What is the expression of chemical equilibrium constant?

The chemical equilibrium constant means that at a certain temperature, the reversible reaction starts from the positive reaction or the reverse reaction, regardless of the initial concentration of reactants, and finally reaches equilibrium. At this time, the ratio of the product of the stoichiometric power of each product concentration to the product of the stoichiometric power of each reactant concentration is a constant, expressed by k, which is called the chemical equilibrium constant.

Equilibrium constants generally include concentration equilibrium constants and pressure equilibrium constants.

When writing the expression of equilibrium constant, we should pay attention to the following questions:

1. When the expression of equilibrium constant is applied, the concentration of water molecules in dilute solution can be omitted. Because the density of dilute solution is close to 1 g/mL. The mass concentration of water is 55.6 mol/L. In the process of chemical change, the change of water quantity has little influence on the change of water concentration, so the concentration of water is a constant and can be incorporated into the equilibrium constant.

For the reaction in solution without water as solvent, the concentration of solvent is also constant.

2. When solid substances participate in the reaction, the collision between molecules can only be carried out on the solid surface, and the quantity and concentration of solid substances have no effect on the reaction rate and equilibrium. Therefore, the "concentration" of solid is taken as a constant, and the concentration of solid is not written in the expression of equilibrium constant.

3. The expression of chemical equilibrium constant is related to the writing method of chemical equation. For the same chemical reaction, due to different writing methods, the stoichiometry of each reactant and product is different, and the equilibrium constant is also different. But these equilibrium constants can be converted to each other.

4. Different chemical equilibrium systems have different equilibrium constants. A large equilibrium constant means that the equilibrium concentration of products is large and that of reactants is relatively small, which means that the reaction is more complete. Therefore, the equilibrium constant can indicate the degree of reaction.

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