Cycle
Definition and meaning of Cycle in chemistry.
Cycle, in chemistry, is a sequence of chemical or physical steps that ultimately returns a system to its original state, so that any catalyst or intermediate consumed in early steps is regenerated by the later ones and the sequence can repeat indefinitely.
In more detail
Because the system ends where it started, the net chemical change equals only the reactants consumed and products released over one full turn, while the catalyst or intermediate species themselves show zero net change. This regeneration is what lets a small amount of catalyst process a large amount of substrate, and it is the organizing principle behind metabolic pathways, industrial catalytic processes, and biogeochemical cycling of elements. Thermodynamic cycles (like the Carnot cycle) apply the same closed-loop idea to state functions such as pressure, volume, and temperature.
Key facts
| Field | General Chemistry |
|---|---|
| Key feature | Net-zero change in catalyst/intermediate per full turn |
| Common examples | Citric acid cycle, catalytic cycle, Carnot cycle, carbon cycle |
| Related metric | Turnover number (cycles completed per catalyst molecule) |
In the citric acid cycle (Krebs cycle), oxaloacetate combines with acetyl-CoA and, after eight enzyme-catalyzed steps that release CO2 and generate NADH, FADH2, and ATP/GTP, oxaloacetate is regenerated so the cycle can turn again.
Frequently asked questions
Is a chemical cycle the same as a chemical equilibrium?
No. An equilibrium is a static state where forward and reverse reaction rates are equal; a cycle is a directional, multi-step process that steadily converts reactants to products while returning the catalyst or intermediate to its starting form.
Why are cycles important in catalysis?
Because the catalyst is regenerated at the end of each turn, a tiny amount can convert vastly more substrate than its own quantity, which is why catalytic cycles (e.g., in the Haber-Bosch or Wacker processes) are so industrially efficient.