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Equilibrium: Chemical and Dynamic

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  • 0:05 Chemical Equilibrium
  • 1:36 Equilibrium Constant
  • 3:44 Examples
  • 5:36 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn the definition of chemical equilibrium and how it is dynamic. Discover what the equilibrium constant is and how it shows whether the reaction favors the reactants or products. Learn how chemists designate equilibrium in an equation and how they show the difference in reaction rate.

Chemical Equilibrium

Chemical equilibrium is when the rate of a forward reaction equals the rate of the reverse reaction and the concentrations of the products and reactants remain unchanged. Equilibrium is a dynamic state, meaning that things are always moving. Products are being broken down into reactants, and reactants are being combined into products. Things are moving, but the concentrations stay the same. When the reaction is written, it is written with a double arrow instead of an equal sign to show that the reaction is reversible. A + B go to C + D.

In some reactions, the forward reaction is almost completed before the reverse reaction starts. In this case, there's a higher concentration of products than reactants, but the reaction can still be in equilibrium because the concentrations of both the reactants and products stay the same. The reaction equilibrium lies to the right because there are more products than reactants. In this case, the reaction is written with two different-length arrows, with the longer arrow pointing to the right, showing that more product is made than reactant. A + B goes to C + D.

The opposite is also true. The forward reaction of making products has barely started, and the reverse reaction is already going like gangbusters. In this case, the equilibrium of the reaction is said to lie to the left and the longer arrow points left. A + B goes to C + D.

Equilibrium Constant

The rate of reactions are often shown in a graph like this one. This graph compares the rate of the forward reaction to the rate of the reverse reaction. To start, the forward reaction has the maximum rate possible, and the reverse reaction has no rate because it hasn't started yet. As the reaction is under way, the forward reaction decreases as the reactants are used up, and the reverse reaction increases as there is more product to turn back into reactants. Eventually, equilibrium is reached, and the graph turns into one straight horizontal line.

Once equilibrium is reached, the concentrations of the reactants and products don't change. When this happens, an equilibrium constant K can be written for the reaction. Only the substances whose concentrations change are included in the equilibrium constant equation. If the reaction equation is nA + mB goes to xC + yD, then the equilibrium constant equation can be written as:


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