If the calculated voltage of the cell is positive, the cell reaction as written will take place
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → spontaneously
A reaction is spontaneous when it can proceed on its own, without a continuous supply of energy from outside. In thermodynamics, a reaction is spontaneous when its Gibbs energy change is negative (ΔrG < 0).
For a cell reaction, the Gibbs energy change is linked to the cell potential (emf):
\( \displaystyle \Delta_r G=-nFE_{cell} \)
Here n is the number of electrons transferred and F is the Faraday constant; both are positive. So the signs of ΔrG and Ecell are always opposite:
\( \displaystyle E_{cell}\gt 0 \quad\Rightarrow\quad \Delta_r G\lt 0 \)
A positive cell voltage gives a negative ΔrG, so the cell reaction as written takes place spontaneously. This is what happens in a galvanic cell such as the Daniell cell (E°cell = 1.1 V):
\( \displaystyle \ce{Zn(s) + Cu^2+(aq) -> Zn^2+(aq) + Cu(s)} \)
Why the other options are incorrect
Option (1) slowly — Incorrect.
The cell voltage is a thermodynamic quantity: it tells whether a reaction can occur, not how fast it occurs.
The speed is a question of chemical kinetics. Ce4+ can oxidise water (E° = +1.74 V), yet the reaction is very slow.
Option (3) reversibly — Incorrect.
A cell works reversibly only when the current drawn is infinitely small, that is, when an opposing voltage almost equal to the emf is applied.
This is a condition on how the current is drawn, used to obtain maximum work; a positive voltage alone does not make the reaction reversible.
Option (4) violently — Incorrect. A positive voltage says nothing about the speed or the vigour of the reaction. A Daniell cell has a positive emf, yet it supplies current steadily and safely.
NCERT (2026-27 edition):
2.2 Galvanic Cells – “As mentioned earlier a galvanic cell is an electrochemical cell that converts the chemical energy of a spontaneous redox reaction into electrical energy.”
2.3.2 Electrochemical Cell and Gibbs Energy of the Reaction – “… The reversible work done by a galvanic cell is equal to decrease in its Gibbs energy and therefore, if the emf of the cell is E and nF is the amount of charge passed and ΔrG is the Gibbs energy of the reaction, then ΔrG = – nFE(cell)”