Weak Acid and Base Equilibria
Introduced| English |
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| acid dissociation constant/ˈæsɪd dɪˌsəʊsɪˈeɪʃn ˈkɒnstənt/ |
The ones that only partly let go
- A strong acid dumps all its protons at once.
- But most acids hold on and release only a fraction.
- That partial release settles into an equilibrium.
- A constant measures just how far it goes.
The acid dissociation constant
- A weak acid ionizes partly: $\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-$.
- The acid dissociation constant 酸解离常数 is $K_a = \dfrac{[\text{H}^+][\text{A}^-]}{[\text{HA}]}$.
- A larger $K_a$ means a stronger weak acid.
A larger $K_a$ means the weak acid is...
A bigger $K_a$ means more ionization -- a stronger weak acid.
The base version
- A weak base has a $K_b$ describing its partial reaction with water.
- A larger $K_b$ means a stronger weak base.
- Both work like any equilibrium constant.
The constant describing a weak base's reaction with water is...
$K_b$ is the base dissociation constant.
Finding the pH
- Set up an ICE table with the small ionization $x$.
- Solve $K_a$ for $x = [\text{H}^+]$, then take $-\log$.
- Percent ionization is $x$ divided by the starting acid.
Strong or weak?
Sort each acid or base by whether it ionises fully or only partly.
For a weak acid, $[\text{H}^+]$ equals the acid's full concentration.
Only strong acids ionize fully; a weak acid needs $K_a$.
Percent ionization is the amount ionized divided by the ____ amount of acid.
It compares $x$ to the initial acid concentration.
A weak acid barely ionizes. How does its pH compare with a strong acid of the same concentration?
- The weak acid releases fewer $\text{H}^+$ ions.
- So its pH is higher (less acidic) than the strong acid.
A weak acid and a dilute acid are the same thing.
Weak is about ionization; dilute is about concentration.
Compared with a strong acid of the same concentration, a weak acid has a...
Less ionization means fewer $\text{H}^+$ and a higher pH.
A weak acid is not the same as a dilute one -- "weak" means partly ionized, "dilute" means low concentration. Do not assume $[\text{H}^+]$ equals the acid's concentration for a weak acid; use $K_a$. And a bigger $K_a$ is a stronger acid, but it is still weak.
A weak acid only partly ionizes, described by the acid dissociation constant $K_a = [\text{H}^+][\text{A}^-]/[\text{HA}]$ (a weak base has a $K_b$). A larger $K_a$ is a stronger weak acid. Find the pH with an ICE table and $K_a$ -- do not assume $[\text{H}^+]$ equals the concentration.