Problem 15 - Chemistry An Atoms First Approach

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.

Step-by-Step Solution

Answer

The mixture of hydrogen and chlorine gas does not react until exposed to ultraviolet light because the activation energy needed for the exothermic reaction is not available. Ultraviolet light provides the necessary high-energy to overcome the activation energy barrier by breaking the H-H and Cl-Cl bonds, allowing the rapid formation of hydrogen chloride (HCl) gas, as described by the reaction: \[ \mathrm{H}_{2}(g)+\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{HCl}(g) \]

Step 1: Understanding Activation Energy

During a chemical reaction, the reactants must overcome a certain threshold energy, called the activation energy (Ea), which is the minimum energy required for the reaction to proceed. Activation energy depends on the chemical species involved in the reaction and their orientation relative to one other.

Step 2: Exothermic Reaction

The reaction of hydrogen gas and chlorine gas to form hydrogen chloride gas is exothermic, which means it releases energy as the reaction progresses. In this case, the reactants (H2 and Cl2) have higher energy than the products (HCl) and need to overcome the activation energy barrier.

Step 3: Involvement of Ultraviolet Light

Ultraviolet light is a form of electromagnetic radiation with high energy. When the hydrogen and chlorine gas mixture is exposed to ultraviolet light from a burning magnesium strip, the energy from the light is absorbed by the mixture. The increased energy provides the necessary activation energy for breaking the H-H and Cl-Cl bonds, allowing the reaction to occur rapidly.

Step 4: Formation of Hydrogen Chloride Gas

Once the H-H and Cl-Cl bonds are broken, the free hydrogen and chlorine atoms can now bond with each other, forming hydrogen chloride (HCl) gas. The overall reaction is: \[ \mathrm{H}_{2}(g)+\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{HCl}(g) \] #Conclusion#: The mixture of hydrogen and chlorine gas remains unreacted until exposed to ultraviolet light because the activation energy needed for the reaction to occur is only provided by the high-energy ultraviolet light. Once the activation energy is supplied, the reaction rapidly takes place, producing hydrogen chloride gas.

Key Concepts

Exothermic Reaction

An exothermic reaction is an engaging and fundamental concept in chemical reactions where energy, primarily in the form of heat, is released into the surrounding environment. It's rather thrilling to imagine that substances when combined, can produce heat as if they're giving a warm embrace to the world around them! Consider the example of hydrogen gas (H2) reacting with chlorine gas (Cl2) to form hydrogen chloride (HCl). This reaction is much like a spirited dance of molecules where, after all the movement and commotion, the dancers (molecules) end up feeling more relaxed and settled (lower energy state).

Key aspects of exothermic reactions that make them noteworthy include the fact that the reactants begin with a higher potential energy level. As they transform into products, they descend into a lower energy level, releasing energy in a triumphant cheer. This released energy often manifests as heat, which can be felt or measured. It's a common exhibit of nature's tendency to prefer lower energy states, a bit like choosing to relax on a comfortable sofa after a long day!

In the remarkableness of this reaction between hydrogen and chlorine gases to form HCl, there's an initial obstacle they need to overcome - the activation energy. This is much like needing a tiny nudge to start a boulder rolling downhill. Once they roll past this energy hill, the reaction cruises forward, releasing energy much like the boulder would rumble down the hill - unstoppable and releasing energy until it comes to a rest.

Chemical Kinetics

Chemical kinetics is an exciting field that deals with the speed or rate at which a chemical reaction proceeds. It's akin to being a timekeeper at a spectacular race between molecules, seeking to understand how quickly these tiny contestants can reach the finish line to form new substances. In our analogy, the reaction between hydrogen gas and chlorine gas to form hydrogen chloride is a sprint, depending on certain conditions. Before the ultraviolet light's 'starting gun,' the two gases are merely loitering at the starting blocks, possessing the potential to react but lacking any urgency to do so.

The pace of a chemical reaction can be influenced by various factors, such as the concentration of reactants, the temperature of the system, the presence of a catalyst, and the physical state of the reactants. This race can be incredibly swift or frustratingly slow, influenced by the path the molecules take, their 'training' (activation energy), and external 'coaching' (catalysts).

In the context of our reaction, ultraviolet light plays the role of a catalyst, reducing the activation energy and consequently speeding up the reaction. This is much like giving our molecular sprinters a pair of supercharged sneakers that allow them to run faster than they ever could barefoot. The brilliance of chemical kinetics is in understanding and controlling these factors to optimize reaction rates, which is critical in industries ranging from pharmaceuticals to energy.

Role of Ultraviolet Light in Chemical Reactions

Ultraviolet light is more than just a component of sunlight responsible for tans and sunburns – it's a powerful actor in the theater of chemical reactions. This electromagnetic radiation has higher energy than visible light, which has fascinating implications in chemistry. When it comes to chemical reactions, think of ultraviolet light as a maestro, orchestrating reactions that otherwise would not budge due to the lack of sufficient energy.

In our example of hydrogen and chlorine gases, ultraviolet light is like the life of the party, providing energy that excites the molecules, giving them the vitality needed to dance together to form hydrogen chloride. Without this radiant input, the molecules are content to exist side by side without engaging with one another, a formal affair with no interactions. With ultraviolet light, however, the reaction accelerates as if the molecules are eager to converse and bond over shared energy.

The kick-start provided by ultraviolet light is crucial for reactions requiring a large supply of energy, especially those with high activation energies. By absorbing ultraviolet light, the molecules gain enough energy to initiate the reaction, which could involve breaking existing bonds and forming new ones. This revelation of light's influence extends beyond textbooks, playing a pivotal role in processes such as photosynthesis in nature, and photochemical reactions used in manufacturing various goods, making ultraviolet light an indispensable character in the narrative of chemistry.