Question

What do you understand about a redox couple?

Answer 1

Hint : When a reducing agent loses one or more electron its oxidation number increases and the product formed is capable of accepting electrons again, the oxidized species is an oxidizing agent Similarly when an oxidizing agent is reduced it can lose an electron behaving as a reducing agent. This is similar to what we see in conjugate acids and conjugate bases: for every oxidizing agent there exists a reducing agent and for every reducing agent there exists an oxidizing agent.

Complete Step By Step Answer:
Redox couple is defined as the combination of the oxidised and reduced form of a same molecule taking part in oxidation half reaction and reduction half reaction. An oxidizing agent and reducing agent which appear on both sides of oxidation-half reaction and reduction-half reaction constitute a redox couple. Redox couples behave similarly to conjugate bases and acids.
The stronger the oxidizing agent the weaker will be the reducing agent and the stronger the reducing agent the weaker will be the oxidising agent. Strength of redox couples is decide by the redox equations

Strong
${F_2} + 2{e^ - } \to 2{F^ - }$
$Mn{O^{4 - }} + 8{H^ + } + 5{e^ - } \to M{n^{2 + }} + {H_2}O$
$C{l_2} + 2{e^ - } \to 2C{l^ - }$
$C{r_2}{O_7}^{2 - } + 14{H^ + } + 6{e^ - } \to 2C{r^{3 + }} + 7{H_2}O$
${O_2} + 4{H^ + } + 4{e^ - } \to 2{H_2}O$

Medium
$OC{l^ - } + {H_2}O + 2{e^ - } \to C{l^ - } + O{H^ - }$
$A{g^ + } + {e^ - } \to Ag$
$F{e^{3 + }} + {e^ - } \to F{e^{2 + }}$
${I_2} + 2{e^ - } \to 2{I^ - }$
$C{u^{2 + }} + 2{e^ - } \to Cu$

Weak
$S{n^{4 + }} + 2{e^ - } \to S{n^{2 + }}$
$2{H^ + } + 2{e^ - } \to {H_2}$
$P{b^{2 + }} + 2{e^ - } \to Pb$
$F{e^{2 + }} + 2{e^ - } \to Fe$

Negligible
$Z{n^{2 + }} + 2{e^ - } \to Zn$
$M{g^{2 + }} + 2{e^ - } \to Mg$
$N{a^ + } + {e^ - } \to Na$
$L{i^ + } + {e^ - } \to Li$

In above reactions the redox couple has been arranged in Decreasing strength of oxidizing agent. Similar to acids and bases, this list of half-reactions can be used to predict the direction in which the redox reaction will proceed.

Note :
The reactions which have the highest tendency are those including strong oxidizing agent from the upper left part of the table and strong reducing agent from the lower right part of the table. The reaction with the lowest tendency will include weak oxidizing agent from the lower left and weak reducing agent from the upper right part of the table.