Water is a polar ,protic inorganic compound that is at room
temperature a tasteless and odourless liquid nearly colourless with hint of blue.
It covers about 71.4% of earth. Water is universal solvent due to its ability
to dissolved many substance .The IUPAC name of is Water and
Oxidane.
On applying the law of mass action at equilibrium,
Since, ionization takes places to a very small extent, so
the concentration of unionized water molecule is regarded as constant. Thus the
product of K[H2O] gives another constant Kw.
The product of concentration of H+ and OH. ion in water
at a particular temperature is known as ionic
product of water.
IMPORTANT POINT ABOUT WATER:
(1) Mass of 1 litre of water = 997 gm.
(2) Molar concentration of water = 55.5 gm-mole / litre.
(3) Number of water molecule in 1 litre of water = 55.5×6.023×1023 = 3.34×1025.
(4) Concentration of H+ ion in one litre of
neutral water = 10-7 moles /
litre.
(5) Concentration of OH- ion in one litre of neutral
water = 10-7 moles /
litre.
(6) Number of H+ ion in one litre of neutral water = 6.023 x 1016.
(6) Number of OH.- ion in one litre of neutral water = 6.023 x 1016.
IONIZATION CONSTANT (Ki) OR
DESSOCIATION CONSTANT (Kd) OF WATER:
At 25c Kw=1x10-14 and conc. of water 1000/18=55.55
mole /liter
Ki or Kd are
also called acid or base constant of water.
Hence Pka and
Pkb of water is equal (Pka=Pkb)
DISSOCIATION CONSTANT OF WATER (DOD):
Pure water is
weak electrolyte and dissociates as
EFFECT OF TEMPERATURE ON IONIC PRODUCT
OF WATER ( Kw):
(1) Ionisation constant of water is endothermic process, so
on increasing temperature Keq increase. Kw increases with increase in
temperature.
(2) During self ionisation of water dissociation concentration of water is remain
constant=55.55 mole /liter.
(3) Kw is a thermodynamics
equilibrium constant for water it depend upon only temperature, if temperature
is increases then value of Kw is increases. and temperature is decreases then Kw
decreases
(4) The numerical
value of Kw increases considerably with temperature from 0.11 × 10–14 at 0°C to
50 × 10–14 at 100°C. It is 1.0 × 10–14 at 25°C which we
will use frequently the variation of ionic product of water with temperature is
given by
On the basis of above equation we calculate the value of kw at different
temperature which are given below .
We know ionic
product of water at 25 c is
The pH scale was marked from 0 to 14 with central point
at 7 at 25ºC, taking water as solvent.
If the temperature and the solvent are changed, the pH range
of the scale will also change. For example
at 25 ºC (Kw = 10–14) Neutral point,
pH= 7
at 80
ºC (Kw = 10–13) Neutral point, pH = 6.5
ILLUSTRATIVE EXAMPLE (1): H2O has its pH 6.5 predict
the nature of solution when
(a) Kw = 10–14 at 25 ºC
(b) Kw = 10–15 at 70 ºC
(c) Kw = 10–12 at 90 ºC
SOLUTION:
(a) acidic (b)
acidic (c) basic
ILLUSTRATIVE EXAMPLE (2):
The ionization
constant for water is 1x10-13.6 at 37ºC.What will be H3O+
and OH-Concentration at that temperature.
(1) 3.75 x10-8 (2) 1.75 x 10-8 (3) 1.58 x 10-7 (4) 1.85 x 10-8
SOLUTION:
ILLUSTRATIVE EXAMPLE (3): Calculate the ionic product of water
at 25c if Kw is 1.0 X10-14 m2 at 25c
(Ans - 5.5x10-14)
ILLUSTRATIVE EXAMPLE
(4): At 25 c the degree of dissociation
of water is 1.8 x 10-9 calculate the dissociation constant and
ionic product of water.
(Ans-3.24x10-18)
ILLUSTRATIVE EXAMPLE
(5): At 30 c
the self ionization constant of liquid ammonia (NH3) is 10-30.the density
of NH3 at -30 c is 0.85 gm /liter.
(1) Calculate the ionic product of liquid ammonia?
(2) Calculate % extent of self ionization of NH3 molecules?
(3) How many NH4+ ions are present in 5ml of liquid
NH3?
(1-
2.5x10-27 m2, 2- 10-13% 3- 25x10-6x6.022x1023
)
ILLUSTRATIVE EXAMPLE
(6): At 25c
the ionic product of heavy water (D2O) is 1.44x 10-15 m2
Calculate it's dissociation constant and degree of dissociation (density of D2O=1.02)
gm/ml)
(Ans-7.44x10-10)
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thanks a lot!
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