Question

A student takes $\mathrm{Cu},\mathrm{Al},\mathrm{Fe}\mathrm{and}\mathrm{Zn}$ strips, separately in four test tubes labelled as I, II, III, and IV respectively. He adds 10 mL of freshly prepared Ferrous Sulphate solution to each test tube and observes the colour of the metal residue in each case.

He would observe a black residue in the test tubes:

• A. (I) and (II)
• B. (I) and (III)
• C. (II) and (III)
• D. (II) and (IV)

Answer 1

The correct option is D

(II) and (IV)

The explanation for the correct answer.

Option D - (II) and (IV)

Part 1:

Case(I)

• When the students take a Copper strip in a test tube and added 10 mL of freshly prepared Ferrous Sulphate solution no reaction will take place.

$$\begin{gathered} \mathbf{Cu}\mathbf{}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}{\mathbf{FeSO}}_{\mathbf{4}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{}\mathbf{\to }\mathbf{No}\mathbf{}\mathbf{reaction} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{Iron}\mathbf{}\mathbf{Sulphate} \end{gathered}$$

• Because Copper is less reactive than Iron hence, no reaction takes place.

Part 2:

Case (II)

• When the students take an Aluminium strip in a test tube and add 10 mL of freshly prepared Iron Sulphate solution, Aluminum Sulphate is formed and Iron is settled in the test tube. So the green colour of ${\mathbf{FeSO}}_{\mathbf{4}}$ disappears.
• Because Aluminium is more reactive than Iron.

$$\begin{gathered} \mathbf{2}\mathbf{Al}\mathbf{}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}\mathbf{3}{\mathbf{FeSO}}_{\mathbf{4}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{}\mathbf{\to }\mathbf{}{\mathbf{Al}}_{\mathbf{2}}{\mathbf{\left(}{\mathbf{SO}}_{\mathbf{4}}\mathbf{\right)}}_{\mathbf{3}}\mathbf{}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}\mathbf{3}\mathbf{Fe}\mathbf{}\mathbf{\left(}\mathbf{s}\mathbf{\right)} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{Iron}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{Aluminium}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{(}\mathbf{Black}\mathbf{}\mathbf{residue}\mathbf{)} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{Sulphate}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{Sulphate} \end{gathered}$$

Part 3:

Case (III)

• When Iron Sulphate is added to Iron no change will be observed as both metals are the same, i.e.Iron.

$$\begin{gathered} \mathbf{Fe}\mathbf{}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}{\mathbf{FeSO}}_{\mathbf{4}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{}\mathbf{\to }\mathbf{No}\mathbf{}\mathbf{reaction} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{Iron}\mathbf{}\mathbf{Sulphate} \end{gathered}$$

Part 4:

Case (IV)

• When Iron Sulphate is added to the Zinc, then Zinc Sulphate is formed, and Iron is deposited in the test tube. Hence, the green colour of Iron Sulphate disappears. Because Zinc is more reactive than Iron.

$$\begin{gathered} \mathbf{Zn}\mathbf{}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}{\mathbf{FeSO}}_{\mathbf{4}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{}\mathbf{\to }\mathbf{}{\mathbf{ZnSO}}_{\mathbf{4}}\mathbf{}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}\mathbf{}\mathbf{Fe}\mathbf{}\mathbf{\left(}\mathbf{s}\mathbf{\right)} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{Iron}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{Zinc}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{(}\mathbf{Black}\mathbf{}\mathbf{residue}\mathbf{)} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{Sulphate}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{Sulphate} \end{gathered}$$

Hence option (D) is correct. II and IV