Equilibrium 5 Flashcards
how does strength of acids depend on bond sttength of acid
In general, when strength of H-A bond
decreases, that is, the energy required to break
the bond decreases, HA becomes a stronger
acid.
But it should be noted that while
comparing elements in the same group of the
periodic table, H-A bond strength is a more
important factor in determining acidity than
its polar nature. As the size of A increases
down the group, H-A bond strength decreases
and so the acid strength increases.
how does strength of acid depend upon polarit of bond
Also, when the H-A bond becomes more
polar i.e., the electronegativity difference
between the atoms H and A increases and
there is marked charge separation, cleavage
of the bond becomes easier thereby increasing
the acidity.
s in the same
row of the periodic table, H-A bond polarity
becomes the deciding factor for determining
the acid strength. As the electronegativity of A
increases, the strength of the acid also
increases.
what can act as acid and base
Some substances like water are unique in their
ability of acting both as an acid and a base.
In presence of an acid, HA it accepts a
proton and acts as the base while in the
presence of a base, B–
it acts as an acid by
donating a proton. In pure water, one H2O
molecule donates proton and acts as an acid
and another water molecules accepts a proton
and acts as a base at the same time
H2O(l) + H2O(l) ⇌ H3O+
(aq) + OH–
(aq)
acid base conjugate conjugate
acid base
this is c/a autoprotolysis
what is ionic product of water
It is the product of molar concentration of h+.and oh- ions in water or in any aqueous solution;The concentration of H
+
has been found
out experimentally as 1.0 × 10–7 M at 298 K.
And, as dissociation of water produces equal
number of H+
and OH
–
ions, the concentration
of hydroxyl ions, [OH
–
] = [H+
] = 1.0 × 10–7 M.
Thus, the value of Kw
at 298K
Kw= 1 x 10-14 M^2
value of Kw increases with increase in temp because ionisation fo water increases
what is pH
it is the negative logarithm to base of 10 of the activity of hydrogen ions or hydrogen ion concentration.
Acidic solution has pH < 7
Basic solution has pH > 7
Neutral solution has pH = 7
Kw
may change with
temperature the variations in pH with
temperature are so small that we often
ignore it
what are the ways to measure ph
The pH of a solution can be found
roughly with the help of pH paper that has
different colour in solutions of different pH.
Now-a-days pH paper is available with four
strips on it. The different strips have different
colours (Fig. 7.11) at the same pH. The pH in
the range of 1-14 can be determined with an
accuracy of ~0.5 using pH paper
For greater accuracy pH meters are used.
pH meter is a device that measures the
pH-dependent electrical potential of the test
solution within 0.001 precision.
limitations of pH
-pH does not give coorect idea
solution with pH 2 has 100 times more hydrogen ion concentration than a solution with pH 3
- Ph of a 1M acid can be 0. It can even be -ve for 2M, 3M acids
-pH of a 10-8 M acid cannot be 8. it will be close to 7 . similarly 10-8M of NaOH cannot be 8.
what is acid dissociation constant
It is the equilibrium constant for the acid dissociation reaction
It is defined product of molar concentrations of the ions divided by the molar conc of acid
Ka= [H+][X] / [HX]
At a given temperature T, Ka
is a
measure of the strength of the acid HX
i.e., larger the value of Ka
, the stronger is
the acid. Ka
is a dimensionless quantity
with the understanding that the standard
state concentration of all species is 1M.’
ka= x^2/ c-x
if x< 5% of c, then ka= x^2/ c (basically only for weak acids)
Ka= cα^2/1-α
pKa
= –log (Ka
)
what is Kb
The equilibrium constant for the equilibrium established when a weak base reacts with water is referred to as the base dissociation constant.
Kb
= (cα)
2
/ c (1-α) = cα
2
/ (1-α)
Kb= [M+][OH–] / [MOH]
what are polybasic acid
Some of the acids like oxalic acid, sulphuric
acid and phosphoric acids have more than one
ionizable proton per molecule of the acid.
Such acids are known as polybasic or
polyprotic acids.
The ionization reactions for example for a
dibasic acid H2
X are represented by the
equations:
H2X(aq) ⇌ H+(aq) + HX–(aq)
HX–(aq) ⇌ H+(aq) + X2– (aq)
for polyprtic acid, why is Ka1> Ka2> Ka3
It can be seen that higher order ionization
constants (K K a a 2 3
, ) are smaller than the
lower order ionization constant ( a1
K ) of a
polyprotic acid. The reason for this is that it is
more difficult to remove a positively charged
proton from a negative ion due to electrostatic
forces. This can be seen in the case of removing
a proton from the uncharged H2CO3
as
compared from a negatively charged HCO3
–
.
Similarly, it is more difficult to remove a proton
from a doubly charged HPO4
2– anion as
compared to H2PO4-
define common ion effect
the degree of dissociation of an electrolyte(weak acid/nase) is suppressed by the addition of another electrolyte having a commonion
explain common ion effect
CH3COOH—->CH3COO- +H+
If to the abv eqbm, another electorlyte is added so that the conc of acetate or h+ ions increases, then to keep ka a constant or to reestablish equilibrium the reactions shifts backward towards ch3cooh, due to which dissociation reduces. this is an effect of le chateliers principle.
what is salt hyrdolysis
the reaction between salt and water to give the back the corrsponding acid and base is called hydrolysis.
The cations/anions formed on ionization of salts either exist as hydrated
ions in aqueous solutions or interact with
water to reform corresponding acids/bases
depending upon the nature of salts.
