Video Transcript
The general equation for the
dissociation of a weak acid is as follows. HA aqueous is in equilibrium with
H+ aqueous plus A‒ aqueous. What is the equation for the acid
dissociation constant, 𝐾 𝑎, of this acid?
An acid dissociation constant is
the equilibrium constant for the reaction of an acid with water. An equilibrium constant is a value
that expresses the relationship between the reactants and products at
equilibrium. Let’s consider a generic
equilibrium reaction equation. In this equation, the lowercase
letters represent stoichiometric coefficients, and the uppercase letters represent
chemical formulas. To calculate the equilibrium
constant for this reaction, we would divide the concentrations of the products
raised to their stoichiometric coefficients by the concentrations of the reactants
raised to their stoichiometric coefficients.
We can apply this understanding of
equilibrium constants to the general equation for the dissociation of a weak acid
and construct an equation for the acid dissociation constant. This reaction equation has two
products, H+ and A‒. We should write H+ and A‒ in the
numerator inside of brackets. As the stoichiometric coefficients
in front of H+ and A‒ are both one, we don’t need to explicitly include these
coefficients in the equation. There’s only one reactant. So we write HA inside of brackets
in the denominator. HA also has a coefficient of
one. That does not need to be explicitly
written in the equation. This gives us our final equation
for the acid dissociation constant. 𝐾 𝑎 equals the concentration of
H+ times the concentration of A‒ divided by the concentration of HA.
