Question

What is chemical equilibrium?

Short answer

Chemical equilibrium is the state in a chemical reaction where the concentrations of the reactants and the products are unchanged over time. This is due to the forward and backward reactions happening at the same rate, keeping the concentrations constant. The equilibrium constant expression defines the relationship between the concentrations of the reactant and product species in the chemical reaction.

Step-by-step solution

Define Chemical Equilibrium

Chemical equilibrium is a state in a chemical reaction where the concentrations of the reactants and the products are unchanged over time. It does not necessarily mean the reactants and the products are equal in concentration, but their concentrations do not change anymore.

Dynamic nature of chemical equilibrium

Chemical equilibrium is dynamic. This means that while the overall concentrations of reactants and products do not change once equilibrium is established, the forward and reverse reactions do continue to happen on a molecular level. Both the forward and backward reactions occur at the same rate, ensuring the concentration of the reactants and products remain constant.

The Balanced Reaction and the Equilibrium Expression

For a reversible reaction depicted with the generic equation \(aA + bB \rightleftharpoons cC + dD\), we can write the equilibrium expression as: \[K_{eq} = \frac{[C]^c[D]^d}{[A]^a[B]^b}\] where \(A\), \(B\), \(C\), and \(D\) are the molar concentrations of the species at equilibrium, raised to the power of their stoichiometric coefficients.

Key concepts

Dynamic Equilibrium

In chemical reactions, the term "dynamic equilibrium" is often used to describe a fascinating phenomenon. Initially, it might seem paradoxical because "equilibrium" implies a state of balance. However, the "dynamic" aspect refers to the continuous, yet balanced, shifting of reactant and product molecules during a reaction. This constant motion happens even though there appears to be no overall change. It is not a static condition because molecules don’t stop reacting with each other.
Instead, **both forward and reverse reactions occur simultaneously** at equal rates. This means, for any reaction at equilibrium:

• Reactants form products,
• Products revert back to reactants.

At this point, the concentrations of products and reactants remain unchanged. It is as if the reaction is "pausing" in a state of balance between opposing processes. Understanding dynamic equilibrium helps illustrate that chemical reactions involve a continuous molecular dance, even when it seems like nothing is happening at a macroscopic level.

Equilibrium Expression

In chemistry, the concept of an equilibrium expression is crucial for understanding how reactants and products relate quantitatively in a balanced system. For a general reversible reaction represented by:\[aA + bB \rightleftharpoons cC + dD\]The equilibrium expression involves the **concentrations** of the reactants and products at equilibrium. It is expressed as:\[K_{eq} = \frac{[C]^c[D]^d}{[A]^a[B]^b}\]Here, the square brackets denote the concentrations of the chemical species:

• \([A]\) and \([B]\) are the concentrations of the reactants.
• \([C]\) and \([D]\) are the concentrations of the products.

Each concentration is raised to the power of its stoichiometric coefficient. This constant, \(K_{eq}\), provides valuable insights into the reaction's behavior. A larger \(K_{eq}\) suggests the products are favored at equilibrium, while a smaller \(K_{eq}\) indicates a preference for the reactants. This expression is a key tool, allowing chemists to predict the direction of the reaction shift in response to different stresses.

Reversible Reactions

Reversible reactions are a fundamental concept in understanding chemical equilibrium. These reactions can proceed in both forward and backward directions, signified by the double-headed arrow (\(\rightleftharpoons\)). Unlike irreversible reactions, where the change goes one way until reactants are exhausted, reversible reactions maintain a balance between transformation and reforming.
This dual nature allows:

• The reactants to form products,
• The products to revert back to reactants.

Such reactions will eventually reach a point of equilibrium, wherein neither side is favored unless external conditions change. For instance, altering temperature, pressure, or concentration shifts the position of equilibrium, favoring either the reactants or the products. This adaptability of reversible reactions highlights their critical role in dynamic systems found throughout nature and industry. Understanding how these reactions behave under different conditions provides insight into everything from metabolic pathways in biology to manufacturing processes in chemistry.