Percent dissociation is a measure of the degree to which a compound breaks down into its constituent parts in a given solution. It quantifies the extent of dissociation, which is the process by which a molecule or compound separates into smaller molecules or ions.
In simpler terms, it tells us how much of a substance dissolves or reacts in a particular environment.
When a molecule or compound separates into smaller molecules or ions, the concentration of dissociated species is compared to the total concentration of the compound.
Calculation of Percentage Dissociation
The percentage dissociation is calculated by dividing the concentration of dissociated molecules by the initial concentration of the compound and multiplying by 100%.
Percent Dissociation = (Concentration of dissociated molecules/Initial concentration of the compound) × 100%
This concept is important in understanding the behavior of acids and bases, as well as the solubility of salts and other compounds in solution. By studying this dissociation of a compound, a chemist can gain insights into a compound’s properties and reactivity.
Percentage dissociation in Acids/Bases
When acids undergo dissociation, they release hydrogen ions (H+). The degree of ionization, denoted as alpha (α), depends on the strength of the acid or base to form ions in solution.
- Strong acids, represented by compounds like hydrochloric acid (HCl) and sulfuric acid (H2SO4), dissociate completely into ions in water. In this case, α = 1, and the percent dissociation is 100%.
- Weak acids or bases, on the other hand, only partially ionize (α < 1), and they establish a dynamic equilibrium between the compounds. For instance, the equilibrium constant Kc defines the acid dissociation constant (Ka), which quantifies the extent of dissociation for weak acids.
The equilibrium expression for the dissociation of a weak acid, HA, can be expressed as:
HA + H2O ⇌ H3O+ + A– Kc<<1
Kc stands for the equilibrium constant which is used to define the acid dissociation constant. Here, the equilibrium constant, Ka, is defined as:
Ka = [H3O+] [A–] / [HA] [H2O]
In this reaction, water is a solvent so it is present in huge excess! that means its concentration barely changes in the reaction so we can treat it as a constant, so we can now define Ka as:
Ka = [H3O+] [A–] / [HA]
Ka is useful for calculating the percent of a given weak acid that is dissociated in a solution with a known acidity, or pH.
% HA dissociation = [HA] dissociated / [HA] Initial ×100%
At equilibrium: [HA] dissociated = [H3O+], thus;
% HA dissociation = [H3O+]/ [HA] Initial ×100%
As the initial concentration of a weak acid decreases, the percentage dissociation of the acid increases.
The degree of dissociation is calculated in the same manner for a weak base.
Example of Dissociation and constants
Consider a weak acid, HA, with a pKa of 5.27. If the solution’s pH is 4.3, you can calculate the percentage of the acid that is dissociated. The reaction and equilibrium expression are as follows:
The equation of the reaction is:
HA ⇌ H+ + A–
The equilibrium expression for that dissociation is:
Ka = ([H+] [A–]) / [HA])
- Convert pKa to Ka and pH to [H+]:
Ka = 10-5.27 = 5.370 x 10-6
[H+] = 10-4.3 = 5.012 x 10-5
- Now calculate [ HA] using the equation Ka= ([H+] [A¯]) / [HA])
[H+] = [A¯] = 5.012 x 10-5
Because of the 1:1 molar ratio in the above equation.
5.370 x 10-6 = [(5.012 x 10-5 M) (5.012 x 10-5 M)] / [HA]
[HA]= 4.678 × 10 -4
- The percent dissociation of acid is [H+] ÷ [HA] multiplied by 100.
α = (5.012 x 10-5 ÷ 4.678 × 10 -4) × 100%
Degree of Ionization = 10.71 %
Factors Affecting the Degree of Dissociation
The extent of dissociation is influenced by several factors:
Nature of Solute
Strong electrolytes have a degree of dissociation of approximately one, while weak electrolytes that only partially dissociate have a degree of dissociation greater than one.
Nature of Solvent
A solvent with a high dielectric constant tends to have a higher degree of dissociation.
The degree of dissociation generally increases as a solution becomes more diluted.
An increase in temperature typically leads to a higher degree of dissociation.
What is percent dissociation and how do you calculate it?
The percent dissociation can be defined as the ratio of the amount of acid or base disassociated to the initial concentration of such acid or base, multiplied by 100.
What are the factors that affect the degree of dissociation?
- Nature of solute: Strong electrolytes have a degree of dissociation of approximately one while weak electrolytes that dissociate partially have a degree of dissociation of more than one.
- Nature of solvent: A solvent with a high dielectric constant has a higher degree of dissociation.
- Dilution: the degree of dissociation increases as the solution becomes more diluted.
- Temperature: As the temperature increases the degree of dissociation increases
What is the percent dissociation in strong acids?
Strong acids dissociate completely into ions in water thus percent dissociation is 100% (α = 1).