AS/A-Level Chemistry- Buffers

we look at buffers here, which is related to the weak acids and bases in A-Level Chemistry

Buffers

• RESIST changes in pHwhen small amounts of acid/alkali are added 
• Doesn’t stop the pH from changing completely  
• They only work when small amounts of acids/alkalis are added 

Acidic buffers

Weak acid + Salt 

1) CH₃COO^-Na⁺(aq) —> CH₃COO^-(aq) + Na⁺(aq)  

This fully dissociates; therefore there are mostly ethanoate ions   

 2) CH₃COOH(aq) <--> CH₃COO^-(aq) +H⁺(aq) 

This only slightly dissociates; therefore there are mostly ethanoic acid

ADDING ACID: (small amount) 

[H⁺] increases which combines with the CH₃COO^- to form CH₃COOH so equilibrium shifts to left, no change in pH.   

ADDING ALKALI: (small amount) 

[OH^-] increases which combines with the H⁺ to form H₂O which removes the H⁺ ions from solution, so more H⁺ dissociate from CH₃COOH so equilibrium shifts to right, no change in pH.

Alkaline buffers

Weak acid + Salt 

1) NH₄Cl(aq)—> NH₄⁺(aq)+ Cl ^-(aq) 

This fully dissociates; therefore mostly there are ammonium ions NH₄⁺(aq) 

 2) H⁺(aq) + NH₃(aq) 

This only slightly dissociates; therefore mostly there are ammonium NH₃

ADDING ACID: (small amount) 

[H⁺] increases which combines with the NH₃ to form NH₄ so equilibrium shifts to left, no change in pH.  

ADDING ALKALI: (small amount) 

[OH^-] increases which combines withthe H⁺ to form H₂O which removes the H⁺ ions from solution, so more H⁺ dissociate from NH₄⁺so equilibrium shifts to right, no change in pH.

Biological environments (don’t need to learn but be aware of) 

1) Cells– need constant pH for biochemical reactions to take place

Controlled by the equilibrium between dihydrogen phosphate and hydrogen phosphate   

H₂PO₄^-  <--> H⁺+ HPO₄²

2) Blood– need to be kept at pH 7.4 

Carbonic acid (H₂CO₃)   

H₂CO₃ <--> H₂O+ CO₂

Lungs - by breathing out CO₂, levels of H₂CO₃  decrease and so equilibrium moves to the right   

H₂CO₃ <-->  H⁺ + HCO₃^-

Kidneys control this equilibrium

3) Food products – changes in pH occur due to fungi and bacteria

Sodium citrate   

Citric acid <--> 

Or 

Phosphoric acid <-->  phosphate ions 

Or 

Benzoic acid <--> benzoate ions 

CALCULATION:

A buffer solution of 0.4 mol dm^-3 of  methanic acid and 0.6  mol dm^-3 sodium methanoate.  

For a methanoic acid with K_a=1.6 x 10^-4 moldm^-3 what is the pH of the buffer?  

K_a= [H⁺] [HCOO^-] / [HCOOH]

[H⁺] = K_a ( [HCOOH] / [HCOO^-] )

pH = -log [H⁺]

pH= 3.87

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