Topic 12: Acid and Base Equilibria Flashcards
What is the definition of an acid?
What is the definition of an base?
A proton donor
A proton acceptor
How can a proton be represented as?
H+
H3O+
What is the definition of a conjugate base?
What is the definition of a conjugate acid?
Conjugate base is the species remaining after an acid loses a proton
Conjugate acid is the species remaining after a base gains a proton
Species with conjugate acid-base pair always differ by one proton
What is an amphiprotic species?
A specie that can act as both an acid and a base
E.g: Water, HCO3-
What is pH?
A measure of concentration of hydrogen ions.
pH = -log[H+]
[H+] = 10^(-pH)
What is pOH?
A measure of concentration of hydroxide ions
pOH= -log[OH-]
[OH-] = 10^(-pOH)
What is the difference between strong acid and weak acid?
Strong acid is an acid which fully dissociates
Weak acid is an acid which partially dissociates into H+ and ion
What is the expression for Kw?
Why is [water] not included and why does Kw increase as temp increase?
Kw = [H+][OH-]
Water only slightly dissociates so [water] is effectively constant. Dissociation is endothermic, so equilibrium moves to the RHS to absorb heat.
Dilutions of Acids by 10, 100 and 1000
If strong acids are diluted by a factor of 10, pH increases by 1
Diluted by 100, pH increases by 2
Diluted by 1000, pH increases by 3
Dilutions cannot take the pH past 7
What is the expression for Ka?
What are the assumptions?
Ka is the extent of dissociation of the acid (dissociation constant)
The greater the value of Ka = more dissociation
Ka = [A-][H+]/[HA]
Assumptions:
Equilibrium concentration = initial concentration as ionisation of weak acid is so small
Ionisation of water is negligible so [H+] = [A-]
What is a buffer solution?
Resist changes in pH when small amount of acid or base are added, as there’s a large reservoir of acid and salt
They contain conjugate acid/base pairs (weak acid and salt/ weak base and salt)
How to form a buffer solution?
Mixing acid and salt directly
Adding strong base to excess weak acid
Blood acting as a buffer
pH of blood maintained with carbonic acid and hydrogencarbonate ions
Large reservoire of HCO3- that can combine with H+
Why is the enthalpy of neutralisation similar for strong acid + strong base?
Because the ionic equation is always the same (H+ + OH- –> H2O)
So the enthalpy of neutralisation is approximately the same
Why are weak acid + strong base enthalpy of neutralisation less exothermic?
You cannot write a single ionic equation because the weak acid doesn’t fully dissociate. So enthalpy of dissociation is added as well (always endothermic). The weaker the acid, the more endothermic the dissociation.
How do indicators work?
They are weak acids with the colour of the conjugate base different to the weak acid.
Relation between pH, pKind and equivalence point
When equivalence point of titration is reached, [HInd] = [Ind-] so Kind = [H+]. So the pH is equal to the pKInd
How to find pKa of a titration curve?
The pH of half the volume of the equivalence point is the pKa. As [A-] = [NaA], and [HA] = [NaA].
Describe and explain the buffer region
There will be weak acid and acid salt ions present. When a excess weak acid from the large reservoir reacts with the strong base, they form acid salt ions to resist changes to H+ and OH-. The concentrations of Weak acid and weak acid salt remain unchanged.
What are the assumptions when calculating the pH of weak acids?
The ionisation of water is negligible, so all [H+] ions comes from weak acid
The ionisation of the acid is so small, so [HA] equilibrium = [HA] initial
How to find the pH at the equivalence point?
pH = pKa
So [H+] = Ka
