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1. What is the concept of oxidation in terms of electron transfer?
ⓐ. Gain of electrons
ⓑ. Loss of protons
ⓒ. Loss of electrons
ⓓ. Gain of protons
Correct Answer: Loss of electrons
Explanation: Oxidation refers to the loss of electrons by an atom, molecule, or ion. In redox reactions, oxidation occurs when an element’s oxidation state increases due to the loss of electrons. This is opposite to reduction, where an element gains electrons and its oxidation state decreases.
2. What is the reduction in terms of electron transfer?
ⓐ. Gain of electrons
ⓑ. Loss of electrons
ⓒ. Gain of protons
ⓓ. Loss of protons
Correct Answer: Gain of electrons
Explanation: Reduction is the process in which an atom, molecule, or ion gains electrons. This results in a decrease in its oxidation state. In a redox reaction, the substance that gains electrons undergoes reduction, while the substance that loses electrons undergoes oxidation.
3. What is the oxidation number of oxygen in $H_2O$?
ⓐ. +1
ⓑ. -1
ⓒ. 0
ⓓ. -2
Correct Answer: -2
Explanation: In most compounds, oxygen has an oxidation number of -2, as it typically gains two electrons when bonding with other elements. In water ($H_2O$), the oxidation number of oxygen is -2, while hydrogen has an oxidation number of +1.
4. Which of the following represents a redox reaction?
ⓐ. $2Na + Cl_2 \rightarrow 2NaCl$
ⓑ. $H_2O \rightarrow H_2 + O_2$
ⓒ. $H_2 + Cl_2 \rightarrow 2HCl$
ⓓ. All of the above
Correct Answer: All of the above
Explanation: All of the reactions listed are redox reactions because they involve the transfer of electrons. In the first reaction, sodium is oxidized (loses electrons), and chlorine is reduced (gains electrons). In the second and third reactions, hydrogen and oxygen undergo changes in oxidation states as electrons are transferred.
5. What is the oxidation number of chlorine in $Cl_2$?
ⓐ. +1
ⓑ. -1
ⓒ. 0
ⓓ. +2
Correct Answer: 0
Explanation: The oxidation number of an element in its pure elemental form (like $Cl_2$) is always zero. This is because there are no net changes in electron distribution, so the oxidation number remains 0.
6. Which of the following is an example of a disproportionation reaction?
ⓐ. $Cl_2 \rightarrow 2Cl^-$
ⓑ. $2H_2 + O_2 \rightarrow 2H_2O$
ⓒ. $2H_2O_2 \rightarrow 2H_2O + O_2$
ⓓ. $Na + Cl_2 \rightarrow NaCl$
Correct Answer: $2H_2O_2 \rightarrow 2H_2O + O_2$
Explanation: In disproportionation reactions, a substance undergoes both oxidation and reduction. In this case, hydrogen peroxide ($H_2O_2$) is both oxidized to oxygen gas and reduced to water, making it a disproportionation reaction.
7. Which method is commonly used to balance redox reactions in acidic media?
ⓐ. Half-reaction method
ⓑ. Oxidation number method
ⓒ. Electron loss method
ⓓ. Ion-electron method
Correct Answer: Ion-electron method
Explanation: The ion-electron method is typically used to balance redox reactions in acidic media. In this method, the reaction is split into two half-reactions (oxidation and reduction), and the number of electrons involved in each half-reaction is balanced by adding electrons.
8. In the oxidation number method of balancing redox reactions, what must be equal on both sides of the reaction?
ⓐ. Mass and energy
ⓑ. Oxidation numbers
ⓒ. Electrons
ⓓ. Both mass and charge
Correct Answer: Both mass and charge
Explanation: In the oxidation number method, both the mass and the charge must be balanced. This is done by adjusting coefficients in the half-reactions to ensure that the total mass and the total charge are the same on both sides of the equation.
9. Which of the following is an example of a redox reaction in everyday life?
ⓐ. Rusting of iron
ⓑ. Photosynthesis
ⓒ. Combustion of fuels
ⓓ. All of the above
Correct Answer: All of the above
Explanation: All of these processes involve redox reactions. In rusting, iron undergoes oxidation while oxygen undergoes reduction. In photosynthesis, carbon dioxide is reduced to glucose, and water is oxidized to oxygen. In combustion, fuels are oxidized to release energy and form carbon dioxide and water.
10. How does an electrochemical cell rely on redox reactions?
ⓐ. By converting chemical energy into mechanical energy
ⓑ. By transferring electrons through an external circuit
ⓒ. By using light energy to drive reactions
ⓓ. By converting heat energy into electrical energy
Correct Answer: By transferring electrons through an external circuit
Explanation: An electrochemical cell works by facilitating redox reactions. Electrons are transferred from the reducing agent (oxidized) to the oxidizing agent (reduced) through an external circuit, which can then be used to power electronic devices.
11. What does oxidation in terms of oxygen gain involve?
ⓐ. A decrease in the number of oxygen atoms
ⓑ. A loss of electrons
ⓒ. A gain of electrons
ⓓ. An increase in the number of oxygen atoms
Correct Answer: An increase in the number of oxygen atoms
Explanation: Oxidation in terms of oxygen gain refers to the process where an atom, molecule, or ion gains oxygen atoms. In this process, the oxidation number of the element increases. This is commonly seen in reactions where oxygen is added to a substance, such as in the formation of oxides.
12. Which of the following is an example of oxidation in terms of oxygen gain?
ⓐ. $2H_2 + O_2 \rightarrow 2H_2O$
ⓑ. $2Na + Cl_2 \rightarrow 2NaCl$
ⓒ. $Mg + O_2 \rightarrow MgO$
ⓓ. $C + O_2 \rightarrow CO$
Correct Answer: $Mg + O_2 \rightarrow MgO$
Explanation: In this reaction, magnesium ($Mg$) gains oxygen to form magnesium oxide ($MgO$), making it an example of oxidation in terms of oxygen gain. The magnesium atom undergoes an increase in oxidation state as it combines with oxygen.
13. What is the oxidation number of carbon in $CO_2$?
ⓐ. +2
ⓑ. -2
ⓒ. +4
ⓓ. 0
Correct Answer: +4
Explanation: In carbon dioxide ($CO_2$), oxygen has an oxidation number of -2, and there are two oxygen atoms, giving a total of -4. To balance the charge, carbon must have an oxidation number of +4. This shows oxidation in terms of the gain of oxygen.
14. Which of the following reactions demonstrates oxidation in terms of hydrogen loss?
ⓐ. $2H_2 + O_2 \rightarrow 2H_2O$
ⓑ. $C + O_2 \rightarrow CO_2$
ⓒ. $Na + Cl_2 \rightarrow NaCl$
ⓓ. $H_2S + O_2 \rightarrow SO_2 + H_2O$
Correct Answer: $H_2S + O_2 \rightarrow SO_2 + H_2O$
Explanation: In this reaction, hydrogen is lost from hydrogen sulfide ($H_2S$) as it reacts with oxygen. The sulfur in $H_2S$ is oxidized (its oxidation state increases), while the hydrogen atoms are lost, making this an example of oxidation in terms of hydrogen loss.
15. In which type of reaction does oxidation occur due to the loss of hydrogen?
ⓐ. Combustion
ⓑ. Decomposition
ⓒ. Reduction
ⓓ. Both A and B
Correct Answer: Both A and B
Explanation: Both combustion and decomposition reactions often involve the loss of hydrogen atoms, leading to oxidation. For example, in the combustion of hydrocarbons, hydrogen is lost, and the carbon is oxidized. In decomposition reactions, a molecule breaks down and loses hydrogen atoms.
16. Which of the following is an example of oxidation in terms of hydrogen loss?
ⓐ. $H_2 + Cl_2 \rightarrow 2HCl$
ⓑ. $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$
ⓒ. $Na + Cl_2 \rightarrow NaCl$
ⓓ. $C + O_2 \rightarrow CO_2$
Correct Answer: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$
Explanation: In this reaction, methane ($CH_4$) loses hydrogen atoms as it reacts with oxygen. The carbon in methane is oxidized, and hydrogen is lost, which is characteristic of oxidation in terms of hydrogen loss. This process releases energy and forms carbon dioxide and water.
17. What is the oxidation number of sulfur in $SO_2$?
ⓐ. +2
ⓑ. +3
ⓒ. +4
ⓓ. 0
Correct Answer: +4
Explanation: In sulfur dioxide ($SO_2$), oxygen has an oxidation number of -2. With two oxygen atoms, the total oxidation number for oxygen is -4. To balance the charge, sulfur must have an oxidation number of +4. This indicates that sulfur has been oxidized by gaining oxygen.
18. Which of the following statements is true regarding oxidation in terms of oxygen gain and hydrogen loss?
ⓐ. It occurs only in the presence of metals.
ⓑ. It involves the addition of oxygen to a substance and/or the removal of hydrogen.
ⓒ. It only happens during combustion reactions.
ⓓ. It only happens in acidic solutions.
Correct Answer: It involves the addition of oxygen to a substance and/or the removal of hydrogen.
Explanation: Oxidation in terms of oxygen gain and hydrogen loss can occur in various types of reactions, including combustion, combination, and decomposition. It involves either the addition of oxygen to a substance or the removal of hydrogen from it.
19. What is the oxidation number of sulfur in $H_2SO_4$ (sulfuric acid)?
ⓐ. +6
ⓑ. +4
ⓒ. +2
ⓓ. 3
Correct Answer: +6
Explanation: In sulfuric acid ($H_2SO_4$), oxygen has an oxidation number of -2, and hydrogen has an oxidation number of +1. The total oxidation number of oxygen and hydrogen is -8 and +2, respectively. To balance the charge, sulfur must have an oxidation number of +6, which indicates oxidation due to the addition of oxygen.
20. In which of the following reactions is oxidation occurring due to the gain of oxygen?
ⓐ. $2H_2 + O_2 \rightarrow 2H_2O$
ⓑ. $Na + Cl_2 \rightarrow NaCl$
ⓒ. $Mg + H_2O \rightarrow Mg(OH)_2 + H_2$
ⓓ. $Fe + O_2 \rightarrow Fe_2O_3$
Correct Answer: $Fe + O_2 \rightarrow Fe_2O_3$
Explanation: In this reaction, iron ($Fe$) gains oxygen to form iron oxide ($Fe_2O_3$). This process represents oxidation in terms of oxygen gain, where iron’s oxidation number increases as it reacts with oxygen to form the oxide.
21. What does reduction in terms of hydrogen gain involve?
ⓐ. A loss of electrons
ⓑ. A gain of electrons
ⓒ. A gain of hydrogen atoms
ⓓ. A loss of hydrogen atoms
Correct Answer: A gain of hydrogen atoms
Explanation: Reduction in terms of hydrogen gain occurs when a substance gains hydrogen atoms. In this process, the oxidation number of the substance decreases. This is the opposite of oxidation, where an element loses hydrogen or gains oxygen.
22. Which of the following is an example of reduction in terms of hydrogen gain?
ⓐ. $H_2 + Cl_2 \rightarrow 2HCl$
ⓑ. $Na + Cl_2 \rightarrow NaCl$
ⓒ. $C + O_2 \rightarrow CO_2$
ⓓ. $H_2O + O_2 \rightarrow H_2O_2$
Correct Answer: $H_2 + Cl_2 \rightarrow 2HCl$
Explanation: In this reaction, hydrogen gas ($H_2$) gains hydrogen atoms to form hydrochloric acid ($HCl$) as chlorine is reduced. Chlorine gains electrons (reduction), while hydrogen is added, demonstrating reduction in terms of hydrogen gain.
23. What is the oxidation number of hydrogen in $H_2$?
ⓐ. +1
ⓑ. -1
ⓒ. 0
ⓓ. +2
Correct Answer: 0
Explanation: In hydrogen gas ($H_2$), hydrogen exists as a diatomic molecule with no net charge, so the oxidation number is 0. This makes it a good example of a substance that can be involved in reduction reactions by providing hydrogen atoms.
24. Which of the following reactions represents reduction in terms of oxygen loss?
ⓐ. $2H_2 + O_2 \rightarrow 2H_2O$
ⓑ. $Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$
ⓒ. $C + O_2 \rightarrow CO_2$
ⓓ. $Na + Cl_2 \rightarrow NaCl$
Correct Answer: $Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$
Explanation: In this reaction, iron(III) oxide ($Fe_2O_3$) loses oxygen atoms as it is reduced to form iron ($Fe$). Carbon monoxide ($CO$) acts as the reducing agent by donating electrons and removing oxygen from the iron oxide, thus demonstrating reduction in terms of oxygen loss.
25. Which of the following is an example of reduction due to the loss of oxygen?
ⓐ. $2Na + Cl_2 \rightarrow 2NaCl$
ⓑ. $H_2O + O_2 \rightarrow H_2O_2$
ⓒ. $CuO + H_2 \rightarrow Cu + H_2O$
ⓓ. $Mg + O_2 \rightarrow MgO$
Correct Answer: $CuO + H_2 \rightarrow Cu + H_2O$
Explanation: In this reaction, copper(II) oxide ($CuO$) loses oxygen to form copper metal ($Cu$), while hydrogen is oxidized to water ($H_2O$). The copper(II) ion undergoes reduction by losing oxygen atoms.
26. What is the oxidation number of oxygen in $H_2O_2$ (hydrogen peroxide)?
ⓐ. +1
ⓑ. -1
ⓒ. 0
ⓓ. +2
Correct Answer: -1
Explanation: In hydrogen peroxide ($H_2O_2$), oxygen has an oxidation number of -1. This is different from most compounds where oxygen typically has an oxidation number of -2. The reduction of hydrogen peroxide often involves the loss of oxygen atoms, making it an example of reduction in terms of oxygen loss.
27. Which of the following represents reduction due to the gain of hydrogen?
ⓐ. $2H_2O_2 \rightarrow 2H_2O + O_2$
ⓑ. $H_2 + Cl_2 \rightarrow 2HCl$
ⓒ. $CuO + H_2 \rightarrow Cu + H_2O$
ⓓ. $Fe + O_2 \rightarrow Fe_2O_3$
Correct Answer: $CuO + H_2 \rightarrow Cu + H_2O$
Explanation: In this reaction, hydrogen ($H_2$) is gained by copper(II) oxide ($CuO$), which is reduced to copper metal ($Cu$). Hydrogen acts as the reducing agent by donating electrons and hydrogen atoms to the copper ion, making it a reduction in terms of hydrogen gain.
28. Which of the following is an example of reduction in terms of hydrogen gain?
ⓐ. $Na + Cl_2 \rightarrow NaCl$
ⓑ. $H_2O + O_2 \rightarrow H_2O_2$
ⓒ. $C + O_2 \rightarrow CO_2$
ⓓ. $C_2H_6 + 3O_2 \rightarrow 2CO_2 + 3H_2O$
Correct Answer: $H_2O + O_2 \rightarrow H_2O_2$
Explanation: In this reaction, oxygen ($O_2$) gains hydrogen atoms to form hydrogen peroxide ($H_2O_2$). Oxygen is reduced as it gains hydrogen, thus demonstrating reduction in terms of hydrogen gain.
29. What is the oxidation number of hydrogen in $H_2O$ (water)?
ⓐ. +2
ⓑ. -1
ⓒ. +1
ⓓ. 0
Correct Answer: +1
Explanation: In water ($H_2O$), hydrogen has an oxidation number of +1. The oxidation number of oxygen is -2. Water is often involved in reduction reactions, where the hydrogen is added to other substances.
30. What type of reaction occurs when hydrogen is added to a substance, resulting in its reduction?
ⓐ. Hydrogenation
ⓑ. Combustion
ⓒ. Decomposition
ⓓ. Neutralization
Correct Answer: Hydrogenation
Explanation: Hydrogenation is a type of reduction reaction where hydrogen is added to a substance, typically an unsaturated compound, to convert it into a saturated compound. This is a reduction process because the substance gains hydrogen atoms, reducing its oxidation state.
31. What was the main limitation of the old definition of oxidation and reduction, based on oxygen and hydrogen?
ⓐ. They only applied to gases
ⓑ. They couldn’t explain all types of reactions
ⓒ. They ignored electron transfer
ⓓ. They were only applicable to combustion reactions
Correct Answer: They couldn’t explain all types of reactions
Explanation: The old definitions of oxidation (gain of oxygen) and reduction (loss of hydrogen) were limited because they couldn’t explain redox reactions that do not involve oxygen or hydrogen. For instance, reactions where electrons are transferred, but no oxygen or hydrogen is involved, could not be accounted for using these definitions.
32. What was a significant shortcoming of the old definition of oxidation in terms of oxygen gain?
ⓐ. It only applied to simple reactions
ⓑ. It did not account for the loss of electrons
ⓒ. It was limited to organic reactions
ⓓ. It failed to explain reactions without oxygen
Correct Answer: It failed to explain reactions without oxygen
Explanation: The old definition of oxidation as the gain of oxygen was limited because many redox reactions do not involve oxygen. For example, in the reaction between metals and halogens (like sodium and chlorine), there is no oxygen involved, yet it is still a redox reaction.
33. Why does the old definition of reduction in terms of hydrogen loss not work for all redox reactions?
ⓐ. It only applies to acidic reactions
ⓑ. It is only valid in organic reactions
ⓒ. It cannot explain reactions that do not involve hydrogen
ⓓ. It ignores the role of electrons in reactions
Correct Answer: It cannot explain reactions that do not involve hydrogen
Explanation: The old definition of reduction as the loss of hydrogen atoms does not account for all redox reactions. Many reactions do not involve hydrogen, yet involve the transfer of electrons. For example, in reactions between metals and oxygen, hydrogen is not involved, but electron transfer still occurs.
34. How did the old definitions of oxidation and reduction limit the understanding of electron transfer?
ⓐ. They ignored the role of electrons
ⓑ. They only considered the movement of protons
ⓒ. They could not explain neutralization reactions
ⓓ. They applied only to ionic compounds
Correct Answer: They ignored the role of electrons
Explanation: The old definitions of oxidation (gain of oxygen) and reduction (loss of hydrogen) focused on atoms or ions rather than the actual transfer of electrons. This limited understanding of redox reactions, especially those in which electron transfer occurs without oxygen or hydrogen involvement.
35. Which of the following is a limitation of the old definitions of oxidation and reduction based on oxygen and hydrogen?
ⓐ. They could only be used in gaseous reactions
ⓑ. They do not account for the oxidation states of elements
ⓒ. They cannot explain reactions involving electron transfer
ⓓ. They are applicable only to acids and bases
Correct Answer: They cannot explain reactions involving electron transfer
Explanation: The old definitions based on oxygen and hydrogen could not explain redox reactions where electron transfer occurs without the involvement of oxygen or hydrogen. For example, in reactions where metals are oxidized (such as in metal displacement reactions), the transfer of electrons is key, but oxygen or hydrogen may not be involved.
36. What is a key limitation of the old definition of reduction as the loss of oxygen?
ⓐ. It does not apply to oxidation reactions
ⓑ. It ignores the involvement of electrons in redox reactions
ⓒ. It fails to explain reactions involving non-metals
ⓓ. It is only valid in acidic environments
Correct Answer: It ignores the involvement of electrons in redox reactions
Explanation: The old definition of reduction as the loss of oxygen does not explain the electron transfer that occurs in redox reactions. For example, in reactions where metals undergo reduction, the metal ions gain electrons, but no oxygen is involved. This electron transfer is central to the redox process.
37. What was a major issue with defining oxidation and reduction solely based on oxygen and hydrogen?
ⓐ. It made the reactions too complex
ⓑ. It was only applicable to non-metal reactions
ⓒ. It failed to account for many redox reactions, such as those involving electron transfer
ⓓ. It was difficult to use in real-world applications
Correct Answer: It failed to account for many redox reactions, such as those involving electron transfer
Explanation: The major issue with defining oxidation and reduction based solely on oxygen and hydrogen is that it cannot account for redox reactions that involve electron transfer but no oxygen or hydrogen, like metal displacement reactions. This limited the scope of the definitions.
38. Why was the old definition of oxidation (gain of oxygen) problematic for reactions where no oxygen is involved?
ⓐ. It was only applicable to organic reactions
ⓑ. It could not explain reactions like corrosion
ⓒ. It did not account for the role of protons in redox reactions
ⓓ. It excluded many non-oxygen-based reactions
Correct Answer: It excluded many non-oxygen-based reactions
Explanation: The old definition of oxidation as the gain of oxygen could not explain redox reactions where oxygen was not involved. For instance, in reactions such as the displacement of metals or the reaction of halogens with metals, oxygen is absent, but electron transfer still takes place.
39. Which of the following reactions cannot be explained using the old definitions of oxidation and reduction based on oxygen and hydrogen?
ⓐ. $2Na + Cl_2 \rightarrow 2NaCl$
ⓑ. $C + O_2 \rightarrow CO_2$
ⓒ. $2H_2O_2 \rightarrow 2H_2O + O_2$
ⓓ. $Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$
Correct Answer: $Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$
Explanation: This reaction involves the reduction of iron(III) oxide ($Fe_2O_3$) by carbon monoxide (CO), which involves electron transfer. The old definitions of oxidation and reduction cannot explain this reaction, as no oxygen or hydrogen is involved in the process.
40. Which of the following is true about the limitations of the old definitions of oxidation and reduction?
ⓐ. They are still widely used in modern chemistry
ⓑ. They cannot explain electron transfer-based reactions
ⓒ. They are only useful in acid-base chemistry
ⓓ. They are valid for all types of chemical reactions
Correct Answer: They cannot explain electron transfer-based reactions
Explanation: The old definitions of oxidation and reduction based on oxygen gain and hydrogen loss cannot explain reactions where electron transfer occurs but oxygen or hydrogen is not involved. This limitation was addressed by the modern definition of redox reactions, which focuses on electron transfer.
41. What is the main characteristic of oxidation in terms of electron loss?
ⓐ. Gain of protons
ⓑ. Loss of electrons
ⓒ. Gain of electrons
ⓓ. Gain of hydrogen
Correct Answer: Loss of electrons
Explanation: Oxidation in terms of electron loss involves the removal of electrons from an atom, molecule, or ion. When an element undergoes oxidation, its oxidation state increases as it loses electrons. This process is always paired with reduction, where another substance gains those electrons.
42. Which of the following reactions demonstrates oxidation as electron loss?
ⓐ. $Zn \rightarrow Zn^{2+} + 2e^-$
ⓑ. $Cu^{2+} + 2e^- \rightarrow Cu$
ⓒ. $NaCl \rightarrow Na + Cl$
ⓓ. $H_2 + Cl_2 \rightarrow 2HCl$
Correct Answer: $Zn \rightarrow Zn^{2+} + 2e^-$
Explanation: In this reaction, zinc metal ($Zn$) loses two electrons to form zinc ions ($Zn^{2+}$), which is an example of oxidation as electron loss. The oxidation state of zinc increases from 0 to +2 as it loses electrons.
43. What is the oxidation number of zinc in $Zn^{2+}$?
ⓐ. 0
ⓑ. +1
ⓒ. +2
ⓓ. -1
Correct Answer: +2
Explanation: In the ion $Zn^{2+}$, zinc has an oxidation number of +2. This indicates that zinc has lost two electrons and is therefore oxidized. The loss of electrons increases the oxidation number, which is characteristic of oxidation.
44. In the reaction $Cu \rightarrow Cu^{2+} + 2e^-$, what type of process is occurring?
ⓐ. Reduction
ⓑ. Oxidation
ⓒ. Neutralization
ⓓ. Decomposition
Correct Answer: Oxidation
Explanation: In this reaction, copper ($Cu$) loses two electrons to form copper ions ($Cu^{2+}$). The loss of electrons is characteristic of oxidation, as the copper is being oxidized from 0 to +2 oxidation state.
45. Which of the following statements is true about oxidation as electron loss?
ⓐ. It always involves the gain of protons
ⓑ. It is a process where the oxidation state decreases
ⓒ. It involves the loss of electrons and an increase in oxidation state
ⓓ. It is the same as reduction
Correct Answer: It involves the loss of electrons and an increase in oxidation state
Explanation: Oxidation as electron loss always involves the loss of electrons and a corresponding increase in the oxidation state of the element. The element that loses electrons is said to be oxidized, and this process occurs simultaneously with reduction, where another species gains the electrons.
46. What is the oxidation number of magnesium in $Mg^{2+}$?
ⓐ. 0
ⓑ. +1
ⓒ. +2
ⓓ. -1
Correct Answer: +2
Explanation: In the ion $Mg^{2+}$, magnesium has an oxidation number of +2. This indicates that magnesium has lost two electrons in the oxidation process, which is why its oxidation number increases from 0 to +2.
47. In the reaction $4Fe + 3O_2 \rightarrow 2Fe_2O_3$, which element is oxidized?
ⓐ. Iron (Fe)
ⓑ. Oxygen (O)
ⓒ. Both iron and oxygen
ⓓ. Neither iron nor oxygen
Correct Answer: Iron (Fe)
Explanation: In this reaction, iron (Fe) loses electrons to form iron(III) oxide ($Fe_2O_3$). The oxidation state of iron increases from 0 to +3, indicating that iron is oxidized. Oxygen gains electrons and is reduced, so it is not oxidized.
48. What is the oxidation state of sulfur in $SO_2$?
ⓐ. +2
ⓑ. +4
ⓒ. +6
ⓓ. -2
Correct Answer: +4
Explanation: In sulfur dioxide ($SO_2$), oxygen has an oxidation number of -2. The total oxidation number for two oxygen atoms is -4, so to balance the charge, sulfur must have an oxidation number of +4. This shows oxidation due to the loss of electrons, as sulfur’s oxidation number increases.
49. Which of the following reactions is an example of oxidation as electron loss?
ⓐ. $Fe^{2+} \rightarrow Fe^{3+} + e^-$
ⓑ. $Cu^{2+} + 2e^- \rightarrow Cu$
ⓒ. $H_2 + Cl_2 \rightarrow 2HCl$
ⓓ. $Mg + O_2 \rightarrow MgO$
Correct Answer: $Fe^{2+} \rightarrow Fe^{3+} + e^-$
Explanation: In this reaction, iron(II) ions ($Fe^{2+}$) lose an electron to become iron(III) ions ($Fe^{3+}$). This is an example of oxidation, where the iron loses an electron, and its oxidation state increases from +2 to +3.
50. In the reaction $2H_2 + O_2 \rightarrow 2H_2O$, which element undergoes oxidation?
ⓐ. Hydrogen
ⓑ. Oxygen
ⓒ. Both hydrogen and oxygen
ⓓ. Neither hydrogen nor oxygen
Correct Answer: Hydrogen
Explanation: In the reaction, hydrogen ($H_2$) loses electrons (oxidation) as it forms water ($H_2O$). Oxygen ($O_2$) gains electrons (reduction). The hydrogen undergoes oxidation, and its oxidation state increases from 0 to +1.
51. What is the oxidation number of chlorine in $Cl_2$?
ⓐ. +1
ⓑ. 0
ⓒ. +2
ⓓ. -1
Correct Answer: 0
Explanation: In chlorine gas ($Cl_2$), chlorine exists in its elemental form, so the oxidation number is 0. When chlorine is involved in a redox reaction, it can gain or lose electrons, undergoing either reduction or oxidation depending on the reaction.
52. What happens to an element when it is oxidized in terms of electron loss?
ⓐ. Its oxidation state decreases
ⓑ. It gains electrons
ⓒ. It loses electrons
ⓓ. It remains unchanged
Correct Answer: It loses electrons
Explanation: When an element is oxidized, it loses electrons, resulting in an increase in its oxidation state. This electron loss is a key characteristic of oxidation and occurs in conjunction with reduction, where another species gains the electrons.
53. In which of the following reactions does copper undergo oxidation as electron loss?
ⓐ. $Cu + 2Ag^+ \rightarrow Cu^{2+} + 2Ag$
ⓑ. $Cu + O_2 \rightarrow CuO$
ⓒ. $Cu^{2+} + 2e^- \rightarrow Cu$
ⓓ. $CuO + H_2 \rightarrow Cu + H_2O$
Correct Answer: $Cu + 2Ag^+ \rightarrow Cu^{2+} + 2Ag$
Explanation: In this reaction, copper ($Cu$) loses two electrons to form copper ions ($Cu^{2+}$). The oxidation state of copper increases from 0 to +2, which is an example of oxidation as electron loss.
54. What is the oxidation state of chlorine in $NaCl$?
ⓐ. +1
ⓑ. -1
ⓒ. 0
ⓓ. +2
Correct Answer: -1
Explanation: In sodium chloride ($NaCl$), chlorine has an oxidation number of -1, while sodium has an oxidation number of +1. The chloride ion gains an electron (reduction), while sodium undergoes oxidation by losing an electron.
55. Which of the following elements undergoes oxidation by electron loss in the reaction $2Fe + 3O_2 \rightarrow 2Fe_2O_3$?
ⓐ. Iron (Fe)
ⓑ. Oxygen (O)
ⓒ. Both iron and oxygen
ⓓ. Neither iron nor oxygen
Correct Answer: Iron (Fe)
Explanation: In this reaction, iron undergoes oxidation by losing electrons, increasing its oxidation state from 0 to +3. Oxygen is reduced by gaining electrons, so iron is the element undergoing oxidation by electron loss.
56. What is the main characteristic of reduction in terms of electron gain?
ⓐ. Loss of electrons
ⓑ. Gain of protons
ⓒ. Gain of electrons
ⓓ. Loss of hydrogen
Correct Answer: Gain of electrons
Explanation: Reduction is the process where an atom, molecule, or ion gains electrons. As a result, the oxidation state of the substance decreases. Reduction always occurs in conjunction with oxidation, where another substance loses electrons.
57. Which of the following reactions demonstrates reduction as electron gain?
ⓐ. $Zn^{2+} + 2e^- \rightarrow Zn$
ⓑ. $Cu + 2e^- \rightarrow Cu^{2+}$
ⓒ. $NaCl \rightarrow Na + Cl$
ⓓ. $C + O_2 \rightarrow CO_2$
Correct Answer: $Zn^{2+} + 2e^- \rightarrow Zn$
Explanation: In this reaction, zinc ions ($Zn^{2+}$) gain two electrons to form zinc metal ($Zn$). The gain of electrons results in a decrease in the oxidation state of zinc, demonstrating reduction as electron gain.
58. What is the oxidation number of chlorine in $Cl^-$?
ⓐ. +1
ⓑ. -1
ⓒ. 0
ⓓ. +2
Correct Answer: -1
Explanation: In the chloride ion ($Cl^-$), chlorine has an oxidation number of -1. This indicates that chlorine has gained one electron and undergone reduction. Chlorine typically gains electrons to form the chloride ion, demonstrating electron gain.
59. In the reaction $2Fe^{3+} + 6e^- \rightarrow 2Fe$, what happens to iron (Fe)?
ⓐ. Iron undergoes oxidation by losing electrons
ⓑ. Iron undergoes reduction by gaining electrons
ⓒ. Iron does not change its oxidation state
ⓓ. Iron gains protons
Correct Answer: Iron undergoes reduction by gaining electrons
Explanation: In this reaction, iron is reduced from $Fe^{3+}$ (oxidation state +3) to iron metal (oxidation state 0) by gaining electrons. This decrease in oxidation state indicates a reduction process.
60. What is the oxidation state of nitrogen in $NO_3^-$?
ⓐ. +2
ⓑ. +3
ⓒ. -3
ⓓ. +5
Correct Answer: +5
Explanation: In the nitrate ion ($NO_3^-$), oxygen has an oxidation number of -2. With three oxygen atoms, the total oxidation number is -6. To balance the charge of -1, nitrogen must have an oxidation number of +5. The reduction of nitrogen occurs when it gains electrons.
61. Which of the following is an example of reduction in terms of electron gain?
ⓐ. $Cu^{2+} + 2e^- \rightarrow Cu$
ⓑ. $Zn \rightarrow Zn^{2+} + 2e^-$
ⓒ. $Fe^{3+} + 3e^- \rightarrow Fe$
ⓓ. $Na + Cl_2 \rightarrow NaCl$
Correct Answer: $Cu^{2+} + 2e^- \rightarrow Cu$
Explanation: In this reaction, copper ions ($Cu^{2+}$) gain two electrons to form copper metal ($Cu$). The gain of electrons results in a decrease in the oxidation state of copper, demonstrating reduction as electron gain.
62. What happens to an element when it is reduced in terms of electron gain?
ⓐ. Its oxidation state increases
ⓑ. It loses electrons
ⓒ. It gains electrons
ⓓ. Its oxidation state remains unchanged
Correct Answer: It gains electrons
Explanation: When an element undergoes reduction, it gains electrons, resulting in a decrease in its oxidation state. This process is always paired with oxidation, where another species loses electrons.
63. In the reaction $2H_2O + 2e^- \rightarrow H_2 + 2OH^-$, which substance is reduced?
ⓐ. Water
ⓑ. Hydrogen
ⓒ. Oxygen
ⓓ. Hydroxide ions
Correct Answer: Water
Explanation: In this reaction, water ($H_2O$) gains electrons to form hydrogen gas ($H_2$) and hydroxide ions ($OH^-$). The water molecules are reduced by gaining electrons, which decreases their oxidation state.
64. What is the oxidation number of oxygen in $O_2$?
ⓐ. -1
ⓑ. 0
ⓒ. +2
ⓓ. -2
Correct Answer: 0
Explanation: In elemental oxygen ($O_2$), oxygen has an oxidation number of 0, as it exists as a diatomic molecule. Oxygen is often reduced in redox reactions by gaining electrons to form oxide ions ($O^{2-}$).
65. Which of the following reactions involves reduction as electron gain?
ⓐ. $2Fe^{3+} + 6e^- \rightarrow 2Fe$
ⓑ. $C + O_2 \rightarrow CO_2$
ⓒ. $Na + Cl_2 \rightarrow NaCl$
ⓓ. $Mg + O_2 \rightarrow MgO$
Correct Answer: $2Fe^{3+} + 6e^- \rightarrow 2Fe$
Explanation: In this reaction, iron ions ($Fe^{3+}$) gain electrons to form iron metal ($Fe$). The reduction occurs as the iron ions are reduced from a +3 oxidation state to a 0 oxidation state.
66. Which element undergoes reduction in the reaction $2Cu^{2+} + 4e^- \rightarrow 2Cu$?
ⓐ. Copper
ⓑ. Oxygen
ⓒ. Hydrogen
ⓓ. Chlorine
Correct Answer: Copper
Explanation: In this reaction, copper ions ($Cu^{2+}$) gain electrons to form copper metal ($Cu$). The copper is reduced because it gains electrons, which decreases its oxidation state from +2 to 0.
67. What is the oxidation number of sulfur in $SO_2$?
ⓐ. +2
ⓑ. 4
ⓒ. +6
ⓓ. +4
Correct Answer: +4
Explanation: In sulfur dioxide ($SO_2$), oxygen has an oxidation number of -2. To balance the charge, sulfur must have an oxidation number of +4. Reduction of sulfur involves the gain of electrons, reducing its oxidation number.
68. In the reaction $Mg + Cl_2 \rightarrow MgCl_2$, which substance is reduced?
ⓐ. Magnesium
ⓑ. Chlorine
ⓒ. Both magnesium and chlorine
ⓓ. Neither magnesium nor chlorine
Correct Answer: Chlorine
Explanation: In this reaction, chlorine ($Cl_2$) gains electrons to form chloride ions ($Cl^-$). The chlorine is reduced, while magnesium is oxidized, losing electrons to form magnesium ions ($Mg^{2+}$).
69. What happens to the oxidation state of a substance that undergoes reduction as electron gain?
ⓐ. The oxidation state increases
ⓑ. The oxidation state remains unchanged
ⓒ. The oxidation state decreases
ⓓ. The substance is oxidized
Correct Answer: The oxidation state decreases
Explanation: When a substance undergoes reduction by gaining electrons, its oxidation state decreases. This decrease in oxidation state is the hallmark of a reduction process.
70. In the reaction $2Na + Cl_2 \rightarrow 2NaCl$, which element undergoes reduction as electron gain?
ⓐ. Sodium
ⓑ. Chlorine
ⓒ. Both sodium and chlorine
ⓓ. Neither sodium nor chlorine
Correct Answer: Chlorine
Explanation: In this reaction, chlorine ($Cl_2$) gains electrons to form chloride ions ($Cl^-$), which reduces its oxidation state from 0 to -1. Chlorine undergoes reduction by gaining electrons, while sodium undergoes oxidation by losing electrons.
71. What is an oxidizing agent?
ⓐ. A substance that gains electrons
ⓑ. A substance that loses electrons
ⓒ. A substance that gains hydrogen
ⓓ. A substance that loses oxygen
Correct Answer: A substance that gains electrons
Explanation: An oxidizing agent (or oxidant) is a substance that gains electrons in a redox reaction and causes the oxidation of another substance. It itself undergoes reduction by gaining electrons.
72. Which of the following is a common example of an oxidizing agent?
ⓐ. Sodium
ⓑ. Chlorine
ⓒ. Hydrogen
ⓓ. Oxygen
Correct Answer: Oxygen
Explanation: Oxygen is a strong oxidizing agent, as it readily gains electrons during redox reactions. For example, in combustion reactions, oxygen is reduced by gaining electrons while oxidizing the substance it reacts with.
73. What is a reducing agent?
ⓐ. A substance that gains electrons
ⓑ. A substance that loses electrons
ⓒ. A substance that gains oxygen
ⓓ. A substance that loses hydrogen
Correct Answer: A substance that loses electrons
Explanation: A reducing agent (or reductant) is a substance that loses electrons in a redox reaction and causes the reduction of another substance. It itself undergoes oxidation by losing electrons.
74. Which of the following is a common example of a reducing agent?
ⓐ. Oxygen
ⓑ. Hydrogen
ⓒ. Chlorine
ⓓ. Nitrogen
Correct Answer: Hydrogen
Explanation: Hydrogen is a common reducing agent, as it donates electrons to other substances during redox reactions. In reactions such as hydrogenation, hydrogen reduces other substances by supplying electrons.
75. Which of the following is a strong oxidizing agent?
ⓐ. $Na$
ⓑ. $Cl_2$
ⓒ. $H_2$
ⓓ. $Fe^{3+}$
Correct Answer: $Cl_2$
Explanation: Chlorine ($Cl_2$) is a strong oxidizing agent because it easily gains electrons to form chloride ions ($Cl^-$). Chlorine is often involved in redox reactions, where it oxidizes other substances by accepting electrons.
76. Which of the following is a strong reducing agent?
ⓐ. $H_2$
ⓑ. $O_2$
ⓒ. $Na^+$
ⓓ. $Cl_2$
Correct Answer: $H_2$
Explanation: Hydrogen gas ($H_2$) is a strong reducing agent, as it readily donates electrons during redox reactions. For example, in hydrogenation reactions, hydrogen reduces other compounds by donating electrons.
77. In the reaction $2H_2O_2 \rightarrow 2H_2O + O_2$, which substance is the oxidizing agent?
ⓐ. $H_2O_2$
ⓑ. $H_2O$
ⓒ. $O_2$
ⓓ. None of the above
Correct Answer: $H_2O_2$
Explanation: In this reaction, hydrogen peroxide ($H_2O_2$) acts as the oxidizing agent, as it gains electrons (undergoes reduction) to form water ($H_2O$) while oxidizing another molecule. Oxygen ($O_2$) is released as a product, but it is not the oxidizing agent in this case.
78. In the reaction $Cu + 2Ag^+ \rightarrow Cu^{2+} + 2Ag$, which substance is the reducing agent?
ⓐ. $Cu$
ⓑ. $Ag^+$
ⓒ. $Cu^{2+}$
ⓓ. $Ag$
Correct Answer: $Cu$
Explanation: In this reaction, copper ($Cu$) is the reducing agent because it donates electrons to silver ions ($Ag^+$), reducing them to silver metal ($Ag$). Copper is oxidized to $Cu^{2+}$ as it loses electrons.
79. In the reaction $2Na + Cl_2 \rightarrow 2NaCl$, which substance is the oxidizing agent?
ⓐ. Sodium ($Na$)
ⓑ. Sodium chloride ($NaCl$)
ⓒ. Chlorine ($Cl_2$)
ⓓ. Both sodium and chlorine
Correct Answer: Chlorine ($Cl_2$)
Explanation: In this reaction, chlorine ($Cl_2$) is the oxidizing agent because it gains electrons (undergoes reduction) and oxidizes sodium ($Na$) by accepting electrons. Sodium is oxidized as it loses electrons.
80. Which of the following statements about oxidizing and reducing agents is true?
ⓐ. Oxidizing agents lose electrons, and reducing agents gain electrons
ⓑ. Oxidizing agents gain electrons, and reducing agents lose electrons
ⓒ. Oxidizing agents and reducing agents both lose electrons
ⓓ. Both oxidizing and reducing agents gain electrons
Correct Answer: Oxidizing agents gain electrons, and reducing agents lose electrons
Explanation: Oxidizing agents gain electrons during redox reactions and are reduced, while reducing agents lose electrons and are oxidized. This electron transfer process defines the roles of these agents in redox reactions.
81. What is the role of potassium permanganate ($KMnO_4$) in redox reactions?
ⓐ. It is a reducing agent
ⓑ. It is an oxidizing agent
ⓒ. It neither gains nor loses electrons
ⓓ. It only acts as a catalyst
Correct Answer: It is an oxidizing agent
Explanation: Potassium permanganate ($KMnO_4$) is a strong oxidizing agent. In redox reactions, it typically gains electrons and gets reduced to manganese dioxide ($MnO_2$) or other reduced forms. It is commonly used in titrations and disinfection processes.
82. Which of the following substances can act as both an oxidizing agent and a reducing agent?
ⓐ. $H_2O$
ⓑ. $O_2$
ⓒ. $Cl_2$
ⓓ. $H_2O_2$
Correct Answer: $H_2O_2$
Explanation: Hydrogen peroxide ($H_2O_2$) can act as both an oxidizing agent and a reducing agent, depending on the reaction. It can reduce other substances by donating electrons (acting as a reducing agent) or oxidize substances by accepting electrons (acting as an oxidizing agent).
83. In the reaction $2Fe^{3+} + 3e^- \rightarrow 2Fe^{2+}$, which ion is the oxidizing agent?
ⓐ. $Fe^{2+}$
ⓑ. $Fe^{3+}$
ⓒ. $e^-$
ⓓ. Both $Fe^{2+}$ and $Fe^{3+}$
Correct Answer: $Fe^{3+}$
Explanation: In this reaction, iron(III) ions ($Fe^{3+}$) are the oxidizing agent, as they accept electrons to be reduced to iron(II) ions ($Fe^{2+}$). The $Fe^{3+}$ ions gain electrons, undergoing reduction.
84. What is the function of an oxidizing agent in a redox reaction?
ⓐ. It donates electrons to reduce another substance
ⓑ. It gains electrons and oxidizes another substance
ⓒ. It provides energy for the reaction
ⓓ. It prevents the reaction from occurring
Correct Answer: It gains electrons and oxidizes another substance
Explanation: The function of an oxidizing agent in a redox reaction is to gain electrons and thereby oxidize another substance. This process decreases the oxidation state of the oxidizing agent while increasing the oxidation state of the substance being oxidized.
85. Which of the following is a strong reducing agent?
ⓐ. $O_2$
ⓑ. $H_2$
ⓒ. $Cl_2$
ⓓ. $Na$
Correct Answer: $Na$
Explanation: Sodium ($Na$) is a strong reducing agent because it readily loses electrons to form sodium ions ($Na^+$). This makes it highly effective at reducing other substances in redox reactions by donating electrons.
86. Which of the following is the rule for assigning oxidation numbers to elements in their elemental form?
ⓐ. The oxidation number is always 0
ⓑ. The oxidation number is always +1
ⓒ. The oxidation number is always -1
ⓓ. The oxidation number is always +2
Correct Answer: The oxidation number is always 0
Explanation: In any element in its pure, elemental form (such as $O_2$, $H_2$, $N_2$, etc.), the oxidation number is always 0. This rule applies because there is no electron transfer or sharing between atoms in elemental molecules.
87. What is the oxidation number of hydrogen in most compounds?
ⓐ. +1
ⓑ. -1
ⓒ. 0
ⓓ. +2
Correct Answer: +1
Explanation: In most compounds, hydrogen has an oxidation number of +1, except in metal hydrides (such as $NaH$), where hydrogen has an oxidation number of -1. In compounds like water ($H_2O$), hydrogen is assigned +1.
88. What is the oxidation number of oxygen in $H_2O$?
ⓐ. +1
ⓑ. +1
ⓒ. 0
ⓓ. -2
Correct Answer: -2
Explanation: In most compounds, oxygen has an oxidation number of -2, except in peroxides (such as $H_2O_2$), where oxygen has an oxidation number of -1. In water ($H_2O$), oxygen has an oxidation number of -2.
89. In the compound $NaCl$, what is the oxidation number of sodium (Na)?
ⓐ. 0
ⓑ. +1
ⓒ. -1
ⓓ. +2
Correct Answer: +1
Explanation: Sodium (Na) is an alkali metal and in ionic compounds like $NaCl$, its oxidation number is always +1. This rule applies to all alkali metals when they form compounds.
90. In the ion $SO_4^{2-}$ (sulfate), what is the oxidation number of sulfur (S)?
ⓐ. +1
ⓑ. +3
ⓒ. +5
ⓓ. +6
Correct Answer: +6
Explanation: In the sulfate ion ($SO_4^{2-}$), the oxidation number of oxygen is -2 (since oxygen typically has an oxidation number of -2). The total oxidation number of the four oxygen atoms is -8. To balance the charge of -2 on the ion, sulfur must have an oxidation number of +6.
91. What is the oxidation number of chlorine in $NaCl$?
ⓐ. +1
ⓑ. -1
ⓒ. 0
ⓓ. +2
Correct Answer: -1
Explanation: Chlorine in $NaCl$ has an oxidation number of -1. Chlorine is a halogen, and in ionic compounds like $NaCl$, halogens typically have an oxidation number of -1.
92. In the compound $K_2SO_4$, what is the oxidation number of potassium (K)?
ⓐ. +1
ⓑ. +2
ⓒ. +3
ⓓ. +4
Correct Answer: +1
Explanation: Potassium (K) is an alkali metal, and in compounds like $K_2SO_4$, it always has an oxidation number of +1. This rule applies to all alkali metals when they form compounds.
93. What is the oxidation number of nitrogen in $NO_3^-$ (nitrate ion)?
ⓐ. +3
ⓑ. +5
ⓒ. +2
ⓓ. 0
Correct Answer: +5
Explanation: In the nitrate ion ($NO_3^-$), oxygen has an oxidation number of -2, and there are three oxygen atoms, giving a total oxidation number of -6. To balance the charge of -1 on the ion, nitrogen must have an oxidation number of +5.
94. What is the oxidation number of hydrogen in $NaH$ (sodium hydride)?
ⓐ. 0
ⓑ. +1
ⓒ. -1
ⓓ. +2
Correct Answer: -1
Explanation: In sodium hydride ($NaH$), hydrogen has an oxidation number of -1. This is because hydrogen, when bonded to metals (such as sodium), can have an oxidation number of -1 instead of its usual +1 oxidation state.
95. What is the oxidation number of sulfur in $H_2SO_4$ (sulfuric acid)?
ⓐ. +4
ⓑ. +6
ⓒ. +2
ⓓ. 0
Correct Answer: +6
Explanation: In sulfuric acid ($H_2SO_4$), oxygen has an oxidation number of -2, and there are four oxygen atoms, giving a total oxidation number of -8. The hydrogen atoms each have an oxidation number of +1, contributing a total of +2. To balance the charges, sulfur must have an oxidation number of +6.
96. In the compound $Na_2O$, what is the oxidation number of sodium (Na)?
ⓐ. 0
ⓑ. -1
ⓒ. +1
ⓓ. -5
Correct Answer: +1
Explanation: Sodium (Na) is an alkali metal, and in compounds like sodium oxide ($Na_2O$), it has an oxidation number of +1. This rule applies to all alkali metals when they form compounds.
97. What is the oxidation number of phosphorus in $PCl_3$ (phosphorus trichloride)?
ⓐ. +3
ⓑ. +5
ⓒ. 0
ⓓ. -3
Correct Answer: +3
Explanation: In $PCl_3$, chlorine (Cl) has an oxidation number of -1 (since halogens typically have an oxidation number of -1 in compounds). With three chlorine atoms, the total oxidation number for chlorine is -3. To balance this, phosphorus must have an oxidation number of +3.
98. What is the oxidation number of carbon in $CO_2$ (carbon dioxide)?
ⓐ. +1
ⓑ. +2
ⓒ. 0
ⓓ. +4
Correct Answer: +4
Explanation: In carbon dioxide ($CO_2$), oxygen has an oxidation number of -2. Since there are two oxygen atoms, the total oxidation number for oxygen is -4. To balance this, carbon must have an oxidation number of +4.
99. In the compound $H_2SO_3$ (sulfurous acid), what is the oxidation number of sulfur (S)?
ⓐ. +4
ⓑ. +6
ⓒ. +2
ⓓ. 0
Correct Answer: +4
Explanation: In sulfurous acid ($H_2SO_3$), oxygen has an oxidation number of -2, and there are three oxygen atoms, giving a total of -6. The hydrogen atoms have an oxidation number of +1, contributing +2 in total. To balance the charge, sulfur must have an oxidation number of +4.
100. What is the oxidation number of nitrogen in $NH_3$ (ammonia)?
ⓐ. -1
ⓑ. +2
ⓒ. -3
ⓓ. -0
Correct Answer: -3
Explanation: In ammonia ($NH_3$), hydrogen has an oxidation number of +1. Since there are three hydrogen atoms, the total oxidation number for hydrogen is +3. To balance the total charge of 0 for the molecule, nitrogen must have an oxidation number of -3.
Welcome to Class 11 Chemistry MCQs – Chapter 8: Redox Reactions (Part of 4). If the words oxidation, reduction, and oxidation number seem confusing, you’re not alone. This page makes Redox Reactions simple and exam-ready with 100 NCERT-based MCQs and clear explanations—perfect for Board exams, JEE Main, NEET, and State entrance tests.
Navigation & parts: This chapter includes 376 MCQs divided into 4 parts (100 + 100 + 100 + 76). You’re viewing Part 1 with the first 100 solved MCQs. Use the Part buttons above to continue through later sets.
What you will learn & practice (Chapter 8 – Redox Reactions)
- Meaning of oxidation and reduction (oxygen/hydrogen/electron transfer concepts)
- Oxidation number and rules for assigning it in compounds & ions
- Oxidizing and reducing agents – how to identify them quickly
- Types of redox reactions: combination, decomposition, displacement, disproportionation
- Balancing redox equations by ion-electron and oxidation-number methods
- Understanding electrode potential and electrochemical series
- Applications in electrochemistry, corrosion prevention, bleaching, and metallurgy
- NCERT, JEE, and NEET-pattern questions with short conceptual explanations
How to use this site to master Redox Reaction MCQs
- Warm-up (10 min): Revise oxidation-number rules and basic formulas from your NCERT book before starting.
- Practice actively: Attempt 10–20 MCQs, check answers instantly, and read the explanations for quick correction.
- Mark favourites: Click the ❤️ Heart beside any MCQ to save it in your Favourite list. Use the Favourite Toggle (beside the Random button) to see only marked questions for focused revision.
- Take notes: Use the Workspace under each MCQ to write oxidation-number steps or balancing hints. Notes auto-save—they’ll be there next time you visit.
- Shuffle & revise: Click Random to mix questions and avoid memorization. Repeat your Favourite list on Day 2, 4 & 7 for long-term recall.
Why Redox Reactions are important for Boards & Competitive Exams
- Boards: 3–5 marks every year from oxidation number, balancing, and redox identification questions.
- JEE Main: Numerical problems on equivalent weight, oxidation number change, and redox titrations.
- NEET: Quick concept MCQs on oxidizing/reducing agents and disproportionation are frequent.
- Your edge: The Favourite + Workspace combo helps you focus on weak areas and retain formulas effortlessly.
Common mistakes to avoid
- Forgetting to multiply by stoichiometric coefficients when balancing oxidation & reduction halves
- Confusing oxidation number with ionic charge
- Ignoring spectator ions in the ion-electron method
- Wrongly identifying oxidizing/reducing agents — remember, the species that gets reduced is the oxidizing agent
- Not checking that the total electrons lost = electrons gained when balancing equations
Redox Reactions build the base for Electrochemistry and later chapters. Use this page as your self-study partner—favourite tough questions, jot hints in Workspace, and see how your accuracy improves every day.
Explore more: Class 11 Chemistry – Chapter Index, Class 11 – Subject Index, All Classes – MCQ Library.
Quick FAQ
Is Redox Reactions a tough chapter?
Once oxidation-number rules and balancing methods are clear, this chapter becomes one of the easiest scoring areas in Physical Chemistry.
How many MCQs should I solve daily?
Start with 25–30 per day; focus on balancing and oxidation-number problems for speed improvement.
How do I revise before the exam?
Switch on the Favourite Toggle and review only your marked questions and Workspace notes for a quick, confident recap.
- 👉 Total MCQs in this chapter: 376 (100 + 100 + 100 + 76)
- 👉 This page: First 100 MCQs with answers & explanations
- 👉 Best for: Boards • JEE/NEET • oxidation-reduction numericals • concept revision • fast practice
- 👉 Next: Use the Part buttons above to continue
FAQs on Redox Reactions ▼
▸ What are Redox Reactions MCQs in Class 11 Chemistry?
These are multiple-choice questions from Chapter 8 of NCERT Class 11 Chemistry – Redox Reactions. They test your understanding of oxidation and reduction, electron transfer, oxidizing and reducing agents, and balancing redox equations.
▸ How many MCQs are available in this chapter?
There are a total of 376 Redox Reactions MCQs. They are divided into 4 structured parts – three sets of 100 questions each and one set of 76 questions.
▸ Are Redox Reactions MCQs important for JEE and NEET?
Yes, this chapter is important for JEE and NEET. Frequently tested topics include oxidation number rules, identification of oxidizing/reducing agents, disproportionation, and balancing using the oxidation number and ion-electron methods.
▸ Do these MCQs include correct answers and explanations?
Yes, every MCQ is accompanied by the correct answer and clear explanations to strengthen concepts and improve problem-solving speed.
▸ Which subtopics are covered in these Redox Reactions MCQs?
Subtopics include oxidation and reduction definitions, electron transfer, oxidation number rules and calculations, identifying oxidizing and reducing agents, disproportionation and comproportionation, and balancing redox reactions by oxidation number and ion-electron methods.