Ch. 18 Aqueous Ionic Equilibrium

buffer solution

A solution that resists changes in pH when a strong acid or base is added.

conjugate base

The chemical species that remains after an acid loses its proton.

common ion effect

The presence of a common ion (from a salt) in a solution of a weak acid or base, which shifts the equilibrium and decreases the concentration of hydronium or hydroxide ions.

Henderson-Hasselbalch equation

pH = pKa + log([conjugate base] / [weak acid])

What condition must be met for the Henderson-Hasselbalch equation to be valid?

The 'x is small' approximation must be valid.

How does the addition of a strong acid affect a buffer system?

The strong acid (H+) reacts with the conjugate base of the buffer, converting it into the weak acid component.

How does the addition of a strong base affect a buffer system?

The strong base (OH-) reacts with the weak acid of the buffer, converting it into the conjugate base component.

ideal buffer

occurs when the concentration of the weak acid equals the concentration of its conjugate base.

What is the relationship between buffer concentration and buffer effectiveness?

A buffer is more effective when the concentrations of both the weak acid and its conjugate base are higher.

buffer range

range of any buffer is said to be 1 pH unit on either side of the pKa

buffer capacity

The amount of strong acid or base that can be added to a buffer before its effectiveness is destroyed.

How can buffer capacity be increased?

By increasing the absolute concentrations of the weak acid and its conjugate base.

What is the solubility product constant (Ksp)?

An equilibrium constant that represents the product of the concentrations of the ions in a saturated solution of a sparingly soluble salt, each raised to the power of its stoichiometric coefficient.

Is Ksp temperature dependent?

yes

molar solubility

The solubility of a substance expressed in units of molarity (M).

If the reaction is Ca(OH)2(s) ↔ Ca2+(aq) + 2OH-(aq), what is the Ksp expression?

Ksp = [Ca2+][OH-]^2

How do you calculate pH from pKa and the ratio of conjugate base to acid?

Use the Henderson-Hasselbalch equation: pH = pKa + log([base]/[acid]).

What happens to the pH of a buffer if the concentration of the acid is equal to the concentration of the base?

pH = pKa of the weak acid.

Can the Henderson-Hasselbalch equation be used for a buffer made of a weak base and its conjugate acid?

Yes, it can be adapted for weak base/conjugate acid systems.

What is the relationship between pKa and Ka?

pKa = -log(Ka).

What does it mean for a substance to be 'soluble' in the context of Ksp?

It means the substance dissolves in water to a significant extent.

If you are given the molar solubility (s) of a salt, how do you find the Ksp?

Write the dissociation equation, express ion concentrations in terms of 's', and substitute these into the Ksp expression.

What is the primary purpose of a buffer?

To maintain a relatively constant pH despite the addition of small amounts of strong acid or base.

Can Ksp be used to predict if compounds with different ratios of ions will precipitate?

No

What is the common ion effect on the solubility of an ionic compound?

The solubility of an ionic compound is lower in a solution containing a common ion than in pure water.

How does increasing acidity (decreasing pH) affect the solubility of an ionic compound with a basic anion?

The solubility increases as the acidity increases because the basic anion reacts with H+ ions.

precipitation

The formation of an insoluble solid (precipitate) from a solution.

What is the difference between Qsp and Ksp?

Ksp is the value at equilibrium only, while Qsp is the reaction quotient calculated under any conditions.

What does Q < K imply regarding precipitation?

No precipitate will form.

Q > K

A precipitate will form.

Q = K

The solution is saturated.

What is the difference between quantitative and qualitative analysis?

Quantitative analysis determines how much of a substance is present, while qualitative analysis determines what substances might form.