Buffers
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Topic Summary & Highlights
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Core Concept
A buffer is a solution that resists changes in pH when small amounts of acid ($H^+$) or base ($OH^−$) are added.
Key Components:
Weak Acid and its Conjugate Base:
Example: Acetic acid ($CH_3COOH$) and acetate ($CH_3COO^−$).
Weak Base and its Conjugate Acid:
Example: Ammonia ($NH_3$) and ammonium ($NH_4^+$).
Practice Tips
Buffers maintain a stable pH by neutralizing small amounts of added acid or base.
They consist of a weak acid and its conjugate base, or a weak base and its conjugate acid.
The Henderson-Hasselbalch equation is a vital tool for calculating buffer pH.
Buffer capacity increases with higher concentrations of buffering components and is optimal when [A−]≈[HA][\text{A}^-] \approx [\text{HA}][A−]≈[HA].
How Do Buffers Work?
Buffers work through equilibrium reactions that neutralize added H+H^+H+ or OH−OH^-OH− ions:
If Acid (H+H^+H+) is Added:
The conjugate base in the buffer reacts with the H+H^+H+, minimizing the increase in H+H^+H+ concentration.
Example: CH3COO−+H+→CH3COOHCH_3COO^- + H^+ \rightarrow CH_3COOHCH3COO−+H+→CH3COOH
If Base (OH−OH^-OH−) is Added:
The weak acid in the buffer reacts with OH−OH^-OH−, minimizing the increase in OH−OH^-OH− concentration.
Example: CH3COOH+OH−→CH3COO−+H2OCH_3COOH + OH^- \rightarrow CH_3COO^- + H_2OCH3COOH+OH−→CH3COO−+H2O
Types of Buffers
Acidic Buffers:
Made from a weak acid and its conjugate base.
Example: Acetic acid (CH3COOHCH_3COOHCH3COOH) and sodium acetate (CH3COONaCH_3COONaCH3COONa).
pH < 7.
Basic Buffers:
Made from a weak base and its conjugate acid.
Example: Ammonia (NH3NH_3NH3) and ammonium chloride (NH4ClNH_4ClNH4Cl).
pH > 7.
The Henderson-Hasselbalch Equation
The Henderson-Hasselbalch equation is used to calculate the pH of a buffer solution:
pH=pKa+log([A−][HA])\text{pH} = \text{p}K_a + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right)pH=pKa+log([HA][A−])
Where:
[A−][\text{A}^-][A−]: Concentration of the conjugate base.
[HA][\text{HA}][HA]: Concentration of the weak acid.
pKa\text{p}K_apKa: Negative logarithm of the acid dissociation constant (KaK_aKa).
Key Points:
Buffers are most effective when [A−]≈[HA][\text{A}^-] \approx [\text{HA}][A−]≈[HA], or when pH≈pKa\text{pH} \approx \text{p}K_apH≈pKa.
The effective buffering range is typically pKa±1\text{p}K_a \pm 1pKa±1.
Buffer Capacity
Definition: The amount of acid or base a buffer can neutralize without a significant change in pH.
Factors Affecting Buffer Capacity:
Higher concentrations of the weak acid and conjugate base increase buffer capacity.
Optimal when [A−]≈[HA][\text{A}^-] \approx [\text{HA}][A−]≈[HA].