IB DP Chemistry: HL复习笔记18.1.4 Buffers

Category: IB课程, 教材笔记, 福利干货 Date: 2022年7月29日下午12:30

Buffers

A buffer solution is a solution which resists changes in pH when small amounts of acid or base are added
- A buffer solution is used to keep the pH almost constant
- A buffer can consist of weak acid – conjugate base or weak base – conjugate acid

Ethanoic acid & sodium ethanoate as a buffer

A common buffer solution is an aqueous mixture of ethanoic acid and sodium ethanoate
Ethanoic acid is a weak acid and partially ionises in solution to form a relatively low concentration of ethanoate ions

CH₃COOH (aq) ⇌ H⁺(aq) + CH₃COO^-(aq)

ethanoic acid ⇌ ethanoate

high conc ⇌ low conc

Sodium ethanoate is a salt which fully ionises in solution

CH₃COONa + aq → Na⁺ (aq) + CH₃COO^-(aq)

sodium ethanoate → ethanoate ion

low conc. → high conc.

There are reserve supplies of the acid (CH₃COOH) and its conjugate base (CH₃COO^–)
- The buffer solution contains relatively high concentrations of CH₃COOH (due to the partial ionisation of ethanoic acid) and CH₃COO^– (due to the full ionisation of sodium ethanoate)
In the buffer solution, the ethanoic acid is in equilibrium with hydrogen and ethanoate ions

CH₃COOH (aq) ⇌ H⁺(aq) + CH₃COO^-(aq)

high conc. high conc.

Adding H⁺ ions to a buffer solution:

The equilibrium position shifts to the left as H⁺ions react with CH₃COO^– ions to form more CH₃COOH until equilibrium is re-established
As there is a large reserve supply of CH₃COO^–, the concentration of CH₃COO^– in solution doesn’t change much as it reacts with the added H⁺ions
As there is a large reserve supply of CH₃COOH, the concentration of CH₃COOH in solution doesn’t change much as CH₃COOH is formed from the reaction of CH₃COO^– with H⁺
As a result, the pH remains reasonably constant

5.5-Equilibria-Effect-of-adding-H

When hydrogen ions are added to the solution the pH of the solution would decrease. However, the ethanoate ions in the buffer solution react with the hydrogen ions to prevent this and keep the pH constant

Adding OH^– ions to a buffer solution:

The OH^– reacts with H⁺ to form water

OH^– (aq) + H⁺(aq) → H₂O (l)

The H⁺ concentration decreases
The equilibrium position shifts to the right and more CH₃COOH molecules ionise to form more H⁺ and CH₃COO^– until equilibrium is re-established

CH₃COOH (aq) ⇌ H⁺ (aq) + CH₃COO^– (aq)

As there is a large reserve supply of CH₃COOH, the concentration of CH₃COOH in solution doesn’t change much when CH₃COOH dissociates to form more H⁺ions
As there is a large reserve supply of CH₃COO^–, the concentration of CH₃COO^– in solution doesn’t change much
As a result, the pH remains reasonably constant

When hydroxide ions are added to the solution, the hydrogen ions react with them to form water; The decrease in hydrogen ions would mean that the pH would increase however the equilibrium moves to the right to replace the removed hydrogen ions and keep the pH constant

Exam Tip

Remember that buffer solutions cannot cope with excessive addition of acids or alkalis as their pH will change significantly.The pH will only remain relatively constant if small amounts of acids or alkalis are added.

The Buffer Region

These are seen on a pH curve and represent the region where small additions in acid or base result in little change in the overall pH of the solution
For a weak acid - strong base titration between sodium hydroxide, NaOH (aq) , and ethanoic acid, CH₃COOH (aq)

NaOH (aq) + CH₃COOH (aq) → CH₃COONa (aq) + H₂O (l)

The initial rise in pH is steep as the neutralisation of the weak acid by the strong base is rapid
Sodium ethanoate is then formed which creates a buffer that will resist changes in pH so the pH rises gradually
- A buffer consists of a weak acid and its conjugate salt
This section of the curve is known as the buffer region
Only small increases in pH are seen when the strong base is added in this part of the titration curve
At this point, the acid is in equilibrium with the salt ion
- CH₃COOH (aq) + H₂O (l) ⇌ CH₃COO^-+ H₃O⁺ (aq)
Continuing to add the strong base will 'use up' all of the acid so the forward reaction will then be favoured

18.1-Weak-acid-Strong-base-buffer

Buffer region shown is a weak acid - strong base titration curve

A buffer region is also observed during a weak base - strong acid titration
- Ammonia, NH₃ (aq), and hydrochloric acid, HCl (aq), is a common example

NH₃ (aq) + HCl (aq) → NH₄Cl (aq)

The starting pH of the weak base will fall as it begins to be neutralised and the conjugate acid, NH₄⁺ (aq), will be produced
This results in a buffer that resists change in pH
Due to this, the pH will only fall gradually

18.1-Weak-base-Strong-acid-buffer