氧化还原反应yǎnghuà huányuán fǎnyìng

Core Concept

A redox reaction (氧化还原反应) is any reaction in which electrons are transferred, shown by a change in oxidation number (化合价).

• Essence (本质): electron transfer
• Characteristic (特征): oxidation number changes

That distinction between essence and characteristic is itself an examined point — 氧化还原反应定义、本质及特征 is a named CSCA knowledge point. The characteristic is what you observe; the essence is what causes it.

$\mathrm{Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu}$

Zinc goes $0 \to +2$ (loses 2 electrons); copper goes $+2 \to 0$ (gains 2 electrons).

The Vocabulary Trap

This is where most marks are lost, because the roles read backwards:

Substance	What it does	What happens to it	Product
Reducing agent 还原剂	gives electrons	is oxidized	oxidation product 氧化产物
Oxidizing agent 氧化剂	takes electrons	is reduced	reduction product 还原产物

The mnemonic that fixes it: 升失氧还原剂 — 化合价升高、失去电子、被氧化，这是还原剂.

So in $\mathrm{Zn + Cu^{2+}}$: zinc is the reducing agent even though it is the thing being oxidized.

Balancing by Electron Conservation

Every redox equation must satisfy:

$\text{electrons lost} = \text{electrons gained}$

Method (化合价升降法):

1. Assign oxidation numbers to every element
2. Identify which rise and which fall
3. Compute total change: (change per atom) × (number of atoms)
4. Scale by least common multiple so rises equal falls
5. Balance remaining atoms, then charge

Common Oxidizing and Reducing Agents

Oxidizing agents: $\mathrm{O_2}$, $\mathrm{Cl_2}$, concentrated $\mathrm{H_2SO_4}$, $\mathrm{HNO_3}$, $\mathrm{KMnO_4}$, $\mathrm{Fe^{3+}}$

Reducing agents: active metals, $\mathrm{H_2}$, $\mathrm{C}$, $\mathrm{CO}$, $\mathrm{Fe^{2+}}$, $\mathrm{I^-}$

Elements in their highest oxidation state can only be oxidizing agents; in their lowest, only reducing agents; in between, they can be either. $\mathrm{Fe^{2+}}$ sits in the middle, which is why it appears in questions on both sides.

CSCA Practice Problems

💡 Note: These problems follow the CSCA exam syllabus and Chinese standardized test formats.

Example 1: Basic (Difficulty ★★☆☆☆)

In $\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}$, the reducing agent is:

Options:

• A. $\mathrm{Fe}$
• B. $\mathrm{CuSO_4}$
• C. $\mathrm{FeSO_4}$
• D. $\mathrm{Cu}$

Detailed Solution:

1. Track oxidation numbers:

   • $\mathrm{Fe}$: $0 \to +2$, rises → loses electrons
   • $\mathrm{Cu}$: $+2 \to 0$, falls → gains electrons

2. Apply 升失氧还原剂: the species whose oxidation number rises is the reducing agent.

3. Therefore $\mathrm{Fe}$ is the reducing agent, and $\mathrm{CuSO_4}$ is the oxidizing agent.

Option B is the trap for students who reason "copper is being reduced, so copper's compound must be the reducing agent" — exactly backwards.

Answer: A

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Example 2: Intermediate (Difficulty ★★★☆☆)

In $\mathrm{Cu + 4HNO_3(\text{conc}) \rightarrow Cu(NO_3)_2 + 2NO_2\uparrow + 2H_2O}$, how many moles of $\mathrm{HNO_3}$ act as the oxidizing agent per mole of $\mathrm{Cu}$?

Options:

• A. $1$
• B. $2$
• C. $3$
• D. $4$

Detailed Solution:

1. Only the nitrogen atoms that change oxidation number are acting as oxidizer. Nitrogen goes $+5 \to +4$ in $\mathrm{NO_2}$ only.

2. Two moles of $\mathrm{NO_2}$ form, so 2 mol of $\mathrm{HNO_3}$ are reduced.

3. The other 2 mol stay as $\mathrm{NO_3^-}$ in $\mathrm{Cu(NO_3)_2}$ — unchanged at $+5$, so they act only as the acid providing the anion (起酸性作用).

4. Verify by electron balance: $\mathrm{Cu}$ loses 2 electrons ($0 \to +2$); each $\mathrm{N}$ gains 1 ($+5 \to +4$), so 2 nitrogen atoms are needed ✓

Answer: B

Why this is a favourite: it separates students who count all the $\mathrm{HNO_3}$ (giving D) from those who track oxidation numbers.

Common Mistakes

❌ Mistake 1: Swapping oxidizing and reducing agent

The reducing agent is the one oxidized. Recite 升失氧还原剂 every time.

❌ Mistake 2: Confusing essence with characteristic

Electron transfer is the essence; oxidation-number change is the observable characteristic.

❌ Mistake 3: Counting all of an acid as the oxidizer

In concentrated $\mathrm{HNO_3}$ reactions, only the portion whose N changes is reduced.

❌ Mistake 4: Assuming oxygen must be involved

Redox is defined by electron transfer. $\mathrm{Fe + Cl_2}$ is redox with no oxygen at all.

Study Tips

1. ✅ Write oxidation numbers above every element before analysing
2. ✅ Chant 升失氧还原剂 — it prevents the single most common error
3. ✅ Balance by electrons lost = electrons gained
4. ✅ In $\mathrm{HNO_3}$/$\mathrm{H_2SO_4}$ problems, split the acid's two roles — oxidizer vs acid
5. ✅ Learn the common agent lists so identification is instant

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💡 Exam Tip: 氧化剂、还原剂的判断 (15 occurrences) and 氧化还原反应定义、本质及特征 are both top CSCA chemistry knowledge points. Nearly every item reduces to assigning oxidation numbers correctly.