standard electrode potentials is shown below. {| class="wikitable" |- ! Half-reaction ! E° (V) ! Ref. |- | Na+ + e− Na | −2.71 || |- | Zn2+ + 2 e− Zn | −0.7618 || |- | 2 H+ + 2 e− H2 | ≡ 0|| |- | Br2 + 2 e− 2 Br− | +1.0873 || |- | O2 + 4 H+ + 4 e− 2 H2O | +1.23 || |- | Cl2 + 2 e− 2 Cl− | +1.36 || |- | + 2 e− 2 | +2.07 || |} In terms of electrolysis, this table should be interpreted as follows: Moving down the table, E° becomes more positive, and species on the left are more likely to be reduced: for example, zinc ions are more likely to be reduced to zinc metal than sodium ions are to be reduced to sodium metal. Moving up the table, E° becomes more negative, and species on the right are more likely to be oxidized: for example, sodium metal is more likely to be oxidized to sodium ions than zinc metal is to be oxidized to zinc ions. Using the Nernst equation the electrode potential can be calculated for a specific concentration of ions, temperature and the number of electrons involved. For pure water (pH 7): the electrode potential for the reduction producing hydrogen is −0.41 V, the electrode potential for the oxidation producing oxygen is +0.82 V. Comparable figures calculated in a similar way, for 1 M zinc bromide, ZnBr2, are −0.76 V for the reduction to Zn metal and +1.10 V for the oxidation producing bromine. The conclusion from these figures is that hydrogen should be produced at the cathode and oxygen at the anode from the electrolysis of water—which is at variance with the experimental observation that zinc metal is deposited and bromine is produced. The explanation is that these calculated potentials only indicate the thermodynamically preferred reaction. In practice, many other factors have to be taken into account such as the kinetics of some of the reaction steps involved. These factors together mean that a higher potential is required for the reduction and oxidation of water than predicted, and these are termed overpotentials. Experimentally it is known that overpotentials depend on the design of the cell and the nature of the electrodes. For the electrolysis of a neutral (pH 7) sodium chloride solution, the reduction of sodium