Question

Consider the following reaction to form the brown gas \(\mathrm{NO}_{2}\) : $$ 2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g) $$ What observation would indicate that the reaction has reached a state of equilibrium?

Short answer

The reaction reaches a state of equilibrium when the brown color of the gas remains constant. That is the indication that both the forward and backward reaction rates are equal, and the concentrations of NO, O_2 and NO_2 are no longer changing.

Step-by-step solution

Understand Chemical Equilibrium

The concept of chemical equilibrium is based on the principle that in a closed system, a chemical reaction will proceed until a state of balance is achieved. This state of balance, referred to as equilibrium, is characterized by the concentrations of all reactants and products remaining constant over time.

Consider the Given Reaction and Changes in Observation

In the given reaction, the brown gas NO_2 is formed from NO and O_2. At the start of the reaction, NO and O_2 will react and the concentration of NO_2 will increase which will be visible due to browning of the gas. As the concentrations of reactants decrease and the concentration of NO_2 increases, the backward reaction will also start to occur. Over time, the rate of the forward and backward reactions will become equal.

Determine the Sign of Equilibrium

The equilibrium will be indicated by the constant brown color. Since the concentration of reactants and products will remain constant, the color of the gas mixture will not change. Due to the formation of NO from NO_2 in the backward reaction, the deepness of the brown color will not increase as it does in the initial stage. This constant color indicates that the system has reached equilibrium.

Key concepts

Chemical Reactions

Understanding chemical reactions is essential for studying chemistry. These reactions involve the transformation of substances through breaking and forming chemical bonds. For instance, when nitrogen monoxide (NO) and oxygen (O2) react to form nitrogen dioxide (NO2), a new substance with different properties emerges. This process, represented by the balanced equation

\(2 NO(g) + O_2(g) \longrightarrow 2 NO_2(g)\)

, shows the stoichiometry of the reaction, indicating that two molecules of NO react with one molecule of O2 to produce two molecules of NO2. The brown color of NO2 in this reaction is a visual cue of its formation. The understanding of chemical reactions extends beyond simple observation—it involves grasping the stoichiometric relationships, energy changes, and rate at which reactions proceed. These are fundamental aspects that dictate whether a reaction is favorable and how it can be manipulated for various applications.

Reaction Rates

The rate of a chemical reaction is an expression of how quickly reactants are converted into products. In the example of the formation of NO2, the reaction starts with a certain speed, visually identifiable by the change in color as the gaseous mixture turns brown. However, it's crucial to understand that reactions can slow down as the concentration of reactants decreases. Moreover, not all reactions proceed in one direction; some revert to form original reactants, known as the reverse reaction.

When it comes to reaction rates, several factors play a role, such as temperature, concentration of reactants, surface area, and the presence of catalysts. Altering these conditions can accelerate or decelerate the reaction. For a balanced perspective, it's also important to acknowledge that external conditions and reaction mechanisms greatly influence how a reaction progresses over time.

Equilibrium Constant

At the heart of chemical equilibrium lies the equilibrium constant, denoted as 'K'. The equilibrium constant is a number that quantifies the ratio of the concentration of products to the concentration of reactants at equilibrium, each raised to the power of their coefficients in the balanced equation. For our reaction

\(K = \frac{[NO_2]^2}{[NO]^2[O_2]}\)

, where the concentrations are those at equilibrium. The magnitude of K gives insight into the position of equilibrium; a high value suggests a product-favored reaction, while a low value indicates that reactants are favored. However, 'K' in itself does not indicate the speed at which equilibrium is reached. This distinction is important for students who might confuse the equilibrium constant with the rate of the reaction. It's also worth noting that 'K' is constant only for a given temperature, and calculating its value can help predict the direction of the reaction when the system is disturbed, referred to as Le Chatelier's principle. Understanding this constant is crucial for predicting the extent of reactions and in the design of chemical processes.