acid and base(urgent)

1a. When HCl is added to Z, gases are produced. Only (C) and (D) can react with HCl to give gases, hence (A) and (B) are incorrect.
for (A): no reaction
for (B) H+ + CH3COO- ---> CH3COOH
but no colourless gas involved
for (C): CO3 2- + 2H+ ---> CO2(colourless gas) + H2O
for (D): CaCO3 + 2H+ ---> CO2(colourless gas) + Ca2+ + H2O
Important note 1 :
(1)NaCO3 is soluble in water and hence its solution consists of mobile Na+ and CO3 2- ions. Only CO3 2- ions react with H+ as given by the ionic equation.
(2) CaCO3 is insoluble in water. It does not have mobile Ca2+ and CO3 2- ions. Thus, we cannot write CO3 2- + 2H+ ---> CO2 + H2O for the reaction.
(3) ethanoic acid is a weak acid, it can only slightly ionize in water
CH3COOH <---> H+ + CH3COO-
the equilibrium lies far on the left hand side.
When strong acid is added to ethanoate ion, it will combine with H+ to give CH3COOH
b. When silver nitrate solution is added, white precipitate appears. Only (A), (B), (C) are correct.
For (A) Ag+ + Cl- ---> AgCl(precipitate)
For (B) Ag+ + CH3COO- --->CH3COO-Ag+ (precipitate)
For (C) 2Ag+ + CO3 2- ---> Ag2CO3 (precipitate)
For (D): no reaction
Important note 2:
(1) the reaction between silver ion and chloride ion to form insoluble silver chloride often appears in exam, you must know it well
(2)the reaction of (B) is for your reference only, you don't need to remember the reaction. Actually, by the method of elimilation, you can get (C) as the ans without considering the reaction for B.
(3) you may wonder why both CaCO3 and Na2CO3 have carbonate ion but only Na2CO3 can give precipitate. I have mentioned the reason before. Na2CO3 is soluble in water and hence the solution consists of Na+ and CO3 2- ions. CO3 2- ions can collide with Ag+ ion to produce silver carbonate.
However, CaCO3 is insoluble in water. There is no mobile CO3 2- ion. Thus no precipitate is produced.









2011-07-06 20:19:48 補充：
2. For (A),
(1) no reaction with HCl
form dirty green precipitate with NaOH.
equation: Fe2+ + 2OH- ----> Fe(OH2) ( dirty green precipitate )
Hence (A) is incorrect.

For (B),
(1) no reaction with HCl
(2) form ammonia gas on heating with NaOH
equation: NH4+ + OH- ---> NH3 + H2O
(B) is incorrect.

2011-07-06 20:20:28 補充：
For (C), it has no reaction with both HCl and NaOH
Hence (C) is correct.
For (D),(1) it reacts with H+ to give sulphur dioxide
equation: SO3 2- + 2H+ ---> SO2 + H2O
(2) no reaction with NaOH
Hence (D) is incorrect.

2011-07-06 20:21:03 補充：
3. For (A), no insoluble compound is produced when ammonium ion is added.
possible combination of compound: K2CO3, (NH4)2CO3 but both are soluble
For (B), insoluble compound MgCO3 appears as precipitate.
possible combination of compoundl K2CO3 ( soluble), MgCO3 (insoluble)

2011-07-06 20:21:27 補充：
For (C), no insoluble compound is produced when Br- is added
possible combination of compound: K2CO3, KBr but both are soluble
For (D), no insoluble compound is produced when SO3 2- is added,
possible combination of compound: K2CO3, K2SO3 but both are soluble

2011-07-06 20:22:54 補充：
Important note 3:
(1) All sodium and potassium compounds are solule in water

2011-07-06 20:23:51 補充：
(2) Precipitation is a famous topic in exam. When two solutions are mixed, you have to list out possible compounds and see whether each of them is soluble or not. If you are experienced enough, you can decide whether precipitation occurs quickly without listing possible compounds.

2011-07-06 20:24:20 補充：
4. This reaction is very similar to the reaction between carbonates and acid, hence it is easy to remember. The reaction is definitely in your syllabus.You will encounter this reaction again in the topic of contact process to make sulphuric acid.The reaction is sometimes used in lab to produced SO2.

2011-07-06 20:24:38 補充：
Look at the reactions below carefully and try to identify the similarities.
(1) CO3 2- + 2H+ ----> CO2 + H2O
(2) SO3 2- + 2H+ ----> SO2 + H2O
the equation represents the reaction between Na2CO3 and H+. This reaction is learnt in the topic of neutralization.

2011-07-06 20:25:10 補充：
Important note 4:
At your level, neutralization refers to the reaction between a base and acid to form salt and water only. The reaction between carbonates and acid is NOT a neutralization reaction AT YOUR LEVEL because CO2 is one of the reaction product.

2011-07-06 20:25:20 補充：
. At higher level, this reaction is regarded as a neutralization reaction.