Class XII Lanthanoids & Actinoids

Class XII Lanthanoids & Actinoids

Sulekha Rani.R,P.G.T Chemistry

KV NTPC Kayamkulam

1. Actinoid contraction is greater from element to element than lanthanoid contraction. Why?

In actinoids, 5f orbitals are filled. These 5forbitals have a poorer shielding effect than 4f orbitals (in lanthanoids). Thus, the effective nuclear charge experienced by electrons in valence shells in case of actinoids is much more that that experienced by lanthanoids. Hence, the size contaction in actinoids is greater as compared to that in lanthanoids.

2. What is lanthanoid contraction? What are the consequences of lanthanoid

contraction?

As we move along the lanthanoid series, the atomic number increases

gradually by one. This means that the number of electrons and protons

present in an atom also increases by one. As electrons are being added to the

same shell, the effective nuclear charge increases. This happens because

the increase in nuclear attraction due to the addition of proton is more

pronounced than the increase in the interelectronic repulsions due to the

addition of electron. Also, with the increase in atomic number, the number of

electrons in the 4f orbital also increases. The 4f electrons have poor shielding

effect. Therefore, the effective nuclear charge experienced by the outer electrons

increases. Consequently, the attraction of the nucleus for the outermost electrons

increases. This results in a steady decrease in the size of lanthanoids with the

increase in the atomic number. This is termed as lanthanoid contraction.

Consequences of lanthanoid contraction

(i) There is similarity in the properties of second and third transition series.

(ii) Separation of lanthanoids not  possible due to lanthanide contraction.

(iii) It is due to lanthanide contraction that there is variation in the basic strength of lanthanide hydroxides. (Basic strength decreases from La(OH)₃ to Lu(OH)₃.)

3. What are the different oxidation states exhibited by the lanthanoids?

In the lanthanide series, +3 oxidation state is most common i.e., Ln(III)

compounds are predominant. However, +2 and +4 oxidation states can

also be found in the solution or in solid compounds.

4. What are alloys? Name an important alloy which contains some of the lanthanoid metals. Mention its uses.

An alloy is a solid solution of two or more elements in a metallic matrix. It can

either be a partial solid solution or a complete solid solution. Alloys are usually

found to possess different physical properties than those of the component

elements.

An important alloy of lanthanoids is Mischmetal. It contains lanthanoids

(94−95%), iron (5%), and traces of S, C, Si, Ca, and Al.

Uses

(1) Mischmetal is used in cigarettes and gas lighters.

(2) It is used in flame throwing tanks.

(3) It is used in tracer bullets and shells.

5. Compare the chemistry of actinoids with that of the lanthanoids with special reference to:

(i) electronic configuration (iii) oxidation state

(ii) atomic and ionic sizes and (iv) chemical reactivity.

(i) Electronic configuration

The general electronic configuration for lanthanoids is [Xe]⁵⁴ 4f^0-14 5d^0-1 6s² and that for

actinoids is [Rn]⁸⁶ 5f^1-14 6d^0-1 7s². Unlike 4f orbitals, 5f orbitals are not deeply buried and

participate in bonding to a greater extent.

(ii) Oxidation states

The principal oxidation state of lanthanoids is (+3). However, sometimes we also

encounter oxidation states of + 2 and + 4. This is because of extra stability of fully-filled

and half-filled orbitals. Actinoids exhibit a greater range of oxidation states. This is

because the 5f, 6d, and 7s levels are of comparable energies. Again, (+3) is the principal

oxidation state for actinoids. Actinoids such as lanthanoids have more compounds in +3

state than in +4 state.

(iii) Atomic and lonic sizes

Similar to lanthanoids, actinoids also exhibit actinoid contraction (overall decrease in

atomic and ionic radii). The contraction is greater due to the poor shielding effect of

5f orbitals.

(iv) Chemical reactivity

In the lanthanide series, the earlier members of the series are more reactive. They have

reactivity that is comparable to Ca. With an increase in the atomic number, the

lanthanides start behaving similar to Al. Actinoids, on the other hand, are highly

reactive metals, especially when they are finely divided. When they are added to boiling

water, they give a mixture of oxide and hydride. Actinoids combine with most of the

non-metals at moderate temperatures. Alkalies have no action on these actinoids. In case

of acids, they are slightly affected by nitric acid (because of the formation of a protective

oxide layer).

6. What are inner transition elements? Decide which of the following atomic numbers are the atomic numbers of the inner transition elements: 29, 59, 74, 95, 102, 104?

Inner transition metals are those elements in which the last electron enters the f-orbital. The elements in which the 4f and the 5f orbitals are progressively filled are called f-block elements. Among the given atomic numbers, the atomic numbers of the inner transition elements are 59, 95, and 102.

7. The chemistry of the actinoid elements is not so smooth as that of the Lanthanoids. Justify this statement by giving some examples from the oxidation state of these elements?

Lanthanoids primarily show three oxidation states (+2, +3, +4). Among these oxidation states, +3 state is the most common. Lanthanoids display a limited number of oxidation states because the energy difference between 4f, 5d, and 6s orbitals is quite large. On the other hand, the energy difference between 5f, 6d, and 7s orbitals is very less. Hence, actinoids display a large number of oxidation states. For example, uranium and plutonium display +3, +4, +5, and +6 oxidation states while neptunium displays +3, +4, +5, and +7. The most common oxidation state in case of actinoids is also +3.

8. Which is the last element in the series of the actinoids? Write the electronic configuration of this element. Comment on the possible oxidation state of this element?

The last element in the actinoid series is lawrencium, Lr. Its atomic number is 103 and its electronic configuration is. The most common oxidation state displayed by it is +3; because after losing 3 electrons it attains stable f¹⁴ configuration.

11. Use Hund’s rule to derive the electronic configuration of Ce³⁺ ion and calculate its magnetic moment on the basis of ‘spin-only’ formula?

Magnetic moment can be calculated as:

Where, n = number of unpaired electrons

In Ce, n = 2

12. Name the members of the lanthanoid series which exhibit +4 oxidation state and those which exhibit +2 oxidation state. Try to correlate this type of behavior with the electronic configurations of these elements?

The lanthanides that exhibit +2 and +4 states are shown in the given table. The atomic numbers of the elements are given in the parenthesis.

+2	+4
Nd (60)	Ce (58)
Sm (62)	Pr (59)
Eu (63)	Nd (60)
Tm (69)	Tb (65)
Yb (70)	Dy (66)

Ce after forming Ce⁴⁺ attains a stable electronic configuration of [Xe].

Tb after forming Tb⁴⁺ attains a stable electronic configuration of [Xe] 4f^7.

Eu after forming Eu²⁺ attains a stable electronic configuration of [Xe] 4f^7.

Yb after forming Yb²⁺ attains a stable electronic configuration of [Xe] 4f^14.

13.

Chemistry model paper 2

Chemistry model paper 2 (class XII)

Sulekha Rani.R,P.G.T Chemistry,KV NTPC Kayamkulam

1. What is the coordination number of each type of ions in a rock-salt type crystal structure?

2. Define the term ‘order of reaction’ for chemical reactions.

3. What causes Brownian movement in a colloidal solution?

4. In which one of the two structures, NO₂ ⁺ and NO2 the bond angle has a higher value?

5. Write the IUPAC name of the following compound:

.

6. Arrange the following compounds in an increasing order of their acid strengths:

(CH3)2 CHCOOH, CH3 CH2 CH (Br) COOH, CH3 CH (Br) CH2 COOH

7. Write a chemical reaction in which the iodide ion replaces the diazonium group

8. Name a substance that can be used as an antiseptic as well as a disinfectant

9. Explain as to why haloarenes are much less reactive than haloalkanes towards nucleophilic

substitution reactions.

OR

Which compound in each of the following pairs will react faster in SN2 reaction with –

OH? Why?

(i) CH3 Br or CH3 I

(ii) (CH3)3 CCl or CH3Cl

10. (a) State the IUPAC name of the following compound:

(b) Complete the following chemical equations:

CH3 CH2 CH=CH2 + HBr Peroxide ……….

11. State Henry’s law correlating the pressure of a gas and its solubility in a solvent and

Mention two applications for the law.

12. A first order decomposition reaction takes 40 minutes for 30% decomposition. Calculate

its t ½ value.

13. What is meant by the ‘rate constant, k’ of a reaction? If the concentration be expressed in

Mol L-1 units and time in seconds, what would be the units for k(i) for a zero order

reaction and (ii) for a first order reaction?

14. Define the following terms in relation to proteins:

(i) Peptide linkage

(ii) Denaturation

15. List the reactions of glucose which cannot be explained by its open chain structure.

16. Assign a reason for each of the following statements:

(i) Ammonia is a stronger base than phosphine.

(ii) Sulphur in vapour state exhibits a paramagnetic behavior.

17. Draw the structures of the following molecules:

(i) SF4

(ii) XeF4

18. What are biodegradable and non-biodegradable detergents? Give one example of each

class.

19. What is semiconductor? Describe the two main types of semiconductors and explain

mechanisms for their conduction.

20. Calculate the temperature at which a solution 54g of glucose, (C6H12O6), in 250g of water

will freeze.

(Kf for water = 1.86 k mol-1kg)

21. What are lyophilic and lyophobic sols? Give one example of each type. Which one of

These two types of sols is easily coagulated and why?

22. State briefly the principles which serve as basis for the following operations in

metallurgy:

(i) Froth floatation process

(ii) Zone refining

(iii) Reining by liquation

23. Write chemical equations for the following processes:

(i) Chlorine reacts with a hot concentrated solution of sodium hydroxide

(ii) Orthophosphorous acid is heated

(iii) PtF6 and xenon are mixed together

OR

Complete the following chemical equations:

(i) Ca3P2(s) + H2O (1)_ …….

(ii) Cu2+ (aq) + NH2 (aq) _……..

(iii) F2 (g) + H2O (1) _…….

24. (a) What is a ligand? Give an example of a bidentate ligand.

(b) Explain as to how the two complexes of nickel, [Ni (CN) 4]2- and Ni (CO)4, have

different structures but do not differ in their magnetic behavior. (Ni = 28)

25. Name the reagents which are used in the following conversions:

(i) A primary alcohol to an aldehyde

(ii) Butan-2-one to butan-2-ol

(iii) Phenol to 2, 4, 6-tribtomophenol

26. Account for the following observations:

(i) pKb for aniline is more than that for methylamine.

(ii) Methylamine solution in water reacts with ferric chloride solution to give a

precipitate of ferric hydroxide.

(iii) Aniline does not undergo Friedel-Crafts reaction.

27. Write the names and structures of the monomers of the following polymers:

(i) Buna-S

(ii) Neoprene

(iii) Nylon-6

28. Conductivity of 0.00241M acetic acid solution is 7.896´ 10-5 Scm-1. Calculate its molar

conductivity in this solution. If 0 m Ù for acetic acid be 390.5 S cm2 mol-1, what

would be its dissociation constant?

OR

Three electrolytic cells A, B and C containing solutions of zinc sulphate, silver nitrate and

copper sulphate, respectively are connected in series. A steady current of 1.5 ampere was

passed through them until 1.45g of silver were deposited at the cathode of cell B. How

long did the current flow? What mass of copper and what mass of zinc were deposited

in the concerned cells? (Atomic masses of Ag = 108, Zn = 65.4, Cu = 63.5)

29. Assign reasons for the following:

(i) The enthalpies of atomisation of transition elements are high.

(ii) The transitions metals and many of their compounds act as good catalyst.

(iii) From element to element the actinoid contraction is greater than the lanthanoid

contraction.

OR

(a) What may be the possible oxidation states of the transition metals with the following d

electronic configuration in the ground state of their atoms:

3d3 4s2, 3d54s2 and 3d64s2. Indicate relative stability of oxidation states in each

case.

(b) Write steps involved in the preparation of (i) Na2 CrO4 from chromite ore and

(ii) K2MnO4 from pyrolusite ore.

30. (a)

(b) Describe the following reactions:

(i) Cannizaro reaction

(ii) Cross aldol condensation

Iii) Etard reaction IV) Stephen reaction V) wurts reaction

OR

(a) How would you account for the following:

(i) Aldehydes are more reactive than ketones towards nucleophiles.

(ii) The boiling points of aldehydes and ketones are lower than of the corresponding acids.

(iii) The aldehydes and ketones undergo a number of addition reactions.

(b) Give chemical tests to distinguish between:

(i) Acetaldehyde and benzldehyde

(ii) Propanone and propanol