Final answer:
Spontaneous oxidation-reduction reactions involve electron transfer resulting in changed oxidation numbers and are accompanied by a positive cell potential, indicating spontaneity. These reactions feature oxidation (loss of electrons) and reduction (gain of electrons) occurring simultaneously. A classic example of a spontaneous redox reaction is the oxidation of lead by copper(II) ions, which has a positive voltage and is thus favorable.
Step-by-step explanation:
Spontaneous oxidation-reduction (redox) reactions are characterized by the transfer of electrons between atoms, which results in a change in the oxidation numbers of at least two elements. A spontaneous reaction implies that the process occurs naturally under given conditions without the need for external energy. In the context of redox reactions, this translates to a positive cell potential. For example, when a cell reaction has a positive voltage, the reaction is spontaneous. Conversely, if the reaction voltage is negative, the process is non-spontaneous in that direction and the reverse reaction would be spontaneous instead.
During a redox reaction, the substance that loses electrons (undergoes increase in oxidation number) is said to be oxidized, while the substance that gains electrons (undergoes decrease in oxidation number) is said to be reduced. These changes in oxidation state are central to any redox process and represent the fundamental basis for understanding such reactions.
To illustrate, the reaction of lead by copper(II) ions is predicted to occur spontaneously due to its positive voltage (+0.47 V). This spontaneous process is demonstrated by the reaction Pb(s) + Cu²+ (aq) → Pb²+ (aq) + Cu(s), which has a positive cell potential and hence is favorable under standard conditions.