3.9 Acid-base Equilibria Flashcards
Acids are
Proton donors
Bases are
Proton acceptors
A strong acid
Dissociates fully in water and at equilibrium will only have ions present
A weak acid
Dissociates only partly in water and at equilibrium has some ions but predominantly acid molecules
A dilute acid has
A low number of particles per unit volume
A concentrated acid has
A high number of particles per unit volume
A conjugate acid
Acts as an acid in the backward reaction
A conjugate base
Acts as a base in the backward reaction
Acid equilibrium constant is
Ka
Ka is
[H+][A-] / [HA]
The Ka of a strong acid is
High
The ionic product of water is
Kw=[H+][OH-]
pH=
-log10[H+]
How to calculate the pH of a strong acid
Since a strong acid completely dissociates the [H+] = [acid] and so pH = -log[acid]
How to calculate the concentration of a strong acid
Using [acid] = [H+] and [H+] = lnvlog(-pH)
pH of weak acids
pH = -log10(ailisradd Ka[HA])
pH of a strong base
[base] = [OH-]
[H+] = Kw/[OH-]
The pH at half-neutralisation is
pKa
What does a buffer solution do
Resists a change in pH when small amounts of acid or alkali are added
What does a buffer solution consist of
A weak acid and its salt
What happens when an acid is added to a buffer solution
• the concentration of H+ initially increases
• this causes the reversible reaction to shift left so that the H+ ions are removed by a reaction with the anion
What happens when an alkali is added to a buffer solution
• OH- ions combine with H+ ions to form water
• this reduces the concentration of H+ and shifts the reversible reaction right
• the acid dissociates to release more H+ ions
What assumptions are done during the calculation of the pH of buffers
• assume that all the salt dissociates
• assume none of the acid dissociates
When are buffer solutions usedv
• in many biochemical experiments where a constant pH is needed
• when enzymes are used or stored
• in the preparation of synthetic and processed food so they can be eaten and digested without undue change in pH
What is at the equivalence point of pH curves
A solution of the salt formed from the acid and base
The pH of this solution is the mid-point of the vertical part of the curve
Equivalence point of a strong acid/strong base
• pH=7
• The solution of the salt is neutral • water is slightly dissociated but there is no tendency for any of the ions yo interact with the ions from the water
Equivalence point of a strong acid/strong base
• pH = 4-6
• the solution and therefore the salt is acidic
• the cation present can dissociate to release H+ ions in a reversible reaction
Equivalence point of weak acid/strong base
• pH= 8-10
• the solution and therefore the salt is basic
• the salt releases all its anions and these react to remove H+ ions from the solution
How is a suitable indicator selected
The pH range of the indicator must lie on the vertical part of the curve
