關於化學的簡單問題(2)

1. Answer : C .. (not B)

Zinc is more reactive than copper, and thus Zinc is oxidized by losing electrons.
Zn(s) → Zn^2+(aq) + 2e^-
Electrons flows from zinc to copper in the external circuit. Hydrogen ions gain electrons to form hydrogen gas at the surface of copper.
2H^+(aq) + 2e^- → H2(g)

A. is incorrect. There is observable change.
B. is incorrect. The zinc plate dissolves without bubbles formed.
C. is correct. Hydrogen gas is formed on copper plate.
D. is incorrect. The solution has no colour change.


2. Answer : A

Firstly, zinc react with hydrochloric acid to form zinc chloride solution and hydrogen gas.
Zn(s) + 2HCl(g) → ZnCl2(aq) + H­2(g) ...... (*)

Then, hydrogen gas reacts with heated copper(II) oxide to give copper and water vapour.
H2(g) + CuO(s) → Cu(s) + H2­O(l) ...... (#)

Both of the above reactions are redox. In (*), the oxidation number of Zn increases from 0 to +2, while that of H decreases from +1 to 0. In (#), the oxidation of H increases from 0 to +1, while that of Cu decreases from +2 to 0.

However, both of the above reactions are neither neutralization nor thermal decomposition.


3. Answer : C

In rusting, iron is firstly oxidized to iron(II), and then to iron(III). There is NO direct oxidation from iron to iron(III).