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Bronsted- Lowry Definition
An Acid is a proton donor, it releases H+ ions
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Strong acids and bases are fully dissociated in water i
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they completely dissociate into their ions
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pH= log10[H+]
The concentration of hydrogen ions in a solution can vary over a huge range, using a logarithmic
scale brings these concentrations to a scale of ~1-14
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-for diprotic acids the concentration of hydrogen ions will double the concentration of the acid
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-Therefore water must dissociate to some extent
-A solution of water dissociates to a very small extent H2O >><< H+ + OH-However the equilibrium lies so far to left that [H2O] can be taken as a constant and incorporated
into kc
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Kw= [H+][OH-] ionic product of water
Kw= 1x10-14 mol2 dm-6 at 25°c
In a sample of water only [H+]=[OH-], therefore we can say kw= [H+]2
So, [H+]=√kw
So…
pH= -log(√kw)
How and Why Does kw Vary with Temperature?
The forward reaction is endothermic as a covalent bond is broken
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Calculate the pH of a Strong Base;
(1x10-14)/(Concentration of base)= Concentration of H+ ions
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The equilibrium lies well over to the left hand side so
there are plenty of acid molecules present and only a small % of H+ ions present
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This can be done by finding the equilibrium constant for the weak acids
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Ka= [H+]2/ [HA]
Strong Acids have high ka values, weak acids have low ka acids
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pKa values can also be calculated