topic 12: acid - base equilibrium
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what is a bronsted lowry acid
a substance that can donate a proton
what is a bronsted lowry base
a substance that can accept a proton
what is each acid linked to
linked to a conjugate base on the other side of the equation
what is the definition of pH
-log[H+]
what do strong acids do
completely dissociate
what is the equation for finding [H+] from pH
10^-pH
what is the Kw expression
= [H+(aq) ][OH-(aq) ] =10^-pkw
at 25 degrees celsius , what is the value of Kw
1x10^-14 mol2dm-6
what are the expression for pKw
=-logKw
what are weak acids
only slightly dissociate when dissolved in water, giving an equilibrium mixture
what is the weak acids dissociation expression (Ka)
[H+(aq)][A-(aq)]/[HA (aq)]
the larger the Ka
the stronger the acid
what are the 2 assumption made to simplify the Ka expression
1) [H+(aq)]eqm = [A(aq)] eqm because they have dissociated according to a 1:1 ratio 2) as the amount of dissociation is small we assume that the initial concentration of the undissociated acid has remained constant. so [HA (aq) ] eqm = [HA(aq) ] initial
what is the Ka expression
[H+(aq)]^2 /[HA (aq)]initial
using one of the assumptions, what is the other expression of pH
pKa
what is a buffer solution
one where the pH does not change significantly if small amounts of acid or alkali are added to it
what is a acidic buffer solution made from
made from a weak acid and a salt of that weak acid ( made from reacting the weak acid with a strong base) example : ethanoic acid and sodium ethanoate
what is a basic buffer solution
made from a weak base and a salt of that weak base (made from reacting the weak base with a strong acid) example :ammonia and ammonium chloride
what happens when small amounts of acid is added to the buffer
the above equilibrium will shift to the left removing nearly all the H+ ions added, CH3CO2 -(aq) + H+ (aq) <—> CH3CO2H (aq) as there is a large concentration of the salt ion in the buffer the ratio [CH3CO2H]/ [CH3CO2 -] stays almost constant, so the pH stays fairly constant
what happens when a small amount of alkali is added to the buffer
the OH- will react with H+ ions to form water. H+ + OH - H2O the equilibrium will then shift to the right to produce more H+ ions. CH3CO2H(aq) <—> CH3CO2-(aq) + H+(aq) some ethanoic acid molecules are changed to ethanoate ions but as there is a large concentration of the salt ion in the buffer the ratio [CH3CO2H]/[CH3CO2-] stays almost constant, so the pH stays fairly constant
what would happen to pH when the acid is diluted by 10
pH increases by 1 unit
H2CO3 (aq) ⇌ H+(aq) + HCO3–(aq) what is the buffer that controls the blood’s pH
H2CO3/HCO3– buffer
what is the value of pH in blood the H2CO3 /HCO3– buffer maintains
7.35 and 7.45
what does the H2CO3 /HCO3– buffer do when the alkali is added
adding alkali reacts with H+ with the equation H+ + OH- —> H2O so the above equilibrium would shift right forming new H+ and more HCO3–