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.

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.