Chemistry XII Karachi Board Scientific Reasons

Give the scientific reasons of the following: (Answers in the end.)




1. Transition elements form complex compounds.

2. Nitric acid is a strong oxidizing agent.

3. Na+ ion is smaller than Na atom.

4. Sulphuric acid has higher boiling point and viscosity.

5. 4s orbital is filled prior to 3d but on ionization the 4s electrons are lost first.

6. Anhydrous CuSO4 is white while hydrous CuSO4 is blue.

7. The heat of hydration of Li+ ion is greater than Cs+ ion.

8. Diamond does not conduct electricity while graphite does.

9. Nascent hydrogen is more reactive than ordinary molecular hydrogen.

10. Elements of group IA and group IIA readily loses their valence electrons.

11. Transition elements are paramagnetic.

12. Melting and boiling point of IA group elements are very low.

13. Ligands are generally called Lewis base.

14. Bromine is displaced from its salts by chlorine.

15. (NH4)+ and H3O+ ions donot act as ligands though H2O and NH3 act as ligands.

16. The salts of alkaline earth metals are hydrated than alkali metal salts.

17. Zn(OH)2 is soluble in excess of NaOH solution.

18. Why do chromium (24) and copper (29) shows an electronic configuration which is out of order.

19. Assign reasons for formation of coloured ion in case of transition elements except zinc.

20. Ionization enthalpies of IIA group elements are higher than IA group elements.

21. Graphite is used as Lubricant.

22. Metallic character of Aluminium is greater than Boron.

23. Plastic sulphur is elastic.

24. Alkaline earth metals are harder than alkali metals.

25. Ordinary hydrogen called as molecular hydrogen.

26. Electropositively increases from top to bottom.

27. Transition elements show variable oxidation state.

28. Na+ ions are discharged at the cathode in preference to H+ in the manufacture NaOH in Castner Kelner cell.

29. Li+/Li couple has exceptionally high negative electrode potential.

Answers to Scientific Reasons

1.


Transition elements have small highly charged ions and vacant d-orbitals of suitable energy. These vacant d-orbitals form coordinate bond by accepting lone electron pairs from ligands. This is the reason why transition elements form coordination compounds.

2.


The oxidizing properties of nitric acid is due to the stability of its molecule and because nitrogen is present in its highest oxidation state, i.e. 5. The degree of oxidation depends upon the concentration of acid and nature of element.

3.


The number of protons in Na+ ion are greater than the number of electrons due to which the nuclear attraction increases. And Na+ has one shell less than Na atom, because the last shell consists of only one electron, which is not present in Na+ ion. These are the reason due to which Na+ ion has smaller radii than Na atom.

4.


Sulphuric acid has higher boiling point and viscosity due to the presence of hydrogen bonding which link the molecules in larger aggregates.

5.


According to Aufbau principle and n+l rule, the sequence of filling in atomic orbitals in dependent on the value of n + l.

Since,

n + l for 4s = 4 + 0 = 4

and,

n + l for 3d = 3 + 2 = 5s

Therefore, electron goes in the orbital having lowest energy i.e. 4s. In the same way, the electron in 4s orbital have less energy, therefore they are lost first during ionization.

6.


Hydrated CuSO4.5H2O contains 5 molecules of water of crystallization and water act as ligand. The lone pair of electron of water molecules influence the 3d orbital of Cu by splitting it into eg and t2g. Thus by absorbing visible light an electron can jump from lower energy set (t2g) to higher energy set (eg). In doing so some of the component wavelength of light is removed, so the remaining component wavelength of light reflected shows the blue colour. An hydrous CuSO4 is colourless due to the absence of water molecule.

7.


The atomic size of Lithium is much smaller than that of Cesium, therefore its charge density is more. Due to more charge density, the electrons are attracted with a greater force of attraction. This is the reason more amount of heat is liberated when one mole of Li+ ion is dissolved in water. This is the reason why heat of hydration decreases down the group.

8.


In diamond, each carbon atom is sp3 hyberidized and is strongly bonded to four other carbon atoms. It utilizes its four unpaired electrons in the formation of four covalent bonds. Since, it has no free electrons; it is a poor conductor of electricity.

Whereas, in graphite, each carbon is sp2 hybridized and covalently linked with three other carbon atoms to give basic hexagonal ring. These hexagonal rings are 3.35 A° away from each other and are held together by weak Vander Waal’s forces. The fourth electron of each carbon forms delocalized p bonds, which are spread uniformly. This is the reason graphite conducts electricity.

9.


Molecular hydrogen is composed of two atoms of hydrogen. They are held together by a covalent bond. The bond energy of this covalent bond is 104 kcal/mole. In order to be reactive, the molecular hydrogen needs an energy to over come the bonding energy i.e. greater than 104 kcal /mole. Whereas in case of atomic hydrogen no such condition is required. This is the reason why atomic hydrogen is more reactive.

10.


The elements in group IA and and IIA have only one or two electron in their valence shell. As we move from right to left in the periodic table, the nuclear charge decreases, which increases the atomic size and reduces the force on valence electron. This is the reason why elements of group IA and IIA readily lose their electrons.

11.


The paramagnetic property of an element depends upon the availability of free electrons. Many compounds of the transition elements are paramagnetic. This is because they have unpaired electrons in their d-orbitals, which becomes active in a magnetic field.

12.


The elements of group IA have large atomic radii and small nuclear charges. This is the reason why melting point, boiling points are lower than other elements in the periods.

13.


Ligands are atoms, molecules or ions that donate a lone pair of electron to metal and form coordinate bond with them. According to Lewis, bases are substances that donate a lone pair of electrons. This is the reason why ligands are called Lewis base.




14.


The electronegativity of chlorine is greater than that of bromine. Because it decreases down the group. Since, chlorine is more electronegative than bromine, it displaces bromine from its salts.

15.


In case of NH3, the central atom nitrogen contains a lone pair of electron, which it can donate to metal. Whereas incase of NH4+, the central atom nitragen has already donated its lone pair to hydrogen so it does not act as ligand. Similarly H2O act as ligand but H3O+ does not act as ligand.

16.


Hydration of cations depend upon the nuclear charge and ionic radii. Smaller the ionic radii, greater would be the nuclear charge and more the salt will show the tendency for hydration. Since, the alkali earth metals have a greater nuclear charge and a stronger electric field than the alkali metals, therefore, they are more hydrated than alkali salts.

17.


Zinc hydroxide I an emphotric substance, i.e. it shows both acidic and basic properties. Therefore, when Zn(OH)2 is dissolved in excess of NaOH solution it does not precipitate out but forms a complex ion and redissolves.

Zn(OH)2 + 2NaOH ® [Zn(OH)4]-2 + 2Na+

Tetra Hydroxozincate

18.


24Cr = 4s23d4 ® (4s13d5)

29Cu = 4s23d9 ® (4s13d10)

The half filled or completely filled orbitals are more stable than other wise filled orbitals. In case of Cromium the one electron of 4s orbital jumps into 3d orbital, as a result 4s and 3d orbitals are half filled and stabized. Similarly in copper one electron of 4s orbital jumps into 3d orbital, as a result 4s is half filled whereas 3d is completely filled.

Hence electronic configuration of Cromium (Cr) is 4s13d5 instead of 4s23d4 and configuration of copper (Cu) is 4s13d10 instead of 4s23d9.

19.


The formation of coloured ion in case of transition elements can be explained by Crystal Field Theory. According to the theory (C.F.T) the bonding between ligand and a metal ion is electrostatic. The ligands surrounding the metal ion create an electrostatic field around its d-orbital. This field splits five degenerated d-orbitals into two sets with different energies.

The energy difference between two sets (eg and t2g) of d orbital is equivalent to a wavelength in a visible region. Thus by absorbing visible light, an electron may be able to move from lower energy set (t2g) to higher energy set eg of d orbitals. In doing so some of the component wavelength of white light is removed, so the remaining component wavelength of light reflected or transmitted shows the colour of ion.

Hence the colour of the ion is due to the half filled 3d orbital, the electron can jump from d orbital of lower energy by the absorption of small amount of energy of a particular wavelength in a visible region. Since in case of zinc, the 3d orbital is completely filled therefore the compounds of Zinc are white or colourless.

20.


The ionization enthalpies IIA (alkaline earth elements) is higher than the corresponding ionization enthalpies of IA group (alkali metals0, because elements of IIA group has an extra nuclear proton which causes an increase in the electrostatic Beautiful force between the nucleus and the outer most electron.

21.


In Graphite, each carbon is Sp2 hyberidized and covalently linked with three other carbon atoms to give basic hexagonal ring. These hexagonal rings then form layers in graphite. These layers are 3.35 A° away from one another and held together by weak Vander Waal’s forces of attraction. Due to large inter planner distance (3.35A°), the layers slide easily over one another that is why it is soft and used as Lubricant.

22.


The maximum capacity of electron accomodation of boron in its outer most shell is eight electrons and that of Aluminium is eighteen electron.

5B = 1s2, 2s2, 2p1

13Al = 1s2, 2s2, 2p6, 3s2, 3p1

It means electron population of aluminium is less than boron. Due to less electron population, the number of neighbouring atoms in the lattice increases, that is why metallic character of aluminium is greater than boron.

23.


The plastic sulphur or g-sulphur is composed of long chains of sulphur atoms. The elasticity of plastic sulphur is due to uncoiling of long sulphur chains and then recoiling of chains by the release of tension.

24.


The alkaline earth metals (II group metals) are appreciably harder than alkali metals (I group) because the presence of divalent cations in their metallic structure produce greater bonding forces.

25.


Ordinary hydrogen exist as diatomic molecule (H2) therefore it is also known as molecular hydrogen.

26.


The tendency of atom to give out electrons is known as electropositivity. Electropositivity is inversely proportional to ionization potential and electron population. Both the factors decreases down the group, hence electropositivity increases from top to bottom.

27.


The transition elemenets show variable oxidation states in their compounds. This variation is due to the very small energy difference in between 3d and 4s orbitals. As a result, electrons of 3d as well as 4s takes part in the bond formation.

28.


In castner-kellner cell, H+ ions are not easily discharged due to high voltage of H+ ion, on the contrary Na+ ions are easily discharged over mercury surface.

29.


Li+/Li couple has exceptionally high negative electrode potential because of its large value for the hydration enthalpy which promotes oxidation of Li to form Li+ ion.