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Redox reactions and reagents

In a redox reaction, one substance is oxidised and another is reduced. The oxidation numbers of the two substances therefore change during the reaction.

Oxidation and reduction occur at the same time in a redox reaction. The substance being oxidised transfers electric charge to the substance that is reduced.

The substance that is oxidised is called the reducing agent because it reduces the other substance. The substance that is reduced is called the oxidising agent.

When copper is placed in a colourless silver nitrate solution for some time, silver metal is found deposited while the solution becomes blue in colour. The blue colour indicates the presence of copper ions.

$$$\ce{Cu {(s)} + 2AgNO3 {(aq)} -> 2Ag {(s)} + Cu(NO3)2 {(aq)}}$$$

The copper metal is oxidised as its oxidation number increases from $$0$$ to $$+2$$. On the other hand, the silver metal is reduced as its oxidation number decreases from $$+1$$ to $$0$$.

Oxidising and reducing agents can be identified by using special indicators.

Aqueous potassium iodide ($$\ce{KI}$$) turns from colourless to brown when an oxidising agent is added.

Potassium iodide dissociates into potassium ($$\ce{K^+}$$) and iodide ($$\ce{I^-}$$) ions. The iodide ions donate electrons to the oxidising agent and are converted to iodine ($$\ce{I2}$$), which turns the solution brown.

$$$\ce{2I^- -> I2 + 2e^-}$$$

Acidified potassium dichromate (VI) ($$\ce{K2Cr2O7}$$) turns from orange to green when a reducing agent is added.

Potassium dichromate (VI) dissociates into potassium ($$\ce{K^+}$$) and dichromate ($$\ce{Cr2O7^2-}$$) ions.

$$\ce{Cr}$$ has an oxidation state of $$ +6$$. $$\ce{Cr}$$ accepts electrons from the reducing agent and is converted to $$\ce{Cr^3+}$$, which turns the solution green.

$$$\ce{Cr2O7^2- + 14H^+ + 6e^- -> 2Cr^3+ + 7H2O}$$$