Chemistry Test - 10

The correct answer is option 3, i.e. Show fixed oxidation state.

Key Points

• Transition elements are those elements whose two outermost shells are incomplete.
• These elements have partially filled d-subshell in the ground state or any of their common oxidation state and are commonly referred to as d-block transition elements.
• The d-block elements are categorized as 1st series transition elements, 2nd series transition elements, 3rd series transition elements, and 4th series transition elements.
• Examples are:- Cu, Zn, Ag, Cd, Au, Hg, etc.
• The f-block elements are termed inner transition elements.
• Some characteristics of Transition elements are;
  • They are hard and have high densities.
  • They always form colored ions and compounds.
  • They have high melting and boiling points.
  • They have more than one oxidation state.

Concept:

• The D-block elements are known as transition elements.
• There total of 4 blocks in the modern periodic table. They are as follows: s block, p block, d block, f block.
• There are 18 groups and 7 periods in the modern periodic table.
• Total elements are 118 out of which 91 are metals, 7 are metalloids, and 20 are non-metals.

Explanation:

• Transition elements are those elements whose two outermost shells are incomplete.
• These elements have partially filled d-subshell in the ground state or any of their common oxidation state and are commonly referred to as d-block transition elements.
• The generalised electronic configuration of these elements is (n-1) d1–10 ns1–2.
• The d-block elements are categorised as 1st series transition elements, 2nd series transition elements, 3rd series transition elements and 4th series transition elements.
• The examples are:- Cu, Zn, Ag, Cd, Au, Hg, etc.

Concept-

​Crystal field theory:

• A metal ion in a complex is surrounded by coordinating ligands anions or negative ends.
• An electric field is produced at the metal ion by the surrounding ligands.
• This electrostatic approach towards the interaction between metals and ligands is known as crystal field theory.
• CFT theory considers the ligands as point charges.
• There is no overlap between the ligand orbitals and metal ion orbitals.
• Ligands donate lone pair of electrons to the metal atoms and form coordinate complexes.

The geometry of the complex formed:

• Placement of the donor atoms at different stereochemical positions will perturb the metal ions to a different extent.
• This will result in the formation of different stereochemistries or structures.
• The stereochemistry also depends on the coordination number and the type of orbitals involved.

Explanation:

• In the complex [CrF6]3-, the central atom is Chromium (Cr).
• It is surrounded by six F^- ligands, which are weak field ligands, the total charge on the complex is -3, so the oxidation state of Cr is +3.
• The electronic configuration of Cr (+3) is: [Ar]3d³4s⁰. 
• Three of the inner 3d orbitals of Cr(+3) are singly occupied and the other two are vacant.
• So, the orbitals involved in hybridization will be 3d,4s, and 4p giving us d²sp³ hybridization.
• Six pairs of electrons are donated in six orbitals of Cr.
• The inner orbitals are involved in the complex formation, hence it is an inner orbital octahedral complex.
• The hybridization is represented as:

Hence, in [CrF6]3-, the central atom Cr is d2sp3 hybridized.

Additional Information

• The geometry of the complex is octahedral and the complex is a high spin complex.

Concept:

Ionization potential (I.P)

• It is the energy required to take out the outermost electron from an isolated gaseous atom.
• The ionization energy of a chemical element is expressed in kilojoules or electron volts.
• The energy required to remove the first electron is called the 1st ionization potential.
• The more stable the atom, the higher is its I.P.

The ionization potential depends on -

•  The size of the atoms- Smaller the size of the atoms, the higher is the I.P value.
•  The penetration power- It is easier to remove an electron from the more diffused shell electron than a less diffused one.The ease of removal follows the order- f > d > p > s.
• The charge of the species- the greater the positive charge, the higher is the I.P value. I.P. value is directly proportional to the positive charge and inversely proportional to the negative charge.
• Electronic configuration- it is harder to remove an electron from a stable electronic configuration- fulfilled and half-filled.

Lanthanides:

• Gadolinium belongs to lanthanoids in the periodic table.
• The lanthanides are f block elements, as they have their last electrons in 4f subshell.
• The general electronic configuration is:

​\(\left[ {Xe} \right]4{f^{1 - 14}}5{d^{0 - 1}}6{s^2}\)

• There are 14 elements in total.
• They have stable oxidation states as +3 because two electrons are removed from the 6s and one electron from the 4f subshell.

Explanation:

• Gadolinium has an atomic number 64.
• The electronic configuration is [Xe]4f⁷5d¹6s².
• The 4f orbital of gadolinium has 7 unpaired electrons.​
• As the energy difference between 4f and 5d orbital is low, the next electron goes into the 5d orbital instead of getting paired into the 4f.
• After losing two electrons from the 6s subshell, the 5d orbital remains with only one electron.
• Losing one more electron from the 5d orbital will leave Gd with a stable half-filled electronic configuration.

Thus the third ionization potential of Gd is very low.

Hence, the Lanthanoid Gd exhibits an oxidation state of +3 only because of low third ionization potential.

Explanation:

• Oxidizing agents change the oxidation number from low to high in a substance it is oxidizing.
• It takes up electrons from the compound and then reduces itself.
• KMnO₄ as an oxidizing agent in basic medium as-

MnO4- + 2 H2O + 3 e- → MnO2(s) + 4OH- (gained 3 electrons)

• KMnO4 acts as an oxidizing agent in acidic medium as-

MnO4- + 8 H+ + 5 e- → Mn2+ + 4 H2O (gained 5 electrons from reductant)

• In a strongly alkaline medium, the ion changes to 

MnO4- + 1 e^- → MnO₄^2-

Equivalent mass:

• It is the amount of substance that combines with 1g of hydrogen or 8 g of oxygen or 35.5g of chlorine.
• The formula for calculating the equivalent mass of a substance undergoing redox reaction is given by:

Equivalent weight = Formula Mass/ no. of electrons exchanged​

In a strongly alkaline medium, the ion changes to 

MnO4- + 1 e- → MnO42-

The oxidation state changes from +7 to +6.

Hence, the number of electrons exchanged = 1.

The molar mass of potassium permanganate is 158g.

Equivalent weight of KMnO₄ = 158/1 = 158

Hence, the equivalent mass of an oxidizing agent, KMnO4, in a strongly alkaline medium is 158.

Explanation:

• When compounds containing chloride are heated with potassium dichromate in presence of sulphuric acid, a red liquid of chromyl chloride is formed.
• It is a test for compounds containing chlorine in them.
• Compounds containing chlorine will produce red-colored vapours of chromylchloride when heated with potassium dichromate in presence of sulphuric acid.
• Sulphuric acid acts as a dehydrating agent.

The net reaction is:

K₂Cr₂O₇ + 6KCl + H₂SO₄ → Cr₂O₂Cl₂

Hence, the formula of the deep red liquid formed on heating potassium dichromate with KCI in conc. H2SO4 is CrO2Cl2.

Key Points

• Chromyl chloride is Cr₂O₂Cl₂, which is a transition metal complex.
• The structure is tetrahedral.​
• It is deep red liquid which is unusual because transition complexes are mostly solids in nature.
• It is volatile at room temperature and the IUPAC name is Chromium (VI) dichloride dioxide.
• It can also be prepared by the reaction of chromium oxide and anhydrous HCl.
• The reaction is:

CrO₃ + HCl →  Cr₂O₂Cl₂.

It is used in Etard's reaction.

Auto catalyst: 

In certain reactions, one of the product acts as a catalyst. In the initial stages, the reaction is slow but as soon as the products come into existences the reaction rate increases. This type of phenomenon is known as auto-catalysis.

In the permanganate titration of oxalic acid in the presence of H₂​SO₄​ (acid medium);

There is a slow discharge of the colour of permanganate solution in the beginning but after some time, the discharge of the colour becomes faster.

Molecular equations

\(\rm 2KMnO_4 \ + \ 3H_2SO_4 \longrightarrow \ K_2SO_4 \ + \ 2MnSO_4 \ + \ 3H_2O \ + \ 5[O]\)

\(\begin{array}{*{20}{c}} {\rm COOH}\\ {| \ \ \,\,\,\,\,\,\,\,\,\,\,}\\ {\rm COOH} \end{array} .\rm 2H_2O \ + \ [O] \ \xrightarrow{60 - 70^\circ C} \ 2CO_2 \ + \ 3H_2O] \ \times \ 5\)

\(\rm 2KMnO_4 \ + \ 2H_2SO_4 + 5 \begin{array}{*{20}{c}} {\rm COOH}\\ {| \ \ \,\,\,\,\,\,\,\,\,\,\,}\\ {\rm COOH} \end{array} . \rm 2H_2O \xrightarrow{ \ \ \ \ \ \ \ \ \ \ \ } \ K_2SO_4 \ + \ 2MnSO_4 \ + \ 18H_2O \ + \ 10CO_2\)

This is due to the formation of MnSO_4​ during the reaction which acts as a catalyst for the same reaction.

So, Mn⁺² is acting like auto catalyst for this reaction.

Concept:

As zinc has no electron to take part in the bond, it has the least enthalpy of atomization among the given elements.

Zn is not a transition element so the transition element having the lowest atomization energy out of the other Cu, V, and Fe is Cu.

For transition metals, ΔH^o_atomization 

ΔHoatomization is directly proportional to the strength of metallic bonding and the number of unpaired electrons in the metal atom.

For the given 3d-transition metals,

From the above table, Zinc has the lowest enthalpy of atomization and among transition metals, it is Copper.

Additional Information

Zinc is a slightly brittle metal at room temperature and has a blue-silvery appearance when oxidation is removed. Zinc is an essential mineral including to prenatal and postnatal development.

Zinc deficiency affects about two billion people in the developing world and is associated with many diseases. In children, deficiency causes growth retardation, delayed sexual maturation, infection susceptibility, and diarrhea.

Zinc oxide is widely used in the manufacture of many products such as paints, rubber, cosmetics, pharmaceuticals, plastics, inks, soaps, batteries, textiles and electrical equipment.

Concept:

Paramagnetic materials:

• Small, positive susceptibility to magnetic fields.
• These materials are slightly attracted by a magnetic field.
• Paramagnetic properties are due to the presence of some unpaired electrons, and from the realignment of the electron paths caused by the external magnetic field
• Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum.

​

Magnetic moment:

• The strength of a magnet and its orientation in presence of a magnetic field is called its magnetic moment.
• The complexes have lone electrons in them, unpaired which contribute to the magnetic moment. It is given by the formula:

​ \(μ = {\sqrt{n(n+2)} }BM\)

Explanation:

• Chromium belongs to the d block. Its electronic configuration is: [Ar]3d⁵4s¹.
• In the Cr⁺² oxidation state, it loses two electrons and has the configuration [Ar]3d⁴.
• The number of unpaired electrons n = 4.

​

• Hence, the magnetic moment is:

\(μ = {\sqrt{n(n+2)} }BM\)

\(μ = {\sqrt{4(4+2)} }BM\)

μ = 4.90 BM

Hence, the magnetic moment of Cr²⁺ is 4.90 BM.

Additional Information

Diamagnetic materials

• Weak, negative susceptibility to magnetic fields
• Diamagnetic materials are slightly repelled by a magnetic field.
• All the electrons are paired so there is no permanent net magnetic moment per atom.
• Most elements in the periodic table, including copper, silver, and gold, are diamagnetic.

Concept:

Crystal field theory:

• A metal ion in a complex is surrounded by coordinating ligands anions or negative ends.
• An electric field is produced at the metal ion by the surrounding ligands.
• This electrostatic approach towards the interaction between metals and ligands is known as crystal field theory.
• CFT theory considers the ligands as point charges.
• There is no overlap between the ligand orbitals and metal ion orbitals.
• Ligands donate lone pair of electrons to the metal atoms and form coordinate complexes.

The geometry of the complex formed:

• Placement of the donor atoms at different stereochemical positions will perturb the metal ions to a different extent.
• This will result in the formation of different stereochemistries or structures.
• The stereochemistry also depends on the coordination number and the type of orbitals involved.

Explanation:

• Nickel (Ni) in the complex NiCl₂(PPh₃)2, Ni is in a +2 oxidation state. Hence the electronic configuration is [Ar]3d⁸.
• Out of eight 3d electrons, 6 are paired and two are unpaired.
• As chlorine is a weak field ligand, it is not able to pair the electrons in Nickel and thus d orbitals of Ni do not participate in hybridization.
• The orbitals participating are 4s and 4p.
• Electron pairs from Cl and PPh₃ are donated into one 4s and three 4p empty orbitals of Nickel giving us sp³ hybridization.

• The structure is tetrahedral.

Hence, the structure of NiCl₂(PPh₃)₂ is tetrahedral.

Additional Information

• Triphenylphosphine is a neutral ligand. We can categorize it as a strong field ligand.
• It is a common phosphine ligand.
• Phosphine ligands are mono-dentate ligands.