Problem 80 - 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

At equilibrium, all species' concentrations remain constant with continuous, dynamic changes at the molecular level.

Step 1: Understanding Chemical Equilibrium

In a chemical reaction at equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. For the decomposition of HI into H2 and I2, the equilibrium state implies that the conversion of HI into hydrogen and iodine is balanced by the recombination of hydrogen and iodine back to HI.

Step 2: Reaction at Equilibrium

The reaction is \( 2\mathrm{HI}(\mathrm{g}) \rightleftharpoons \mathrm{H}_{2}(\mathrm{g}) + \mathrm{I}_{2}(\mathrm{g}) \). At equilibrium, the concentrations of HI, H2, and I2 remain constant over time, although the individual molecules continuously react to form products and then recompose to form reactants.

Step 3: Molecular Activity at Equilibrium

At the molecular level, even though the macroscopic properties (such as concentration) of the system appear constant, dynamic changes are still happening. Molecules of \(\mathrm{HI}\) are continuously decomposing into \(\mathrm{H}_{2}\) and \(\mathrm{I}_{2}\), while simultaneously, \(\mathrm{H}_{2}\) and \(\mathrm{I}_{2}\) molecules are recombining to form \(\mathrm{HI}\).

Key Concepts

Decomposition Reaction

A decomposition reaction is a type of chemical reaction in which a single compound breaks down into two or more simpler substances. In the reaction given, hydrogen iodide (HI) decomposes upon heating to form hydrogen gas (H₂) and iodine gas (I₂).
This can be represented as follows:

• The general form of a decomposition reaction: \[ AB \rightarrow A + B \]
• For our specific example: \[ 2 ext{HI(g)} \rightarrow ext{H}_{2} ext{(g)} + ext{I}_{2} ext{(g)} \]

In this reaction, HI acts as the single reactant, which splits into two different products. Decomposition reactions are often driven by the input of energy, such as heat.

Molecular Level Explanation

At the molecular level, a chemical reaction process involves individual molecules undergoing changes to form new substances. In the context of the decomposition of HI:

• The molecules of hydrogen iodide (HI) absorb energy, which promotes their decomposition.
• The bonds between hydrogen (H) and iodine (I) atoms are broken.
• This leads to the formation of H₂ and I₂ as the atom rearrange into diatomic forms.

Even when the reaction reaches equilibrium, molecular collisions and reactions continue. Molecules of H₂ and I₂ continue to form from HI molecules, and such activities remain consistent on a microscopic scale.

Dynamic Equilibrium

Dynamic equilibrium refers to a condition in chemical reactions where the forward and reverse reaction rates are equal. Even though the overall concentrations of the substances do not change with time, on the molecular level, reactions are continually occurring.
In the HI decomposition reaction:

• The forward reaction involves the break-up of HI molecules into H₂ and I₂.
• The reverse reaction sees H₂ and I₂ molecules recombine to form HI.

At equilibrium, both processes occur at the same rate, resulting in no net change in the concentration of reactants and products. This ongoing process at the molecular level is why equilibrium is considered "dynamic."

Reaction Rates

Reaction rates refer to the speed at which reactants turn into products. Understanding the rate of decomposition of HI requires considering both the forward and reverse reactions.

• Initially, the concentration of HI is higher, so the decomposition rate is high.
• As products (H₂ and I₂) build up, the reverse reaction's rate increases.
• Equilibrium is achieved when these rates balance out, resulting in a steady state.

Reaction rates are influenced by several factors, including temperature, concentration, and the presence of a catalyst. In this reaction, heat acts as a principal factor, providing the necessary energy to promote the decomposition process.