easy CHEM question~~~

1.
The answer is b.

a. The oxidation number of Cr decreases from +6 to +3.
a. Cr 的氧化數由 +6 降至 +3。

b. The oxidation number of Mn decreases from +7 to +2.
a. Mn 的氧化數由 +7 降至 +2。

c. The oxidation number of N decreases from +5 to +2.
a. N 的氧化數由 +5 降至 +2。

d. The oxidation number of S increases from +4 to +6.
a. S 的氧化數由 +4 升至 +6。

2.
(1) is not a redox reaction. There is no change in oxidation number.
(1) 不是氧化還原反應，因為沒有氧化數改變。

(2) is a redox reaction. MnO2 is reduced, because the oxidation number of Mn decreases from +4 to +2; HCl is oxidized, because the oxidation number of Cl increases from -1 to 0.
(2) 是氧化還原反應。MnO2 被還原，因為 Mn 的氧化數由 +4 降至 +2； HCl 被氧化，因為 Cl 的氧化數由 -1 升至 0。

3.
The answer is C.

HCl is a very weak reducing agent, but acidified KMnO4 is a very strong oxidizing agent. A redox reaction can occur. Therefore, A is incorrect, but C is correct.
HCl 是很弱的還原劑，但酸化 KMnO4 是很強的氧化劑，發生氧化還原反應。因此，A 錯誤而 C 正確。

Both HCl and H2SO­4 are strong acid. Therefore, B is incorrect.
HCl 和 H2SO4 均為強酸。因此，B 錯誤。