d-Block Elements

The elements whose last electron goes into any one of the five d-orbitals of their respective penultimate shell are called d-block elements. They are also known as transition elements. They are divided into ten groups and four series.

Q:- What are transition elements?

Ans:- Those elements which have incomplete d-subshells in their ground state or any of their oxidation states.

Q:- What is the cause of exceptional electronic configuration of transition elements?

Ans:-

1)symmetrical distribution of electrons

2) Exchange energy

3) Electronic-Electronic interactions.

4) Electronic-nuclear interactions.

Q:- What are transition elements?

Ans:- Those elements which have incomplete d-subshells either in elemental form or in combined form are called transition elements. E.g. Zn, Cd and Hg are not considered transition elements. It is because atoms and ions of these elements do not have incomplete d-subshell.

Q:- why are Zn, Cd and Hg not considered transition elements ?

Ans:- These elements do not fulfill the condition of definition of a transition element.It is because atoms and ions of these elements do not have incomplete d-subshell. Let us consider the electronic configuration of Zn and Zn ion. Write their electronic configurations to show their completely filled d-subshells.

Q:- Write their General Electronic configuration.

Ans:-[Noble gas] (n – 1) d1-10 ns0-2

EXCEPTIONAL ELECTRONIC CONFIGURATION

The actual electronic configurations of some elements like chromium and copper are different from their expected configurations. They have stable half-filled and completely filled configurations. Their actual configurations are :

Cause of Exceptional Electronic Configuration

The extra stability of these exceptional configurations is due to the following two reasons:-

1) SYMMETRY

The half-filled or completely filled configurations are more symmetrical and symmetry leads to greater stability

2) EXCHANGE ENERGY

The energy released when electrons present in the degenerate orbitals exchange their positions is known as exchange energy. Greater the number of exchanges more will be the exchange energy and hence more stable will be the configuration. Now in half filled and completely filled configurations maximum number of exchanges take place. Therefore such configurations are extra stable in nature.

1) BOHR-BURY RULE

This rule is also known as (n + l) rule.This rule can be used to calculate increasing order of energy of orbitals. This rule states that lower the value of (n + l) for an orbital the lower will be its value. If two orbitals have same value of (n + l) then the orbital with lower value of n will have lower energy and is to be filled first.

Q:- What is the cause of exceptional electronic configuration of transition elements?

Ans:-1) symmetrical distribution of electrons

2) Exchange energy

3) Electronic-Electronic interactions.

4) Electronic-nuclear interactions.

METALLIC CHARACTER

Q:- Explain the metallic character of transition elements ?

1.Metallic lustre

2.Ductile

3.Malleable

4.Good conductor of heat and electricity.

5.High density

6.High melting and boiling points

7.High tensile strength

8.Form colored compounds

9.Form complexes, alloys and interstitial compounds.

10.Show variable oxidation states.

11.Act as catalysts

ATOMIC RADIUS

The distance from the center of the nucleus to the outermost shell containing electrons.

In a particular transition series with the increase in atomic number the atomic radius first decreases till the middle and becomes almost constant and then slightly increases towards the end of the series.

REASON

In a transition series the electrons enter into penultimate d-subshell which screen the valence shell electrons from the nucleus.

IONIC RADIUS

It is the effective distance from the centre of nucleus of an ion upto which it exerts an influence on its electron cloud.

Ionic radius is smaller than atomic radius. It decreases along the series .

Reason

along the series with the increase in atomic number the effective nuclear charge increases thereby decreasing the radius of cation .

Q :- why do Zr and Hf exhibit almost similar properties ?

Ans :-The phenomenon of lanthanide contraction is the reason behind their similar properties. for example their atomic sizes are almost similar and they exhibit similar properties.

OXIDATION NUMBER OR STATE

1.It is a charge present on an ion .

2.It is the charge which an atom has on its ion.

3.Or it is the charge which an atom has in the combined state.

4.An atom can have positive , zero or negative oxidation number. E.g. Mn ,Fe, Co etc.

Transition metals exhibit variable oxidation states

Q:- Explain the variable oxidation states of transition elements ? Or Q:- why do transition elements exhibit variable oxidation states ?

Ans :-variable oxidation states mean their atoms can lose different number of electrons due to :

1) participation of electrons of inner d-orbitals and outer ns-orbitals.

2) nearly similar energy of d-subshell and next higher s-subshell. E.g. Mn and Ru show oxidation states from +2 to +7.

Q:- the higher oxidation states are usually exhibited by the members present in the middle of the series of transition elements ?

Ans :- because these elements have maximum number of unpaired d-electrons.

Q:- what is the maximum oxidation state shown by d-block elements ?

Ans :- The highest oxidation state is the sum of inner (n-1) d electrons and outer ns-electrons .the highest oxidation state of +8 is shown by Os and Ru . E.g. Osmium exits in +8 oxidation state in OsO4

DISPROPORTIONATION REACTIONS

The reaction in which an element undergoes oxidation and reduction simultaneously are called disproportionation reactions. That element exits in three different oxidation states.E.g. hydrogen peroxide is very unstable and undergoes disproportionation.

2H2O2 → 2H2O + O2

𝐶𝑢2𝐶𝑙2 → 𝐶𝑢𝐶𝑙2 + Cu

MAGNETIC PROPERTIES

On the basis of magnetic behavior the transition metal compounds are divided into two parts

PARA MAGNETIC SUBSTANCES

The substances which are weakly attracted by the magnetic field are called paramagnetic substances and the phenomenon is known as paramagnetism. Paramagnetism occurs due to the presence of unpaired electrons in the d-subshell of transition metal ions . E.g. Cu2+,Fe3+,Cr3+ etc.

DIAMAGNETIC SUBSTANCES

The substances which are weakly repelled by the magnetic field are called diamagnetic substances and the phenomenon is known as diamagnetism. Diamagnetism occurs due to the presence of paired electrons in the d-subshell of transition metal ions. E.g. Sc3+ , Zn2+, Cd2+ ,Cu+

Q:-why do transition metals form colored compounds?

Or Q:- why are the compounds of transition metals generally colored ?

Ans:-The formation of colored compounds by the transition metals can be explained on the basis of crystal field splitting theory. According to this theory, under the influence of approaching ions, partially filled five degenerate d-orbitals of transition metal ion split into two sets. This phenomenon is called CFS.

1. One set of triply degenerate d-orbitals with lower energy called t2g orbitals.
2. Second set of Doubly degenerate d-orbitals with higher energy called eg orbitals.

Due to the small energy difference between these two sets of orbitals ,the electrons present in the lower set of orbitals can absorb the radiations from the visible region of electromagnetic spectrum and jump to the higher set of orbitals. This is called d-d transition. Due to the absorption of certain radiations from the visible region of spectrum, the complementary color is reflected or transmitted. The color of a solid complex or its solution is due to the reflected or transmitted colors.

E.g. The solution of [Ti(H2O)6]3+ is purple in color. This is due to the reason that Ti3+ has one electron in d-sub shell

Ti :- [Ar] 3d24s2

Ti3+ :-[Ar] 3d14s0

In the ground state this electron is present in one of the lower set of orbitals(t2g). When this electron absorbs radiations of green and yellow wavelengths from the visible region it gets excited to one of the higher set of orbitals (eg).This is known as d-d transition.

The reflected or transmitted colors are blue and red which are complementary to the absorbed colors that is green and yellow. The solution of the complex [Ti(H2O)6]3+ therefore, looks purple.

IONISATION ENERGY

The energy required to remove an electron from a neutral isolated gaseous atom or a cation is called ionization energy. Its units are KJ/mol. The process of removing an electron from an atom is always endothermic in nature. It means energy is supplied from surrounding to the system. It is represented by prefixing positive sign with enthalpy change(∆𝑯).

CATALYTIC PROPERTIES

Many transition metals and their compounds are used as catalysts due to the following reasons :-

REASON :-

1) They have vacant or incompletely filled d-orbitals for the exchange of electrons with reactants to form reaction intermediates.

2)Ability to show variable oxidation state.

3)Ability to provide alternate reaction path with lower activation energy via the formation of intermediate compound.

4)Ability to provide large surface area for adsorption

Let us consider two examples of homogenous and heterogeneous transition metal catalysts.

1. the reaction between persulphate ion and iodide ion in liquid state is very slow because both the ions are negatively charged. But in the presence of ferric ions as homogeneous catalyst this reactions becomes fast.

Partial equation method to understand the complete reaction

2) Ability to adsorb reactants and variable oxidation state

The ability of transition metals to provide large surface area for the adsorption of reactants makes them effective heterogeneous catalysts. For example : vanadium petaoxide is used as a heterogeneous catalyst in the manufacture of sulphuric acid by contact process.Here it adsorbs sulphur dioxide molecules on its surface . Then vanadium undergoes change in oxidation state from +5 to +4 due to its tendency to show variable oxidation states.For example : vanadium petaoxide is used as a heterogeneous catalyst in the manufacture of sulphuric acid by contact process. Here it adsorbs sulphur dioxide molecules on its surface . Then vanadium undergoes change in oxidation state from +5 to +4 due to its tendency to show variable oxidation states.

In this reaction the catalyst vanadium pentaoxide reacts with sulphar dioxide and forms an intermediate compound 𝑉2 𝑂4 and 𝑆𝑂3. The intermediate compound 𝑉2𝑂4 provides a new reaction path with lower activation energy.

Q:-The first ionization energies of elements of first transition series do not vary much with increase in atomic number ?

Ans :-due to shielding effect of d-electrons on the nucleus, effective nuclear charge changes very little and hence atomic sizes do not vary much.

Nature of Oxides

With the increase in oxidation number of transition metals acidic character of oxides increases With the increase in oxidation number of transition metals the covalent character of transition metal oxides increases.

Q:- Give one example each of basic, amphoteric and acidic oxides of transition metals.

Ans :-

Amphoteric oxide :𝑀𝑛𝑂2

Acidic oxide :- 𝑀𝑛2 𝑂7

Q:- Give one example each of ionic and covalent oxides of transition metals.

Ans :-

ionic oxide :MnO

covalent oxide :- 𝑀𝑛2 𝑂7

POSTASSIUM DICHROMATE

Q: -write the name of the ore form which potassium dichromate is prepared ?

Ans :- Iron chromite ore (Fe𝐶𝑟2 𝑂4)

EFFECT OF INCREASING pH

The aqueous solution of potassium dichromate has pH around 4. At this pH both dichromate and chromate ions exist in equilibrium. By increasing pH orange color of solution will turn yellow due to the formation of chromate ions. Hence by changing the pH both the ions can be interconverted

Q:- color of potassium dichromate solution changes with pH of solution why ?

Ans :- The aqueous solution of potassium dichromate has orange color. Its pH is about 4. At this pH both dichromate ion and chromate ion exist in equilibrium with each other.

𝐶𝑟2 𝑂72- + 𝐻_2O ⇌ 2𝐶𝑟𝑂42- + 2𝐻+

(orange) (yellow)

However they are interconvertible by changing pH of the solution.

By increasing the pH that is in basic medium the equilibrium will shift in the forward direction producing yellow colored solution of chromate ions.

𝐶𝑟2𝑂72- + 2𝑂𝐻^− → 2𝐶𝑟42- + 𝐻2O

(yellow)

On the other hand by decreasing the pH that is in acidic medium the equilibrium will shift in the backward direction producing orange colored solution of dichromate ions.

2𝐶𝑟𝑂42- + 2𝐻+ → 𝐶𝑟2𝑂72− + 𝐻2O

(orange)

Hence color of pot dichromate sol depends on pH of the solution.

IMPORTANT REACTIONS OF K2Cr2O7 AS OXIDIZING AGENT (BOTH MLECULAR AND IONIC)

Q:- Explain oxidizing nature of pot dichromate in acidic medium with the help of examples ?

Ans :-pot dichromate is a powerful oxidizing agent in acidic medium . In the presence of dilute sulphuric acid it gives three atoms of nascent oxygen. which helps in the oxidation of several compounds:

Q:- Write balanced molecular and ionic equations for what happens when pot dichromate reacts with dilute sulphuric acid sol ?

1. OXIDATION OF KI
2. OXIDATION OF FeSO4
3. OXIDATION OF H2S
4. OXIDATION OF C2H5OH
5. OXIDATION OF 𝐻𝐵𝑟
6. OXIDATION OF Na2CO3

Ionic reaction with KI

Q:- Find out the equivalent weight of potassium dichromate in acidic medium ?

USES OF K2Cr2O7

1) It is used as a laboratory reagent.

2) for the preparation of dyes in industries

3) in organic chemistry as an oxidizing agent for the preparation of azo dyes.

4) it is used as strong oxidizing agent in many chemical reactions.

POTASSIUM PERMANGANATE(𝐾𝑀𝑛𝑂4)

Q:- Name the ore form which potassium permanganate is prepared ?

Ans :-Pyrolusite - 𝑴𝒏𝑶𝟐

PREPARATION OF KMnO4

It is prepared from Pyrolusite ore.Its preparation involves following steps :

1) Conversion of Pyrolusite ore into pot Manganate : In this step powdered ore is heated with potassium hydroxide in the presence of air to form potassium manganate :-

2𝑀𝑛𝑂2+ 4KOH + 𝑂2 → 2𝐾2𝑀𝑛𝑂4 + 2𝐻2O

2) OXIDATIO OF POTASSIUM MANGANATE TO POTASSIUM PERMANGANATE :- Two methods :

(a) In this step 𝐾2𝑀𝑛𝑂4 reacts with chlorine gas to form potassium permanganate :

2𝐾2𝑀𝑛𝑂4 + 𝐶𝑙2 → 2𝐾𝑀𝑛𝑂4 + 2KCl

(b) In this method potassium manganate obtained in first step is made to electrolyzed .: The solution contains following ions :-

𝐾2𝑀𝑛𝑂4 ⇌2𝐾+ + 𝑀𝑛𝑂42-

𝐻2O ⇌ 𝐻^+ + 𝑂𝐻^−

(b) In this method potassium manganate obtained in first step is made to electrolyzed .The solution contains following ions :-

𝐾2𝑀𝑛4 ⇌ 2𝐾^+ + 𝑀𝑛𝑂42-

𝐻2O ⇌ 𝐻^+ + 𝑂𝐻^−

At anode :- 𝑀𝑛𝑂42- → 𝑀𝑛𝑂〗4- + e-

At cathode : - 𝐻^+ + e- → H

2H → 𝑯𝟐

Hence during electrolysis manganate ions are oxidized to permanganate ions at anode and hydrogen gas is evolved at cathode. The solution is filtered and evaporated to obtain dark purple crystals of pot permanganate.

Physical Properties

1. It exits as deep purple crystals.
2. It exits as solid at room temperature.
3. Moderately soluble in cold water and freely soluble in hot water.
4. It dissolves in water to form ionic solution

CHEMICAL PROPERTIES

Q:- Explain oxidizing nature of pot permanganate with the help of examples ?

Ans :-pot permanganate is a powerful oxidizing agent in acidic medium . In the presence of dilute sulphuric acid it gives five atoms of nascent oxygen. which helps in the oxidation of several compounds:

Q:- Write balanced molecular and ionic equations for what happens when pot permanganate reacts with dilute sulphuric acid sol ?

IMPORTANT REACTIONS OF KMnO4 AS OXIDIZING AGENT (MOLECULAR AND IONIC)

1. OXIDATION OF OXALIC ACID
2. OXIDATION OF (KI)
3. OXIDATION OF FeSO4
4. OXIDATION OF H2S
5. OXIDATION OF Na2SO3
6. OXIDATION OF THIOSULPHATE
7. OXIDATION OFH2O2
8. OXIDATION OF ETHYL ALCOHOL

Q:- Write balanced molecular and ionic equations for what happens when acidified pot permanganate solution is treated with oxalic acid solution?

MOLECULAR EQUATION

IONIC EQUATION

Q:- Find out the equivalent weight of KMnO4 in acidic , basic and neutral medium ?

In Acidic medium

In Basic medium

In Neutral medium

Q :- how many carat is considered to be the purest form of gold ?

Ans :- 24 carat gold

Q:- what is the percentage of gold in 22 carat gold ?

Ans :- used proportional method(91%)

Q:- what are coinage metals ?

Ans :-Some metals were used to make coins in the ancient times . Those metals are called coinage metals. E.g. copper, gold, silver.