Match each redox reagent mixture or reaction system with its corresponding characteristic … — JAMB UTME

Match each redox reagent mixture or reaction system with its corresponding characteristic diagnostic observation.

Acidified potassium heptaoxodichromate(VI) solution ( $K_2Cr_2O_7 / H^+$ ) treated with sulfur(IV) oxide gas ( $SO_2$ )Solution changes color from orange to green due to the reduction of chromium from oxidation state $+6$ to $+3$
Freshly prepared iron(II) tetraoxosulfate(VI) solution ( $FeSO_4$ ) treated with concentrated trioxonitrate(V) acid ( $HNO_3$ )Solution turns from pale green to reddish-brown due to the oxidation of iron from oxidation state $+2$ to $+3$
Aqueous potassium iodide solution ( $KI$ ) treated with chlorine gas ( $Cl_2$ ) in the presence of starch indicatorFormation of a dark blue-black complex due to the oxidation of iodide ions ( $I^-$ ) to elemental iodine ( $I_2$ )
Acidified potassium tetraoxomanganate(VII) solution ( $KMnO_4 / H^+$ ) treated with hydrogen peroxide ( $H_2O_2$ )Solution decolorizes from purple to colorless accompanied by effervescence of oxygen gas ( $O_2$ )

Answer

Acidified potassium heptaoxodichromate(VI) with sulfur(IV) oxide matches the orange to green color change. Freshly prepared iron(II) tetraoxosulfate(VI) with concentrated trioxonitrate(V) acid matches the pale green to reddish-brown change. Aqueous potassium iodide with chlorine gas and starch matches the blue-black complex formation. Acidified potassium tetraoxomanganate(VII) with hydrogen peroxide matches decolorization from purple to colorless with oxygen gas effervescence.

Each test reagent matches its specific chemical observation based on electron transfer and oxidation state changes:

K_2Cr_2O_7

turns green when reduced by

SO_2

;

Fe^{2+}

turns brown when oxidized by conc.

HNO_3

;

KI

yields free

I_2

which gives a blue-black color with starch when oxidized by

Cl_2

; and acidified

KMnO_4

is decolorized with

O_2

evolution when reduced by

H_2O_2

Step-by-Step Solution

Analyze the reduction of acidified

K_2Cr_2O_7

SO_2

Chromium decreases in oxidation number from

+6

Cr_2O_7^{2-}

+3

Cr^{3+}

, causing a distinct color shift from orange to green.

SO_2

acts as a reductant and

K_2Cr_2O_7

as an oxidant.

Analyze the oxidation of

FeSO_4

by concentrated

HNO_3

Iron increases in oxidation state from

+2

(

Fe^{2+}

, pale green) to

+3

(

Fe^{3+}

, brown/yellowish-brown).

Concentrated

HNO_3

is a strong oxidizing agent.

Analyze halogen displacement of

KI

Cl_2

Chlorine oxidizes

I^-

I_2

. Liberated

I_2

forms a blue-black starch-iodine inclusion complex.

Chlorine has a higher standard reduction potential than iodine.

Analyze the redox reaction between acidified

KMnO_4

and

H_2O_2

Manganese is reduced from

+7

(

MnO_4^-

, purple) to

+2

(

Mn^{2+}

, colorless), while peroxide oxygen is oxidized from

-1

0

(

O_2

gas bubbles).

In the presence of a stronger oxidant (

KMnO_4

H_2O_2

behaves as a reducing agent.

Key Concept

Oxidizing and Reducing Agents and Diagnostic Chemical Tests

Estimated Time:1m 30s

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