Cambridge O Level Chemistry · Syllabus 5070 · Chemical Reactions
Reducing Agent
What is Reducing Agent?
A substance that reduces another substance and is itself oxidised in the process. It gives electrons to the other species, or takes oxygen from it, and its own oxidation number therefore increases.
This definition is part of the Chemical Reactions chapter in Cambridge O Level Chemistry.
Reducing Agent in context
A reversible reaction, shown with the symbol \(\rightleftharpoons\), can run in both directions at once, and in a closed system it reaches dynamic equilibrium when the forward and reverse rates are equal, so the concentrations of reactants and products stop changing. Changing temperature, pressure or concentration shifts the position of equilibrium, but a catalyst never does — it speeds up both directions equally, reaching the same equilibrium sooner rather than reaching a different one. The Haber and Contact processes each pick operating conditions that trade off rate, yield, safety and cost rather than chasing the highest possible yield alone. In redox terms, oxidation is loss of electrons and reduction is gain of electrons, so an oxidising agent is itself reduced and a reducing agent is itself oxidised.
Common mistakes with Reducing Agent
- 19. “The species oxidised is the oxidising agent.” Why temptingThe two phrases share a word and differ by three letters. RepairThe species oxidised gave electrons away, causing reduction elsewhere, so it is the reducing agent. An oxidising agent is itself reduced. TransferIn \(\mathrm{2Mg + O_2 \rightarrow 2MgO}\), name both agents. Magnesium reducing; oxygen oxidising.
Questions students ask about Reducing Agent
How do you identify the oxidising agent and reducing agent in a reaction?
Track the oxidation number of each species. The species whose oxidation number falls has been reduced, so it is the oxidising agent; the species whose oxidation number rises has been oxidised, so it is the reducing agent. For \(\mathrm{Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)}\), zinc rises from \(0\) to \(+2\) so it is the reducing agent, and copper falls from \(+2\) to \(0\) so copper(II) ions are the oxidising agent.