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Difficulty: Very hardDefinitions and Classical vs Modern Concepts of Redox

Match each chemical transformation on the left with the correct classical or modern redox definition that specifically describes it on the right.

  • Conversion of methane (CH4\text{CH}_4) to methanol (CH3OH\text{CH}_3\text{OH})Classical oxidation defined by the direct addition of oxygen to a compound
  • Reaction of mercury(II) chloride (HgCl2\text{HgCl}_2) to mercury(I) chloride (Hg2Cl2\text{Hg}_2\text{Cl}_2) in 2HgCl2(aq)+SnCl2(aq)→Hg2Cl2(s)+SnCl4(aq)2\text{HgCl}_{2(aq)} + \text{SnCl}_{2(aq)} \rightarrow \text{Hg}_2\text{Cl}_{2(s)} + \text{SnCl}_{4(aq)}Classical reduction defined by the removal of an electronegative element (chlorine) relative to mercury
  • Transformation of magnesium metal (Mg(s)\text{Mg}_{(s)}) to magnesium fluoride (MgF2(s)\text{MgF}_{2(s)})Modern oxidation defined by the loss of valence electrons and an increase in oxidation number from 00 to +2+2
  • Conversion of dichromate ion (Cr2O72−\text{Cr}_2\text{O}_7^{2-}) to chromium(III) ion (Cr3+\text{Cr}^{3+}) in acidic solutionModern reduction defined by a decrease in oxidation number from +6+6 to +3+3

Answer

Conversion of methane to methanol matches classical oxidation via direct addition of oxygen; conversion of mercury(II) chloride to mercury(I) chloride matches classical reduction via removal of an electronegative element; transformation of magnesium to magnesium fluoride matches modern oxidation via electron loss (00 to +2+2); conversion of dichromate to chromium(III) ion matches modern reduction via decrease in oxidation number (+6+6 to +3+3).
Each chemical transformation is matched to its corresponding classical or modern definition based on fundamental redox principles: oxygen addition represents classical oxidation; removal of an electronegative element represents classical reduction; electron loss and oxidation number increase represent modern oxidation; and oxidation number decrease represents modern reduction.

Step-by-Step Solution

1
Analyze the conversion of CH4\text{CH}_4 to CH3OH\text{CH}_3\text{OH} using classical definitions.
An oxygen atom is directly added to methane without removing hydrogen. Under classical rules, addition of oxygen is defined as oxidation.
Classical redox concepts classify oxygen gain as oxidation.
2
Analyze 2HgCl2+SnCl2→Hg2Cl2+SnCl42\text{HgCl}_2 + \text{SnCl}_2 \rightarrow \text{Hg}_2\text{Cl}_2 + \text{SnCl}_4 for HgCl2\text{HgCl}_2.
In HgCl2\text{HgCl}_2, mercury is bound to two chlorines per atom. In Hg2Cl2\text{Hg}_2\text{Cl}_2, mercury is bound to one chlorine per atom. The removal/loss of an electronegative element (Cl\text{Cl}) is classical reduction.
Classical definitions extend reduction to include the removal of electronegative elements or addition of electropositive elements.
3
Examine Mg(s)→MgF2(s)\text{Mg}_{(s)} \rightarrow \text{MgF}_{2(s)} under modern concepts.
Elemental magnesium (Mg0\text{Mg}^0) forms Mg2+\text{Mg}^{2+} ions by losing 22 electrons. Loss of electrons and an increase in oxidation state from 00 to +2+2 represents modern oxidation.
Modern redox theory defines oxidation as loss of electrons (OIL) or increase in oxidation number.
4
Examine Cr2O72−→Cr3+\text{Cr}_2\text{O}_7^{2-} \rightarrow \text{Cr}^{3+} using oxidation states.
In Cr2O72−\text{Cr}_2\text{O}_7^{2-}, 2x+7(−2)=−2⇒x=+62x + 7(-2) = -2 \Rightarrow x = +6. In Cr3+\text{Cr}^{3+}, the oxidation state is +3+3. The decrease in oxidation number from +6+6 to +3+3 represents modern reduction.
A decrease in oxidation state is the defining feature of reduction under modern IUPAC guidelines.

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Definitions and Classical vs Modern Concepts of Redox
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Definitions and Classical vs Modern Concepts of Redox

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