Chemistry Objective Questions and Answers: Redox Reactions
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Multiple Choice Questions
(a) Addition of oxygen
(b) Addition of hydrogen
(c) Removal of oxygen
(d) Addition of an electropositive element
Show Answer & Explanation
Answer: (a) Addition of oxygen
Explanation:
1. Oxidation is addition of oxygen (or an electronegative element), removal of hydrogen (or an electropositive element), or loss of electrons.
2. Addition of hydrogen, removal of oxygen and addition of an electropositive element are all reduction, so only (a) is oxidation.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) oxidation
(b) reduction
(c) combustion
(d) neutralisation
Show Answer & Explanation
Answer: (a) oxidation
Explanation:
1. By the electronic concept, oxidation is loss of electrons (OIL RIG).
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) \( \ce{MnO4- -> Mn^{2+}} \)
(b) \( \ce{CrO4^{2-} -> Cr^{3+}} \)
(c) \( \ce{MnO4^{2-} -> MnO2} \)
(d) \( \ce{Cr2O7^{2-} -> Cr^{3+}} \)
Show Answer & Explanation
Answer: (a) \( \ce{MnO4- -> Mn^{2+}} \)
Explanation:
1. Mn changes from +7 to +2, which is a gain of 5 electrons.
2. The others: +6 to +3 is 3 e-, +6 to +4 is 2 e-, and \( \ce{Cr2O7^{2-}} \) takes 6 e- for two Cr.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) \( \ce{CrO4^{2-} -> Cr2O7^{2-}} \)
(b) \( \ce{Cr -> CrCl3} \)
(c) \( \ce{Na -> Na+} \)
(d) \( \ce{2S2O3^{2-} -> S4O6^{2-}} \)
Show Answer & Explanation
Answer: (a) \( \ce{CrO4^{2-} -> Cr2O7^{2-}} \)
Explanation:
1. Cr is +6 in both chromate and dichromate, so no oxidation state changes.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) phosphorus is both oxidised and reduced
(b) only phosphorus is reduced
(c) phosphorus is not oxidised
(d) none of these
Show Answer & Explanation
Answer: (a) phosphorus is both oxidised and reduced
Explanation:
1. P goes from 0 to +1 in \( \ce{KH2PO2} \) (oxidation) and from 0 to -3 in \( \ce{PH3} \) (reduction), a disproportionation.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) an oxidising agent
(b) a reducing agent
(c) both of these
(d) none of these
Show Answer & Explanation
Answer: (b) a reducing agent
Explanation:
1. Sodium loses an electron to oxygen, so it is oxidised and acts as the reductant.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) -1
(b) -3
(c) +3
(d) +5
Show Answer & Explanation
Answer: (c) +3
Explanation:
1. N in \( \ce{N2H4} \) is -2; two N atoms lose 10 e- in total, so each loses 5 and the oxidation state becomes -2 + 5 = +3.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) Cu > Zn > Ag
(b) Zn > Ag > Cu
(c) Ag > Zn > Cu
(d) Zn > Cu > Ag
Show Answer & Explanation
Answer: (d) Zn > Cu > Ag
Explanation:
1. In the electrochemical series Zn is above Cu, which is above Ag, so Zn releases electrons most readily.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) +1, +1, -1
(b) -1, +1, +1
(c) +1, +1, +1
(d) -1, -1, -1
Show Answer & Explanation
Answer: (d) -1, -1, -1
Explanation:
1. In ionic (saline) hydrides hydrogen is more electronegative than the metal and exists as \( \ce{H-} \).
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) \( \ce{4KClO3 -> 3KClO4 + KCl} \)
(b) \( \ce{SO2 + 2H2S -> 2H2O + 3S} \)
(c) \( \ce{BaO2 + H2SO4 -> BaSO4 + H2O2} \)
(d) \( \ce{3BaO + O2 -> 2BaO2} \)
Show Answer & Explanation
Answer: (c) \( \ce{BaO2 + H2SO4 -> BaSO4 + H2O2} \)
Explanation:
1. In (c) every element keeps its oxidation state (O is -1 in both \( \ce{BaO2} \) and \( \ce{H2O2} \)).
2. The other reactions involve changes in oxidation numbers.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) +6
(b) -5
(c) -2
(d) +2
Show Answer & Explanation
Answer: (a) +6
Explanation:
1. \( \ce{K2Cr2O7} \): 2(+1) + 2x + 7(-2) = 0, so x = +6.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) 0, +1 and -2
(b) +2, +1 and -2
(c) 0, +1 and +2
(d) -2, +1 and -2
Show Answer & Explanation
Answer: (a) 0, +1 and -2
Explanation:
1. Sulphur is 0 in the free element, +1 in \( \ce{S2F2} \) (F is -1) and -2 in \( \ce{H2S} \).
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) +1
(b) +3
(c) -1
(d) -3
Show Answer & Explanation
Answer: (c) -1
Explanation:
1. K is +1 and CO is neutral, so the complex ion carries charge -1 and Co is -1.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) \( \ce{K3[Fe(CN)6]} \)
(b) \( \ce{K4[Fe(CN)6]} \)
(c) \( \ce{FeSO4.(NH4)2SO4.6H2O} \)
(d) \( \ce{Fe(CO)5} \)
Show Answer & Explanation
Answer: (d) \( \ce{Fe(CO)5} \)
Explanation:
1. The oxidation states of Fe are +3, +2, +2 and 0; in the carbonyl, CO is neutral so Fe is 0.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) +7
(b) +6
(c) +4
(d) +8
Show Answer & Explanation
Answer: (d) +8
Explanation:
1. x + 4(-2) = 0 gives x = +8.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) \( \ce{MnO4-} \)
(b) \( \ce{Cr(CN)6^{3-}} \)
(c) \( \ce{NiF6^{2-}} \)
(d) \( \ce{CrO2Cl2} \)
Show Answer & Explanation
Answer: (d) \( \ce{CrO2Cl2} \)
Explanation:
1. The oxidation states are Mn +7, Cr +3, Ni +4 and, in \( \ce{CrO2Cl2} \), Cr = +6.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) HCHO
(b) \( \ce{CH3COOH} \)
(c) \( \ce{C12H22O11} \)
(d) \( \ce{CH3CHO} \)
Show Answer & Explanation
Answer: (d) \( \ce{CH3CHO} \)
Explanation:
1. The average oxidation number of C is 0 in HCHO, acetic acid and sucrose, but -1 in acetaldehyde (C2: -3 + 1 = -2 over two carbons).
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) +3
(b) 8/3
(c) +6
(d) +2
Show Answer & Explanation
Answer: (b) 8/3
Explanation:
1. 3x + 4(-2) = 0 gives x = 8/3 (a mixed Fe(II) and Fe(III) oxide).
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) -2
(b) -1
(c) +2
(d) +1, -2
Show Answer & Explanation
Answer: (c) +2
Explanation:
1. Fluorine is more electronegative than oxygen and is -1, so oxygen is +2.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) +2.5
(b) +2
(c) +3.0
(d) +3.5
Show Answer & Explanation
Answer: (a) +2.5
Explanation:
1. 2(+1) + 4x + 6(-2) = 0 gives x = +2.5.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) \( \ce{Cl-} \)
(b) \( \ce{ClO4-} \)
(c) \( \ce{ClO-} \)
(d) \( \ce{MnO4-} \)
Show Answer & Explanation
Answer: (c) \( \ce{ClO-} \)
Explanation:
1. Cl in \( \ce{ClO-} \) is +1, an intermediate state between -1 and +7, so it can be oxidised or reduced.
2. \( \ce{Cl-} \) is at the lowest, and \( \ce{ClO4-} \), \( \ce{MnO4-} \) at the highest oxidation states.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) \( \ce{ClO2-} \), \( \ce{ClO4-} \) and \( \ce{ClO-} \)
(b) \( \ce{F2} \) only
(c) \( \ce{F2} \) and \( \ce{ClO4-} \)
(d) \( \ce{ClO4-} \) only
Show Answer & Explanation
Answer: (c) \( \ce{F2} \) and \( \ce{ClO4-} \)
Explanation:
1. \( \ce{ClO4-} \) has Cl in its highest state (+7) and fluorine is never positive, so neither can disproportionate.
2. The others have intermediate oxidation states and can.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) \( \ce{2H2SO4 + Cu -> CuSO4 + 2H2O + SO2} \)
(b) \( \ce{As2O3 + 3H2S -> As2S3 + 3H2O} \)
(c) \( \ce{2KOH + Cl2 -> KCl + KOCl + H2O} \)
(d) \( \ce{Ca3P2 + 6H2O -> 3Ca(OH)2 + 2PH3} \)
Show Answer & Explanation
Answer: (c) \( \ce{2KOH + Cl2 -> KCl + KOCl + H2O} \)
Explanation:
1. In (c) chlorine goes from 0 to -1 (KCl) and to +1 (KOCl), so the same species is oxidised and reduced.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) Cl
(b) Br
(c) F
(d) I
Show Answer & Explanation
Answer: (c) F
Explanation:
1. Fluorine, the most electronegative element, has no positive oxidation state.
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
(a) acidic
(b) alkaline
(c) neutral
(d) both acidic and alkaline
Show Answer & Explanation
Answer: (b) alkaline
Explanation:
1. P4, S8 and Cl2 disproportionate on heating with alkali, e.g. \( \ce{Cl2 + 2OH- -> Cl- + ClO- + H2O} \).
Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.
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Multiple Choice Questions (MCQs) for JEE Chemistry Redox Reactions
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FAQs
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