The oxidation number is a theoretical concept. It tells about the hypothetical charge of an atom if all of its bonds are ionic. It tells us about the extent of oxidation of an atom in a molecule. It can be either positive, negative or neutral.
Definition: The oxidation number is the charge that a central metal atom will have even after all the ligands have been removed from that atom. |
Oxidation Number Chemistry Questions with Solutions
Q1. What is the oxidation number of sulfur in sulphuric acid (H_{2}SO_{4})?
- 4
- 6
- 8
- None of the above
Answer: (b), The oxidation number of sulphur in sulphuric acid (H_{2}SO_{4}) is 6.
Calculation:
1 X 2 + x + 4 X – 2 = 0
2 + x – 8 = 0
x = 6
So, option (b) is correct.
Q2. What is the oxidation number of chromium in calcium dichromate (CaCr_{2}O_{7})?
- 4
- 6
- 8
- None of the above
Answer: (b), The oxidation number of chromium in calcium dichromate (CaCr_{2}O_{7}) is 6.
Calculation:
2 + 2x + 7 X (- 2) = 0
2 + 2x – 14 = 0
2x = 12
x = 6
So, option (b) is correct.
Q3. What is the oxidation number of nitrogen in nitric acid (HNO_{3})?
- 3
- 5
- 7
- None of the above
Answer: (b), The oxidation number of nitrogen in nitric acid (HNO_{3}) is 5.
Calculation:
1 + x + 3 X ( -2) = 0
1 + x – 6 = 0
x – 5 = 0
x = 5
So, option (b) is correct.
Q4. Which of the following statements between HClO_{4} and HClO_{3} is true?
- The oxidation number of chlorine in HClO_{4} has been decreased in HClO_{3}
- The oxidation numbers for all atoms are the same in both molecules
- The oxidation number for chlorine in HClO_{4} has increased in HClO_{3}
- The oxidation number of oxygen in HClO_{4} has been decreased in HClO_{3}
Answer: (a), The oxidation number of chlorine in HClO_{4} has been decreased in HClO_{3} is true.
Explanation: To understand it better, foremost we will calculate the oxidation number of HClO_{4} and HClO_{3.}
The oxidation number of chlorine in HClO_{4} will be:
1 + x + 4 X ( -2) = 0
1 + x – 8 = 0
x – 7 = 0
x = 7
The oxidation number of chlorine in HClO_{3} will be:
1 + x + 3 X ( -2) = 0
1 + x – 6 = 0
x – 5 = 0
x = 5
Thus, we can clearly see that oxidation number in HClO_{4} has been decreased in HClO_{3.} So option (a) is correct.
Q5. What is the oxidation number of carbon in CH_{2}Cl_{2}?
- 2
- 4
- 6
- None of the above.
Answer: (d), The oxidation number of carbon in CH_{2}Cl_{2} is zero.
Calculation:
x + 2 X 1 + 2 X ( -1) = 0
x + 2 – 2 = 0
x = 0
The oxidation number of carbon in CH_{2}Cl_{2} is zero.
So, option (d) is correct.
Q6. Chlorine is in +1 oxidation number in
- HCl
- HClO_{4}
- ICl
- Cl_{2}O
- None of the above
Answer: (d), Chlorine is in +1 oxidation number in Cl_{2}O.
Explanation: To understand it better, foremost we will calculate the oxidation number of HCl, HClO_{4}, ICl and Cl_{2}O.
The oxidation number of chlorine in HCl will be:
1 + x = 0
x = -1
The oxidation number of chlorine in HClO_{4} will be:
1 + x + 4 X ( -2) = 0
1 + x – 8 = 0
x – 7 = 0
x = 7
The oxidation number of chlorine in ICl will be:
1 + x = 0
x = -1
The oxidation number of chlorine in Cl_{2}O will be:
2x + ( -2) = 0
2x = 2
x = 1
Thus, chlorine is in +1 oxidation number in Cl_{2}O.
So, option (d) is correct.
Q7. When K_{2}Cr_{2}O_{7} is converted to K_{2}CrO_{4}, the change in the oxidation number of chromium is
- 0
- 2
- 4
- None of the above
Answer: (a), When K_{2}Cr_{2}O_{7} is converted to K_{2}CrO_{4}, the change in the oxidation number of chromium is zero.
Explanation: To understand it better, foremost we will calculate the oxidation of K_{2}Cr_{2}O_{7} and K_{2}CrO_{4}.
The oxidation number of chromium in K_{2}Cr_{2}O_{7} is:
2 X 1 + 2x + 7 X ( -2) = 0
2 + 2x – 14 = 0
2x – 12 = 0
2x = 12
x = 6
The oxidation number of chromium in K_{2}CrO_{4 }is:
2 X 1 + x + 4 X ( -2) = 0
2 + x – 8 = 0
x – 6 = 0
x = 6
Thus, the oxidation number of chromium does not change when K_{2}Cr_{2}O_{7} is converted to K_{2}CrO_{4}.
So, option (a) is correct.
Q8. What is the oxidation number of chlorine in HOCl?
- 1
- 3
- 5
- None of the above
Answer: (a), The oxidation number of chlorine in HOCl is 1.
Calculation:
The oxidation number of chlorine in HOCl will be:
1 + ( -2) + x = 0
-1 + x = 0
x = 1
The oxidation number of chlorine in HOCl is one.
So, option (a) is correct.
Q9. Oxidation number of oxygen in O_{2} molecule is
- 0
- 1
- 2
- None of the above
Answer: (a), Oxidation number of oxygen in O_{2} molecule is zero.
Q10. The process in which oxidation number increases is known as
- Oxidation
- Reduction
- Catalysis
- None of the above
Answer: (a), The process in which oxidation number increases is known as oxidation.
Q11. Which element in the given compounds has the highest oxidation number?
- Sulphur in SO_{3}
- Carbon in CO_{2}
- Aluminum in AlCl_{3}
- Sulphur in CaS
Answer: (a) Sulphur in SO_{3 }will have the highest oxidation number.
Explanation: To understand it better, foremost we will calculate the oxidation number of SO_{3}, CO_{2}, AlCl_{3} and CaS.
The oxidation number of sulphur in SO_{3 }is
x + 3 X (- 2) = 0
x – 6 = 0
x = 6
The oxidation number of carbon in CO_{2 }is
x + 2 X (- 2) = 0
x – 4 = 0
x = 4
The oxidation number of aluminum in AlCl_{3} is
x + 3 X (- 1) = 0
x – 3 = 0
x = 3
The oxidation number of sulphur in CaS is
2 + x = 0
x = – 2
Thus, the oxidation number of sulphur in SO_{3 }is highest.
So, option (a) is correct.
Q12. Name an element that always shows a negative oxidation number.
Answer: Fluorine always shows a negative ( -1) oxidation number.
Q13. What is the oxidation state?
Answer: The oxidation state is the number of electrons that a specific atom can gain, lose or share with another atom. It explains the degree of oxidation of an atom in a molecule.
Q14. What is the oxidation number?
Answer: The oxidation number is the charge that a central metal atom will have even after all the ligands have been removed from that atom.
Q15. What are the rules for finding oxidation numbers?
Answer: The oxidation number is the charge that a central metal atom will have even after all the ligands have been removed from that atom.
Rules for finding the oxidation number:
Rule 1: An atom has a zero oxidation number in its elemental form.
Example: The oxidation number of chlorine in the Cl_{2} molecule is zero.
Rule 2: The oxidation number of an ion is equivalent to its charge.
Example: The charge of chlorine ion is -1, so the oxidation number of chlorine ion will be -1.
Rule 3: The oxidation number of alkali metals is +1, and alkaline earth metal is +2.
Example: The oxidation number of sodium is +1, while the oxidation number of calcium is +2.
Rule 4: Hydrogen has two probable oxidation numbers, i.e. +1 and -1.
Example: The oxidation number of hydrogen in NaH is -1, while the oxidation number in HCl is +1.
Rule 5: Oxygen has three probable oxidation numbers: +2, -2 and -1.
Example: The oxidation number of oxygen in H_{2}O is -2, while the oxidation number in OF_{2} is +2. In contrast, the oxidation number of oxygen is -1 in H_{2}O_{2}.
Rule 6: The oxidation number of fluorine in any compound is -1.
Example: The oxidation number of fluorine in HF is -1.
Rule 7: The oxidation number of halogen is typically equal to -1 except when bonded with oxygen or fluorine atom.
Example: The oxidation number of chlorine in HCl is -1, while the oxidation number of chlorine in HClO_{4} is +7.
Rule 8: The sum of the oxidation numbers of neutral compounds equals zero.
Example: The oxidation number of chlorine is -1 in HCl, while the oxidation number of hydrogen is +1, and their sum is equal to zero.
Practise Questions on Oxidation Number
Q1. Fluorine always shows a -1 oxidation number. Why?
Q2. Differentiate between oxidation state and oxidation number.
Q3. What is the oxidation number of chlorine in the perchlorate ion?
- 5
- 7
- 9
- None of the above
Q4. What is the oxidation number of carbon in carbon suboxide (C_{3}O_{2})?
- 4 / 3
- 2 / 3
- 1 / 3
- None of the above
Q5. When Chlorine gas reacts with hot and concentrated sodium hydroxide solution, the oxidation number of chlorine changes from
- Zero to -1 and Zero to +3
- Zero to +1 and Zero to -3
- Zero to +1 and Zero to -5
- Zero to -1 and Zero to +5
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