COLOR OF COMPOUNDS OF TRANSITION ELEMENTS ( D-BLOCK) - MCQ
IIT JEE  - NEET

1) Which of the following compounds is expected to be colored? 

(IIT JEE 2004)

a) Ag₂SO₄ 

b) CuF₂

c) MgF₂ 

d) CuCl

Logic:

The transition metal ions with partially filled d-orbitals exhibit colors in aqueous solutions and also in crystals due to d-d transitions. However, these transitions are not possible with empty or full-filled i.e. d⁰ and d¹⁰ configurations and metal ions with these configurations usually does not show any color (said to be white).

Solution:

Conclusion:

Correct option is: 'b'

Related questions

2) The aqueous solutions of following salts will be colored in the case of: 

(IIT JEE 1990) 

a) CrCl₃ 

b) Zn(NO₃)₂ 

c) Co(NO₃)₂ 

d) LiNO₃ 

e) Potash alum

Solution:

Conclusion:

Correct options are: 'a' and 'c'.

3) Which one of the following ions exhibits colour in aqueous solution?

(EAMCET 2000 M) 

1) Sc³⁺ 

2) Ni²⁺ 

3) Ti⁴⁺ 

4) Zn²⁺

Solution:

Conclusion:

Correct option: '2'.

Note:

Above ions are in aqueous solutions. Hence they are coordinated to water molecules acting as ligands, mostly in octahedral geometry. As a result, the degeneracy in the d-orbitals is lifted i.e. the energy of d_xy, d_xz and d_yz orbitals (t_2g set) is lowered while the d_x2_-y2 and d_z2 orbitals (e_g set) get higher energy. This is called crystal field splitting.

In general, the energy gap between these two sets of d-orbitals is in the visible region. The electrons in the t_2g set are excited to higher levels by absorbing any one color of the VIBGYOR  and transmit out the remaining colors. The combination color of transmitted radiation is called complementary color. The ion appears in that complementary color.

One should note that the excitation (d-d transition) is only possible when there is vacancy in the higher levels. Therefore, the ions with partially filled d-electrons can only show color. This is the case only in Ni²⁺ ion among the given options. In other cases, d-orbitals are either empty or full-filled.

4) Which one of the following ions is colourless ? 

(MANIPAL BVP 2010) 

A) Cu⁺

B) Co²⁺

C) Ni²⁺ 

D) Fe³⁺

Solution:

Conclusion:

Correct option: 'A'.

5) The colour of zinc sulphide is:

(MP PMT 2010) 

(a) white

(b) black

(c) brown

(d) red 

Solution:

Both the Zn²⁺ & S^2- ions are colorless.

Conclusion:

Correct option: 'a'.

6) Out of TiF₆^2– , CoF₆^3– , Cu₂Cl₂   and NiCl₄^2– (Z of Ti = 22,  Co = 27, Cu = 29, Ni = 28), the colourless species are :

(CBSE AIPMT 2009) 

A) TiF₆^2– & CoF₆^3–

B) Cu₂Cl₂ & NiCl₄^2–

C) TiF₆^2– & Cu₂Cl₂

D) CoF₆^3– & NiCl₄^2–

Solution:

Conclusion:

Correct options are: 'A'.

Extra information:

The color of the complexes are influenced by the strength of the ligands.

7) Which compound is expected to be coloured ?    

 (CPMT 2009) 

a) CuCl

b) CuF₂

c) Ag₂SO₄

d) MgF₂

Solution:

Only in Cu²⁺ ion, the d-orbitals are partially filled. Hence only CuF₂ is colored.

Conclusion:

Correct option is:  b

Extra information:

The electrons in Mg²⁺ ion are paired up. The electronic transitions require energy in the UV region. Hence it is colorless.

8) Which of the following compounds is coloured ? 

 (AIIMS 2009) 

A) TiCl₃

B) FeCl₃

C) CoCl₂      

D) All of the above

Solution:

Ti³⁺ is a d¹ ion, Fe³⁺ ion is a d⁵ ion and Co²⁺ ion is a d⁷ ion. All are with partially filled d-orbitals. Hence all the ions are colored.

Conclusion:

Correct option is: D

Extra information:

A) TiCl₃ - blue

B) FeCl₃ - The colour of iron(III) chloride crystals depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red.  The aqueous solutions of Ferric chloride are yellowish in color.

C) CoCl₂ - anhydrous is blue in color while the hexahydrate is pink in color

9) An aqueous solution of CoCl₂ on addition of excess of conc. HCl turns blue due to the formation of:

 (DUMET 2008) 

A) [Co(H₂O)]Cl₂

 B) [Co(H₂O)₂Cl₄]^2-

C) [CoCl₄]^2-      

D) [Co(H₂O)₂]Cl₂

Solution:

In aqueous solutions, Cobalt chloride, CoCl₂ exists as octahedral complex, [Co(H₂O)₆]Cl₂ , which is pink in color.  It exists in equilibrium with small amount of a tetrahedral complex, [CoCl₄]^2- that is intense blue in color. The equilibrium is shown below. 

According to le Chatelier's principle, when HCl is added, the equilibrium will be shifted to the right side and more [CoCl₄]^2- is formed and thus by turning the solution into deep blue.

Conclusion:

Correct option is: C

Extra information:

The d-d transitions in octahdedral [Co(H₂O)₆]Cl₂ complex are both spin and  Laporte forbidden. Hence the color is not intense. However, in the tetrahdral complex, [CoCl₄]^2- the d-d transitions are allowed and hence the absorptions are intense.

10) The dark blue colour of the solution formed when excess of ammonia is added to a solution of copper(II) sulphate is due to the presence of the ion :                                          

[Kerala CEE 2008]   

A) [Cu(OH)₄(H₂O)₂]^2-       

B) [Cu(H₂O)₆]²⁺

C) [Cu(NH₃)₂(H₂O)₄]²⁺

D) [Cu(NH₃)₄(H₂O)₂]²⁺ 

Solution:

Initially, a pale blue colored precipitate of Cu(OH)₂ is formed that is dissolved in excess of ammonia to give a soluble intense blue colored complex, [Cu(NH₃)₄(H₂O)₂]²⁺.

CuSO₄ + 2OH^-  -------> Cu(OH)₂ + SO₄^2-

Cu(OH)₂ + 4NH₃  + 2H₂O -------->  [Cu(NH₃)₄(H₂O)₂]²⁺ + 2OH^-

Conclusion:

Correct option is: D

Homework

1) Why transition metal ions show colors?

2) Why anhydrous CuSO₄ is colorless, whereas CuSO₄.5H₂O is colored?