cu(s) (reduction). …
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CBSE 2018Set ANNUAL1 mark
$Q. What is salt bridge?$
›Reveal solutionSolution
The salt bridge completes the electrical circuit of a galvanic cell by letting ions migrate between the two half-cells (maintaining charge neutrality) while physically keeping the two electrolyte solutions from mixing.

A salt bridge is typically a U-shaped glass tube filled with a gel (agar-agar or gelatin) containing a concentrated solution of an inert electrolyte (one whose ions do not react with the cell's other ions and are not involved in the electrode reactions), commonly KCl, KNO $_3$ or NH $_4$ NO $_3$ . Its two functions:

It completes the internal electrical circuit of the cell, allowing ions to flow between the two half-cells, so that current continues to flow through the external circuit.

As oxidation at the anode generates excess positive ions and reduction at the cathode depletes positive ions (or vice-versa), the salt bridge's ions migrate to maintain electrical neutrality in both half-cell solutions (anions flow toward the anode compartment, cations toward the cathode compartment) — without this, charge would build up and stop the current flow almost immediately. …
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Q.Assertion: Current stops flowing when $E_{Cell} = 0$ .
Reason: Equilibrium of the cell reaction is attained.

Concept understanding — Cell Representation Nernst Equation

Cell Representation and the Nernst Equation: From Intuition to Precision

The Intuition First

The Precise Statement

Cell Representation: How We Write It

Applying the Nernst Equation to a Cell Representation

A Worked Example

Why this formula?

Cell Representation & the Nernst Equation: Why It Works

The Core Question

1. The Fundamental Link: Gibbs Free Energy & Cell Potential

2. The Chemical Side: $\Delta G$ Depends on Concentration

3. Combining Both Sides: The Derivation

4. The "Why" in Plain Terms

5. The Common Form (log base 10)

Method: Thermodynamic Equilibrium Analysis of Electrochemical Cells

Steps:

Common Mistakes & How to Avoid Them

Mistake 1: Thinking current completely stops at equilibrium

Mistake 2: Confusing $E_{Cell} = 0$ with a "dead" battery

Mistake 3: Misapplying the Nernst equation

Mistake 4: Thinking the Reason is false

Q.Assertion: Current stops flowing when ECell=0E_{Cell} = 0ECell​=0. Reason: Equilibrium of the cell reaction is attained.

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Q.Assertion: Current stops flowing when $E_{Cell} = 0$ .
Reason: Equilibrium of the cell reaction is attained.

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