Video Transcript
The given table shows the 𝐾 𝑎
values for a selection of acids. Which acid is the strongest?
𝐾 𝑎 is the acid dissociation
constant. The acid dissociation constant is
the equilibrium constant for the reaction of an acid with water. In its most basic form, we can
state that the acid dissociation constant is equal to the concentration of the
products divided by the concentration of the reactants at equilibrium.
Let’s consider a generic weak acid,
represented by HA. When the acid reacts with water,
some of the acid molecules dissociate. A− ions and hydronium ions are
produced. When equilibrium is established,
the concentration of the water will be virtually the same as it was initially. So this equation is often
abbreviated by removing the water to HA is in equilibrium with H+ plus A−. Using the abbreviated equation, we
can write an expression for the acid dissociation constant. 𝐾 𝑎 equals the concentration of
H+ times the concentration of A− divided by the concentration of HA.
We have an equation for the acid
dissociation constant. But how does this constant
correspond to acid strength? When we say that substance is a
stronger acid, what we mean is that more of the acid molecules dissociate in aqueous
solution, and the equilibrium is shifted to the right. So at equilibrium, a stronger acid
will have a lower HA concentration and higher H+ and A− concentrations than a weaker
acid. This means that the numerator of
the 𝐾 𝑎 equation will be larger, while the denominator will be smaller. As such, stronger acids will have a
larger acid dissociation constant.
We can say then that as acid’s
strength increases, the acid dissociation constant will also increase. So to identify the acid that is the
strongest, we need to identify which acid has the largest 𝐾 𝑎 value. Of the acids provided, the acid
with the largest acid dissociation constant is chloroethanoic acid. In conclusion, the acid that is the
strongest is chloroethanoic acid.
