What Are Equilibrium Constants?

Under specific conditions (such as temperature, pressure, solvent properties, ionic strength, etc.), when the reversible chemical reaction reaches an equilibrium state, the concentration of the product and the reactant (the power of the equation coefficient) product ratio or the concentration of the reaction product and the substrate (Equation Coefficient Power) Product ratio. It is represented by the symbol "K". In terms of thermodynamics, all reactions have reverse reactions, that is to say, all reactions have thermodynamic equilibrium and have equilibrium constants. The larger the equilibrium constant, the more thorough the reaction.

For a reversible reaction, at a certain temperature, regardless of the initial concentration of the reactants, after the reaction reaches an equilibrium state, the ratio of the reactant to the product concentration coefficient power is a constant, called the chemical equilibrium constant, expressed by K .

For the following reactions:

, Then:

, Where c is the equilibrium concentration of each component, the temperature is constant, and K is a fixed value.

(1) K is only related to temperature, so the temperature should be noted when using;

(2) In the expression of K value, do not write solid and liquid water;

(3) The expression of the equilibrium constant is related to the way the equation is written;

(4) The equilibrium constant indicates the degree of reaction, but does not indicate the speed of the reaction, that is, the rate is large, and the value of K is not necessarily large;

(5) Use the value of K to determine whether a state is in equilibrium: When it is not balanced,

. If K '= K, the reaction is in equilibrium, v _positive = v _inverse ; if K' <K, the reaction proceeds in the positive direction, v _positive > v _inverse ; if K '> K, the reaction proceeds in the reverse direction, v _positive <v _Inverse .

The balance constant is a sign of the balance. The reaction is generally considered to be complete when K> 10 ^{5 and} incomplete. The numerical value of the equilibrium constant can judge the degree of the reaction and estimate the possibility of the reaction. Because the equilibrium state is the maximum of the reaction.

The value of the equilibrium constant value can only tell us roughly the maximum degree of the forward reaction of a reversible reaction, and it does not predict the time required for the reaction to reach equilibrium. In addition, a reaction with a very small equilibrium constant value indicates that a positive reaction cannot proceed under these conditions.

(1) See if it can be concluded that v _positive = v _inverse : the same substance: the generation rate of the substance is equal to its consumption rate; different substances: the positive reaction rate of one substance and the reverse reaction rate of another substance The ratio is equal to the ratio of the number of measurements in the equation; analysis of the bond angle: the amount of the substance that breaks with a unit of chemical bond per unit time is equal.

(2) See if it can be concluded that the content of each component in the reaction mixture remains unchanged: The content of each component (concentration, amount of substance, volume fraction, etc.) remains unchanged; When the volume of the container is constant, colored The color of the gas no longer changes, such as I ₂ , Br ₂ , NO ₂ and so on.

(3) See if it can be concluded from the changes in P _total , n _total , , M: If the _total , n _total , , and M will change when the reaction proceeds in the forward or reverse direction, it will not change. When the balance is reached; if it is always the same, you cannot judge.

The value of K indicates the degree of reaction (the limit of the reaction) when equilibrium is reached at this temperature. The larger the value of K, the greater the degree of reaction; otherwise, the smaller the value.

When the reversible chemical reaction reaches equilibrium, the continuous product of the power of the concentration coefficient of each product is proportional to the continuous product of the power of the concentration coefficient of each reactant. This ratio is called the equilibrium constant. The more complete the reaction, the larger the equilibrium constant. When a reversible reaction reaches equilibrium, the product of the power of the product concentration or partial pressure and the power of the reactant concentration (the power index is the

(1) The equilibrium constant is a characteristic constant of a chemical reaction. It does not change with the initial concentration (or partial pressure) of the substance, it depends only on the nature of the reaction. For a certain reaction, as long as the temperature is constant, the equilibrium constant is a fixed value, and any other conditions change will not affect its value. The equilibrium constant K is only affected by temperature, and has nothing to do with the concentration change of any one of the reactants or products, and it has nothing to do with the change of pressure. Because the catalyst changes the forward and reverse reaction rate to the same extent, the equilibrium constant is not affected by the catalyst.

(2) The magnitude of the equilibrium constant value is a sign of the progress of the reaction. It is a good indication of how well the reaction has progressed. The larger the K value of a reaction, the larger the concentration of the product at equilibrium, the smaller the remaining concentration of the reactant, the greater the conversion of the reactant, that is, the stronger the tendency of the positive reaction. vice versa. For a reversible reaction, when the temperature remains unchanged and other conditions affecting chemical equilibrium are changed, the K value does not change even if the equilibrium moves.

(3) When other conditions are unchanged, if the positive reaction is an endothermic reaction, K will increase (or decrease) because the equilibrium moves to the positive (or reverse) reaction direction when the temperature is raised (or decreased); if the positive reaction is For exothermic reactions, as the equilibrium moves toward the reverse (or positive) reaction direction when the temperature is increased (or decreased), K decreases (or increases); therefore, the equilibrium constant may increase or decrease when the temperature increases, but not Will not change.