What is Henry's Law?

1. Introduction to the Law

      Henry's law, one of the fundamental laws of physical chemistry, was discovered by Henry in England in 1803 while studying the solubility of gases in liquids. It can be expressed as: "At isothermal and isopressure, the solubility of a gas in solution is proportional to the equilibrium pressure at the liquid surface." "This law is equally useful for volatile solutes in dilute solutions. Its formula is

       Pg=Hx
　　

      Where: H is Henry's constant, x is the solubility of the gas's molar fraction, and Pg is the partial pressure of the gas. H can well represent the amount of gas dissolved, but Henry's Law only applies to systems with very low solubility. Strictly speaking, Henry's Law is only an approximate law and cannot be applied to systems with higher pressure. In this sense, the Henry constant is simply a function of temperature and is unrelated to pressure.

2. Detailed content

       In experiments involving volatile solutes in dilute solutions, the experiment shows that this law is only true when the solubility of a gas in the liquid is not very high; at this point, the gas is actually the volatile solute in the dilute solution, and the gas pressure is the vapor pressure of the solute. Therefore, Henry's Law can also be expressed as: at a certain temperature, the vapor partial pressure of the solute in a dilute solution is proportional to the solution concentration.

      Generally speaking, gases have very low solubility in solvents, so the resulting solution falls within the range of dilute solutions. The composition of gas B solution in solvent A, whether expressed by B's molar fraction xB, mass molar concentration bB, concentration cB, etc., is approximately proportional to the pressure of gas solute B. When expressed as a formula, Henry's Law can take many forms. For example:

        PB=Kx,B·xB

        PB=Kb,B·bB

       PB=Kc,B·cB

       In the formula, pB is the vapor partial pressure of the solute in a dilute solution; xB is the fractional amount of the solute substance; k is the Henry constant, whose value depends on temperature, pressure, and the properties of the solute and solvent. Due to differences in the scale of solution composition in Henry's law, the units of the Henley coefficient differ. At certain temperatures, the values of the same solute in the same solvent also vary. When expressed in the above formula xB (molar fraction of solute B), bB (molar concentration mass), or cB (concentration of substance), the k value will change accordingly. The units for Kx, Kb, and Kc are Pa, Pa·mol^-1·kg, and Pa·mo^1-1·dm^3, respectively.　

       Henry's law applies only when the molecular states of the solute in the gas phase and the liquid phase are the same. If solute molecules undergo dissociation or association in the solution, then xB (or mB, cB, etc.) in the above formula should refer to the portion of the molecule in the gas phase that matches its state; When the total pressure is not high, if multiple gases dissolve simultaneously in the same liquid, Henry's law can apply to any one of them; Generally, the dilute the solution, the more accurate Henry's Law is; at xB→0, the solute energy strictly obeys the law.