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AQA A Level Chemistry復習筆記5.6.5 Buffers

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年6月30日下午5:43

Acid & Basic Buffers

A?buffer solution?is a solution which resists changes in pH when small amounts of acids or alkalis are added
- A buffer solution is used to keep the pH almost constant
- A buffer can consists 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

5.-Buffers-equation-1

Sodium ethanoate is a?salt?which fully ionises in solution

5.-Buffers-equation-2

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

5.-Buffers-equation-3

When H+?ions are added:
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

When OH-?ions are added:
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

5.5-Equilibria-Effect-of-adding-OH

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.

Buffer Calculations

The pH of a?buffer?solution?can be calculated using:
- The?Ka?of the?weak acid
- The?equilibrium concentration?of the?weak acid?and its?conjugate base?(salt)
To determine the pH, the concentration of?hydrogen?ions?is needed which can be found using the equilibrium expression

6.-Calculating-pH-of-Buffer-Solutions-equation-1

To simplify the calculations,?logarithms?are used such that the expression becomes:

6.-Calculating-pH-of-Buffer-Solutions-equation-2

Since -log10?[H+] = pH, the expression can also be rewritten as:

5.6.5-Hendersen-equation-3

This is known as the Hendersen-Hasselbalch equation

Exam Tip

Note that when you change the sign of the log, the terms become inverted so the salt concentration is on the top - be careful it is very easy to get this the wrong way around.

-log (A/B) = +log(B/A)

Worked Example

Calculating the pH of a buffer solutionCalculate the pH of a buffer solution containing 0.305 mol dm-3?of ethanoic acid and 0.520 mol dm-3?sodium ethanoate.The?Ka?of ethanoic acid? = 1.74 × 10-5?mol dm-3

Answer

Ethanoic acid is a weak acid that ionises as follows:

CH3COOH (aq) ? H+?(aq) + CH3COO-?(aq)

Step 1:?Write down the equilibrium expression to find?Ka

6.-Calculating-pH-of-Buffer-Solutions-equation-4

Step 2:?Rearrange the equation to find [H+]

6.-Calculating-pH-of-Buffer-Solutions-equation-5

Step 3:?Substitute the values into the expression

5.6.5

= 1.02 x 10-5?mol dm-3

Step 4:?Calculate the pH

pH = - log [H+]

= -log 1.02 x 10-5

= 4.99

Applications of Buffers

Uses of buffer solutions in controlling the pH of blood

In humans, HCO3-?ions act as a buffer to keep the blood pH between 7.35 and 7.45
Body cells produce CO2?during?aerobic respiration
This CO2?will combine with water in blood to form a solution containing H+?ions

CO2?(g) + H2O (l) ? H+?(aq) + HCO3-?(aq)

This equilibrium between CO2?and HCO3-?is extremely important
If the concentration of H+?ions is not regulated, the blood pH would drop and cause ‘acidosis’
- Acidosis?refers to a condition in which there is too much acid in the body fluids such as blood
- This could cause body malfunctioning and eventually lead to coma
If there is an increase in H+?ions
The equilibrium position shifts to the?left?until equilibrium is restored

H+?(aq) + HCO3-?(aq) ? CO2?(g) + H2O (l)

This reduces the concentration of H+?and keeps the pH of the blood?constant
If there is a decrease in H+?ions
- The equilibrium position shifts to the?right?until equilibrium is restored

CO2?(g) + H2O (l) ? H+?(aq) + HCO3-?(aq)