5.1.3 Acids, bases and buffers Flashcards
what is an acid (B-L)?
a proton donor
what is a base (B-L)?
a proton acceptor
what is a hydronium ion?
H3O+
when a water molecule form a dative bond with a proton
water exists in equilibrium with hydronium ions
what is the conjugate base pair of an acid?
an acid is a proton donor
the conjugate base is a product of the acid that needs a proton to become the acid again and so would act as a base
what is the conjugate acid pair of a base?
a base is a proton acceptor
the conjugate acid is a product of the base that needs to lose a proton to become the base again and so would act as an acid
what is the equation to calculate pH?
pH = - log [H+]
what is the equation to calculate [H+]?
[H+] = 10^-pH
what is the difference between strong and weak acids?
strong acids fully dissociate into their ions whereas weak acids only partly dissociate into their ions
what is the equation of Ka?
Ka = [products] / [reactants]
what does the Ka show?
the higher Ka value the stronger the acid
what is the dissociation of weak acids?
< 5% dissociation of [H+]
what does Ka equal for weak acids?
Ka = [H+]^2 / [HA]
how to calculate the value of [H+] from Ka?
[H+] = square root of (Ka x [HA])
why is pKa used for weak acids?
for weak acids there is a large range of very small Ka values
how to calculate pKa from Ka?
pKa = - log Ka
why does Kw exist?
water ionises partially and contains a very low concentration of ions. since very little water ionises, the conc of water molecules is virtually constant compared with the conc of the ions
so: Kc x [H2O] = Kw
what is the expression for Kw?
Kw = [H+] [OH-]
how to find [H+] from Kw? (strong base)
Kw / [OH-] = [H+]
why is the value of Kw temperature dependent?
value of Kc and Kw is temperature dependent as dissociation increases as temperature increases
What is a buffer solution?
- a solution that can minimise small changes in pH when small quantities of acid or base are added
How does ethanoic acid / sodium ethanoate act as a buffer?
- acid is added: H+ + CH3COO-
- equilibrium shifts to the left
- alkali is added: OH- + H+
- equilibrium shifts to the right as H+ is used up
Does the salt in a buffer fully dissociate?
- salt fully dissociates
- weak acid partially dissociates
What is the link between Ka and buffers?
Ka = [H+] [salt] / [acid]
[H+] = Ka [acid] / [salt]
What is the blood buffer made up of?
H2CO3 <=> HCO3 + H+
H2CO3 <=> H2O + CO2
- keeps the pH of the blood between 7.35 and 7.45
- prevent acidiosis
How does the carbonic acid-hydrogencarbonate ion system acts as a buffer?
- acid is added: H+ + HCO3-
- equilibrium will shift to the left
- alkali is added: OH- + H+
- equilibrium will shift to the right
What is the end point and equivalence point of a titration?
- end: equal concentrations of weak acid and conjugate base of indicator (when the indicator changes colour)
- equivalence: volume of one reactant has reacted completely with volume of second solution
How to choose correct indicator?
- changes colour in the pH corresponding to the sharp rise in the titration curve
- disqualify one - changes colour before sharp rise
Why can indicator not be used for weak acid weak base titrations?
- no sharp pH change at equivalence point
- indicator would change colour gradually
Where is the equivalence point on a titration curve?
- centre of the vertical sections of the titration curve
- volume of alkali added (cm3) on x axis
- pH on y axis
