✔ 最佳答案
1.
equivalence point is where the acid and base (or other pairs of chemicals for different reactions) completely and merely neutralize (react) each other, not with any excess.
at such a point, the pH of product mixture may or may not be 7. it depends on nature of the salt formed.
end point is where you stop the titration as there's color change of indicators. like when basic methyl orange turns from yellow to red, colorless phenolphthalein to pink etc.
end point is very close, yet behind, the equivalence point. usually a drop or two of solution behind the equivalence point is the end point.
2.
simply speaking,
strong acid + strong base ----> neutral salt (HCl + NaOH ----> NaCl )
strong acid + weak base ----> acidic salt (HCl + NH3 ----> NH4Cl )
weak acid + strong base ----> alkaline salt (H2CO3 + NaOH ----> Na2CO3 )
in higher level, you'll consider the hydrolysis of the ions of the salt, as well as the equilibrium constants for ionization / dissociation of the acid & base.
5.
standard solution is solution that concentration of solute in it is Accurately known and calibrated.
3.
primary standard is solute suitable for direct preparation of standard solution, from pure solute into solution.
they're relatively stable, not absorbing / decomposing chemicals like oxygen, CO2 and water. also molar mass of such should be high.
oxalic acid (C2H2O4.2H2O), hydrated sodium carbonate (Na2CO3.7H2O), potassium bromate (KBrO3), etc.
4.
secondary standard is solute NOT suitable for direct preparation of std. sol., owing to its unstable chemical properties.
NaOH -- solid absorbs water and CO2 from air
H2SO4 -- liquid strongly absorbs water from air
KI -- iodide is unstable in air, easily oxidized
Cl2 -- volatile, decomposes water to give side products
2011-04-03 15:54:49 補充：
take NH4Cl as example.
each ion can undergo reversible hydrolysis:
NH4(+) + H(+) <====> NH3 + H3O(+)
Cl(-) + H2O <====> HCl + OH(-)
2011-04-03 15:55:15 補充：
the upper reaction can undergo to some extent, so some H(+) is produced;
the lower rxn. tends not to go, so not much OH(-) is formed.
thus, there's excess H(+), leading to acidic solution.
2011-04-03 15:58:46 補充：
equilibrium constant, K, is used to describe the extent of these reversible.
say, for upper reaction, K = [NH3][H(+)] / [NH4(+)]
where [A] means concentration of A in solution.
when K is large, the forward rxn. is dominant, vice versa.
2011-04-03 15:59:43 補充：
K for upper rxn. is large, so concentration of H(+) is higher;
K for lower rxn. is small, so concentration of OH(-) is lower.
