Buffers

Define buffer solution

A solution that minimises changes in pH when small amounts of acid or alkali are added to it.

What is an acidic buffer made from (+example)

From a mixture of weak acid and the salt of its conjugate base (eg CH3COOH/CH3COONa)

What is a basic buffer made from (+ example)

A mixture of a weak base and the salt of its conjugate acid (eg. NH3/NH4Cl)

Write the equation for an acidic buffer (the weak acid and its salt)

• CH3COOH ←→ CH3COO- + H+

• CH3COONa ←→ CH3COO- + Na+

What is the effect of adding a small amount of acid to an acidic buffer

H+ + CH3COO- → CH3COOH

What is the effect of adding a small amount of alkali to an acidic buffer

• CH3COOH + OH- → CH3COO- + H2O

• H+ + OH- → H2O, equilibrium shifts right to replace H+

Write the equation for an basic buffer (the base acid and its salt)

• NH3 + H2O → NH4+ + OH-

• NH4Cl → NH4 + Cl-

What is the effect of adding a small amount of acidic to an basic buffer

• H+ + NH3 → NH4+

• H+ + OH- → H2O, equilibrium shifts right to replace OH-

What is the effect of adding a small amount of alkali to an basic buffer

NH4+ + OH- → NH3 + H2O

How can you make an acidic buffer

1. Mix a weak acid and the salt of its conjugate base

2. Mix a strong base with an excess of weak acid, therefore forming the salt of its conjugate base with leftover of the weak acid

How is the blood pH controlled (give equations)

a buffer of carbonic acid and hydrogen carbonate anion

• H+ + HCO3- → H2CO3

• OH- + H2CO3 → HCO3- + H2O

What happens if there is excess carbon dioxide in the blood

• CO2 reacts with excess carbon dioxide in the blood plasma

• Forms H2CO3

• H+ + HCO3- from H2CO3

• H+ reacts with the HCO3- reforming H2CO3 therefore suppressing the formation of H+

What happens if there is too much carbonic acid in the blood stream

It will convert to water and carbon dioxide which will be released in the lungs

Why does adding base/acid hardly change the pH of a solution

• large reservoir of salt/conjugate base

• React with the H+/salt

• Small change in [acid]:[salt] stays constant