Equation of dissociation equilibrium (explanation of physical chemistry)

　When dealing with dissociation equilibrium problems, the "law of mass action" is followed. That is, the equilibrium constant is expressed as the ratio of the concentration product of the original and product forms of the reaction equation.

　In the dissociation equilibrium equation, the substance before dissociation is placed on the left side and the substance after dissociation is placed on the right side.

【H₂CO₃】↔【HCO₃^-】+【H⁺】 (a)

【HCO₃^-】↔【CO₃^2-】+【H⁺】 (b)

　In a chemical reaction equation, the left side is called the original form, and the right side is called the product form.

　The equilibrium constant (K) of the dissociation equilibrium is equal to the ratio of the concentration product of the product form (active product) to the concentration product of the original form (active product). (numerator is the product form, denominator is the original form)

K = （Concentration product of product form）／（Concentration product of original form）

　This is called the law of mass action.

　The concentration product of a product form is the product of the concentrations of substances in the product form. If the product form is 【HCO₃^-】+【H⁺】, then 【HCO₃^-】×【H⁺】.

(Although not included in this course, if the product form is 【Ca²⁺】+2【Cl^-】, then it is 【Ca²⁺】×【Cl^-】²)

The mass action equation is written for the dissociation equilibrium of carbonic acid components.

(a)　→ K1 = 【HCO₃^-】・【H⁺】／【H₂CO₃】

(b)　→ K2 = 【CO₃^2-】・【H⁺】／【HCO₃^-】

If you see 【H⁺】 in a dissociation equilibrium problem, write down the equation for the dissociation equilibrium of water or determine 【H⁺】 if the solution pH is specified.

H₂O ↔【OH-】+【H⁺】

Kw = 【OH-】・【H⁺】

（For pure water, the dissociation equilibrium constant of waterKw = 10^-14）

(Kw = 10^-14 is the value when 【OH-】×【H⁺】／【H₂O 】, where【H₂O 】=1 in the denominator. Note that the dissociation equilibrium constant for water may be expressed using the number of moles of 1L of water as the denominator, so be careful when studying on your own.)

　In the above three equations, the mass action law equation is less for the number of unknown parameters (【CO₃^2-】や【H⁺】、【HCO₃^-】). To solve this, certain approximations or conditions (such as giving concentration) are set. Sometimes the conservation of charge or law of mass action is used. You have to get used to each case, so solve examples to get used to it.