Question
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Answer
i. $Zn _{( s )}+ NO _{3( aq )}^{-} \longrightarrow NH _{3( aq )}+ Zn ( OH )_{6( aq )}^{2-}$
Step 1: Write skeletal equation and balance the elements other than 0 and $H$.
$
Zn _{( s )}+ NO _{3( aq )}^{-} \longrightarrow NH _{3( aq )}+ Zn ( OH )_{6( aq )}^{2-}
$
Step 2: Assign oxidation number to $Zn$ and $N$. Calculate the increase and decrease in the oxidation number and make them equal.
![]()
https://pg-data.sgp1.digitaloceanspaces.com/chapter_wise/15982/y26.png" alt="Image" width="330" height="">
To make the net increase and decrease equal, we must take 2 atoms of $Zn$.
$
2 Zn _{( s )}+ NO _{3( aq )}^{-} \longrightarrow NH _{3( aq )}+2 Zn ( OH )_{6( aq )}^{2-}
$
Step 3: Balance 'O' atoms by adding $9 H _2 O$ to the left-hand side.
$
2 Zn _{( s )}+ NO _{3( aq )}^{-}+9 H _2 O _{(l)} \longrightarrow NH _{3( aq )}+2 Zn ( OH )_{6( aq )}^{2-}
$ Step 4: The medium is basic. To make hydrogen atoms on the two sides equal, add $3 H$ on the right-hand side.![]()
https://pg-data.sgp1.digitaloceanspaces.com/chapter_wise/15982/s176.png" alt="Image" width="630" height="">
Step 5: Check two sides for balance of atoms and charges.
Hence, balanced equation:
$
2 Zn _{( s )}+ NO _{3( aq )}^{-}+6 H _2 O _{( f )}+3 OH _{(a q)}^{-} \longrightarrow NH _{3( aq )}+2 Zn ( OH )_{6( aq )}^{2-}
$
ii. $MnCl _{2( aq )}+ HO _{2( aq )}^{-} \longrightarrow Mn ( OH )_{3( s )}+ Cl _{( aq )}^{-}$
Step 1: Write skeletal equation and balance the elements other than $O$ and $H$.
$
MnCl _{2( aq )}+ HO _{2( aq )}^{-} \longrightarrow Mn ( OH )_{3( s )}+ Cl _{( aq )}^{-}
$
Step 2: Assign oxidation number to $Mn$ and $O$. Calculate the increase and decrease in the oxidation number and make them equal.
![]()
https://pg-data.sgp1.digitaloceanspaces.com/chapter_wise/15982/y27.png" alt="Image" width="340" height="">
![]()
https://pg-data.sgp1.digitaloceanspaces.com/chapter_wise/15982/y28.png" alt="Image" width="530" height="">$
\text { iii. } Cu ( OH )_{2( s )}+ N _2 H _{4( aq )} \rightarrow Cu _{( s )}+ N _{2( g )}
$
Step 1: Write skeletal equation and balance the elements other that $O$ and $H$.
$
Cu ( OH )_{2( s )}+ N _2 H _{4( aq )} \rightarrow Cu _{( s )}+ N _{2( g )}
$
Step 2: Assign oxidation number to $Cu$ and $N$. Calculate the increase and decrease in the oxidation number and make them equal.
![]()
https://pg-data.sgp1.digitaloceanspaces.com/chapter_wise/15982/y29.png" alt="Image" width="340" height="">
To make the net increase and decrease equal, we must take 2 atoms of $Cu$.
$
2 Cu ( OH )_{2( s )}+ N _2 H _{4( aq )} \rightarrow 2 Cu _{( s )}+ N _{2( g )}
$
Step 3: Balance ' $O$ ' atoms by adding $4 H 20$ to the right-hand side.
$
2 Cu ( OH )_{2( s )}+ N _2 H _{4( aq )} \rightarrow 2 Cu _{( s )}+ N _{2( g )}+4 H _2 O _{(l)}
$
Step 4: The equation is balanced for both atoms as well as charges.
Step 5: Check two sides for balance of atoms and charges.
Hence, balanced equation: $2 Cu ( OH )_{2( s )}+ N _2 H _{4( aq )} \rightarrow 2 Cu _{( s )}+ N _{2( g )}+4 H _2 O _{( ) )}$
Step 1: Write skeletal equation and balance the elements other than 0 and $H$.
$
Zn _{( s )}+ NO _{3( aq )}^{-} \longrightarrow NH _{3( aq )}+ Zn ( OH )_{6( aq )}^{2-}
$
Step 2: Assign oxidation number to $Zn$ and $N$. Calculate the increase and decrease in the oxidation number and make them equal.
To make the net increase and decrease equal, we must take 2 atoms of $Zn$.
$
2 Zn _{( s )}+ NO _{3( aq )}^{-} \longrightarrow NH _{3( aq )}+2 Zn ( OH )_{6( aq )}^{2-}
$
Step 3: Balance 'O' atoms by adding $9 H _2 O$ to the left-hand side.
$
2 Zn _{( s )}+ NO _{3( aq )}^{-}+9 H _2 O _{(l)} \longrightarrow NH _{3( aq )}+2 Zn ( OH )_{6( aq )}^{2-}
$ Step 4: The medium is basic. To make hydrogen atoms on the two sides equal, add $3 H$ on the right-hand side.
Step 5: Check two sides for balance of atoms and charges.
Hence, balanced equation:
$
2 Zn _{( s )}+ NO _{3( aq )}^{-}+6 H _2 O _{( f )}+3 OH _{(a q)}^{-} \longrightarrow NH _{3( aq )}+2 Zn ( OH )_{6( aq )}^{2-}
$
ii. $MnCl _{2( aq )}+ HO _{2( aq )}^{-} \longrightarrow Mn ( OH )_{3( s )}+ Cl _{( aq )}^{-}$
Step 1: Write skeletal equation and balance the elements other than $O$ and $H$.
$
MnCl _{2( aq )}+ HO _{2( aq )}^{-} \longrightarrow Mn ( OH )_{3( s )}+ Cl _{( aq )}^{-}
$
Step 2: Assign oxidation number to $Mn$ and $O$. Calculate the increase and decrease in the oxidation number and make them equal.
\text { iii. } Cu ( OH )_{2( s )}+ N _2 H _{4( aq )} \rightarrow Cu _{( s )}+ N _{2( g )}
$
Step 1: Write skeletal equation and balance the elements other that $O$ and $H$.
$
Cu ( OH )_{2( s )}+ N _2 H _{4( aq )} \rightarrow Cu _{( s )}+ N _{2( g )}
$
Step 2: Assign oxidation number to $Cu$ and $N$. Calculate the increase and decrease in the oxidation number and make them equal.
To make the net increase and decrease equal, we must take 2 atoms of $Cu$.
$
2 Cu ( OH )_{2( s )}+ N _2 H _{4( aq )} \rightarrow 2 Cu _{( s )}+ N _{2( g )}
$
Step 3: Balance ' $O$ ' atoms by adding $4 H 20$ to the right-hand side.
$
2 Cu ( OH )_{2( s )}+ N _2 H _{4( aq )} \rightarrow 2 Cu _{( s )}+ N _{2( g )}+4 H _2 O _{(l)}
$
Step 4: The equation is balanced for both atoms as well as charges.
Step 5: Check two sides for balance of atoms and charges.
Hence, balanced equation: $2 Cu ( OH )_{2( s )}+ N _2 H _{4( aq )} \rightarrow 2 Cu _{( s )}+ N _{2( g )}+4 H _2 O _{( ) )}$
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