An equilibrium is a state of a system where the system is balanced. A system that is in equilibrium does not change. The word has been used for different concepts from different fields of study.
Chemical equilibrium is an idea which describes the behavior of chemical reactions over time. In reactions that have finished reacting and have attained equlibrium, the concentrations of the reactant and product chemicals do not change. Instead, the amount of product chemicals to reactant chemicals remains a fixed value; this ratio is called the equilibrium constant (K). For some reactions, the equilibrium constant is very large, in which case nearly all of the reactant chemical is turned into products, such as when paper is burned. For other reactions, it is very small, which results in very little product being formed. If the equilibrium constant is close to 1, a mixture of both products and reactants is expected at equilibrium. When chemists talk about equilibrium constants, they call reactions with large K values favorable and reactions with small K values unfavorable. It is important to note, however, these terms are not judgements about the value or usefulness of a particular chemical reaction. In fact, some very important reactions are actually "unfavorable"; one such example is the synthesis of ammonia from nitrogen and hydrogen gas (Haber process), which has a rather small equilibrium constant.
Understanding the concept of chemical equilibrium in general and finding the equilibrium constant for specific reactions in particular is very important to chemists and other scientists. Discovering the equilibrium constant for a reaction is very helpful for understanding that reaction. Knowing the equilibrium constant of a reaction is important because it allows you to calculate how much product will ultimately be formed during a reaction. Moreover, it also tells you how a particular mixture of chemicals will react. This is because chemical reactions always occur in the direction which will make the ratio of their products to reactants equal to the equilibrium constant. This is known as Le Chattelier's Principal. The exception to this rule is when an outside energy source forces the reaction to occur in the opposite direction, such as when a battery is recharged. Knowing the equilibrium constant of a reaction also helps you to understand the flow of energy that occurs during the reaction (thermodynamics).
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