अर्ध-अभिक्रिया

किसी रेडॉक्स अभिक्रिया के केवल आक्सीकरण या केवल अपचयन वाली अभिक्रिया को अर्ध-अभिक्रिया (half reaction) कहते हैं। यह ध्यान रखने योग्य है कि आक्सीकरण या अपचयन अकेले नहीं होते बल्कि साथ-साथ होते हैं, और इसीलिये इसे 'रेडॉक्स अभिक्रिया' कहते हैं।

Consider the Galvanic cell shown in the image to the right: it is constructed with a piece of zinc (Zn) submerged in a solution of zinc sulfate (ZnSO₄) and a piece of copper (Cu) submerged in a solution of copper(II) sulfate (CuSO₄). The overall reaction is:

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

At the Zn anode, oxidation takes place (the metal loses electrons). This is represented in the following oxidation half-reaction (note that the electrons are on the products side):

Zn(s) → Zn²⁺ + 2e⁻

At the Cu cathode, reduction takes place (electrons are accepted). This is represented in the following reduction half-reaction (note that the electrons are on the reactants side):

Cu²⁺ + 2e⁻ → Cu(s)