Question

Which of the following is not homogeneous equilibrium? (1) \(\mathrm{PCl}_{5}(\mathrm{~g}) \rightleftharpoons \mathrm{PCl}_{3}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g})\) (2) \(\mathrm{C}(\mathrm{s})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{CO}_{2}(\mathrm{~g})\) (3) \(\mathrm{H}_{2}(\mathrm{~g})+\mathrm{I}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{HI}(\mathrm{g})\) (4) \(2 \mathrm{H}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{g})\)

Short answer

Reaction 2 is not homogeneous equilibrium.

Step-by-step solution

- Understand Homogeneous Equilibrium

Homogeneous equilibrium occurs when all the reactants and products in a chemical reaction are in the same phase. This can be all gases, all liquids, or all solutions.

- Identify Phases of Each Reaction

Review the phases of each reaction's reactants and products to see if they are all in the same phase.

- Examine Reaction 1

Equation: \[\begin{equation}\mathrm{PCl}_{5}(\rmm{~g}) \rightleftharpoons \mathrm{PCl}_{3}(\rmm{~g})+ \mathrm{Cl}_{2}(\rmm{~g})\end{equation}\]This reaction consists entirely of gases.

- Examine Reaction 2

Equation: \(\mathrm{C}(\mathrm{s})+ \mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{CO}_{2}(\mathrm{~g}) \)This reaction has both a solid (\(\mathrm{C}\)) and gases (\(\mathrm{O}_{2}\) and \(\mathrm{CO}_{2}\)).

- Examine Reaction 3

Equation: \(\mathrm{H}_{2}(\mathrm{~g})+ \mathrm{I}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{HI}(\mathrm{g}) \)This reaction consists entirely of gases.

- Examine Reaction 4

Equation: \(2 \mathrm{H}_{2}(\mathrm{~g})+ \mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \)This reaction consists entirely of gases.

- Conclusion

Out of the given reactions, Reaction 2 has both a solid and gases. Thus, it is not a homogeneous equilibrium, while the other reactions are homogeneous equilibria.

Key concepts

chemical phases

In chemistry, all materials exist in one of four primary phases: solid, liquid, gas, and plasma. Plasma, however, is less commonly encountered in everyday chemical reactions, so we often focus on solids, liquids, and gases. These phases play a vital role in understanding how substances interact and react with each other. For example, water can be in the form of ice (solid), liquid water, and water vapor (gas), each exhibiting different properties. The phase a substance is in also determines how its molecules interact, influencing reaction rates and equilibrium states.
Phase differences can significantly impact a chemical process. For example, a solid might react differently when it is dissolved in a liquid versus when it is in its solid form. Recognizing and distinguishing these phases is crucial for understanding various chemical behaviors and predicting the outcomes of reactions.

homogeneous vs heterogeneous equilibrium

Chemical equilibria can be broadly classified into two categories: homogeneous and heterogeneous equilibrium.

• In a homogeneous equilibrium, all the reactants and products are in the same phase. For example, a reaction occurring entirely in the gas phase, like \(\text{N}_{2}\text{O}_4(\text{g}) \rightleftharpoons 2 \text{NO}_2(\text{g}) \), is homogeneous.
• In contrast, a heterogeneous equilibrium involves reactants and products in different phases. An example is the reaction \(\text{CaCO}_3(\text{s}) \rightleftharpoons \text{CaO}(\text{s}) + \text{CO}_2(\text{g})\), where solids and gases are present.

The distinction is essential because the phase interaction can affect the equilibrium state and constants. Homogeneous reactions are often simpler to analyze, as the uniform phase allows easier calculation and prediction using established equations. Heterogeneous equilibria require consideration of phase interfaces and the differing behaviors of substances in those phases.

phase identification in reactions

Accurately identifying the phases of reactants and products in a chemical reaction is fundamental to understanding and predicting its behavior. Here is how you can identify and utilize phase information:

• Look at the chemical notation: The phase of each substance is often indicated in parentheses next to the chemical formula, such as (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution.
• Consider the context: Some substances commonly exist in certain phases under typical laboratory conditions. For example, \(\text{H}_2\text{O}\) is a liquid at room temperature, emphasizing the importance of context in phase identification.

Let's take the example reaction \(\text{C(s)} + \text{O}_2(\text{g}) \rightleftharpoons \text{CO}_2(\text{g})\):

• The carbon (\text{C}) is a solid (s).
• Oxygen (\text{O}_2}) is a gas (g).
• Carbon dioxide (\text{CO}_2}) is also a gas (g).

This reaction shows a heterogeneous equilibrium due to the presence of different phases (solid and gas). Correct phase identification aids in understanding the nature of the equilibrium and the balance of the reaction.