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Dynamic Equilibrium: Definition, Function & Examples

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  • 0:00 Dynamic Equilibrium
  • 1:20 Equilibrium Constant…
  • 2:36 Reactions With Liquids…
  • 3:07 Equilibrium Calculations
  • 3:53 Lesson Summary
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Lesson Transcript
Instructor: LaRita Williams

LaRita holds a master's degree and is currently an adjunct professor of Chemistry.

In this lesson, we will discuss dynamic equilibrium and define the equilibrium constant expression, K, for a given reaction. We will also use an example to practice calculating the equilibrium constant.

Dynamic Equilibrium

Have you ever been stuck in traffic on a bridge between two cities and noticed that traffic in the other direction was flowing smoothly while your side was at a standstill? It's a bummer. What if traffic were flowing smoothly on both sides? Better yet, what if traffic were flowing at the same rate in both directions, causing the number of cars in both cities to remain constant? Well, when the rate of cars entering and leaving both cities is the same, and the net number of cars in each city is steady over time, the entire process is in a dynamic equilibrium.

Equilibrium occurs when a reversible reaction, a reaction that proceeds in both directions, has an unchanging ratio of products and reactants. The big picture of chemical equilibrium is static, as the concentration of products and reactants is constant. A closer look, however, reveals that equilibrium is actually a very dynamic process as the reaction is moving in both directions at an equal pace.

Dynamic equilibrium is an example of a steady state function. For a system in a steady state, presently observed behavior continues into the future. Therefore, once a reaction has reached equilibrium, the ratio of product and reactant concentrations will remain the same even as the reaction continues.

The Equilibrium Constant Expression

We can express the ratio of product and reactant concentrations at equilibrium for any reversible reaction. The value of this ratio is represented by the capital letter K, the equilibrium constant.

This is the equilibrium constant expression where reactants A and B yield products C and D.

Equilibrium Equation

Equilibrium Constant Expression

In the equilibrium constant expression, product concentrations (capital C and D) are raised to an exponent based on their coefficients from the balanced chemical equation (lower case c and d) and divided by the reactant concentrations (A and B), which are also raised to an exponent based on their coefficients from the balanced chemical equation (lower case a and b).

Let's look at an example of an equilibrium constant expression. Here is the reversible reaction between hydrogen gas and nitrogen gas to form ammonia gas.

Equation 2

Equilibrium Constant Expression Example

Here, the concentration of ammonia product (NHsub3) is raised to the exponent 2, its coefficient from the balanced chemical equation. It is then divided by reactants hydrogen gas (Hsub2) and nitrogen gas (Nsub2), which are also raised to exponents based on their coefficients from the balanced equation (3 and 1, respectively).

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