Which of the following reactions is not an example of an oxidation-reduction reaction? (2 points) Select one: a. 3Br2 + 2Ga yields 2GaBr3 b. H2S + Cl2yields 2HCl + S c. 2PbS + 3O2yields 2PbO + 2SO2 d. K2Cr2O7 + KOH yields 2 K2CrO4 + H2O

The correct answer is : d) K₂Cr₂O₇ + KOH ⇒ 2 K₂CrO₄ + H₂O

Explanation:

An oxidation-reduction reaction (redox reaction) is the one in which the number of oxidation of the atoms change, and electrons are shared.

Oxidation: the atom loses electrons and its oxidation number is increased.

Reduction: the atom gains electrons and its oxidation number is decreased.

In the following reactions, the atoms which loses or gains electrons are:

a) 3Br₂ + 2Ga ⇒ 2GaBr₃

Br: its oxidation number changes from 0 to -1 (reduction)

Ga: its oxidation number changes from 0 to +3 (oxidation)

IT IS A REDOX REACTION.

b) H₂S + Cl₂ ⇒ 2HCl + S

Cl: its oxidation number changes from 0 to -1 (reduction)

S: its oxidation number changes from -2 to 0 (oxidation)

IT IS A REDOX REACTION

c) 2PbS + 3O₂ ⇒ 2PbO + 2SO₂

O: its oxidation number changes from 0 to -2 (reduction)

S: its oxidation number changes from -2 to +4 (oxidation)

IT IS A REDOX REACTION

d) K₂Cr₂O₇ + KOH ⇒ 2 K₂CrO₄ + H₂O

There are not changes in oxidation numbers of the atoms:

K : +1

Cr: +6

H: +1

O: -2

IT IS NOT A REDOX REACTION.

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