Summary of knowledge points related to chemical balance

1864, G.M. Goldberg put forward the chemical equilibrium constant, which became the dynamic reality of chemical equilibrium. The following summarizes some knowledge points related to chemical balance, hoping to help you.

One. chemical equilibrium constant

It 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.

Step 2 pay attention to this problem

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

① When using the expression of equilibrium constant, 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 solids is taken as a constant, and the concentration of solids 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.

⑤ It is generally believed that the reaction of K> 10 5 is complete (i.e. irreversible reaction), while K

The value of equilibrium constant can judge the degree of reaction.

Three. assay method

The equilibrium constant can be measured by experimental methods or calculated by thermodynamic data.

Experimental methods usually include chemical methods and physical methods.

Chemical method

The chemical method is to determine the concentration of each substance when the reaction reaches equilibrium through chemical analysis. However, it is necessary to prevent chemical reagents from disturbing the chemical balance during the determination. Therefore, before chemical analysis, chemical equilibrium must be "frozen" in the original equilibrium state. The usual method is to quench, dilute or add inhibitors to stop the reaction, and then analyze it. For example, determine the reaction 2H2+O2? The equilibrium constant of 2H2O at 2 000℃ can be obtained by heating a certain amount of water in a high temperature resistant alloy tube and keeping it at 2 000℃ for a period of time to achieve chemical equilibrium. Then, the test tube was suddenly cooled, and then the contents of H2O, H2 and O2 were analyzed, so that the equilibrium constant of the reaction at 2 000℃ could be calculated.

physical method

The physical method is to measure the concentration change of each substance when it reaches equilibrium by using the change of its physical properties, for example, by measuring the refractive index, conductivity, color, pressure or volume of the system. The advantage of physical method is that it will not interfere with or destroy the equilibrium state of the system during the determination.

Four. unit

The unit of equilibrium constant is complex, which can be divided into standard equilibrium constant and nonstandard equilibrium constant. The dimension of the former is 1, which means that it is calculated according to the thermodynamic data △G, so it is a unified data, a data that indirectly reflects the degree of reaction, and a unified unit meaning. The dimension of the latter depends on the physical quantity of the substance used in the expression of equilibrium constant.

The equilibrium constant calculated according to the standard thermodynamic function is a standard equilibrium constant, called k, also called thermodynamic equilibrium constant; The equilibrium constant expressed by the pressure quotient or concentration quotient of products to reactants at equilibrium is an empirical equilibrium constant (Kp or Kc), or it is called a nonstandard equilibrium constant. The equilibrium constants involved in middle school textbooks are all empirical equilibrium constants.

As we all know, the absolute values of some important thermodynamic functions are uncertain, such as u, h, f and g. In order to calculate their relative size, it is necessary to define a state as a comparison standard to avoid confusion. The so-called standard configuration refers to the specific state of a substance at temperature T and standard pressure (100kPa), which is referred to as standard configuration for short. The standard configuration in thermodynamic data sheet has been clearly defined internationally.

For the reaction in which the sum of the stoichiometric coefficients of reactants is equal to the sum of the stoichiometric coefficients of products, the empirical equilibrium constant is a dimensionless pure number, which has nothing to do with the units used in pressure and concentration, and is also equal to the value of the standard equilibrium constant.

For the reaction in which the sum of the stoichiometric coefficients of reactants is not equal to the sum of the stoichiometric coefficients of products, the empirical equilibrium constant is a dimensional quantity, and its value is related to the units used for pressure and concentration. Only when the units of pressure and concentration are the same as those corresponding to the standard configuration in the thermodynamic data table can the calculated empirical equilibrium constant be the same as the standard equilibrium constant.

Influencing factors of verb (abbreviation of verb)

In middle school, it is generally believed that the chemical equilibrium constant is only related to temperature, and the equilibrium constant of endothermic reaction increases with the increase of temperature, while the exothermic reaction is the opposite. But strictly speaking, the chemical reaction equilibrium constant is a function of temperature and pressure, and the situation is different for different chemical equilibrium constants.

In gas phase reaction, all standard equilibrium constants are only functions of temperature. If a gas is an ideal gas, its empirical equilibrium constant is only a function of temperature. But for non-ideal gases, the equilibrium constant is affected by temperature and pressure.

Theoretically, any reaction involving condensed phase (solid or liquid) is a function of temperature and pressure. However, in the case of small pressure change range, the influence of pressure on the volume change of condensed phase can be ignored, that is, the influence of pressure on equilibrium constant can be ignored.

Intransitive verb catalyst

For a reversible reaction, because the chemical composition and mass of the catalyst are unchanged before and after the reaction, the initial and final state of the reaction are the same whether the catalyst is used or not, so the standard Gibbs function of the reaction changes equally, and the catalyst has no effect on the chemical equilibrium. That is to say, catalysts only change the kinetic path of chemical reactions, while thermodynamics only pays attention to the beginning and end of changes.

Using the same catalyst in chemical reaction can change the forward and reverse reaction rates to the same extent (increase the forward and reverse reaction rates by the same number of times, but keep the ratio unchanged), thus keeping the chemical equilibrium in the original state. Therefore, the catalyst has no effect on the chemical equilibrium constant.

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