Ncert Exercises & Solutions

On the basis of standard electrode potential values, suggest which of the following reactions would take place?

$1 . Cu + {Zn}^{2 +} \to {Cu}^{2 +} + Zn 2 . Mg + {Fe}^{2 +} \to {Mg}^{2 +} + Fe 3 . {Br}_{2} + 2 {Cl}^{-} \to {Cl}_{2} + 2 Br 4 . Fe + {Cd}^{2 +} \to Cd + {Fe}^{2 +}$

As we know that,

E °_{{Cu}^{2 +} / Cu} = 0.34 V, E °_{{Zn}^{2 +} / Zn} = - 0.76 V,

E °_{{Mg}^{2 +} / Mg} = - 2.37 V, E °_{{Fe}^{2 +} / Fe} = - 0.74 V,

E °_{{Br}_{2} / {Br}^{-}} = + 1.08 V, E °_{{Cl}_{2} / {Cl}^{-}} = + 1.36 V

E °_{{Fe}^{2 +} / Fe} = - 0.74 V, E °_{{Cd}^{2 +} / Cd} = - 0.44 V

(a)

E °_{{Cu}^{2 +} / Cu} = 0.34 V and E °_{{Zn}^{2 +} / Zn} = - 0.76 V

Cu + {Zn}^{2 +} \to {Cu}^{2 +} + Zn

In the given cell reaction, Cu is oxidised to Cu²⁺, therefore, Cu²⁺/Cu couple acts as anode and Zn²⁺ is reduced to Zn, therefore, Zn²⁺/Zn couple acts as cathode.

E °_{cell} = E °_{cathode} - E °_{anode}

E °_{cell} = - 0.74 - (+ 0.34) = - 1.10 V

Positive value of

E °_{cell}

indicates that the reaction will not occur.

(a)

Mg + {Fe}^{2 +} \to {Mg}^{2 +} + Fe

E °_{{Mg}^{2 +} / Mg} = - 2.37 V and E °_{{Fe}^{2 +} / Fe} = - 0.74 V

In the given cell reaction, Mg is oxidised to Mg²⁺ hence, Mg²⁺/Mg couple acts as anode and Fe²⁺ is reduced to Fe hence, Fe²⁺/Fe couple acts as cathode.

E °_{cell} = E °_{cathode} - E °_{anode}

E °_{cell} = - 0.74 - (- 2.37) = + 1.63 V

Positive value of

E °_{cell}

5 indicates that the reaction will occur.

(c)

{Br}_{2} + 2 {Cl}^{-} \to {Cl}_{2} + 2 {Br}^{-}

E °_{{Br}_{2} / {Br}^{-}} = + 1.08 V and E °_{{Cl}_{2} / {Cl}^{-}} = + 1.36 V

In the given cell reaction, Cl^- is oxidised to Cl₂ hence, Cl^-/Cl₂ couple acts as anode and Br₂ is reduced to Br^- hence; Br^-/Br₂ couple acts as cathode.

E °_{cell} = E °_{cathode} - E °_{anode}

E °_{cell} = + 1.08 - (+ 1.36) = - 0.28 V

Negative value of

E °_{cell}

indicates that the reaction will occur.

(d)

Fe + {Cd}^{2 +} \to Cd + {Fe}^{2 +}

E °_{{Fe}^{2 +} / Fe} = - 0.74 V and E °_{{Cd}^{2 +} / Cd} = - 0.44 V

In the given cell reaction, Fe is oxidised to Fe²⁺ hence, Fe²⁺/Fe couple acts as anode and Cd²⁺ is reduced to Cd hence, Cd²⁺/Cd couple acts as cathode.

E °_{cell} = E °_{cathode} - E °_{anode}

E °_{cell} = - 0.44 - (- 0.74) = + 0.30 V

Positive value

E °_{cell}

indicates that the reaction will occur.

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