Chemical Equilibrium refers to a state where the forward and reverse reaction rates of a chemical reaction are equal in a reversible reaction with certain macroscopic conditions. The concentration of each component of the reactant and product no longer changes.

It can be judged by ΔrGm = ΣνΑμΑ = 0, μA is the chemical potential of substance A in the reaction. According to Le Chatelier’s principle, if a balanced system is changed, the system will change accordingly to counteract the change.

Equilibrium Constant

The chemical equilibrium constant means that at a certain temperature, the reversible reaction starts from the forward reaction or the reverse reaction, and regardless of the initial concentration of the reactants, the equilibrium is finally reached.

At this time, the stoichiometric power of each product concentration is several times The ratio of the product divided by the product of the stoichiometric power of each reactant concentration is a constant, denoted by K. This constant is called the chemical equilibrium constant.

Reaction aA (g) + bB (g) = cC (g) + dD (g)

K = (Cc × Dd) / (Aa × Bb)

Balance Movement

Under chemical reaction conditions, the process of changing the reversible reaction from one equilibrium state to another equilibrium state due to changes in the reaction conditions is called the shift of chemical equilibrium.

The fundamental reason for the shift of chemical equilibrium is that the forward and reverse reaction rates are not equal, and the result of the equilibrium movement is that the reversible reaction reaches a new equilibrium state, and the forward and reverse reaction rates are equal again.

The factors that affect the chemical equilibrium movement mainly include concentration, temperature, and pressure.

Balancing Process

(1) Process (dynamic angle)

From a kinetic point of view, at the beginning of the reaction, the concentration of the reactant is larger and the concentration of the product is smaller, so the positive reaction rate is greater than the reverse reaction rate. As the reaction progresses, the concentration of the reactant decreases and the concentration of the product increases, so the rate of the positive reaction decreases and the rate of the reverse reaction increases.

When the forward and reverse reaction rates are equal, the concentration of each substance in the system no longer changes, and the reaction reaches equilibrium. At this time, the system is in a state of dynamic equilibrium, and it does not mean that the reaction will stop completely at this point.

(2) Process (micro angle)

From a microscopic point of view, it is because of the equilibrium phenomenon caused by the breaking rate of chemical bonds in the reactant molecules is equal to the breaking rate of the chemical bonds of the products in the reversible reaction.

Equivalent balance


The same reversible reaction, under certain conditions, when the amount of reactant or product substance or the concentration of a substance is changed at the beginning to reach equilibrium, the percentage composition of each component in the mixture is equal. Such a balance is called equivalent equilibrium.

Cause: The balance is only related to temperature, pressure, and concentration, and has nothing to do with the order of addition.

According to the gas state equation, pV = nRT, it can be found that if the temperature is kept constant, the constant volume system, as long as the “one-sided”, the components n are the same, the pressure is the same, and the equilibrium state is also the same.

If the temperature is kept constant and the constant pressure system, as long as the component n is in the same proportion and the concentration is the same after “one-sided”, then the equilibrium state is also the same.

Laws and judgments

(1) General reversible reaction, temperature constant volume when the starting reactants or the product when the amount of the substance by the stoichiometric number are the same, equilibrium is established in terms of equivalent.


If the reaction 2SO₂ + O₂ = (reversible) = 2SO₃ Equivalent equilibrium is established under the conditions of (A) and (B)

(A) Add at the beginning: 2mol SO₂ + 1mol O₂

(B) Initially add: 2mol SO₃

Note: In this case, regardless of the reactants or products, the number of substances at the beginning must be the same as the stoichiometric ratio.

(2) In general reversible reactions, at constant temperature and pressure, when the quantity ratio of the starting reactant or product (not necessarily the same as the stoichiometric ratio) is the same, an equivalent balance is established.

For example, 2SO₂ + O₂ = (reversible) = 2SO₃ Equivalent balance is established at (C) and (D)

(C) Add at the beginning: 1mol SO₂ + 1mol O₂

(D) Initially add: 2mol SO₂ + 2mol O₂

(3) For a constant volume of the reaction gas before and after the reaction, the temperature constant volume, the ratio when the starting materials or product of reactants (not necessarily require the stoichiometry ratio the same) are the same, equivalent establish equilibrium.

For example, the reaction H₂ + I₂ (gas) 2HI establishes an equivalent balance between (E) and (F)

(E) Add at the beginning: 1mol H₂ + 2mol I₂

(F) Add at the beginning: 2mol H₂ + 4mol I₂

Limit judgment

(1) The concentration or volume fraction of the amount of substances of each component in the system, and the amount of substances remain unchanged.

(2) All the reversible reactions in which the gas participates and the stoichiometry changes before and after, the pressure remains unchanged.

(3) All reversible reactions involving gas and changing the stoichiometric number before and after, the average relative molecular mass remains unchanged.

(4) For reversible reactions involving or generated by colored substances, the color does not change with time.

(5) For the same substance, the amount of the substance that breaks the chemical bond is equal to the amount of the substance that forms the chemical bond.

(6) V positive = V inverse.