21.6- BUFFER SOLUTIONS Flashcards
What are buffers?
solutions that can resist changes in pH when small amount of acid or alkali added to them
What are buffers designed to do?
keep conc. of hydrogen ions + hydroxide ions in solution almost unchanged
What are buffers based on?
equilibrium reaction which will move in direction to remove either additional hydrogen ions or hydroxide ions if these are added
Why do acidic buffers work?
as the dissociation of a weak acid is an equilibrium reaction
HA(aq) = H+(aq) + A-(aq)
What happens when alkali is added to an acidic buffer?
OH- ions from alkali will react with HA to produce water molecules and A-
HA(aq) + OH-(aq) -> H20(aq) + A-(aq)
this removes the added OH- so pH tends to remain almost same
What happens when acid is added to an acidic buffer?
equilibrium shifts to left- H+ ions combining with A- ions to produce undissociated HA
but, since [A-] small, supply of A- soon runs out + there’s no A- left to combine added H+
so solution not buffer
What is an acidic buffer made from?
mixture of weak acid + soluble salt of that acid it will remain at pH below 7 (acidic)
What is the function of the weak acid component of a buffer?
act as source of HA which can remove any added OH-
HA(aq) + OH-(aq) -> A-(aq) + H20(l)
What is the function of the salt component of a buffer?
act as source of A- ions which can remove any added H+ ions
A-(aq) + H+(aq) -> HA(aq)
How can a buffer be saturated?
add so much acid or alkali that all the available HA or A- used up
What is another way of achieving a mixture of weak acid + its salt?
by neutralising some of the weak acid with alkali i.e. sodium hydroxide
What happens if you neutralise half the acid? (acidic buffer)
end up with buffer whose pH equal to pKa of acid as it has an equal supply of HA and A-
What is the pH equal to at half-neutralisation?
pH=pKa
Whys is the buffer useful when the pH=pKa?
as it’s equally efficient at resisting a change in pH whether acid or alkali added
What pH do basic buffers maintain?
pH above 7
