chemical question [急! 20點!!!]

This is mainly due to the hydrolysis of the salt (ions) formed.

Example 1 :
Take the neutralization between HCl and NH3 to be an example, whereHCl is a strong acid and NH3 is a weak base.
HCl(aq) + NH3(aq) → NH4Cl(aq)
When equal moles of HCl and NH3 react, it completely gives NH4Cl,existing as aqueous NH4^+ and Cl^- ions.
Cl^- ions come from the strong acid HCl, and thus Cl^- ions cannot be hydrolyzedback to HCl.
However, NH4^+ ions come from the weak base NH3, and thusNH­4^+(aq) ions are hydrolyzed back to NH3 to a certainextent.
NH4^+(aq) ⇌ NH3(aq) +H^+(aq)
Due to the formation of H^+(aq) ions, the solution is acidic (pH < 7) whenthe neutralization completes.

Example 2 :
Take the neutralization between NaOH and CH3COOH to be an example,where NaOH is a strong base and CH3COOH is a weak acid.
NaOH(aq) + CH3COOH(aq) → CH3COONa(aq) +H2O(l)
When equal moles of NaOH and CH3COOH react, It completely gives CH3COONa,existing as aqueous Na^+ and CH3COO^- ions.
Na^+ ions come from the strong base NaOH, and thus Na^+ ions cannot behydrolyzed back to NaOH.
However, CH3COO^- ions come from the weak acid CH3COOH,and thus CH3COO^-(aq) ions are hydrolyzed back to CH3COOHto a certain extent.
CH3COO^-(aq) + H2O ⇌ CH3COOH(aq)+ OH^-(aq)
Due to the formation of OH^-(aq) ions, the solution is alkaline (pH > 7)when the neutralization completes.

Temperature can also affect the pH. In an aqueous solution pH = 7,[H^+], concentration of H^+(aq) ions, is 1 x 10^7 M at 25°C.
H2O(l) ⇌ H^+(aq) + OH^-(aq)
When the temperature changes, the self-ionized of water is changed accordingly.As the changes in both [H^+] and [OH^-] are equal, the solution keeps neutral. However,[H^+] is not 1 x 10^-7 M now, and thus pH = -log[H^+] ≠ 7