Van 't Hoff factor
Encyclopedia
The van 't Hoff factor 
(named after J. H. van 't Hoff) is a measure of the effect of a solute upon colligative properties
, such as vapor pressure
, osmotic pressure
and freezing point depression. The van 't Hoff factor is the ratio between the actual concentration of particles produced when the substance is dissolved, and the concentration
of a substance as calculated from its mass. For most non-electrolyte
s dissolved in water, the van' t Hoff factor is essentially 1. For most ionic compound
s dissolved in water, the van 't Hoff factor is equal to the number of discrete ions in a formula unit
of the substance. This is true for ideal solutions
only. Sometime ion pairing
occurs in solution. At a given instant a small percentage of the ions are paired and count as a single particle. Ion pairing occurs to some extent in all electrolyte solutions. This causes deviation from the van 't Hoff factor. The deviation for the van 't Hoff factor tends to be greatest where the ions have multiple charges.
. It is usually indicated by the Greek symbol
. There is a simple relationship between this parameter and the van 't Hoff factor. If the solute substance dissociates into 
ions, then
For instance, for the following dissociation
As
, we would have that 
Physical significance of
The value of
is ;
= the actual number of particles in solution after dissociation ÷ the number of formula units initially dissolved in solution.
Means the number of particles per formula unit of the solute when a solution is dilute.
(named after J. H. van 't Hoff) is a measure of the effect of a solute upon colligative propertiesColligative properties
Colligative properties are properties of solutions that depend on the number of molecules in a given volume of solvent and not on the properties/identity of the molecules. Colligative properties include: relative lowering of vapor pressure; elevation of boiling point; depression of freezing point...
, such as vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
, osmotic pressure
Osmotic pressure
Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....
and freezing point depression. The van 't Hoff factor is the ratio between the actual concentration of particles produced when the substance is dissolved, and the concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
of a substance as calculated from its mass. For most non-electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
s dissolved in water, the van' t Hoff factor is essentially 1. For most ionic compound
Ionic compound
In chemistry, an ionic compound is a chemical compound in which ions are held together in a lattice structure by ionic bonds. Usually, the positively charged portion consists of metal cations and the negatively charged portion is an anion or polyatomic ion. Ions in ionic compounds are held together...
s dissolved in water, the van 't Hoff factor is equal to the number of discrete ions in a formula unit
Formula unit
A formula unit in chemistry is the empirical formula of an ionic or covalent network solid compound used as an independent entity for stoichiometric calculations. It is the lowest whole number ratio of ions represented in an ionic compound...
of the substance. This is true for ideal solutions
Ideal solution
In chemistry, an ideal solution or ideal mixture is a solution with thermodynamic properties analogous to those of a mixture of ideal gases. The enthalpy of solution is zero as is the volume change on mixing; the closer to zero the enthalpy of solution is, the more "ideal" the behavior of the...
only. Sometime ion pairing
Ion-association
Ion-association is a chemical reaction whereby ions of opposite electrical charge come together in solution to form a distinct chemical entity. Ion-associates are classified according to the number of ions that associate with each other, and the nature of the interaction. The most important factor...
occurs in solution. At a given instant a small percentage of the ions are paired and count as a single particle. Ion pairing occurs to some extent in all electrolyte solutions. This causes deviation from the van 't Hoff factor. The deviation for the van 't Hoff factor tends to be greatest where the ions have multiple charges.
Relationship with the dissociation degree
The dissociation degree is the fraction of original solute molecules that have dissociatedDissociation (chemistry)
Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...
. It is usually indicated by the Greek symbol
. There is a simple relationship between this parameter and the van 't Hoff factor. If the solute substance dissociates into ions, then
For instance, for the following dissociation
-
-
- KCl K+ + Cl-
-
As
, we would have that Physical significance of 
- When solute particles associateAssociateAssociate may refer to:* A business valuation concept.* A title used by some companies instead of employee.* A title used to signify an independent person working as if directly employed by the company of which they are an associate...
in solution,
is less than 1. (e.g. ethanoic acid in benzene, benzoic acid in benzene) - When solute particles dissociate in solution,
is greater than 1. (e.g. sodium chloride in water, potassium chloride in water, magnesium chloride in water) - When solute particles neither dissociate nor associate in solution,
equals 1. (e.g. Glucose in water)
The value of
is ; = the actual number of particles in solution after dissociation ÷ the number of formula units initially dissolved in solution.Means the number of particles per formula unit of the solute when a solution is dilute.
See also
- Colligative propertiesColligative propertiesColligative properties are properties of solutions that depend on the number of molecules in a given volume of solvent and not on the properties/identity of the molecules. Colligative properties include: relative lowering of vapor pressure; elevation of boiling point; depression of freezing point...
- Solutions
- Raoult's law
- Van 't Hoff equation
- Dissociation (chemistry)Dissociation (chemistry)Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...