Consider the following ionic equation for the redox reaction between dichromate(VI) ions a… — JAMB UTME

Consider the following ionic equation for the redox reaction between dichromate(VI) ions and iodide ions in an acidic medium:

\text{Cr}_2\text{O}_7^{2-}(\text{aq}) + x\text{I}^-(\text{aq}) + y\text{H}^+(\text{aq}) \rightarrow 2\text{Cr}^{3+}(\text{aq}) + z\text{I}_2(\text{aq}) + w\text{H}_2\text{O}(\text{l})

What are the correct values of the stoichiometric coefficients

x

y

, and

z

respectively when the equation is completely balanced?

The correct stoichiometric coefficients for x, y, and z are 6, 14, and 3 respectively.

The reduction of one dichromate ion (

\text{Cr}_2\text{O}_7^{2-}

) requires 6 electrons and 14 hydrogen ions to yield two

\text{Cr}^{3+}

ions and 7 water molecules. Oxidation of iodide ions (

\text{I}^-

) to iodine (

\text{I}_2

) releases 2 electrons per molecule formed. To equalize electron exchange at 6 electrons, 6 moles of iodide ions (

x = 6

) produce 3 moles of iodine molecules (

z = 3

), requiring 14 moles of

\text{H}^+

(

y = 14

Write and balance the reduction half-reaction for dichromate(VI) ions

\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}

Chromium changes oxidation state from +6 to +3 (total 6 electrons gained for two Cr atoms), and 7 oxygen atoms require

14\text{H}^+

to form

7\text{H}_2\text{O}

Write and balance the oxidation half-reaction for iodide ions

2\text{I}^- \rightarrow \text{I}_2 + 2e^-

Iodine changes oxidation state from -1 to 0, losing 1 electron per iodide ion.

Equalize the number of electrons transferred in both half-reactions

Multiply the oxidation half-reaction by 3:

6\text{I}^- \rightarrow 3\text{I}_2 + 6e^-

6 electrons lost must equal 6 electrons gained.

Combine the half-reactions and determine coefficients

\text{Cr}_2\text{O}_7^{2-} + 6\text{I}^- + 14\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 3\text{I}_2 + 7\text{H}_2\text{O}

Matching coefficients gives

x = 6

y = 14

, and

z = 3

Balancing Redox Equations using Half-Reactions in Acidic Medium

Question