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Dynamic equilibrium is a state in which the ratio of products to reactants in a chemical reaction remains constant. The constant ratio of products to reactants does not mean that the reaction is completely static or that reactants do not react to form products, and vice-versa. This type of equilibrium is "dynamic" because there is a constant exchange between reactants and products as the reaction continues to consume reactants to form products and to consume products to form reactants. The ratio between products and reactants does not change because the rate of the reaction that favors products and the rate of the reaction that favors reactants are equal. The equal rates result in no change in the product-to-reactant ratio over time, so the reaction is said to be at equilibrium, or at a steady state.
A reaction must be reversible if it is to reach any form of dynamic equilibrium. A reversible reaction is a chemical reaction in which reactants can react to form products that can react with each other to form the original reactants. Reversible reactions often favor either the reactants or the products, so while it is possible for the reaction to proceed both ways, the reaction tends to produce more of one than the other. This means that the "position" of a dynamic equilibrium may involve a greater quantity of either products or reactants. An "equilibrium constant" is used to describe and to make calculations relating to the position of the equilibrium of a given reaction.
Many different changes in variables relating to a chemical reaction can shift the position of the dynamic equilibrium. Le Châtelier's Principle provides a general way for chemists to think about shifts in dynamic equilibrium. The principle states, generally, that when stress is placed on a reaction at equilibrium, the reaction will shift in the direction that minimizes the change in equilibrium. If one adds more reactant to a reaction at equilibrium, the equilibrium will shift toward products and achieve a new equilibrium based on the new reactant and product concentrations.
Dynamic equilibrium is intrinsically linked to the rates at which chemical reactions occur. One can think of a dynamic equilibrium as two distinct chemical reactions. One occurs in the forward direction, with reactants forming products, while the other occurs in the reverse direction, with the products of the first reaction forming reactants. Eventually, these two reactions settle into an equilibrium based on their equilibrium constants. In the equilibrium, the forward reaction and the reverse reaction occur at the same rate.