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Chemical Equilibrium

Chemical Equilibrium in Chemical Reactions

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At equilibrium the ratio between the reactants and the products remains unchanged.

When a reaction is at equilibrium the ratio between the reactants and the products remains unchanged.

Nick Koudis/Getty Images
Learn about the basics of chemical equilibrium, including how to write the expression for chemical equilibrium.

What Is Chemical Equilibrium?

Chemical equilibrium is the condition which occurs when the concentration of reactants and products participating in a chemical reaction exhibit no net change over time. Chemical equilibrium may also be called a "steady state reaction." This does not mean the chemical reaction has necessarily stopped occurring, but that the consumption and formation of substances has reached a balanced condition. The quantities of reactants and products have achieved a constant ratio, but they are almost never equal. There may be much more product or much more reactant.

Dynamic Equilibrium

Dynamic equilibrium occurs when the chemical reaction continues to proceed, but the amount of products and reactants remain constant. This is one type of chemical equilibrium.

Writing the Equilibrium Expression

The equilibrium expression for a chemical reaction may be expressed in terms of the concentration of the products and reactants. Only chemical species in the aqueous and gaseous phases are included in the equilibrium expression because the concentrations of liquids and solids does not change. For the chemical reaction:

jA + kB → lC + mD

The equilibrium expression is

K = ([C]l[D]m) / ([A]j[B]k)

K is the equilibrium constant
[A], [B], [C], [D] etc. are the molar concentrations of A, B, C, D etc.
j, k, l, m etc. are coefficients in a balanced chemical equation

More about the equilibrium constant...

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