Question

A mixture of hydrogen gas and chlorine gas remains unreacted until it is exposed to ultraviolet light from a burning magnesium strip. Then the following reaction occurs very rapidly: $$\mathrm{H}_{2}(g)+\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{HCl}(g)$$ Explain.

Short answer

The reaction between hydrogen gas (H₂) and chlorine gas (Cl₂) remains unreacted until exposed to ultraviolet (UV) light because the reaction requires activation energy to break the H-H and Cl-Cl bonds in the reactants. UV light carries energy, which provides the activation energy needed for the reaction to proceed rapidly. When the mixture is exposed to the UV light from a burning magnesium strip, the energy absorbed by the gases breaks the bonds in the hydrogen and chlorine molecules, allowing them to rapidly react to form hydrogen chloride gas (HCl).

Step-by-step solution

Understanding the reaction equation

The given reaction equation is: \[\mathrm{H}_2(g) + \mathrm{Cl}_2(g) \longrightarrow 2\ \mathrm{HCl}(g)\] This equation tells us that one molecule of hydrogen gas (H₂) reacts with one molecule of chlorine gas (Cl₂) to produce two molecules of hydrogen chloride (HCl) gas.

Understanding activation energy

Activation energy is the minimum amount of energy required for a chemical reaction to occur. It is the energy barrier that the reactants must overcome for the reaction to proceed. In this case, the activation energy is the energy needed to break the H-H and Cl-Cl bonds in the hydrogen and chlorine molecules.

The role of ultraviolet light

Ultraviolet (UV) light is an electromagnetic wave that carries energy. When the hydrogen and chlorine gases are exposed to UV light, the energy carried by the UV photons is absorbed by the gases' molecules. This energy provides the activation energy needed to break the bonds between H-H and Cl-Cl, allowing the reaction to proceed rapidly.

Burning magnesium strip as a UV light source

A burning magnesium strip emits ultraviolet light. When the mixture of hydrogen and chlorine gases is exposed to this UV light source, the energy carried by the UV photons initiates the reaction, breaking the bonds in the hydrogen and chlorine molecules and allowing them to react rapidly to form hydrogen chloride gas.

Conclusion

In summary, the reaction between hydrogen gas and chlorine gas remains unreacted until exposed to ultraviolet light because the reaction requires an activation energy to break the H-H and Cl-Cl bonds in the reactants. The ultraviolet light from a burning magnesium strip provides this activation energy, enabling the reaction to occur rapidly and form hydrogen chloride gas as the product.

Key concepts

Activation Energy

Understanding the concept of activation energy is central to comprehending why certain chemical reactions require a start-up boost. In the simplest terms, activation energy is the initial push needed to get a chemical reaction going. Imagine a boulder perched atop a hill; it has the potential to roll down, but it needs a nudge to overcome the resistance of being at a standstill. Similarly, chemical reactants often require an input of energy to commence a reaction.

Picture the molecules of hydrogen and chlorine. They are stable in their current form, and to react to form hydrogen chloride (HCl), the bonds within the hydrogen (H-H) and chlorine (Cl-Cl) molecules must be broken. This bond breaking is not spontaneous; it necessitates a boost of energy. Activation energy, thus, serves as that crucial nudge that enables the reactants to get to a state where a reaction can occur by allowing them to overcome the energy barrier that stands in the way.

Ultraviolet Light in Chemical Reactions

When we delve into the role of ultraviolet (UV) light in chemical reactions, we tap into an intriguing aspect of chemistry where light doesn't just illuminate; it transforms. UV light is a form of electromagnetic radiation that carries enough energy to affect the bonds within molecules. It falls in the spectrum beyond the violet end of visible light, hence the name 'ultraviolet'.

As for its role in chemical reactions, ultraviolet light can supply the necessary activation energy to the reactants. It works like a very precise pair of scissors, cutting the bonds within molecules at the right wavelength. When the mixture of hydrogen and chlorine gases is exposed to UV light, the photons, which are particles of light, have just the right amount of energy to snap the H-H and Cl-Cl bonds. This energy instigation prompts the atoms to recombine into HCl molecules. In essence, the UV light is the proverbial finger that flicks the boulder, providing the initial push (activation energy) for the chemical reaction.

Hydrogen and Chlorine Reaction

The specific reaction between hydrogen gas (H₂) and chlorine gas (Cl₂) that produces hydrogen chloride (HCl) is a vivid demonstration of chemical synergy at work. The equation \(H_2(g) + Cl_2(g) \rightarrow 2 HCl(g)\) showcases a simple yet profound transformation.

Under normal conditions, without any external energy source, this reaction mixture is like an unlit fuse—full of potential but inactive. This is where the activation energy and UV light come into the picture. A burning magnesium strip used in the experiment emits UV light, which is absorbed by H₂ and Cl₂ molecules, breaking them apart and spurring them to reorganize into HCl. It's a dynamic chemical dance where the initial stillness is broken by the UV light's energetic rhythm, leading to a flurry of activity that results in the formation of new bonds and the release of energy in the form of heat. In a classroom or a laboratory, witnessing this reaction is a powerful reminder of how the invisible hand of energy can orchestrate the movement of atoms to create something entirely new.