Chem question

[匿名]

2012-07-21 6:58 am

A student was asked to prepare copper(II) nitrate solution. It was found that copper(II) nitrate solution could be prepared by the following three reactions.
Reaction I: Put copper granules into concentrated nitric acid.
Reaction II: Put copper granules into dilute nitric acid.
Reaction III: Put copper(II) oxide into dilute nitric acid.
(a) For each of the reactions,
(1) write the chemical equation involved.
(2) briefly describe any observable changes.
(3) state the change in the oxidation number of nitrogen.
(b) Which reaction is most suitable for preparing copper(II) nitrate solution? Explain your choice.
(c) Why concentrated nitric acid is usually kept in a brown bottle?

回答 (1)

賣女孩的火柴

2012-07-21 8:46 am

✔ 最佳答案

(a)
(1)
Reaction I : Cu + 4HNO3 → Cu(NO3)2+ 2H2O + 2NO2
Reaction II : 3Cu + 8HNO3 → 3Cu(NO3)2+ 4H2O + 2NO
Reaction III : CuO + 2HNO3 → Cu(NO3)2+ H2O

(2)
Reaction I : Copper dissolves. Thecolourless solution turns blue. A browngas is given off.
Reaction II : Copper dissolves. Thecolourless solution turns blue. A colourlessgas is given off which turns brown in air immediately.
Reaction III : Copper(II) oxide dissolves. The colourless solution turns blue.

(3)
Reaction I : The oxidation number of N decreases from +5 (in HNO3) to+4 (in NO2).
Reaction II : The oxidation number of N decreases from +5 (in HNO3)to +2 (in NO).
Reaction III : The oxidation number of N does not change, and keeps in +5.

(b)
Reaction III is most suitable.
In reaction I, toxic nitrogen dioxide is given off. In reaction II, nitrogen monoxide is givenoff which changes to toxic nitrogen dioxide in air. In reaction III, no toxic gas is given off.

(c)
Concentrated nitric acid decomposes in light to given water, nitrogen dioxideand oxygen.
4HNO3 → 2H2O + 4NO2 + O2

參考: 賣女孩的火柴

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