CHEM CE問題1(已有答案, 只求解答) (20pt)

2009-07-09 12:12 am

以下問題已知答案, 但不太明白, 求高人解釋

1. Which of the following oxides dissolves in both dilute sulphuric acid and sodium hydroxide solution?
A. CuO
B. MgO
C. Fe2O3
D. Al2O3
Ans: D

2. Which of the following reaction sequences is most suitable for preparing magnesium carbonate from magnesium in the laboratory?
A. Mg(s) + dil. HCl→MgCl2(aq) + Na2CO3(aq)→MgCO3(s)
B. Mg(s) + dil. H2SO4→MgSO4(aq) + CaCO3(s)→MgCO3(s)
C. Mg(s) + conc. HNO3→Mg(NO3)2(aq) + (NH4)2CO3(aq)→MgCO3(s)
D. Mg(s) + O2→MgO (s) + CO2 + H2O →MgCO3(s)
Ans: A

3. Silver nitrate + excess ammonia會出乜野?
係唔係silver hydroxide? 咁係咩色?

4. If a glass reagent bottle containing sodium hydroxide solution is allowed to stand in air for a few weeks, it is often difficult to remove the glass stopper due to the formation of a solid round the bottle neck. The solid is probably
A. Sodium oxide
B. Sodium peroxide
C. Sodium carbonate
D. Sodium hydrogencarbonate
Ans: C

THZ!!

回答 (1)

老爺子

2009-07-09 6:51 pm

✔ 最佳答案

1. D

All of A, B and C are basic oxides which can dissolve in dilute strong acids but not in dilute strong alkalis.
CuO(s) + 2H+(aq) → Cu2+(aq) + H2O(l)
MgO(s) + 2H+(aq) → Mg2+(aq) + H2O(l)
Fe2O3(s) + 6H+(aq) → 2Fe3+(aq) + H2O(l)

Al2O3 is an amphoteric oxide which can dissolve in both dilute strong acids and dilute strong alkalis.
Al2O3(s) + 6H+(aq) → 2Al3+(aq) + H2O(l)
Al2O3(s) + 3H2O(l) + 2OH-(aq) → 2[Al(OH)4]-(aq)

2. A

In B, no reaction occurs. This is because it is not practical to precipitate a solid by reaction a solution and a solid. Even if the reaction occurs, the solids would mix together.

In C, MgCO3(s) would react with the excess acid and dissolves.

In D, the reaction is the second step is extremely slow.

3.
When silver nitrate solution reacts with a small amount of ammonia solution, a dark precipitate of silver oxide is formed. Silver hydroxide is unstable, which decomposes to give silver oxide readily.
NH3(aq) + H2O(l) ≒ NH4+(aq) + OH-(aq)
2Ag+(aq) + 2OH-(aq) → Ag2O(l) + H2O(l)

When an excess of ammonia solution is added, the precipitate re-dissolves to give a colourless solution.
Ag2O(s) + H2O(l) + 4NH3(aq) → 2[Ag(NH3)2]+(aq) + 2OH-(aq)

4. C

Sodium hydroxide reacts with carbon dioxide in air to give sodium carbonate.
2NaOH + CO2 → Na2CO3 + H2O

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