<P> The standard reduction potential (E 0 (\ displaystyle E_ (0))) is measured under standard conditions: 25 ° C, a 1 activity for each ion participating in the reaction, a partial pressure of 1 bar for each gas that is part of the reaction, and metals in their pure state . The standard reduction potential is defined relative to a standard hydrogen electrode (SHE) reference electrode, which is arbitrarily given a potential of 0.00 V. However, because these can also be referred to as "redox potentials", the terms "reduction potentials" and "oxidation potentials" are preferred by the IUPAC . The two may be explicitly distinguished in symbols as E 0 r (\ displaystyle E_ (0) ^ (r)) and E 0 o (\ displaystyle E_ (0) ^ (o)). </P> <P> The relative reactivities of different half cells can be compared to predict the direction of electron flow . A higher E 0 (\ displaystyle E_ (0)) means there is a greater tendency for reduction to occur, while a lower one means there is a greater tendency for oxidation to occur . </P> <P> Any system or environment that accepts electrons from a normal hydrogen electrode is a half cell that is defined as having a positive redox potential; any system donating electrons to the hydrogen electrode is defined as having a negative redox potential . E h (\ displaystyle E_ (h)) is measured in millivolts (mV). A high positive E h (\ displaystyle E_ (h)) indicates an environment that favors oxidation reaction such as free oxygen . A low negative E h (\ displaystyle E_ (h)) indicates a strong reducing environment, such as free metals . </P> <P> Sometimes when electrolysis is carried out in an aqueous solution, water, rather than the solute, is oxidized or reduced . For example, if an aqueous solution of NaCl is electrolyzed, water may be reduced at the cathode to produce H and OH ions, instead of Na being reduced to Na, as occurs in the absence of water . It is the reduction potential of each species present that will determine which species will be oxidized or reduced . </P>

When will a molecule be assigned a negative standard reduction potential
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