First slide

Chemical reactions and calculations

Question

Which of the following reactions involve neither oxidation, non reduction ?

Moderate

A

${CrO}_{4}^{- 2} ⟶ {Cr}_{2} O_{7}^{- 2}$
B

$Cr ⟶ {CrCl}_{3}$
C

$Na ⟶ {Na}^{+}$
D

$2 {SO}_{3}^{- 2} ⟶ S_{4} O_{6}^{- 2}$

Solution

Considering that oxidation involves increase in oxidation number while reduction involves decrease in oxidation number, in a redox reaction, we have:
(a) ${Cr}^{+ 6} O_{4}^{- 2} ⟶ {Cr}_{2}^{+ 6} O_{7}^{- 2}$
(b) ${Cr}^{2} ⟶ {Cr}^{+ 3} {Cl}_{3}$
(c) ${Na}^{0} ⟶ {Na}^{+}$
(d) $2 S_{2}^{+ 2} O_{3}^{- 2} ⟶ S_{4}^{2 .5} O_{6}^{- 2}$
From above, we find that in reaction (a), neither the oxidation number of Cr increases nor decreases from +6.

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