Equilibrium Conditions

conditions for equilibrium thione to thiol

Core Concepts

In this article, you’ll learn the conditions for a chemical system at equilibrium, equilibrium constant concentrations, and their applications in kinetics.

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What is Chemical Equilibrium?

Chemical equilibrium refers to a condition where the speed of the forward reaction matches that of the reverse reaction. In other words, there is no overall alteration in the concentrations of the reactants and products. Chemists often call this dynamic equilibrium.

Conditions and Properties of Equilibrium

  1. The system should be isolated, ensuring that no substances can enter or exit.
  2. Equilibrium embodies a dynamic progression. Although the reactions may not be readily observable, both the forward and reverse reactions are occurring.
  3. The rates of the forward and reverse reactions must be equal.
  4. The quantities of reactants and products aren’t required to match. However, once equilibrium is reached, the amounts of reactants and products will remain consistent.

Equilibrium Concentrations and Application of Kinetics

equilibrium conditions graph
Equilibrium is reached at approximately t=2s, since the concentrations of A and B do not change.

The equilibrium constant, K, expresses the relationship between products and reactants of a reaction at equilibrium. It’s also a relationship between the rate constants of the forward reaction (kf) and the reverse reaction (kr).

K = \frac{k_{f}}{k_{r}}

Homogeneous Reactions under Equilibrium Conditions

A homogeneous reaction occurs when all the substances in the reaction, both the ones you begin with and the ones you create, are in the same state, like all being liquids or all being gases. In many cases, the choice of solvent, the liquid used in the reaction, determines the state in which everything ends up.

For example, the process of creating methanol from a blend of carbon monoxide and hydrogen involves a gaseous homogeneous mixture. This mixture encompasses two or more distinct substances that are uniformly distributed at a molecular level.

 CO\text{(g)}+2H_{2}\text{(g)} \rightleftarrows CH_{3}OH\text{(g)}

In a state of equilibrium, the rates at which the forward and reverse reactions occur are balanced, as indicated by the arrows. The associated constant (K), on the other hand, provides insight into the relationship between the quantities of products and reactants in terms of units such as pressure or concentration, specifically when the reaction reaches equilibrium.

An additional illustration of a gaseous homogeneous mixture is found in the process of synthesizing ammonia via the Haber Process:

 N_{2}\text{(g)}+3H_{2}\text{(g)} \rightleftarrows 2NH_{3}\text{(g)}

Heterogeneous Reactions under Equilibrium Conditions

A heterogeneous reaction is characterized by differing states within the reaction itself, where the term “heteros” from Greek signifies “different.”

For example, when lead(II) iodide in solid form interacts to generate an aqueous solution, it results in a heterogeneous mixture involving particles existing in both the solid and aqueous phases:

 PbI_{2}\text{(s)} \rightleftarrows Pb^{+2}\text{(aq)}+2I^{−}\text{(aq)}

The contrast between homogeneous and heterogeneous reactions is highlighted in order to emphasize that solids, pure liquids, and solvents necessitate distinct treatment compared to gases and solutes when estimating the influences of substances within equilibrium constant expressions.

Writing Equilibrium Constant Expressions

The numerical value of an equilibrium constant is acquired by measuring the relative amounts of all reacting substances at equilibrium. In particular, the computation involves determining the ratio of product concentrations to reactant concentrations. Crucially, since the concentrations are gauged when equilibrium is achieved, the equilibrium constant remains constant for a specific reaction, regardless of the initial concentrations. This foundational understanding enabled researchers to formulate a model expression that universally applies as a “template” for any reaction. This fundamental “template” structure of an equilibrium constant expression is explored in the following discussion.

Equilibrium Constant of Concentration

To simplify experimental measurements, the equilibrium constant of concentration involves the molarities of solutes and gases in diluted solutions as reasonable estimates. Yet, when it comes to solids, pure liquids, and solvents, we consequently assign these activities a fixed value of 1 (one). This therefore brings us to the equilibrium constant expression, denoted as Kc:

     \begin{gather*} {HF \text{(aq)}+H_{2}O \text{(l)} \rightleftarrows H_{3}O^{+} \text{(aq)} +F^{−} \text{(aq)}} \\ {K_{c}=\frac{[H_{3}O^{+}][F^{−}]}{[HF](1)}=\frac{[H_{3}O^{+}][F^{−}]}{[HF]}} \end{gather*}

In this context, the letters enclosed within the brackets symbolize the concentration, measured in molarity, of each substance. It’s important to observe that the multiplication of the chemical products, each raised to the power of their corresponding coefficients, constitutes the numerator of the ratio. Similarly, the multiplication of the reactants, raised to the power of their corresponding coefficients, forms the denominator. Importantly, this pattern holds true for all equilibrium constants. When the majority of the participating species are dissolved in water, a ratio of molarities between products and reactants is typically employed. Alternatively, in reactions involving gases, a ratio of concentrations can be used if the volume of the container is known.