Question

Chlorine gas is exposed to ultraviolet light in a closed container. The reactive species expected to be formed are (a) \(\mathrm{Cl}\) (b) \(\mathrm{Cl}^{+}\) (c) \(\mathrm{Cl}^{-}\) (d) \(\mathrm{Cl}\)

Short answer

The reactive species formed is \( \mathrm{Cl} \).

Step-by-step solution

Understand the Context

The problem involves the exposure of chlorine gas to ultraviolet (UV) light in a closed container. We need to identify the reactive species that are formed as a result of this exposure.

Recall Key Chemistry Concepts

Chlorine gas (\( \mathrm{Cl_2} \)) can dissociate under UV light. UV light provides enough energy to break the bond in the \( \mathrm{Cl_2} \) molecule, creating reactive chlorine atoms.

Analyze the Dissociation Process

When \( \mathrm{Cl_2} \) is exposed to UV light, the energy absorbed can break the covalent bond between chlorine atoms, resulting in the formation of two chlorine radicals: \( \mathrm{Cl_2} \xrightarrow{UV} 2\mathrm{Cl}. \) The chlorine radical (\( \mathrm{Cl} \)) is a reactive species due to its unpaired electron.

Eliminate Other Options

Options \( \mathrm{Cl}^+ \) and \( \mathrm{Cl}^- \) involve the creation of ions, which are less likely in this dissociation process without any electron transfer medium or additional chemical reactions.

Confirm the Answer

The correct answer is \( \mathrm{Cl} \) radicals. After breaking down the options, it's clear that the reactive species formed are chlorine atoms rather than ions.

Key concepts

Ultraviolet Light

Ultraviolet (UV) light is a type of electromagnetic radiation that is not visible to the human eye. It lies between visible light and X-rays in the electromagnetic spectrum. UV light is known for having enough energy to cause significant chemical changes.
It is often responsible for initiating a variety of chemical reactions by providing sufficient energy to break chemical bonds.
When chlorine gas (\( \mathrm{Cl_2} \)) is exposed to UV light, it absorbs this energy, which can be enough to break the bond holding the two chlorine atoms together. This process is known as photodissociation, where light energy leads to the dissociation of molecules.

• This energy absorption is crucial for many reactions occurring in the atmosphere.
• Due to its high energy, UV light can break the covalent bonds in molecules, such as the chlorine-chlorine bond in chlorine gas.
• The specific bond dissociation energy must be overcome for this reaction to occur, demonstrating UV light's potency.

Chlorine Radicals

Chlorine radicals (\( \mathrm{Cl} \)) are highly reactive species formed under specific conditions. A radical is generally an atom, molecule, or ion that has an unpaired electron, which makes it very reactive due to its instability.
When \( \mathrm{Cl_2} \) gas is exposed to UV light, the photon energy absorbed is enough to break the covalent bond between the two chlorine atoms. As a result, two chlorine radicals are formed.

• This process can be represented by the equation: \( \mathrm{Cl_2} \xrightarrow{UV} 2\mathrm{Cl} \)
• Chlorine radicals tend to react quickly with other substances or further react to form stable compounds.
• They are instrumental in the chemistry of many environmental systems, particularly in the stratosphere, where they participate in the breakdown of ozone.

Bond Dissociation Energy

Bond dissociation energy is a key concept in understanding the stability and reactivity of molecules. It is defined as the energy required to break a bond in a molecule and separate the elements in their gaseous state.
The chlorine-chlorine bond in \( \mathrm{Cl_2} \) has a specific bond dissociation energy, which must be provided to break this bond. UV light provides precisely the amount of energy needed.

• The measurement of this energy plays a crucial role in predicting whether a molecule will undergo a reaction under certain conditions.
• In the case of chlorine gas, knowing the bond dissociation energy helps explain why exposure to UV light results in the formation of two chlorine radicals.
• This concept is critical in various scientific fields, including chemistry, physics, and materials science.

Understanding bond dissociation energy helps us predict and control chemical reactions, especially those involving radical formation.