A Level Chemistry Chapter 21 Equilibria Detailed Summary Note

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21 equilibria date


conjugate acid-base pairs
pair of reactants and products that are linked to each other by the
transfer of a proton



pH indicates the acidity or basicity of an acid or alkali
1
pH -log10 [H ) , so [1
-




H 10
+
-




Ka is the acid dissociation constant for weak acid at 298K
HA(aq) H (aq) A (aq)
=
+
[H ][A-]
I
only partially dissociates
&


so Ka -




[HA]



high value of Ka means low value of Ka means
-the equilibrium lies to the right -the equilibrium lies to the left
-the acid is almost -the acid is only slightly ionised
completely ionised -the acid is weakly acidic
-the acid is strongly acidic
as Ka value for many acids are very low, pKa value are used
instead to compare their strengths
pKa -log10Ka (less positive pKa more acidic)
-
-




1426
Kw is the ionic product of water (1.00 x 10 mol dm )
equilibrium constant for the dissociation of water at 298K
H2O(l) H (aq) OH (aq)
= -


so Kw [H ][]
-




OH
+


or Kw [Hja since molar ratio is 1 1
-

·




4[] H2O can be regarded as constant and can be removed form
the Kw expression as the extent of ionisation is very low
buffer solution
a solution in which the pH does not change a lot when small amounts
of acids or alkalise are added
-consists of weak acid-conjugate base/weak base-conjugate acid
-there's a large initial supply of those compounds so the pH remains
relatively constant when H or OH are added
&




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