Question

The halogen which oxidizes water to \(\mathrm{O}_{2}\) exothermally is (a) \(\mathrm{I}_{2}\) (b) \(\mathrm{Br}_{2}\) (c) \(\mathrm{Cl}_{2}\) (d) \(\mathrm{F}_{2}\)

Short answer

(d) \( \mathrm{F}_{2} \)

Step-by-step solution

Understand Halogen and Water Reaction

Halogens can react with water in oxidation-reduction reactions. Some halogens can oxidize water to form oxygen i.e., converting water (H2O) to oxygen (O2). We are asked to find which halogen out of those given can do this exothermally, meaning the reaction releases energy.

Evaluate Halogen Properties

Fluorine (F2) is the most electronegative element, making it the strongest oxidizing agent among the halogens. It can readily oxidize water. Chlorine (Cl2), bromine (Br2), and iodine (I2) are less effective oxidizers in this context, with iodine being the weakest.

Reaction of Halogens with Water

The reaction of fluorine with water is \[ 2F_{2} + 2H_{2}O \rightarrow 4HF + O_{2} \] This reaction is highly exothermic, releasing energy. On the other hand, reactions of Cl2, Br2, and I2 with water are either non-spontaneous or less exothermic.

Identify Exothermic Reaction

Since the fluorine reaction with water is known to be highly exothermic, fluorine (F2) is the halogen that most readily oxidizes water to O2 exothermally. Chlorine, bromine, or iodine do not have exothermic reactions with water that produce oxygen.

Key concepts

Halogen Reactivity

Halogens are a group of highly reactive nonmetals located in Group 17 of the periodic table. The members include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Their reactivity stems from their need to gain one electron to achieve a stable, filled outer electron shell, like noble gases.

Among the halogens, the reactivity decreases as you move down the group from fluorine to iodine. This is because:

• The atomic size increases, making it harder for the atoms to attract additional electrons.
• The effective nuclear charge that influences incoming electrons becomes weaker.
• Fluorine, being at the top of the group, exhibits the highest reactivity due to its small size and high electronegativity, making it an excellent oxidizing agent.

Understanding this trend of halogen reactivity is crucial in predicting the outcome of chemical reactions, especially those involving oxidation-reduction processes.

Exothermic Reactions

Exothermic reactions are chemical processes that release energy in the form of heat. Such reactions are usually characterized by the temperature of the surroundings increasing, which is why they often feel warm or hot to the touch.

In the context of halogens reacting with water, if a reaction is exothermic, it means that energy is released when the halogen oxidizes water. Fluorine exhibits this property particularly well:

• The reaction of fluorine with water \[ 2F_{2} + 2H_{2}O \rightarrow 4HF + O_{2} \] is an excellent example of an exothermic reaction.
• The release of energy is a result of the formation of strong bonds in the product molecules (HF and O\(_2\)).
• Other halogens, like chlorine, bromine, and iodine, tend to have less exothermic or even non-exothermic reactions with water under similar conditions.

Recognizing exothermic reactions is essential for safely conducting many chemical processes, as the heat released can influence the physical state of the reactants or products.

Properties of Halogens

Each halogen possesses unique properties, yet they share several similar traits due to their placement in the periodic table. Some common properties include high electronegativity, the ability to form salts when reacting with metals, and participation in various oxidation-reduction reactions.

Specific properties differ between the halogens:

• Fluorine (F): This is the most reactive of all halogens. It is a pale yellow gas at room temperature and is the most electronegative and reactive element known.
• Chlorine (Cl): It is a greenish-yellow gas. Due to its strong bleaching properties, it is widely used for disinfecting purposes.
• Bromine (Br): This is a dark, reddish-brown liquid at room temperature. It easily vaporizes, forming a reddish vapor.
• Iodine (I): It appears as a dark, shiny solid. Iodine is less reactive, but crucial for biological functions, particularly in the human thyroid gland.

Halogens can be found in nature most commonly as compounds, rather than in their elemental forms, due to their high reactivity. Understanding these properties helps predict how they might behave in different chemical environments.