5B4 Acid-Base Equilibrium Flashcards
Analyze the properties and equilibria of strong and weak acids and bases, buffers, and polyprotic acids.
Define:
autoionization
Self-ionization, where water molecules react with each other to form ions.
Autoionization of water results in hydronium (H₃O⁺) and hydroxide (OH⁻) ions in a 1:1 molar ratio.
These ions form when water molecules act as both acids and bases, donating and accepting protons.
Fill in the blank:
The chemical equation for the autoionization of water is _______.
H₂O + H₂O ⇌ H₃O⁺ + OH⁻
This equation represents the reversible reaction of water molecules forming hydronium and hydroxide ions.
True or False:
The concentration of H₃O⁺ and OH⁻ is equal in pure water.
True
In pure water, both ions are present in equal concentrations of 10⁻⁷ M at 25°C.
Define:
Kw
It is the autoionization constant of water, equal to [H₃O⁺][OH⁻].
It is calculated by multiplying the concentrations of H₃O⁺ and OH⁻ ions.
For pure water at 25°C: [H₃O⁺][OH⁻] = (10⁻⁷)(10⁻⁷) = 10⁻¹⁴.
How does the concentration of H₃O⁺ affect the pH of a solution?
Higher H₃O⁺ concentration decreases pH, making the solution acidic.
pH is the negative logarithm of H₃O⁺ concentration: pH = -log[H₃O⁺].
Fill in the blank:
When the concentration of OH⁻ increases, the solution becomes more _____.
basic
Basic solutions have higher OH⁻ concentrations and lower H₃O⁺ concentrations.
What is the relationship between pKw and Kw?
pKw = -log(Kw)
At 25°C, pKw = 14.
How is pKw calculated?
pKw = pH + pOH
At 25°C, pH + pOH = 14.
Give a factor that affect the value of Kw.
Temperature
Kw is temperature-dependent and increases with rising temperature.
How is the pH scale range determined?
By taking the negative logarithm of Kw.
Kw = 10⁻¹⁴; pKw = 14, so the pH scale generally ranges from 0 to 14.
What happens to the concentrations of H₃O⁺ and OH⁻ when an acid is added to water?
H₃O⁺ concentration increases, OH⁻ concentration decreases.
Le Chatelier’s principle explains the shift in equilibrium to restore the product [H₃O⁺][OH⁻] = 10⁻¹⁴.
What is the pH of a solution where [H₃O⁺] = 1 × 10⁻⁵ M?
pH = 5
pH = -log[H₃O⁺], so pH = -log(1 × 10⁻⁵) = 5.
True or False:
The product [H₃O⁺][OH⁻] is always 10⁻¹⁴, regardless of solution conditions.
False
While Kw = 10⁻¹⁴ at 25°C, Kw varies with temperature.
List the ions present in a basic solution.
- Hydroxide ions (OH⁻)
- Some hydronium ions (H₃O⁺)
In a basic solution, OH⁻ ions outnumber H₃O⁺ ions, but both are present.
What is a dissociation constant?
It is the equilibrium constant for the dissociation of a substance in a solution.
The dissociation constant indicates how much of the substance dissociates into its ions at equilibrium.
What is Ka in chemistry?
Ka is the acid dissociation constant.
It is a measure of how much an acid dissociates in solution.
How is Ka used in acid-base equilibria?
Ka helps determine the strength of an acid and its dissociation in a solution.
Ka is a critical factor in determining how much an acid dissociates in water and its behavior in acid-base reactions.
True or False:
Strong acids always have Ka > 1.
True
A Ka > 1 indicates complete dissociation, characteristic of strong acids.
How is Ka calculated?
Ka = [H⁺][A⁻] / [HA]
This formula uses the concentrations of hydrogen ions (H⁺), conjugate base (A⁻), and the undissociated acid (HA).
What is the difference between strong and weak acids based on Ka?
- Strong acids: They have high Ka values.
- Weak acids: They have low Ka values.
Strong acids dissociate completely, while weak acids dissociate partially, leading to different Ka values.
What is the relationship between Ka and pKa?
pKa = -log(Ka)
pKa is the negative logarithm of Ka, providing a different scale to express the strength of acids.
What is the Ka expression for acetic acid?
Ka = [H⁺][C₂H₃O₂⁻] / [HC₂H₃O₂]
The Ka measures the extent of acid dissociation in water.
Calculate the Ka for acetic acid if the concentration of H⁺ is 0.0006 M and that of the acid is 1.2 M.
Ka = 3.0 x 10⁻⁷
This calculation uses the concentrations of H⁺ and acetic acid at equilibrium to find the Ka value.
Ka = (0.0006)(0.0006) / 1.2 = 3.0 x 10⁻⁷
What is the relationship between pH and Ka for weak acids?
A lower pH indicates a stronger acid with a higher Ka.
pH measures the concentration of hydrogen ions, which is related to the dissociation constant of an acid.
What does Kb represent in chemistry?
Kb is the base dissociation constant, a measure of how much a base dissociates.
Strong bases have high Kb values, while weak bases have low Kb values.
How is Kb calculated?
Kb = [BH⁺][OH⁻] / [B]
Kb is calculated similarly to Ka, using the concentrations of the conjugate acid (BH⁺), hydroxide ions (OH⁻), and the base (B).
How is Kb related to Ka for conjugate acid-base pairs?
Kb = Kw / Ka
The product of Ka and Kb for a conjugate acid-base pair is equal to the ionization constant of water (Kw), which is 1.0 x 10⁻¹⁴ at 25°C.
What is the pKa of a weak acid with a Ka of 1.0 x 10⁻⁵?
pKa = 5
pKa is the negative logarithm of Ka, so pKa = -log(1.0 x 10⁻⁵) = 5.
Which acid dissociates completely in water:
HCl or CH₃COOH
HCl
HCl is a strong acid, while acetic acid (CH₃COOH) is a weak acid that dissociates partially.
Define:
buffer solution
It is a solution that resists changes in pH.
A buffer is made of a weak acid and its conjugate base or vice versa.
Buffers help maintain stable pH levels in chemical and biological systems.
What type of acids and bases cannot form buffers?
Strong acids and bases.
Strong acids/bases completely dissociate in water, leaving no conjugate acid/base to establish equilibrium.
True or False:
A buffer is most effective when the acid-to-base ratio is 1:1.
True
At a 1:1 ratio, the buffer has maximum capacity to neutralize added acid or base.
What does the Henderson-Hasselbalch equation calculate?
The pH of a buffer solution.
It uses pKa and the ratio of the concentrations of conjugate acid and base.
What is the Henderson-Hasselbalch equation?
pH=pKa+log([base]/[acid])
This equation relates pH, pKa, and the ratio of base to acid concentrations in a buffer solution.
What is the pH of a buffer with equal concentrations of acid and base?
The pH is equal to the pKa.
At a 1:1 ratio, log(1)=0, so pH=pKa.
Which principle explains why buffers resist pH changes?
Le Chatelier’s principle
Adding acid or base shifts the equilibrium to minimize changes in hydrogen ion concentration.
True or False:
A strong base can be used to create a buffer by reacting with a weak acid.
True
This reaction forms the conjugate base of the weak acid, which helps maintain buffer equilibrium.
How does the pH change when there is more base than acid in a buffer?
The pH becomes higher than the pKa.
The logarithmic term in the Henderson-Hasselbalch equation increases the pH.
True or False:
A buffer solution can have a neutral pH.
False
Buffers never have a neutral pH because they consist of a weak acid/base pair.
What happens when a strong acid is added to a buffer?
The buffer neutralizes the acid by reacting with the conjugate base.
This minimizes the change in pH.
Define:
buffer capacity
The amount of acid or base a buffer can neutralize before its pH changes significantly.
Buffer capacity depends on the concentrations of the buffer components.
Which graph feature indicates the buffer region during titration?
A flat slope on the titration curve.
The flat slope corresponds to the buffer’s resistance to pH change.
