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Question


If the balanced reaction is as given below, find the value of x, y and z.

Cu(NH3)42+(aq)+S2O42(aq)+xOHySO32(aq)+Cu(s)+zNH3(aq)+2H2O

A
x=4, y=2, z=4
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B
x=3, y=2, z=3
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C
x=4, y=3, z=4
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D
None of the above
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Solution

The correct option is A x=4, y=2, z=4
The unbalanced chemical equation is,

Cu(NH3)42+(aq)+S2O42(aq)SO32(aq)+Cu(s)+NH3(aq)

Balance N and S atoms.

Cu(NH3)42+(aq)+S2O42(aq)2SO32(aq)+Cu(s)+4NH3(aq)

The oxidation number of Cu decreases from +2 to 0. Net decrease in the oxidation number is 2.
The oxidation number of S atom increases from +3 to +4. For 2S atoms, the oxidation number increases by 2.
Thus, an increase in the oxidation number is balanced with a decrease in oxidation number.

To balance O atoms, add 2 water molecules on LHS.

Cu(NH3)42+(aq)+S2O42(aq)+2H2O(l)2SO32(aq)+Cu(s)+4NH3(aq)

To balance H atoms, add 4 protons on RHS.

Cu(NH3)42+(aq)+S2O42(aq)+2H2O(l)2SO32(aq)+Cu(s)+4NH3(aq)+4H+(aq)

Since the reaction is in basic medium, add 4 hydroxide ions on either side.

Cu(NH3)42+(aq)+S2O42(aq)+2H2O(l)+4OH(aq)2SO32(aq)+Cu(s)+4NH3(aq)+4H+(aq)+4OH(aq)

On RHS, 4 hydrogen ions combine with 4 hydroxide ions to form 4 water molecules out of which 2 water molecules cancel with 2 water molecules on LHS.

Cu(NH3)42+(aq)+S2O42(aq)+4OH(aq)2SO32(aq)+Cu(s)+4NH3(aq)+2H2O(aq)

This is the balanced chemical equation in basic medium.

The coefficients x,y and z are 4,2 and 4 respectively.

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