an external opposite potential is applied to the galvanic cell and increased slowly till opposing potential reaches 1.1 V. What does opposite potential mean ?
The galvanic cell made up of Zn and Cu electrodes have potential difference of 1.1 V when the concentration of Zn2+ and Cu2+ is unity.
Now, when this cell is connected to some external source of electrical energy whose potential difference is lesser than 1.1 V ethen the chemical reaction continues till the external potential reaches the value of 1.1V.
If the external energy source have same potential as the cell then flow of current will stop and there will be no reaction.
So here opposite potential means the potential difference of external energy source whose value is lesser than 1.1V.
Now, when this cell is connected to some external source of electrical energy whose potential difference is lesser than 1.1 V ethen the chemical reaction continues till the external potential reaches the value of 1.1V.
If the external energy source have same potential as the cell then flow of current will stop and there will be no reaction.
So here opposite potential means the potential difference of external energy source whose value is lesser than 1.1V.