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.
