Equilibrium, Acids, Bases & Buffers Flashcards
Equilibrium constant, Kc for the reaction A + 2B –> C + D is
[C][D] / [A][B]^2
What effect does an increase in temperature have on Kc for an exothermic reaction?
An increase in temperature shifts the equilibrium in the endothermic direction. This means the number of moles of the reactants increases, and so Kc decreases
Kc is unaffected by….
Changes in concentration or pressure, or by the presence of a catalyst
Define acid
A species capable of donating a proton
Define base
A species capable of accepting a proton
Define a weak acid
Partially dissociates in solution
Define conjugate acid-base pairs and give an example
A pair of two species that transform into each other by gain or loss of a proton
Eg HNO2 and NO2-
Define a strong acid
Fully dissociates in solution
Define pH
-log[H+]
Give some examples of strong acids
Hydrochloric, nitric, sulphuric, hydrobromic
Give some examples of weak acids
Phosphoric, sulfurous, methanoic, ethanoic
Define Ka, the acid dissociation constant
[H+][A-] / [HA]
For a weak acid, it can be assumed that…
[H+] = [A-]
What does a large Ka value tell us?
A large Ka means the equilibrium is primarily to the right, which indicates a large extent of dissociation, and so a strong acid
pKa =
-log[Ka]
For a strong acid it can be assumed…
[H+] = [HA]
What is Kw, the ionic product of water
[H+][OH-]
For a strong base, it can be assumed…
[OH-] = [base]
Describe a buffer solution
A system that minimises pH changes on addition of small amounts of an acid or a base
A buffer solution can be made up of…
A weak acid and the salt of the weak acid
Eg CH3COOH and CH3COONa
Describe how a buffer solution minimises changes in pH on the addition of an acid
HA –> A- + H+
[H+] is increased
The conjugate base, A-, reacts with H+ ions
The equilibrium shifts to the left, removing most of the added [H+] ions
pH of a buffer solution =
pKa + log ([A-]/[HA])
Describe how a buffer solution minimises changes in pH on the addition of an alkali
HA –> A- + H+
[OH-] is increased
Some of the H+ ions react with the OH- ions
OH- + H+ –> H2O
This causes more of HA to dissociate, shifting the equilibrium to the right, to restore most of the H+ ions that have reacted
What is the equivalence point of a titration curve?
The point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution
(The centre of the vertical section of the titration curve)
What buffer is used to control blood pH?
Carbonic acid acts as the weak acid
H2CO3 –> HCO3- + H+
Define the end point of a titration
The point in a titration at which there are equal concentrations of the weak acid and conjugate base forms of the indicator
There are no indicators suitable for a titration of a…
Weak acid with a weak base. There is no vertical section to the titration curve, an indicator would change colour gradually over a few cm^3
Define standard enthalpy change of neutralisation
The energy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form one mole of H2O under standard conditions
Definition of an alkali
A soluble base that realises OH- ions in solution
What acids have a Ka value?
Weak acids
What is the percentage molar dissociation of an acid?
[H+] / [HA] x 100
