Redox Reactions Answer key

✅ SECTION – A

(2 Points Each – Each point is a full sentence.)

1. What does the abbreviation ‘redox’ stand for?

Redox stands for reduction and oxidation.
It represents reactions in which oxidation and reduction occur simultaneously.

2. Define oxidation according to the classical concept.

Oxidation is defined as the addition of oxygen to a substance.
It is also defined as the removal of hydrogen from a substance.

3. Define oxidation in terms of electron transfer.

Oxidation is the loss of electrons by an atom or ion.
It results in an increase in oxidation number.

4. Define reduction in terms of electron transfer.

Reduction is the gain of electrons by an atom or ion.
It results in a decrease in oxidation number.

5. Define an oxidising agent in terms of oxidation number.

An oxidising agent is a substance that increases the oxidation number of another substance.
It itself undergoes reduction during the reaction.

6. Define a reducing agent in terms of oxidation number.

A reducing agent is a substance that decreases the oxidation number of another substance.
It itself undergoes oxidation during the reaction.

7. Define a reductant in terms of electron transfer.

A reductant is a substance that donates electrons to another substance.
It causes reduction and itself gets oxidised.

8. Define an oxidant in terms of electron transfer.

An oxidant is a substance that accepts electrons from another substance.
It causes oxidation and itself gets reduced.

9. State the oxidation number of an atom in its free state.

The oxidation number of an atom in its free state is zero.
This is because there is no transfer of electrons in the elemental form.

10. What is the oxidation number of Fluorine in all its compounds?

The oxidation number of fluorine in all its compounds is –1.
This is because fluorine is the most electronegative element.

11. State any one rule for assigning oxidation number to Hydrogen.

Hydrogen generally has an oxidation number of +1 in its compounds.
It has an oxidation number of –1 when it forms metal hydrides.

12. State any one rule for assigning oxidation number to Halogens.

Halogens usually have an oxidation number of –1 in their compounds.
They show positive oxidation states when combined with oxygen or fluorine.

13. What is a displacement reaction?

A displacement reaction is a reaction in which one element replaces another element in a compound.
It occurs when a more reactive element displaces a less reactive element.

14. What is meant by a metal displacement reaction?

A metal displacement reaction occurs when a more reactive metal displaces a less reactive metal from its compound.
This type of reaction follows the metal reactivity series.

15. What is meant by a non-metal displacement reaction?

A non-metal displacement reaction occurs when a more reactive non-metal displaces a less reactive non-metal.
This commonly occurs among halogens.

16. Define combustion in terms of oxygen.

Combustion is a chemical reaction in which a substance reacts with oxygen.
It produces heat and often light energy.

17. What is a salt bridge?

A salt bridge is a device that connects the two half-cells in an electrochemical cell.
It allows the flow of ions to maintain electrical neutrality.

18. Name the enzyme responsible for the browning of a cut apple.

The enzyme responsible for browning of a cut apple is polyphenol oxidase.
This enzyme catalyzes the oxidation of phenolic compounds.

19. What is the driving force behind the reactions of elements?

The driving force behind reactions is the tendency of atoms to achieve stable electronic configuration.
Atoms gain, lose, or share electrons to attain stability.

20. What is meant by oxidation number?

Oxidation number is the hypothetical charge assigned to an atom in a compound.
It indicates the number of electrons lost or gained by the atom.

21. What is the function of a voltmeter in an electrochemical cell?

A voltmeter measures the potential difference between two electrodes.
It indicates the electrical energy produced by the cell.

22. What is corrosion?

Corrosion is the slow deterioration of a metal due to chemical reactions with the environment.
It generally involves oxidation in the presence of air and moisture.

23. What is meant by electrode potential?

Electrode potential is the tendency of an electrode to gain or lose electrons.
It is measured with respect to a standard reference electrode.

24. What is a combination reaction?

A combination reaction is a reaction in which two or more substances combine to form a single product.
It may involve changes in oxidation number.

25. What is a decomposition reaction?

A decomposition reaction is a reaction in which a compound breaks down into simpler substances.
It may involve oxidation and reduction processes.

✅ SECTION – B

(4 Points Each – All points are full sentences.)

1. Rules for assigning oxidation numbers to Hydrogen and Halogens

Hydrogen generally has an oxidation number of +1 in its compounds.
Hydrogen has an oxidation number of –1 in metal hydrides.
Halogens generally have an oxidation number of –1 in their compounds.
Halogens show positive oxidation states when combined with oxygen or fluorine.

2. Distinguish between metal and non-metal displacement reactions

A metal displacement reaction involves a more reactive metal displacing a less reactive metal from its compound.
For example, zinc displaces copper from copper sulphate solution.
A non-metal displacement reaction involves a more reactive non-metal displacing a less reactive non-metal.
For example, chlorine displaces bromine from potassium bromide solution.

3. Why is the browning of a cut apple considered a redox reaction?

When an apple is cut, it reacts with oxygen in the air.
The enzyme polyphenol oxidase catalyzes the reaction.
The phenolic compounds present in the apple undergo oxidation.
Since oxidation occurs along with reduction of oxygen, it is a redox reaction.

4. Why does an old car bumper change its colour?

The metal in the bumper undergoes corrosion over time.
Iron reacts with oxygen and moisture in the air.
Iron gets oxidised to form rust.
Rust has a reddish-brown colour, which changes the appearance of the bumper.

5. Why do new batteries become useless after some days?

Batteries generate electricity through redox reactions.
The reactants inside the battery get consumed over time.
When the chemicals are exhausted, the redox reaction stops.
As a result, the battery can no longer produce electricity.

6. What is a salt bridge and what is its role in a Daniel cell?

A salt bridge is a tube containing an electrolyte solution.
It connects the two half-cells of the Daniel cell.
It allows the movement of ions between the half-cells.
It maintains electrical neutrality and completes the circuit.

7. What is the oxidation number of Fluorine in its compounds? Explain why.

Fluorine always has an oxidation number of –1 in its compounds.
This is because it is the most electronegative element.
It always gains one electron in chemical reactions.
Therefore, it does not show positive oxidation states.

8. Arrange Zinc, Copper, and Silver in decreasing order of electron-releasing tendency.

Zinc has the highest tendency to release electrons.
Copper has a lower tendency than zinc.
Silver has the least tendency among the three.
Therefore, the correct order is Zn > Cu > Ag.

9. Define and distinguish between Combination and Decomposition redox reactions.

A combination reaction is one in which two or more substances combine to form a single product.
For example, magnesium reacts with oxygen to form magnesium oxide.
A decomposition reaction is one in which a compound breaks down into simpler substances.
For example, mercuric oxide decomposes to give mercury and oxygen.

10. What is meant by corrosion? Explain with one example.

Corrosion is the slow deterioration of a metal due to chemical reactions with the environment.
It usually involves oxidation of the metal.
Rusting of iron is a common example of corrosion.
In rusting, iron reacts with oxygen and moisture to form hydrated iron oxide.

11. Define standard hydrogen electrode (SHE).

The standard hydrogen electrode is a reference electrode used to measure electrode potentials.
It consists of a platinum electrode dipped in 1 M acid solution.
Hydrogen gas is bubbled over the platinum electrode at 1 atmosphere pressure.
Its standard electrode potential is taken as zero volts.

12. What is meant by electrochemical series?

The electrochemical series is a list of elements arranged according to their standard electrode potentials.
It is based on their tendency to gain or lose electrons.
It helps in predicting displacement reactions.
It also helps in determining the strength of oxidising and reducing agents.

✅ SECTION – C

(6 Points Each – All points are full sentences.)

1. Explain the construction of a Daniel cell with a focus on the electrical circuit.

A Daniel cell consists of a zinc rod dipped in zinc sulphate solution.
It also contains a copper rod dipped in copper sulphate solution.
The two solutions are connected by a salt bridge.
The zinc and copper rods are connected externally by a conducting wire.
A voltmeter is connected in the circuit to measure the potential difference.
Electrons flow from zinc to copper through the external circuit.

2. Define standard hydrogen electrode (SHE) and explain why it is used as a reference electrode.

The standard hydrogen electrode is a reference electrode used to measure electrode potentials.
It consists of a platinum electrode immersed in 1 M hydrochloric acid solution.
Hydrogen gas is bubbled over the platinum electrode at 1 atmosphere pressure.
The electrode potential of SHE is taken as zero volts.
It provides a standard for comparing other electrode potentials.
It is used because absolute electrode potentials cannot be measured directly.

3. Write the chemical formulae for:
(i) Thallium(I) sulphate
(ii) Chromium(III) oxide
(iii) Bismuth(V) chloride

The formula of Thallium(I) sulphate is Tl₂SO₄.
The formula of Chromium(III) oxide is Cr₂O₃.
The formula of Bismuth(V) chloride is BiCl₅.
These formulae are written based on their oxidation numbers.
The charges of the ions are balanced to make the compound neutral.
Roman numerals indicate the oxidation state of the metal.

4. Describe the change in color of the Statue of Liberty in the context of redox reactions.

The Statue of Liberty is made of copper.
Copper undergoes oxidation when exposed to air and moisture.
Copper reacts with oxygen, carbon dioxide, and water.
It forms a green compound called basic copper carbonate.
The oxidation of copper causes the color change.
The green layer protects the metal from further corrosion.

5. Explain the process of rusting of iron in terms of oxidation and reduction.

Rusting is an electrochemical process involving oxidation and reduction.
Iron loses electrons and gets oxidised to form iron ions.
Oxygen gains electrons and gets reduced in the presence of water.
The iron ions react with hydroxide ions to form iron hydroxide.
Iron hydroxide further oxidises to form hydrated iron(III) oxide.
Hydrated iron oxide is commonly known as rust.

6. Explain the concept of oxidation number with suitable examples.

Oxidation number is the hypothetical charge assigned to an atom in a compound.
It indicates the number of electrons lost or gained by an atom.
The oxidation number of an element in free state is zero.
The sum of oxidation numbers in a neutral compound is zero.
In H₂SO₄, the oxidation number of sulfur is +6.
Oxidation numbers help in identifying redox reactions.

7. Write a short note on corrosion and its prevention methods.

Corrosion is the slow deterioration of metals due to chemical reactions with the environment.
It usually involves oxidation of the metal.
Rusting of iron is a common example of corrosion.
Corrosion can be prevented by painting the metal surface.
It can also be prevented by galvanization using a layer of zinc.
Cathodic protection and alloying are other effective prevention methods.

8. Explain the working of a galvanic cell with a neat diagram.

A galvanic cell converts chemical energy into electrical energy.
It consists of two half-cells connected by a salt bridge.
Oxidation occurs at the anode.
Reduction occurs at the cathode.
Electrons flow from the anode to the cathode through the external circuit.
The flow of electrons produces electric current.

9. What is the driving force behind redox reactions? Explain briefly.

The driving force behind redox reactions is the tendency to achieve a stable electronic configuration.
Atoms tend to gain or lose electrons to reach stability.
The difference in electrode potential also drives the reaction.
The reaction proceeds in the direction of energy decrease.
Formation of stable products contributes to spontaneity.
Energy released during the reaction makes it feasible.

10. Differentiate between oxidising and reducing agents with examples.

An oxidising agent is a substance that gains electrons during a reaction.
An oxidising agent undergoes reduction.
Potassium permanganate is an example of an oxidising agent.
A reducing agent is a substance that loses electrons during a reaction.
A reducing agent undergoes oxidation.
Zinc metal is an example of a reducing agent.

11. Explain metal displacement reaction with a balanced chemical equation.

A metal displacement reaction occurs when a more reactive metal displaces a less reactive metal.
The reaction is based on the reactivity series of metals.
For example, zinc reacts with copper sulphate solution.
Zinc displaces copper from copper sulphate.
The balanced equation is Zn + CuSO₄ → ZnSO₄ + Cu.
In this reaction, zinc is oxidised and copper is reduced.

12. Explain non-metal displacement reaction with a balanced chemical equation.

A non-metal displacement reaction occurs when a more reactive non-metal displaces a less reactive one.
This type of reaction is common among halogens.
For example, chlorine reacts with potassium bromide solution.
Chlorine displaces bromine from the compound.
The balanced equation is Cl₂ + 2KBr → 2KCl + Br₂.
In this reaction, bromide ions are oxidised and chlorine is reduced.

13. Describe the role of oxidation number in balancing redox reactions.

Oxidation number helps in identifying the species that are oxidised and reduced.
The increase and decrease in oxidation numbers are calculated.
The total increase and decrease are made equal.
Coefficients are added to balance the equation.
Charge balance is also maintained in the reaction.
This method is known as the oxidation number method.

14. Explain combustion as a redox reaction with an example.

Combustion is a reaction in which a substance reacts with oxygen.
During combustion, the fuel undergoes oxidation.
Oxygen undergoes reduction in the process.
Methane combustion is a common example.
The balanced equation is CH₄ + 2O₂ → CO₂ + 2H₂O.
Heat and light are produced during combustion.

✅ SECTION – D

(10 Points Each – All points are full sentences.)

1. Explain redox reactions in terms of electron transfer and oxidation number changes with suitable examples.

A redox reaction is a reaction in which oxidation and reduction occur simultaneously.
Oxidation is the loss of electrons by a substance.
Reduction is the gain of electrons by a substance.
Oxidation results in an increase in oxidation number.
Reduction results in a decrease in oxidation number.
Both oxidation and reduction occur together in the same reaction.
For example, zinc reacts with copper sulphate solution.
Zinc loses electrons and gets oxidised to zinc ions.
Copper ions gain electrons and get reduced to copper metal.
The balanced reaction is Zn + CuSO₄ → ZnSO₄ + Cu.

2. Describe the construction and working of a Daniel cell with a labelled diagram and chemical reactions involved.

A Daniel cell consists of a zinc electrode placed in zinc sulphate solution.
It also consists of a copper electrode placed in copper sulphate solution.
The two half-cells are connected by a salt bridge.
The electrodes are connected externally by a conducting wire.
Oxidation takes place at the zinc electrode, which acts as the anode.
The oxidation reaction at the anode is Zn → Zn²⁺ + 2e^-.
Reduction takes place at the copper electrode, which acts as the cathode.
The reduction reaction at the cathode is Cu²⁺ + 2e^- → Cu.
Electrons flow from zinc to copper through the external circuit.
The Daniel cell converts chemical energy into electrical energy.

3. Explain corrosion as a redox reaction. Discuss rusting of iron and methods to prevent it.

Corrosion is a redox reaction involving oxidation of metals.
Rusting of iron is a common example of corrosion.
During rusting, iron loses electrons and gets oxidised.
Oxygen gains electrons and gets reduced in the presence of water.
Moisture acts as a medium for the electrochemical reaction.
Iron reacts with oxygen to form iron oxide.
The product formed is hydrated iron(III) oxide, commonly known as rust.
Rust weakens the metal structure over time.
Corrosion can be prevented by painting or coating the metal surface.
Galvanization and cathodic protection are also effective methods of prevention.

4. Define oxidation and reduction according to classical and electronic concepts and compare both with examples.

According to the classical concept, oxidation is the addition of oxygen.
According to the classical concept, reduction is the removal of oxygen.
According to the electronic concept, oxidation is the loss of electrons.
According to the electronic concept, reduction is the gain of electrons.
The classical concept is limited to reactions involving oxygen and hydrogen.
The electronic concept is broader and applies to all redox reactions.
For example, magnesium reacts with oxygen to form magnesium oxide.
Magnesium undergoes oxidation by losing electrons.
Oxygen undergoes reduction by gaining electrons.
The electronic concept explains redox reactions more completely than the classical concept.

5. Explain the electrochemical series and its applications in predicting displacement reactions.

The electrochemical series is a list of elements arranged according to their standard electrode potentials.
It shows the tendency of elements to gain or lose electrons.
Elements with lower reduction potential act as strong reducing agents.
Elements with higher reduction potential act as strong oxidising agents.
The series helps in predicting displacement reactions.
A more reactive metal can displace a less reactive metal from its compound.
The electrochemical series helps in calculating the cell potential of electrochemical cells.
It is useful in understanding the tendency of metals to undergo corrosion.
The series is important in processes such as electroplating and metallurgy.
It provides valuable information about chemical reactivity of elements.

6. Discuss the rules for assigning oxidation numbers with suitable examples.

The oxidation number of an element in its free state is zero.
The oxidation number of a monoatomic ion is equal to its charge.
Oxygen usually has an oxidation number of –2 in most compounds.
Hydrogen usually has an oxidation number of +1 in most compounds.
Halogens usually have an oxidation number of –1 in their compounds.
The sum of oxidation numbers in a neutral compound is zero.
The sum of oxidation numbers in a polyatomic ion is equal to its overall charge.
In H₂SO₄, the oxidation number of sulfur is +6.
In KMnO₄, the oxidation number of manganese is +7.
These rules help in balancing redox reactions.

7. Explain combination, decomposition, displacement and combustion reactions as types of redox reactions with balanced equations.

A combination reaction is one in which two or more substances combine to form a single product.
An example of a combination reaction is 2Mg + O₂ → 2MgO.
A decomposition reaction is one in which a compound breaks into simpler substances.
An example of a decomposition reaction is 2HgO → 2Hg + O₂.
A displacement reaction is one in which one element replaces another element in a compound.
An example of a metal displacement reaction is Zn + CuSO₄ → ZnSO₄ + Cu.
A non-metal displacement reaction occurs when a more reactive non-metal displaces a less reactive non-metal.
An example of non-metal displacement reaction is Cl₂ + 2KBr → 2KCl + Br₂.
A combustion reaction involves the reaction of a substance with oxygen.
An example of a combustion reaction is CH₄ + 2O₂ → CO₂ + 2H₂O.