Oxidation and reduction by oxygen

Gain and loss of oxygen, and the agents that cause them

1. Properties of iron

2. Black, then pink again

Heat a copper wire in a flame and it turns black. Heat the black wire in hydrogen, and the copper comes back.

Pick the right answer

Predict: what did the hydrogen do to the black solid?

A. It took oxygen away from it
B. It added hydrogen to the copper
C. It just cleaned off soot

Explanation

The black solid is copper (II) oxide, formed when copper gained oxygen from the air in the flame. Heating it in hydrogen took that oxygen away, so the copper came back.

3. Oxidation

Oxidation

Gain of oxygen, or loss of hydrogen.

2Cu + O2 \to[Δ] 2CuO 2 Cu + O X_{2} Δ 2 CuO

Copper gains oxygen: copper is oxidized

Burning and rusting are oxidation too: a substance gains oxygen.

4. Reduction

Reduction

Loss of oxygen, or gain of hydrogen.

CuO + H2 \to[Δ] Cu + H2O CuO + H X_{2} Δ Cu + H X_{2} O

$CuO C u O$ → $Cu C u$ (loses oxygen: reduced)
$H₂ H_{2}$ → $H₂O H_{2} O$ (gains oxygen: oxidized)

Oxidation and reduction happen together. Here

CuO C u O

loses oxygen while

H₂ H_{2}

gains it.

5. The two agents

H₂ H_{2}

takes the oxygen: it is the reducing agent.

CuO C u O

gives the oxygen: it is the oxidizing agent.

	What it does	What happens to it
Oxidizing agent (e.g. CuO)	supplies oxygen	it is reduced
Reducing agent (e.g. $H₂ H_{2}$ , CO, C)	takes oxygen away	it is oxidized

True or false?

The oxidizing agent is the substance that gets oxidized.

Explanation

Each agent is named for what it does to the other substance, not for what happens to it.

Contrast:

H₂ H_{2}

is oxidized to water, yet we call it the reducing agent.

6. New reactions

Pick the right answer

2Mg + CO2 \to 2MgO + C 2 Mg + CO X_{2} 2 MgO + C

, which substance is the reducing agent?

A. Mg
B. $CO₂ C O_{2}$
C. MgO
D. C

Explanation

Mg takes the oxygen from

CO₂ C O_{2}

and becomes MgO, so Mg is oxidized and acts as the reducing agent.

CO₂ C O_{2}

loses oxygen, so it is reduced (the oxidizing agent); MgO and C are products.

Pick the right answer

Fe2O3 + 2Al \to[Δ] Al2O3 + 2Fe Fe X_{2} O X_{3} + 2 Al Δ Al X_{2} O X_{3} + 2 Fe

, what happens to aluminum?

A. It is oxidized, so it is the reducing agent
B. It is reduced, so it is the oxidizing agent
C. It is oxidized, so it is the oxidizing agent

Explanation

Aluminum gains oxygen from

Fe₂O₃ F e_{2} O_{3}

and becomes

Al₂O₃ A l_{2} O_{3}

, so it is oxidized. The substance that is oxidized does the reducing, so aluminum is the reducing agent.

Fill in the blanks

H2S + Cl2 \to 2HCl + S H X_{2} S + Cl X_{2} 2 HCl + S

H₂S H_{2} S

loses hydrogen, so

H₂S H_{2} S

is _____.

Explanation

Loss of hydrogen is oxidation, so

H₂S H_{2} S

is oxidized (and acts as the reducing agent).

7. Summary

Oxidation: gain of oxygen or loss of hydrogen. Reduction: loss of oxygen or gain of hydrogen.
The oxidizing agent supplies oxygen and is itself reduced.
The reducing agent (e.g. $H₂ H_{2}$ , CO, C) takes oxygen away and is itself oxidized.

$Cu Cu$ (pink) → $CuO CuO$ (black) (+ $O2 O X_{2}$ , heat: Cu oxidized)
$CuO CuO$ (black) → $Cu Cu$ (pink again) (+ $H2 H X_{2}$ (reducing agent), heat: CuO reduced)

The hook, step by step: oxidation, then reduction.

Back to the hook: the flame oxidized copper to CuO; hydrogen, the reducing agent, reduced it back.

Can you explain why CO is called a reducing agent, although it is oxidized?

Concepts in this lesson