What is the name for systems which convert stored chemical energy to electrical energy through a spontaneous irreversible redox reaction?
There’re quite a few terms to define here. So I’m gonna start in reverse. A redox reaction is a reaction where there is simultaneous reduction of one species and oxidation of another. So one species is gaining electrons, undergoing reduction, and another is losing electrons, undergoing oxidation. What about irreversible? Another word for irreversible is one way. This means that a reaction can occur forwards but not back. A reaction being described as irreversible is usually a practical consideration rather than a theoretical one. The products of the forward reaction may undergo different reactions if the conditions are reversed? Or, is it impractical to get them into a position where they would react? For instance, if one of the products is a gas and escapes into the room, you’ll never going to get it back into the reaction vessel.
What about spontaneous? A spontaneous reaction occurs on its own, without external intervention. So far, we’ve explored that the reaction involved occurs on its own, happens one way, and involves a simultaneous reduction and oxidation. The rest of the question is a strong clue as to the type of system we’re looking for. One that converts chemical energy to electrical energy must involve some form of external circuit. The electrons consumed or produced in the reduction and oxidation processes must be extracted from one system and provided to the other.
Hopefully, all this brings to mind is the galvanic cell. A system where a difference in activities between two chemicals generates an electric current, where the preference of one species allows it to be oxidised releasing electrons into an external circuit, allowing for the reduction of a second species. So a galvanic cell is a spontaneous redox reaction where chemical energy is converted into electrical energy.
But what about the irreversible part? A galvanic cell can either be reversible or irreversible. If it’s reversible, it’s otherwise said to be rechargeable. Another name for a rechargeable galvanic cell is a secondary cell. Hopefully, this has reminded you of the name we’re looking for, primary cells. Primary cells involve the conversion of chemical energy to electrical energy through a spontaneous irreversible redox reaction.