Acid base equilibrium

2.
Citric acid is a tribasic (triprotic) acid. Each citric acid molecule has 3 ionizable H atoms. Denote citric acid as H3A. The pH of citric acid is mainly due to the first ionization of the acid, an endothermic reaction, i.e.
H3A(aq) ≒ H+(aq) + H2A-(aq) .. ΔH = +ve

When temperature decreases, according to Le Chatelier's principle, the equilibrium position will shifts to the left because the backward reaction exothermic. Due to the decrease in the concentration of H+(aq) ions, the fruit tastes less sour.


3.
The self-ionization of pure water is endothermic, i.e.
H2O(l) ≒ H+(aq) + OH-(aq) .. ΔH = +ve

At 298 K, [H+] = 1 x 10-7 M, and thus pH = -log(1 x 10-7) = 7

When temperature is increased to 323 K, according to Le Chatelier's principle, the equilibrium position will shifts to the right because the forward reaction is endothermic. Due to the increases in H+(aq) ions, [H+] > 1 x 10-7 M.
Hence, pH = -log[H+] < -log(1 x 10-7) = 7.
pH < 7


4.
(a)
Bronsted acids: CH3COOH, H+, H2O
Bronsted bases: CH3COO-, OH-, H2O

(b)
CH3COOH(aq) ≒ CH3COO-(aq) + H+(aq)

At equilibrium:
[CH3COOH] ≈ [CH3COOH]o = 0.2 M
[CH3COO-] ≈ [CH3COO-]o = 0.2 M
Ka­ = (0.2)[H+]/(0.2) = 1.76 x 10-5
[H+] = 1.76 x 10-5 M
[OH-] = Kw/[H+] = (1 x 10-14)/(1.76 x 10-5) = 5.68 x 10-10 M

Ans: [CH3COOH] = 0.2 M
Ans: [CH3COO-] = 0.2 M
Ans: [H+] = 1.76 x 10-5 M
Ans: [OH-] = 5.68 x 10-10 M
=

H+ and OH- itself is not Bronsted acids or Bronsted bases

the concentration is also wrong

they should be

[CH3COOH] = 0.1 M
[CH3COO-] = 0.1 M

as equal amount of them is mixed ,that 's mean the concentration has halfed

Less hydrogen ions are ionized as the temperature decreases from 30 degree celcius to 5 degree celcius

At 323 K, more hydrogen ions are releasesd, decreasing pH value

No reaction between

2009-03-20 17:20:50 補充：
A-level~~~