Problem 88 - Chemistry and Chemical Reactivity

Question

Heating \(\mathrm{HI}(\mathrm{g})\) at \(425^{\circ} \mathrm{C}\) causes some of this compound to decompose, forming \(\mathrm{H}_{2}(\mathrm{g})\) and \(\mathrm{I}_{2}(\mathrm{g})\) Eventually, the amounts of the three species do not change further; the system has reached equilibrium. (At this point, approximately \(22 \%\) of the HI has decomposed.) Describe what is happening in this system at the molecular level.

Step-by-Step Solution

Answer

The reaction is dynamic but balanced at equilibrium, where 22% of HI has decomposed into H₂ and I₂, and concentrations remain stable.

Step 1: Understanding the Decomposition Reaction

The reaction is represented as \( 2 \text{HI} \rightleftharpoons \text{H}_2 + \text{I}_2 \). This means that two molecules of hydrogen iodide (HI) decompose into one molecule of hydrogen gas (H₂) and one molecule of iodine gas (I₂).

Step 2: Initial State of Reaction

Initially, the system contains only molecules of hydrogen iodide (HI), no molecules of hydrogen (H₂) or iodine (I₂). With time and upon heating to \(425^{\circ} \text{C}\), HI molecules start to decompose, forming H₂ and I₂ molecules.

Step 3: Equilibrium State

The system reaches equilibrium when the rate of formation of HI from H₂ and I₂ equals the rate of decomposition of HI to H₂ and I₂. This balance means the concentrations of HI, H₂, and I₂ remain constant over time despite the ongoing forward and reverse reactions.

Step 4: Extent of Reaction at Equilibrium

It is given that at equilibrium, approximately 22% of HI has decomposed. This means that only 78% of the original HI remains in the system. The rest has reacted to form equivalent amounts of H₂ and I₂.

Step 5: Molecular Level Description

At the molecular level, the system is dynamic. Even at equilibrium, HI molecules continue to decompose to become H₂ and I₂, and simultaneously, H₂ and I₂ react to form HI. However, these rates are equal, making the macroscopic concentrations stable over time.

Key Concepts

Decomposition Reaction

In the context of chemical reactions, a decomposition reaction is one where a single compound breaks down into two or more simpler substances. In our exercise, the decomposition reaction can be represented by:

• \( 2 \text{HI} \rightarrow \text{H}_2 + \text{I}_2 \)

This means two molecules of hydrogen iodide (HI) split to produce one molecule of hydrogen gas (H₂) and one molecule of iodine gas (I₂).
This reaction usually requires an external energy source like heat to occur, which is why heating to \( 425^{\circ} \text{C} \) initiates the decomposition.
The decomposition process is a common way in which complex molecules are broken down into simpler ones, and it's crucial in understanding how chemical changes occur at the molecular level.

Le Chatelier's Principle

Le Chatelier's principle is a key concept in chemistry that helps us predict how a reaction at equilibrium responds to changes in concentration, temperature, or pressure. It states that if a system at equilibrium experiences a change, it will adjust to counteract that change and re-establish equilibrium.

• If the concentration of a substance is altered, the system shifts to restore balance.
• Increasing temperature typically increases the rate of endothermic reactions, encouraging decomposition in our example.
• Volume and pressure changes can also shift the balance in gaseous reactions.

In the HI decomposition, heating the reaction shifts the equilibrium towards the formation of H₂ and I₂, as the reaction absorbs heat. Understanding this principle is essential for manipulating conditions to favor desired products in chemical processes.

Dynamic Equilibrium

Dynamic equilibrium occurs in a closed system when the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant, although both reactions are still occurring at the molecular level.
For our reaction:

• \( 2 \text{HI} \rightleftharpoons \text{H}_2 + \text{I}_2 \)

Even though approximately 22% of HI decomposes, there is continuous formation and decomposition of HI molecules.
This state of balance means no net change in the system, highlighting the difference between dynamic and static equilibrium.
In dynamic equilibrium, reactants and products are constantly interchanging, maintaining the concentration ratios despite ongoing reactions.

Reaction Rates

Reaction rates measure how quickly reactants convert into products in a chemical reaction. These rates are influenced by several factors, such as temperature, concentration, and the presence of catalysts.

• Temperature: Increasing it generally speeds up reactions, providing energy for bonds to break.
• Concentration: A higher concentration of reactants typically boosts the rate, as more molecules collide with each other.
• Catalysts: These substances increase the rate without being used up in the reaction.

In the decomposition of HI:

• The heated environment at \( 425^{\circ} \text{C} \) increases the reaction rate by providing energy needed for bond breaking.
• Once equilibrium is reached, the reaction rates of the forward and reverse reactions are equal, as seen in dynamic equilibrium.

Understanding reaction rates is vital for controlling how fast or slow a reaction occurs, particularly in industrial and laboratory settings.