(blue) buffers indicators pH titation curves Flashcards
(blue) buffers indicators pH titation curves
what is a buffer
can resist changes in pH despite the addition of small amounts of acid or alkali.
a buffer is a solution containing
A weak acid and the salt of that weak acid eg CH3COOH(aq) + CH3COONa(aq)
OR: A weak alkali and the salt of that weak alkali. eg NH4OH(aq) + NH4Cl(aq)
how does a buffer work
Buffers are examples of equilibrium systems. A typical buffer consists of a weak acid AND its salt, for buffers below 7 or a weak base AND its salt for buffers above 7
in a Ethanoic acid (acid) and Sodium ethanoate (salt)
If you add Acid to the system, What happen?
If you add alkali to the system what happens?
acid :
A buffer contains a large reservoir of CH3COOH and CH3COO- . On addition of a small amount of acid (H+) the equilibria position shifts to the LHS the H+ reacting with the CH3COO- therefore the ration of [H+] / [CH3COO-] hardly changes and pH stays roughly constant.
base
A buffer contains a large reservoir of CH3COOH and CH3COO- . On addition of a small amount of alkai (OH-) the H+ ions react with the OH- ions. the equilibria position shifts to the RHS, more acid then dissociates to maintain the concentration of H+ , therefore the ration of [H+] / [CH3COO-] hardly changes and pH stays roughly constant.
expalin how this :
CH3COOH CH3COO- + H+
acts as a buffer solution
Show that the H+ concentration does not vary.
On addition of H+ ions the above equilibrium shifts to the LHS, H+ combine with ethanoate ions to form ethanoic acid so removing the added H+ from solution and keeping [H+] relatively constant.
On addition of OH- they combine with the H+ in solution. The [H+] is kept relatively constant by dissociation of more ethanoic acid (equilibrium shifts to RHS).
[CH3COOH(aq)]original = [CH3COOH(aq)]equilibrium
why can we make this assumption
Ethanoic acid is a weak acid with very little/ no dissociation, dissociation is further suppressed by the high concentration of CH3COO- from the salt.
because its also soluable
what is the Ka expression
Ka = [salt] x [H+] / [acid]
how do you find pH from Ka
when salt = acid , which is at half equivulance point then just minus log it and then you get pH
what is the equivulance point
The equivalence point is the point at which: no. moles H+(aq) = no. moles OH-(aq)
what is an indicator
An indicator is a weak acid or a weak base, which has one colour in the undissociated form and a different colour as a dissociated ion.
what is the ideal indicator
The ideal indicator for a given acid - base titration is one that is in the middle of its colour change at the pH of the equivalence point of the titration. i.e. end point = equivalence point
