Of the reactions involved in the photodecomposition of ozone (shown below) , which are exothermic?
1) O₂(g) + hv → O (g) + O (g)
2) O (g) + O₂(g) + M (g) → O₃(g) + M^*(g)
3) O₃(g) + hv →O₂(g) + O (g)
4) O (g) + O (g) + M (g) →O₂(g) + M^*(g)

Question

Quizplus · Accepted Answer

The term "exothermic" implies that the reaction releases heat energy. We can determine whether a reaction is exothermic or endothermic by looking at the change in enthalpy (ΔH) of the reaction. If ΔH is negative, the reaction is exothermic, and if ΔH is positive, the reaction is endothermic.

For the given reactions, ΔH values are not provided, but we can use some general knowledge of the photochemistry of ozone to determine which reactions are exothermic.

Reaction (1) involves the photodissociation of ozone into two oxygen atoms via absorption of UV light, and is therefore endothermic.

Reaction (2) is the reaction of an oxygen atom and an ozone molecule to form two oxygen molecules and a free radical. This reaction is highly exothermic and is responsible for the absorption of UV radiation in the stratosphere.

Reaction (3) is similar to Reaction (1), but the absorption of UV light leads to the formation of an energized ozone molecule with extra vibrational energy in one of its bonds. The excess energy is subsequently released as heat, making this reaction exothermic.

In reaction (4), two oxygen atoms react with a molecule M (often nitrogen or another gas in the atmosphere) to form two ozone molecules and M in an exothermic manner.

Therefore, only reactions (2) and (4) are exothermic. The correct answer is D.

Of the Reactions Involved in the Photodecomposition of Ozone (Shown