Theories of Acids and Bases 08 Flashcards
Arrhenius theory of acids and bases
An acid is a substance that dissolves in water to produce H+
A base is a substance that dissolves in water to produce OH-
Bronsted Theory of acids and bases
An acid is defined as any species that donates a proton, H+
A base is defined as any species that accepts a proton OH-
A bronsted acid-base reaction
involves the transfer of an electron from an acid to a base
Where do bronsted reactions occur
aqueous solution, gases, non-aqueous solutions and homogenous mixtures
how to form a conjugate base
an acid after donating a proton
how to form a conjugate acid
a base after accepting a proton
Why is the reversible reaction of a conjugate base of a strong acid (HCl) negligible?
Conjugate bases of strong acids have a lower tendency to accept protons
Why is does the conjugate base of a weak acid have a reversible reaction?
Conjugate bases of weak acids have a higher tendency of accepting protons
formula to find pH
-lg[H+]
formula to find concentration of H+
[H+]=10^-pH
formula to find pOH
-lg[OH-]
14-(-lg[OH-])
formula to find concentration of [OH-]
10^-pOH
Equilibrium constant, Kc is
[H+][OH-]/[H2O]
Kw, ionic product of water is:
Kw=[H+][OH-]
=Kc x [H2O]
Kw depends only on temperature, and as always constant at a constant temperature
acid dissociation constant, Ka
pKa
-lg[Ka]
what does Ka and Kb depend on?
temperature only
pKb
-lg Kb
degree of dissociation, a
a
=amount ionised/initial amount
=[acid]dissociated/[acid]initial
for strong acids, a is close to 1
for weak acids, a is «_space;1
*how to determine the POE and determination of the weaker/stronger acid/base
if the Kc value is very small, the POE lies on the EXTREME LEFT, the reaction favours the backward reaction, hence the reactants on the right side is the stronger base/acid and is a better proton acceptor/donor
*definition of a buffer solution
it resists pH changes upon addition of a small amount of acid or base.
an acidic buffer contains a weak acid and its conjugate base
a buffer contains a WEAK bronsted acid and a WEAK bronsted base, forming a conjugate acid-base pair.
- how does a buffer work?
when a SMALL amount of H+ is added to the solution, the H+ ions react with the large reservoir of xxxOH-, the conjugate base, hence, the added H+ ions are removed and the pH remains almost constant.
strong acid-strong base titration suitable indicator
methyl orange or thymol blue
weak acid-strong base indicator
thymol blue
strong acid-weak base indicator
methyl orange
at the equivalence point:
there is a sharp increase in pH as the acid has been completely neutralised
why is there a buffer solution formed in weak-acid strong base but no buffer solution formed in strong acid-weak base?
once the strong base is added and some acid is neutralised, a weak acid and its salt (conjugate base) forms, therefore buffer solution is formed. the buffer solution causes the part before the steep increase at the equivalence point have a gentle upward gradient.
when the weak base is added, the H+ ions from the strong acid neutralises it, hence there is no weak base to form a buffer solution
working range of methyl orange and thymol blue
MO: 3.1-4.4, from red to yellow
TB: 8.0-9.6, from yellow to blue
Kw
[H+][OH-]
pKw= pH + pOH
affected only by temperature
