This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Werner’s Theory of Coordination Compounds”.
1. What was the term proposed by Werner for the number of groups bound directly to the metal ion in a coordination complex?
a) Primary valence
b) Secondary valence
c) Oxidation number
Explanation: Primary valence of a metal ion is also known as its oxidation number. The secondary valence is equal to coordination number and it is fixed for a metal. Coordination polyhedra is the term for the spatial arrangements of the groups/ions around the metal.
2. What is the colour of the compound CoCl3.5NH3?
Explanation: The different compounds of cobalt(III) chloride with ammonia show distinct properties, most notably with respect to colour and conductivity measurements in solution.
3. If the secondary valence in CoCl3.4NH3 is six, the solution conductivity in silver nitrate corresponds to ________ electrolyte.
Explanation: Werner observed that ‘x’ mol of AgCl was precipitated per mole of CoCl3.4NH3 with excess silver nitrate. Currently, there are seven groups attached to the cobalt atom. For the secondary valence to be six, one of the chloride (x=1) has to be precipitated as AgCl, therefore 1:1 electrolyte.
4. Which is the counter ion in [Pd(NH3)4]2+2Cl–?
d) It does not have a counter ion
Explanation: In coordination compounds, the ions outside the square brackets are called counter ions, and the species within the square bracket are the coordination entities.
5. Werner proposed that the primary valences are ionizable and are satisfied only by positive ions.
Explanation: According to Werner’s postulates, primary valences are ionizable and are satisfied by negative ions. Whereas, secondary valences are non-ionizable, and these are satisfied by neutral molecules or negative ions.
6. Werner postulated that octahedral, tetrahedral and square planar geometrical shapes are more common in coordination compounds of ________
a) alkali metals
d) transition metals
Explanation: Transition metals bond a bit differently from main group elements. They form coordinate covalent bonds and are often the central metal ion/atom of coordination complexes. Thus, octahedral, tetrahedral and square planar shapes are more common in transition metal central atoms.
7. Given that 1 mol of NiCl2.6H2O with excess AgNO3 precipitates 2 mols of AgCl, what is the secondary valence of Ni?
Explanation: After the reaction, it will form [Ni(H2O)6]2- and 2Cl– in which the former is the coordination complex. There are six water molecules bound to Ni, which makes the secondary valence of Ni as 6.
8. If Pt in PtCl4.2HCl has a secondary valence of 6, how many mols of AgCl will 1 mol of the compound precipitate with excess AgNO3?
Explanation: Since the secondary valence is 6, the compound should have six groups linked to Pt atom. But it already has six groups linked to it and cannot lose any Cl atoms to form AgCl. Hence, no AgCl will be produced.
9. What is the sum of the oxidation number of cobalt in [Co(H2O)(CN)(en)2]2+and [CoBr2(en)2]+?
Explanation: In [Co(H2O)(CN)(en)2]2+, the overall charge is +2 and 0, -1 and 0 on H2O, CN and en groups respectively. If ‘x’ is the oxidation number of Co, then (x + 0 – 1 + 0 = +2), which implies x=+3. Similarly, in [CoBr2(en)2]+, the charge on Br is -1, so if ‘y’ is the oxidation number of Co, then (y – 2 + 0 = +1), which implies y=+3. Therefore, the sum of the oxidation numbers is 3 + 3 = 6.
10. Predict the secondary valence of Pt within the complex ion from the equation shown below.
Explanation: It is given that 3 parts of the platinum chloride compounds in excess silver nitrate gives 9 parts of AgCl and 3 parts of the complex ion. There are two approaches to this:
Method 1 (By balancing): It is seen that (3 x 4 = 12) atoms of Cl on the left give (9 x 1 = 9) on the right, which leaves 3 Cl atoms, that is one in each complex ion. Now, each complex ion has 1 Cl and 5 NH3 linked to Pt, giving a secondary valence of 6.
Method 2 (By proportioning): 3 mols gives 9 mols, which implies, 1 part of compound will give 3 parts of AgCl. Thus, only one Cl atom remains with the compound along with 5 NH3 molecules, giving Pt a secondary valence of 6.
11. What is the primary valence of iron in the coordination entity of compound Fe4[Fe(CN)6]3?
Explanation: This compound, also known as Prussian blue, dissociates into 4 Fe3+ and 3[Fe(CN)6]4- ions, of which the latter is the coordination entity having a charge of -4. The Fe atom in question is the one inside the bracket. Since CN has a charge of -1 and there are 6 of them, the primary valence of Fe will be 6 – 4 = +2.
12. [Fe(CN)6]4- is an example of a _______
a) Coordination compound
b) Complex ion
c) Double salt
d) Complex salt
Explanation: [Fe(CN)6]4- is the coordination complex ion of the coordination compound K4[Fe(CN6)], or potassium ferrocyanide, which is also a complex salt. Coordination complexes are also known as coordination entities.
13. Which of the following is a complex salt?
a) Potassium aluminium sulphate
b) Ammonium iron(ll) sulphate
d) Potassium ferrocyanide
Explanation: Potassium ferrocyanide, K4[Fe(CN6)], is a complex salt that does not dissociate into Fe2+ and CN– ions when dissolved in water. The other three, i.e., potash alum, carnallite and Mohr’s salt are double salt that dissociate into simple ions completely when dissolved in water.
14. The compound tetraamminecopper(II) sulphate, or [Cu(NH3)4]SO4.5H2O, a double salt.
Explanation: The given compound is a coordination compound made of a complex ion, i.e., [Cu(NH3)4]2+ which does not dissociate into its constituent ions when dissolved. Hence, it is a complex salt and not a double salt.
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