IB Physics EMF of a cell

Which of the following is/are correct regarding the emf of a cell?

1. The voltage of the battery.
2. The potential difference across the terminals of the cell when zero current passes through it.
3. The work done in moving unit charge through the cell.
4. The work done in moving unit charge around the circuit.
5. The chemical energy changed to thermal energy when unit charge moves through the cell.
6. The power per unit current passing through the cell.