In the reaction CuO(s) + H₂(g) \rightarrow Cu(s) + H₂O(g), which element is reduced?

Correct Option: A. Solution: In the reaction, copper (Cu) is reduced from CuO to Cu.

Redox Reactions

Objectives Notes Exam Tips Practice Qs Ask AI

Learning Objectives

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Learning Objectives

Identify redox reactions as a class of reactions in which oxidation and reduction occur simultaneously.
Define the terms oxidation, reduction, oxidant (oxidising agent), and reductant (reducing agent).
Explain the mechanism of redox reactions by electron transfer processes.
Use oxidation number to identify oxidant and reductant in a reaction.
Classify redox reactions into combination (synthesis), decomposition, displacement, and disproportionation reactions.
Suggest a comparative order among various reductants and oxidants.
Balance chemical equations using (i) oxidation number (ii) half reaction method.
Learn the concept of redox reactions in terms of electrode processes.

Revision Notes & Summary

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UNIT 7: REDOX REACTIONS

Overview

Redox reactions involve simultaneous oxidation and reduction processes.
Important in various fields: pharmaceutical, biological, industrial, metallurgical, and agricultural.
Applications include energy production, electrochemical processes, and environmental issues.

Objectives

After studying this unit, you will be able to:

Identify redox reactions as a class of reactions in which oxidation and reduction occur simultaneously.
Define the terms oxidation, reduction, oxidant (oxidising agent), and reductant (reducing agent).
Explain the mechanism of redox reactions by electron transfer processes.
Classify redox reactions into:
- Combination (synthesis)
- Decomposition
- Displacement
- Disproportionation reactions
Balance chemical equations using:
- Oxidation number
- Half-reaction method
Learn the concept of redox reactions in terms of electrode processes.

Classical Idea of Redox Reactions

Originally, oxidation referred to the addition of oxygen to an element or compound.
Example reactions:
- 2 Mg (s) + O₂ (g) → 2 MgO (s)
- S (s) + O₂ (g) → SO₂ (g)

Key Concepts

Oxidation and Reduction

Oxidation: Loss of electrons or increase in oxidation state.
Reduction: Gain of electrons or decrease in oxidation state.

Oxidation Numbers

Assigned according to a consistent set of rules.
Useful for writing equations for redox reactions.

Redox Couples and Electrode Processes

Introduction to redox couples and their significance in electrode processes and cells.

Classification of Redox Reactions

Combination Reactions: Two or more reactants combine to form a single product.
Decomposition Reactions: A single compound breaks down into two or more products.
Displacement Reactions: An element displaces another in a compound.
Disproportionation Reactions: A single substance is both oxidized and reduced.

Balancing Redox Reactions

Methods

Oxidation Number Method: Assign oxidation numbers to elements and balance the changes.
Half-Reaction Method: Split the reaction into oxidation and reduction half-reactions.

Important Reactions and Examples

Example of a redox reaction: CuO(s) + H₂(g) → Cu(s) + H₂O(g)
Justification of reactions as redox based on oxidation state changes.

Standard Electrode Potentials

Table of standard electrode potentials at 298 K for various reactions, indicating the strength of oxidizing and reducing agents.

Diagram Descriptions

Daniell Cell

Anode: Zinc electrode, oxidation reaction: Zn → Zn²⁺ + 2e⁻.
Cathode: Copper electrode, reduction reaction: Cu²⁺ + 2e⁻ → Cu.
Salt Bridge: Connects two solutions, allowing ion flow.
Electron Flow: From anode to cathode through an external circuit.

Chemical Reactions

Decomposition of hydrogen peroxide: 2 H₂O₂(aq) → 2 H₂O(l) + O₂(g).
Oxidation states labeled for clarity in reactions.

Conclusion

Understanding redox reactions is crucial for various chemical processes and applications.

Exam Tips & Common Mistakes

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Common Mistakes and Exam Tips for Redox Reactions

Common Pitfalls

Misidentifying Oxidation and Reduction: Students often confuse which species is oxidized and which is reduced. Remember, oxidation involves loss of electrons, while reduction involves gain of electrons.
Incorrect Assignment of Oxidation Numbers: Failing to correctly assign oxidation numbers can lead to errors in identifying redox reactions. Always follow the established rules for assigning oxidation states.
Neglecting to Balance Equations: Many students forget to balance the number of atoms and charges in redox equations. Use the half-reaction method to ensure both mass and charge are balanced.
Overlooking the Role of the Medium: The reaction conditions (acidic or basic) can affect the balancing of redox reactions. Be mindful of the medium when applying the ion-electron method.

Tips for Success

Practice Assigning Oxidation Numbers: Regularly practice assigning oxidation numbers to different compounds to build confidence.
Use Half-Reaction Method: When balancing redox reactions, break them down into half-reactions for oxidation and reduction to simplify the process.
Familiarize with Common Redox Reactions: Study and memorize common redox reactions and their products to recognize patterns and facilitate quicker problem-solving during exams.
Check Your Work: After balancing, double-check that both mass and charge are conserved in your final equations.

AI Learning Assistant

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Practice Test – MCQs, True/False

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Multiple Choice Questions

2A^{2-} + 3B^0 \rightarrow 2A^0 + 3B^{3-}

A^{2-} + 3B^0 \rightarrow A^0 + 3B^{3-}

3A^{2-} + 2B^0 \rightarrow 3A^0 + 2B^{3-}

A^{2-} + B^0 \rightarrow A^0 + B^{3-}

Correct Answer: A

Solution:

The balanced equation

2A^{2-} + 3B^0 \rightarrow 2A^0 + 3B^{3-}

ensures that both mass and charge are conserved.

Potassium (K)

Fluorine (F₂)

Potassium fluoride (KF)

None of the above

Correct Answer: B

Solution:

Fluorine (F₂) is the oxidizing agent because it gains electrons from potassium, causing potassium to be oxidized.

Assign oxidation numbers to all elements, identify changes, and balance electron transfer.

Balance the charges first, then balance the atoms.

Balance the atoms first, then balance the charges.

Use trial and error to balance the equation.

Correct Answer: A

Solution:

The oxidation number method involves assigning oxidation numbers to all elements, identifying changes in oxidation states, and balancing the electron transfer to ensure the reaction is balanced.

Electrons flow from

D

C

through the external circuit.

Electrons flow from

C

D

through the external circuit.

Electrons flow from the cathode to the anode internally.

Electrons do not flow in the external circuit.

Correct Answer: B

Solution:

In an electrochemical cell, electrons flow from the anode to the cathode through the external circuit. Therefore, electrons flow from

C

D

CuO(s) + H₂(g) \rightarrow Cu(s) + H₂O(g)

H₂O(l) \rightarrow H₂(g) + O₂(g)

CaCO₃(s) \rightarrow CaO(s) + CO₂(g)

NaCl(s) \rightarrow Na⁺(aq) + Cl⁻(aq)

Correct Answer: A

Solution:

The reaction CuO(s) + H₂(g) \rightarrow Cu(s) + H₂O(g) is a displacement reaction where hydrogen displaces copper.

4NH_3(g) + 5O_2(g) \rightarrow 4NO(g) + 6H_2O(g)

2NH_3(g) + 3O_2(g) \rightarrow 2NO(g) + 3H_2O(g)

NH_3(g) + O_2(g) \rightarrow NO(g) + H_2O(g)

4NH_3(g) + 3O_2(g) \rightarrow 2N_2(g) + 6H_2O(g)

Correct Answer: A

Solution:

The balanced equation for the oxidation of ammonia in the Ostwald process is

4NH_3(g) + 5O_2(g) \rightarrow 4NO(g) + 6H_2O(g)

They involve only the transfer of protons.

They involve the simultaneous occurrence of oxidation and reduction.

They only occur in acidic solutions.

They do not involve electron transfer.

Correct Answer: B

Solution:

Redox reactions involve the simultaneous occurrence of oxidation and reduction processes, where electrons are transferred between species.

It gains electrons.

It loses electrons.

It is oxidized.

It is reduced.

Correct Answer: B

Solution:

The reducing agent donates electrons to another substance, thus it is oxidized itself.

2H_2 + O_2 \rightarrow 2H_2O

2Fe + 3Cl_2 \rightarrow 2FeCl_3

K_2Cr_2O_7 + 6FeSO_4 + 7H_2SO_4 \rightarrow Cr_2(SO_4)_3 + 3Fe_2(SO_4)_3 + 7H_2O

C + O_2 \rightarrow CO_2

Correct Answer: C

Solution:

The reaction

K_2Cr_2O_7 + 6FeSO_4 + 7H_2SO_4 \rightarrow Cr_2(SO_4)_3 + 3Fe_2(SO_4)_3 + 7H_2O

involves changes in oxidation numbers and is balanced using the oxidation number method.

Copper

Silver nitrate

Copper nitrate

Silver

Correct Answer: D

Solution:

In the reaction between copper and silver nitrate, silver ions (Ag⁺) are reduced to silver metal (Ag).

Z + Y^{3+} \rightarrow Z^{2+} + Y

Z + 2Y^{3+} \rightarrow Z^{2+} + 2Y^{2+}

Z + 3Y^{3+} \rightarrow 3Z^{2+} + 3Y

2Z + 3Y^{3+} \rightarrow 2Z^{2+} + 3Y

Correct Answer: D

Solution:

Metal

Z

is oxidized from 0 to +2, and

Y^{3+}

is reduced to

Y

. The balanced reaction is

2Z + 3Y^{3+} \rightarrow 2Z^{2+} + 3Y

Copper dissolves and silver metal precipitates.

The solution turns blue.

The temperature of the solution increases.

No visible change is observed.

Correct Answer: A

Solution:

In the redox reaction between copper and silver nitrate, copper dissolves to form copper ions, and silver ions are reduced to form silver metal, which precipitates.

Hydrogen is the oxidizing agent, and oxygen is the reducing agent.

Oxygen is the oxidizing agent, and hydrogen is the reducing agent.

Both hydrogen and oxygen act as oxidizing agents.

Both hydrogen and oxygen act as reducing agents.

Correct Answer: B

Solution:

In this reaction, hydrogen is oxidized (loses electrons) and oxygen is reduced (gains electrons). Therefore, oxygen is the oxidizing agent, and hydrogen is the reducing agent.

A substance that gains electrons in a redox reaction.

A substance that loses electrons in a redox reaction.

A substance that is oxidized in a redox reaction.

A substance that is reduced in a redox reaction.

Correct Answer: B

Solution:

A reducing agent is a substance that loses electrons and is oxidized in a redox reaction.

Metal

M

is oxidized, and

X_2

is reduced.

X_2

is oxidized, and metal

M

is reduced.

Both metal

M

and

X_2

are oxidized.

Both metal

M

and

X_2

are reduced.

Correct Answer: A

Solution:

In the reaction, metal

M

goes from an oxidation state of 0 to +2, indicating it loses electrons and is oxidized.

X_2

gains electrons to form

2X^-

, indicating it is reduced.

Oxidation and reduction occur in separate reactions.

Oxidation and reduction occur simultaneously.

Only oxidation occurs in redox reactions.

Only reduction occurs in redox reactions.

Correct Answer: B

Solution:

In redox reactions, oxidation and reduction occur simultaneously as electrons are transferred from one species to another.

Nitrogen

Oxygen

Hydrogen

None

Correct Answer: A

Solution:

In this reaction, nitrogen in NH₃ is oxidized from -3 to +2, while oxygen is reduced from 0 to -2.

Fe^{3+}

Fe^{2+}

I^-

I_2

Correct Answer: A

Solution:

In this reaction,

Fe^{3+}

is reduced to

Fe^{2+}

, meaning it gains electrons. Therefore,

Fe^{3+}

is the oxidizing agent.

They gain electrons and are reduced.

They lose electrons and are oxidized.

They neither gain nor lose electrons.

They are always in a gaseous state.

Correct Answer: A

Solution:

Oxidizing agents gain electrons and are reduced in the process.

G_2H_6

is oxidized.

G_2H_6

is reduced.

G_2H_6

acts as a catalyst.

G_2H_6

is neither oxidized nor reduced.

Correct Answer: A

Solution:

In the reaction,

G_2H_6

is oxidized to

CO_2

, indicating that it loses electrons and acts as a reducing agent.

Oxidizing agent

Reducing agent

Catalyst

Inert gas

Correct Answer: A

Solution:

Oxygen acts as an oxidizing agent as it gains electrons from hydrogen in the formation of water.

Correct Answer: A

Solution:

K_2Cr_2O_7

, the oxidation number of Cr is +6. This is calculated by considering the oxidation states of K (+1) and O (-2), and the fact that the sum of oxidation numbers in a neutral compound is zero.

F^{3+}

acts as an oxidizing agent.

F^{3+}

acts as a reducing agent.

F^{3+}

does not participate in the reaction.

F^{3+}

acts as a catalyst.

Correct Answer: A

Solution:

F^{3+}

is reduced to

F^{2+}

, indicating that it gains electrons and thus acts as an oxidizing agent.

Correct Answer: A

Solution:

\text{H}_2\text{SO}_4

, the oxidation number of sulfur is +6, calculated by considering the oxidation states of hydrogen (+1) and oxygen (-2).

Copper is oxidized and silver ions are reduced.

Copper is reduced and silver ions are oxidized.

Both copper and silver ions are oxidized.

Both copper and silver ions are reduced.

Correct Answer: A

Solution:

In the reaction, copper loses electrons to form

Cu^{2+}

ions, indicating oxidation, while

Ag^+

ions gain electrons to form silver metal, indicating reduction.

Electrons flow from

A

B

through the external circuit.

Electrons flow from

B

A

through the external circuit.

Electrons flow from the cathode to the anode internally.

Electrons do not flow in this electrochemical cell.

Correct Answer: A

Solution:

In an electrochemical cell, electrons flow from the anode to the cathode through the external circuit. Since

A

is the anode, electrons flow from

A

B

It acts as a reducing agent.

It acts as an oxidizing agent.

It acts as a catalyst.

It acts as a spectator ion.

Correct Answer: B

Solution:

MnO_4^-

gains electrons and is reduced to

Mn^{2+}

, thus acting as an oxidizing agent.

It donates electrons.

It accepts electrons.

It neither donates nor accepts electrons.

It acts as a catalyst.

Correct Answer: B

Solution:

An oxidizing agent accepts electrons from another species, causing the other species to be oxidized.

Copper (Cu)

Oxygen (O)

Hydrogen (H)

None

Correct Answer: A

Solution:

In the reaction, copper (Cu) is reduced from CuO to Cu.

Ozone has a high electron affinity and readily gains electrons.

Ozone can donate electrons to form stable O₂ molecules.

Ozone decomposes into oxygen and free radicals.

Ozone has a low ionization energy.

Correct Answer: A

Solution:

Ozone (O₃) acts only as an oxidant because it has a high electron affinity, making it effective at gaining electrons and thus oxidizing other substances.

2H_2(g) + O_2(g) \rightarrow 2H_2O(l)

2Mg(s) + O_2(g) \rightarrow 2MgO(s)

Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)

Cl_2(g) + 2KBr(aq) \rightarrow 2KCl(aq) + Br_2(l)

Correct Answer: A

Solution:

In the reaction

2H_2(g) + O_2(g) \rightarrow 2H_2O(l)

, hydrogen acts as a reducing agent by donating electrons to oxygen, which is reduced.

Oxidizing agent

Reducing agent

Catalyst

Precipitating agent

Correct Answer: B

Solution:

Sulfur dioxide acts as a reducing agent to remove excess chlorine by converting it to chloride ions.

C_2H_4

acts as a reducing agent.

C_2H_4

acts as an oxidizing agent.

C_2H_4

is neither oxidized nor reduced.

C_2H_4

acts as a catalyst.

Correct Answer: A

Solution:

C_2H_4

is oxidized to

CO_2

, meaning it loses electrons and acts as a reducing agent.

Potassium (K)

Fluorine (F)

Both are oxidized

Neither is oxidized

Correct Answer: A

Solution:

Potassium (K) is oxidized as it loses electrons to form

\text{K}^+

ions.

2H₂O₂(aq) \rightarrow 2H₂O(l) + O₂(g)

2Mg(s) + O₂(g) \rightarrow 2MgO(s)

C(s) + O₂(g) \rightarrow CO₂(g)

H₂(g) + Cl₂(g) \rightarrow 2HCl(g)

Correct Answer: A

Solution:

The reaction 2H₂O₂(aq) \rightarrow 2H₂O(l) + O₂(g) is a decomposition reaction where hydrogen peroxide breaks down into water and oxygen, involving a redox process.

\text{NaHSO}_4 \rightarrow \text{Na}_2\text{SO}_4

\text{H}_2\text{S} \rightarrow \text{SO}_2

\text{S} (s) + \text{O}_2 (g) \rightarrow \text{SO}_2 (g)

\text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{S}

Correct Answer: B

Solution:

In the reaction

\text{H}_2\text{S} \rightarrow \text{SO}_2

, sulfur is oxidized from an oxidation state of -2 in

\text{H}_2\text{S}

to +4 in

\text{SO}_2

K_2Cr_2O_7 + 3Na_2SO_3 + 4H_2O \rightarrow 2Cr^{3+} + 3Na_2SO_4 + 2K^+ + 8H^+

K_2Cr_2O_7 + 6Na_2SO_3 + 7H_2O \rightarrow 2Cr^{3+} + 3Na_2SO_4 + 2K^+ + 14H^+

K_2Cr_2O_7 + 3Na_2SO_3 + 7H_2O \rightarrow 2Cr^{3+} + 3Na_2SO_4 + 2K^+ + 6H^+

K_2Cr_2O_7 + 3Na_2SO_3 + 4H_2O \rightarrow 2Cr^{3+} + 3Na_2SO_4 + 2K^+ + 6H^+

Correct Answer: A

Solution:

The balanced equation is

K_2Cr_2O_7 + 3Na_2SO_3 + 4H_2O \rightarrow 2Cr^{3+} + 3Na_2SO_4 + 2K^+ + 8H^+

based on balancing the redox reaction using the ion-electron method.

Ozone (O₃)

Sulfur dioxide (SO₂)

Nitric acid (HNO₃)

Hydrochloric acid (HCl)

Correct Answer: B

Solution:

Sulfur dioxide (SO₂) can act as both an oxidizing and a reducing agent in different reactions.

Copper

Zinc

Both

Neither

Correct Answer: B

Solution:

In a galvanic cell, the anode is where oxidation occurs. Zinc is oxidized to

Zn^{2+}

, thus it acts as the anode.

Hydrogen atoms gain electrons and are reduced.

Oxygen atoms gain electrons and are reduced.

Oxygen atoms lose electrons and are oxidized.

Hydrogen atoms lose electrons and are oxidized.

Correct Answer: B

Solution:

In the reaction

2H_2(g) + O_2(g) \rightarrow 2H_2O(l)

, oxygen atoms gain electrons from hydrogen and are reduced.

2H₂(g) + O₂(g) \rightarrow 2H₂O(l)

Pb₃O₄ + 8HCl \rightarrow 3PbCl₂ + Cl₂ + 4H₂O

H₂O₂(aq) + Fe²⁺(aq) \rightarrow Fe³⁺(aq) + H₂O(l)

Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ \rightarrow 2Cr³⁺ + 6Fe³⁺ + 7H₂O

Correct Answer: D

Solution:

The reaction involves balancing using oxidation numbers, as it includes changes in the oxidation states of chromium and iron.

Oxidation and reduction occur simultaneously.

Only reduction occurs.

Only oxidation occurs.

Neither oxidation nor reduction occurs.

Correct Answer: A

Solution:

In a redox reaction, oxidation and reduction occur simultaneously as one species loses electrons and another gains electrons.

Neon

Fluorine

Iodine

Cesium

Correct Answer: C

Solution:

Iodine can exhibit both positive and negative oxidation states, unlike neon, fluorine, and cesium.

Potassium (K)

Hydrogen (H)

Oxygen (O)

None

Correct Answer: B

Solution:

In this reaction, hydrogen is reduced as it gains electrons to form hydrogen gas.

Hydrogen

Oxygen

Water

None

Correct Answer: A

Solution:

Hydrogen is oxidized as it loses electrons to form water.

Neon

Iodine

Cesium

Fluorine

Correct Answer: B

Solution:

Iodine can exhibit both positive and negative oxidation states, unlike neon which is inert, cesium which is typically positive, and fluorine which is typically negative.

Correct Answer: C

Solution:

In H₂SO₄, sulfur has an oxidation number of +6. The oxidation numbers of hydrogen and oxygen are +1 and -2, respectively.

X_2

+ 6

Y^{3+}

→ 3

XY_3

X_2

+ 2

Y^{3+}

→ 2

XY_3

X_2

+ 3

Y^{3+}

→

XY_3

X_2

+ 3

Y^{3+}

→ 2

XY_3

Correct Answer: A

Solution:

In the balanced equation, 2 moles of

X_2

react with 6 moles of

Y^{3+}

to form 3 moles of

XY_3

, ensuring that both mass and charge are conserved.

2H₂O₂(aq) \rightarrow 2H₂O(l) + O₂(g)

2Mg(s) + O₂(g) \rightarrow 2MgO(s)

CuO(s) + H₂(g) \rightarrow Cu(s) + H₂O(g)

Fe₂O₃(s) + 3CO(g) \rightarrow 2Fe(s) + 3CO₂(g)

Correct Answer: A

Solution:

In a disproportionation reaction, a single substance is simultaneously oxidized and reduced. In the reaction of hydrogen peroxide, it decomposes into water and oxygen, with oxygen being both reduced and oxidized.

A substance that donates electrons.

A substance that accepts electrons.

A substance that increases the oxidation number of another substance.

A substance that decreases the oxidation number of another substance.

Correct Answer: B

Solution:

An oxidizing agent is a substance that accepts electrons from another substance, thereby getting reduced itself.

Electrons flow from copper to zinc.

Electrons flow from zinc to copper.

Electrons flow from the salt bridge to the anode.

Electrons flow from the cathode to the anode.

Correct Answer: B

Solution:

In a galvanic cell, electrons flow from the anode to the cathode. Zinc, being more reactive, loses electrons to copper, thus electrons flow from zinc to copper.

W^{4+}

acts as an oxidizing agent.

W^{4+}

acts as a reducing agent.

W^{4+}

is neither oxidized nor reduced.

W^{4+}

acts as a catalyst.

Correct Answer: A

Solution:

W^{4+}

is reduced to

W^{2+}

, meaning it gains electrons and acts as an oxidizing agent.

A^{3+}

gains 3 electrons and

B

loses 2 electrons.

A^{3+}

gains 2 electrons and

B

loses 3 electrons.

A^{3+}

gains 1 electron and

B

loses 1 electron.

A^{3+}

gains 3 electrons and

B

loses 3 electrons.

Correct Answer: A

Solution:

A^{3+}

gains 3 electrons to become

A

, and

B

loses 2 electrons to become

B^{2+}

. Therefore,

A^{3+}

gains 3 electrons and

B

loses 2 electrons.

Potassium (K)

Fluorine (F)

Potassium fluoride (KF)

None of the above

Correct Answer: A

Solution:

In this reaction, potassium (K) loses electrons and is oxidized, thus acting as the reducing agent.

An element in one oxidation state is simultaneously oxidized and reduced.

An element is only oxidized.

An element is only reduced.

No change in oxidation state occurs.

Correct Answer: A

Solution:

In a disproportionation reaction, an element in one oxidation state is simultaneously oxidized and reduced.

Copper acts as a reducing agent and silver ions are reduced.

Copper acts as an oxidizing agent and silver ions are oxidized.

Copper ions are formed and silver metal is oxidized.

Silver nitrate decomposes into nitrogen gas and silver metal.

Correct Answer: A

Solution:

In the reaction, copper metal loses electrons (is oxidized) and acts as a reducing agent. Silver ions gain electrons (are reduced) to form silver metal.

X + Y_2 \rightarrow XY_2

2X + Y_2 \rightarrow 2XY

X + 2Y_2 \rightarrow XY_4

2X + 3Y_2 \rightarrow 2XY_3

Correct Answer: A

Solution:

In the reaction,

X

is oxidized from 0 to +2, and

Y

is reduced from 0 to -1. Therefore, the balanced equation is

X + Y_2 \rightarrow XY_2

Oxidizing agent

Reducing agent

Catalyst

Inert gas

Correct Answer: A

Solution:

In the combustion of

C_2H_6

O_2

gains electrons and is reduced to form

H_2O

, acting as the oxidizing agent.

Ammonia acts as an oxidizing agent.

Ammonia acts as a reducing agent.

Ammonia is neither oxidized nor reduced.

Ammonia acts as a catalyst.

Correct Answer: B

Solution:

Ammonia (NH₃) is oxidized to form nitric oxide (NO), thus acting as a reducing agent by donating electrons to oxygen.

\text{Zn}(s) + \text{CuSO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{Cu}(s)

\text{H}_2(g) + \text{Cl}_2(g) \rightarrow 2\text{HCl}(g)

\text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g)

\text{2H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)

Correct Answer: A

Solution:

In the reaction

\text{Zn}(s) + \text{CuSO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{Cu}(s)

, zinc displaces copper from copper sulfate, making it a displacement redox reaction.

It involves assigning oxidation numbers to all elements and ensuring the total increase in oxidation numbers equals the total decrease.

It requires balancing hydrogen atoms first, followed by oxygen atoms.

The method is only applicable to reactions in acidic solutions.

It does not account for ions present in the solution.

Correct Answer: A

Solution:

The oxidation number method involves assigning oxidation numbers to all elements and ensuring the total increase in oxidation numbers equals the total decrease, thus balancing the redox reaction.

Oxidizing agent

Reducing agent

Catalyst

Spectator ion

Correct Answer: A

Solution:

\text{MnO}_4^-

acts as an oxidizing agent because it gains electrons and is reduced to

\text{Mn}^{2+}

2 Mg (s) + O₂ (g) \rightarrow 2 MgO (s)

H₂O (l) \rightarrow H₂ (g) + O₂ (g)

NaCl (s) \rightarrow Na⁺ (aq) + Cl⁻ (aq)

CaCO₃ (s) \rightarrow CaO (s) + CO₂ (g)

Correct Answer: A

Solution:

In the reaction 2 Mg (s) + O₂ (g) \rightarrow 2 MgO (s), magnesium is oxidized and oxygen is reduced, making it a redox reaction.

The oxidation state of oxygen in

M_2O_3

is -2.

The oxidation state of

M

M_2O_3

is +2.

The reaction is not a redox reaction.

The oxidation state of oxygen in

M_2O_3

is -1.

Correct Answer: A

Solution:

M_2O_3

, the oxidation state of

M

is +3, and since the compound is neutral, the oxidation state of oxygen must be -2 to balance the charge.

MnO_4^-

SO_2

Mn^{2+}

HSO_4^-

Correct Answer: A

Solution:

In the reaction,

MnO_4^-

acts as the oxidizing agent as it gains electrons and is reduced to

Mn^{2+}

It gains electrons and is reduced.

It loses electrons and is oxidized.

It gains electrons and is oxidized.

It loses electrons and is reduced.

Correct Answer: A

Solution:

An oxidizing agent gains electrons in a redox reaction and is reduced in the process.

ext{H}_2 ext{O}

ext{F}_2

ext{HF}

ext{O}_2

Correct Answer: B

Solution:

In this reaction,

ext{F}_2

is reduced to

ext{HF}

, indicating that it gains electrons and acts as the oxidizing agent.

Reducing agent

Oxidizing agent

Spectator ion

Catalyst

Correct Answer: B

Solution:

In this reaction,

MnO_4^-

is reduced to

Mn^{2+}

, indicating that it acts as the oxidizing agent.

Oxidation number method

Half-reaction method

Both A and B

Neither A nor B

Correct Answer: C

Solution:

Both the oxidation number method and the half-reaction method can be used to balance redox reactions in acidic solutions.

Correct Answer: C

Solution:

ext{K}_2 ext{Cr}_2 ext{O}_7

, potassium has an oxidation number of +1 and oxygen has an oxidation number of -2. The compound is neutral, so the sum of oxidation numbers is zero:

2(+1) + 2(x) + 7(-2) = 0

, solving gives

x = +6

for chromium.

True or False

Correct Answer: False

Solution:

Oxidation numbers are a key tool in identifying oxidants and reductants in chemical reactions.

Correct Answer: False

Solution:

Ozone acts only as an oxidizing agent, not as a reducing agent.

Correct Answer: True

Solution:

Some substances, like sulfur dioxide and hydrogen peroxide, can act as both oxidizing and reducing agents in different reactions.

Correct Answer: True

Solution:

The concept of oxidation number is used to identify oxidants and reductants in redox reactions.

Correct Answer: False

Solution:

Redox reactions are crucial in the operation of both dry and wet batteries.

Correct Answer: True

Solution:

Redox reactions are categorized into these four types based on the nature of the chemical changes involved.

Correct Answer: True

Solution:

The oxidizing agent is the substance that gains electrons and is reduced in a redox reaction.

Correct Answer: True

Solution:

Originally, oxidation was defined as the addition of oxygen to an element or compound.

Correct Answer: True

Solution:

The reaction involves the transfer of electrons from copper to silver ions, resulting in the deposition of silver metal.

Correct Answer: True

Solution:

The reaction between hydrogen and chlorine involves the transfer of electrons, thus it is a redox reaction.

Correct Answer: True

Solution:

In the reaction between hydrogen and chlorine, hydrogen is oxidized and chlorine is reduced, making it a redox reaction.

Correct Answer: True

Solution:

In the reaction between hydrogen and oxygen to form water, hydrogen is oxidized and oxygen is reduced, making it a redox reaction.

Correct Answer: False

Solution:

Environmental issues like the Hydrogen Economy and the development of the Ozone Hole are related to redox reactions.

Correct Answer: True

Solution:

Galvanic cells use chemical reactions to produce electrical energy through electron transfer processes.

Correct Answer: True

Solution:

The oxidising agent is reduced because it gains electrons during the redox process.

Correct Answer: True

Solution:

Burning of fuels involves oxidation and reduction processes, making it a redox reaction.

Correct Answer: True

Solution:

Redox reactions are indeed classified into these four categories, as they encompass the various ways oxidation and reduction can occur.

Correct Answer: False

Solution:

The oxidation number method can be used to balance redox reactions in both acidic and basic solutions.

Correct Answer: False

Solution:

Oxidation numbers are a crucial tool for balancing redox reactions as they help track electron transfer.

Correct Answer: True

Solution:

Redox reactions involve both oxidation and reduction processes happening at the same time.

Correct Answer: False

Solution:

While the classical definition of oxidation involves the addition of oxygen, modern definitions also include electron transfer processes.

Correct Answer: True

Solution:

The oxidation number method is one of the methods used to balance chemical equations for redox reactions.

Correct Answer: True

Solution:

By definition, redox reactions are characterized by the simultaneous occurrence of oxidation and reduction processes.

Correct Answer: False

Solution:

In a redox reaction, the substance that is oxidized is the reducing agent, not the oxidizing agent.

Correct Answer: True

Solution:

The use of oxidation numbers helps in identifying oxidants and reductants, as described in the excerpts.

Correct Answer: False

Solution:

In a redox reaction, the substance that gets oxidized is called the reductant, not the oxidant. The oxidant is the substance that gets reduced.

Correct Answer: False

Solution:

The reaction between hydrogen and chlorine is a redox reaction because it involves the transfer of electrons.

Correct Answer: True

Solution:

In redox reactions, the oxidant gains electrons and is reduced, while the reductant loses electrons and is oxidized.

Correct Answer: False

Solution:

Ozone acts only as an oxidizing agent in reactions.

Correct Answer: True

Solution:

Redox reactions are characterized by the simultaneous occurrence of oxidation and reduction processes.

Correct Answer: False

Solution:

An oxidising agent is a substance that gains electrons and is reduced in a redox reaction.

Correct Answer: True

Solution:

In the reaction, silver ions gain electrons from copper, thus undergoing reduction.

Correct Answer: True

Solution:

An oxidizing agent gains electrons and is reduced, while a reducing agent loses electrons and is oxidized.

Correct Answer: False

Solution:

Redox reactions have extensive applications in various fields including pharmaceuticals, biology, industry, metallurgy, and agriculture.

Correct Answer: True

Solution:

Redox reactions are defined as a class of reactions in which oxidation and reduction occur simultaneously.

Correct Answer: True

Solution:

In the reaction

2H_2(g) + O_2(g) \rightarrow 2H_2O(l)

, hydrogen is oxidized as it loses electrons, and oxygen is reduced as it gains electrons.

Correct Answer: True

Solution:

Oxidation numbers help in keeping track of electron shifts by assigning a consistent set of rules to elements in compounds.

Correct Answer: False

Solution:

There are two methods used to balance redox reactions: the oxidation number method and the half-reaction method.

Correct Answer: True

Solution:

Ozone is known to act solely as an oxidant, not as a reductant, in chemical reactions.

Correct Answer: True

Solution:

Burning of fuels involves the transfer of electrons, which is a hallmark of redox reactions.

Correct Answer: True

Solution:

Historically, oxidation was defined as the addition of oxygen to a substance, as seen in reactions like 2 Mg (s) + O₂ (g) → 2 MgO (s).

Correct Answer: False

Solution:

Both Hydrogen Economy and Ozone Hole development are related to redox reactions as they involve oxidation and reduction processes.

Correct Answer: True

Solution:

Electrochemical processes, such as the extraction of metals, involve redox reactions where electron transfer occurs.

Correct Answer: True

Solution:

Oxidation numbers help in identifying which elements are oxidized and which are reduced, thus identifying oxidants and reductants.

Correct Answer: False

Solution:

Originally, oxidation referred to the addition of oxygen to an element or compound, not the loss of electrons.

Correct Answer: False

Solution:

The oxidation number method is a useful technique for balancing redox reactions, as mentioned in the excerpts.

Correct Answer: True

Solution:

Both the oxidation number method and the half-reaction method are valid techniques for balancing redox reactions.

Correct Answer: True

Solution:

The oxidation number method is used to track changes in oxidation states by assigning oxidation numbers to elements in a reaction.

Correct Answer: True

Solution:

Electrochemical processes involve redox reactions where metals are extracted by reducing metal ions to metal atoms.

Correct Answer: False

Solution:

The burning of fuels is a redox process as it involves oxidation reactions that release energy.

Correct Answer: False

Solution:

The reaction between copper and silver nitrate is a redox reaction, as it involves electron transfer.

Correct Answer: True

Solution:

The oxidation number method involves assigning oxidation numbers to elements and ensuring the increase and decrease in oxidation numbers are balanced.

Correct Answer: True

Solution:

The oxidation number method is one of the methods used to balance redox reactions by tracking changes in oxidation states.

Correct Answer: True

Solution:

Oxidation involves the loss of electrons, so the substance that is oxidised loses electrons.

Correct Answer: False

Solution:

Redox reactions are significant in both physical and biological phenomena, as they are involved in various biological processes.

Correct Answer: True

Solution:

In covalent compounds, electron transfer is conceptualized as shifts in electron density, which is useful for understanding redox reactions.

Correct Answer: True

Solution:

Oxidation numbers help identify which species are oxidized and reduced, thus identifying the oxidants and reductants.

Correct Answer: True

Solution:

Originally, oxidation was defined as the addition of oxygen to an element or compound due to the presence of dioxygen in the atmosphere.

Correct Answer: True

Solution:

By definition, redox reactions involve the simultaneous occurrence of oxidation and reduction processes.

Correct Answer: True

Solution:

Oxidation numbers help in identifying the electron shifts by assigning a numerical value to the oxidation state of elements in a reaction.

Correct Answer: True

Solution:

Electrochemical cells use redox reactions as a source of electrical energy, as explained in the excerpts.

Correct Answer: False

Solution:

Redox reactions are significant in both physical and biological phenomena, as mentioned in the excerpts.

Redox Reactions

Learning Objectives

Learning Objectives

Revision Notes & Summary

UNIT 7: REDOX REACTIONS

Overview

Objectives

Classical Idea of Redox Reactions

Key Concepts

Oxidation and Reduction

Oxidation Numbers

Redox Couples and Electrode Processes

Classification of Redox Reactions

Balancing Redox Reactions

Methods

Important Reactions and Examples

Standard Electrode Potentials

Diagram Descriptions

Daniell Cell

Chemical Reactions

Conclusion

Exam Tips & Common Mistakes

Common Mistakes and Exam Tips for Redox Reactions

Common Pitfalls

Tips for Success

AI Learning Assistant

Practice Test – MCQs, True/False

Multiple Choice Questions

Q1.Consider a redox reaction where A2−A^{2-}A2− is oxidized to A0A^0A0 and B0B^0B0 is reduced to B3−B^{3-}B3−. Which of the following represents the correct balanced equation?

Correct Answer: A

Q2.In the reaction 2K(s) + F₂(g) \rightarrow 2KF(s), which element is the oxidizing agent?

Correct Answer: B

Q3.Which of the following is a correct representation of the oxidation number method for balancing redox reactions?

Correct Answer: A

Q4.In a hypothetical electrochemical cell, metal CCC is used as the anode and metal DDD as the cathode. Which of the following statements is true regarding the flow of electrons in the cell?

Correct Answer: B

Q5.Which of the following is a redox reaction involving displacement?

Correct Answer: A

Q6.In the Ostwald process for the manufacture of nitric acid, ammonia is oxidized by oxygen to form nitric oxide. What is the balanced equation for this reaction?

Correct Answer: A

Q7.Which of the following is a characteristic of redox reactions?

Correct Answer: B

Q8.In a redox reaction, which of the following is true about the reducing agent?

Correct Answer: B

Q9.Which of the following reactions involves the use of the oxidation number method for balancing?

Correct Answer: C

Q10.In the reaction of copper with silver nitrate, which substance is reduced?

Correct Answer: D

Q11.In an experiment, a metal ZZZ is placed in a solution containing Y3+Y^{3+}Y3+ ions. After some time, Z2+Z^{2+}Z2+ ions are found in the solution. Which of the following is the correct redox reaction?

Correct Answer: D

Q12.In a redox reaction, copper metal is placed in a solution of silver nitrate. Which of the following observations supports that a redox reaction has occurred?

Correct Answer: A

Q13.Consider the reaction: 2H2(g)+O2(g)→2H2O(l)2H_2(g) + O_2(g) \rightarrow 2H_2O(l)2H2​(g)+O2​(g)→2H2​O(l). Which of the following statements correctly identifies the oxidizing and reducing agents?

Correct Answer: B

Q14.Which of the following statements correctly defines a reducing agent?

Correct Answer: B

Q15.In a hypothetical redox reaction, metal MMM reacts with X2X_2X2​ gas to form MX2MX_2MX2​. If MMM has an oxidation state of +2 in MX2MX_2MX2​, which of the following statements is correct?

Correct Answer: A

Q16.Which of the following statements is true about redox reactions?

Correct Answer: B

Q17.In the reaction 4 NH₃(g) + 5 O₂(g) \rightarrow 4 NO(g) + 6 H₂O(g), which element is reduced?

Correct Answer: A

Q18.Consider the redox reaction: 2Fe3++2I−→2Fe2++I22Fe^{3+} + 2I^- \rightarrow 2Fe^{2+} + I_22Fe3++2I−→2Fe2++I2​. Which species is the oxidizing agent?

Correct Answer: A

Q19.Which of the following statements is true about oxidizing agents?

Correct Answer: A

Q20.In a novel chemical process, compound G2H6G_2H_6G2​H6​ reacts with O2O_2O2​ to produce CO2CO_2CO2​ and H2OH_2OH2​O. Which of the following statements correctly describes the role of G2H6G_2H_6G2​H6​ in this reaction?

Correct Answer: A

Q21.What is the role of oxygen in the reaction 2H₂(g) + O₂(g) \rightarrow 2H₂O(l)?

Correct Answer: A

Q22.What is the oxidation number of chromium in K2Cr2O7K_2Cr_2O_7K2​Cr2​O7​?

Correct Answer: A

Q23.A solution of E2+E^{2+}E2+ ions is mixed with a solution of F3+F^{3+}F3+ ions. After the reaction, E3+E^{3+}E3+ ions and F2+F^{2+}F2+ ions are observed. Which of the following best describes the role of F3+F^{3+}F3+ in this reaction?

Correct Answer: A

Q24.What is the oxidation number of sulfur in H2SO4\text{H}_2\text{SO}_4H2​SO4​?

Correct Answer: A

Q25.Consider the redox reaction between copper and silver nitrate: Cu(s)+2AgNO3(aq)→Cu(NO3)2(aq)+2Ag(s)Cu(s) + 2AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2Ag(s)Cu(s)+2AgNO3​(aq)→Cu(NO3​)2​(aq)+2Ag(s). Which of the following statements is true about this reaction?

Correct Answer: A

Q26.In a novel electrochemical cell, metal AAA is used as the anode and metal BBB as the cathode. Which of the following statements is true regarding the electron flow in the cell?

Correct Answer: A