If the standard cell potential (E° cell) is negative, it indicates that the overall cell reaction is not spontaneous under standard conditions. In other words, the reaction does not proceed in the forward direction without external energy input.
The standard cell potential is a measure of the tendency of a redox reaction to occur. It represents the potential difference between the two half-cells in an electrochemical cell when they are connected by a wire and a salt bridge, and the concentrations of all species are at standard conditions (1 M for aqueous solutions and 1 atm for gases).
If E° cell is negative, it means that the reduction potential of the cathode half-cell is lower than the oxidation potential of the anode half-cell. In a spontaneous redox reaction, the electrons flow from the anode (where oxidation occurs) to the cathode (where reduction occurs). However, if the cathode has a lower reduction potential than the anode, the reaction will not occur spontaneously in the forward direction.
To make such a reaction proceed, an external energy source, such as a battery or an electrical power supply, is required to drive the reaction in the desired direction. By applying an external potential greater than the E° cell value, the reaction can be forced to proceed in the reverse direction, from the cathode to the anode.
It's important to note that E° cell represents the standard conditions, and actual reaction conditions may differ. Factors such as temperature, concentration, and pressure can influence the actual cell potential, and it is possible for a reaction with a negative E° cell to still occur under non-standard conditions if these factors are adjusted accordingly.