Easychemistry by Aisha Anjum

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Question: why CO2 is gas while SiO2 is solid even both are oxides of IV A group elements?

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Question:
What is Fajan's rule?

Answer:
Fajan's rule predict covalent nature of compounds.
According to this
" Compounds with Smaller cations with high charge density ( relatively high electronegative character ) and bigger anion with more polarizability will have more covalent character."

For example:
Sn+4 ion is smaller than Sn+2, compounds of Sn+4 are more covalent in nature while those of Sn+2 are ionic.

Why Na2O is more basic than MgO?

Answer: All metal oxides are basic in nature because when they have oxide ions which get hydrogen ions (H+) from water give hydroxide ions OH-1.
Both sodium and magnesium oxides are basic but sodium oxide is more basic because it produces more oxide ions O-2. As in sodium oxide Na+ and O-2 held together by weak attraction between ions hence less energy is required to break the crystal lattice in Na2O. In contrast magnesium oxides crystal lattice required more energy to free the Mg+2 and Oxides O-2 ions of smaller sizes.

Note: Difference in strength lattice energy is due to ionic sizes and charge density. Smaller ionic sizes and more charge density give tightly packed crystal lattice hence more energy required to free ions( as on case of MgO)

Question:
BeO is amphoteric in nature? Why
Or
Why BeO is covalent in nature?

Answer:
In BeO, Be is smallest in it's group hence has highest positive charge density and high electronegativity values in the group. As it's electronegativity value Increses the difference in electronegativity in BeO decreases which make it covalent in nature rather ionic( other all alkaline metal oxides are ionic) . Covalent compounds are insoluble in water hence not able to produce Be(OH)2 hence not a base rate it is amphoteric it's reacts with base as well as with acid.so amphoteric in nature

Question: justify why AlCl3 is non conductor in both solid and molten state ( at high pressure) whereas NaCl is conductor in molten state.

NaCl is ionic crystalline solid at room temperature. To be conductor of electricity free electrons or free ions are needed . In solid state NaCl has not free ions hence non conductor. In molten state NaCl has free ions hence it is conductor of electricity.

AlCl3 is ionic solid at room temperature hence dur to absence of free ions AlCl3 is non conductor in solid state.
In molten state AlCl3 ( only at high pressure possible) is non conductor of electricity. It is due to the tricky nature ok AlCl3. On melting it changes it's ionic nature onto covalent nature. covalent substances are non conductor of electricity hence it is non conductor even in molten state.

Important questions from past papers
12th S and P block elements

1. Generally ionization energy Increases along the period but ionization energy of "Al" is less than Mg and "S" is less than "P"

2. Write the reactions of oxygen with Li, Na and K , also write the names of oxides.

3.justify why AlCl3 is non conductor in both solid and molten state ( under high pressure) whereas NaCl is conductor in molten state.

4. Give reason for
(A) BeO is amphoteric (b) BeO is covalent in nature bus has high melting point.

5. Why thermal stability of carbonates of group II elements Increses down the group. Describe by examples.

6. Explain the trend of density in alkali metals.

7. Na2O is more basic than MgO give reason.

8. Upon moving left to right in 3rd period nature of oxides changes from basic to amphoteric why?

9. Discuss the trends in thermal stability of Carbonates and nitrates of group I elements.

10. Ionization energy of Pb is greater than tin , although it decrease down the group, why?

11. (a) Why PbCl4 is thermally unstable whereas PbCl2 is stable ?
(b) Why CCl4 doesn't undergo hydrolysis?

12. CO2 is gas whereas SiO2 is solid explain with help of structures?

13. How tetrahalides of group VI elements react with H2O? Write down the mechanism of this reaction. Why this reaction is not shown by the CCl4 under normal conditions.

14. Electron affinity of fluorine is less than that of other helogens why?

B) fluorine has least bond enthalpy among halogens Why?

15. Why are the tetrahalides of C not hydrolyzed while those of So, Ge, and Sn readily hydrolysed.

16. Define inert pair effect . How does it explain the stability of +2 and +4 oxidation state in elements of group IV A?

17. BeO and Al2O3 are amphoteric oxides give their chemical equation with both acid and base.

18. What is Fajan's rule give example to explain?

A double is not twice as stronger as single bond or triple bond is not thrice as strong. As a single bond why?

During formation of double and triple bonds, first bond is sigma next all are pi bonds. Sigma bond is stronger than pi bond that's why double bond is not as strong as twice of single bond and triple bond is not as strong as thrice of single bond.

Question:
Bond energies decreases as bond length increases, in Halogens
Bond energies are
F2= 15KJ/mol
Cl2 = 242kj/mol
Br2= 193KJ/mol
I2= 151KJ/mol

Why F2 is deviating the normal trend of decreasing bond energies?

From chlorine to iodine halogens are showing normal trend in decrease of bond energies as bond lengths are increasing down the group. Keep in mind from top to bottom in a group atomic size Increses hence bond length increases, as bond lengths Increses bond energies decreases. But here Florine is showing less bond energy ( abnormal in trend) this is due to Small size of Florine which cause interelectronic repulsion which decrease the bond energy of Florine molecule.

Question:
Give reason that why the structure of BF3 is trigonal planar and NH3 is trigonal pyramidal.

Answer:
According to VSEPR theory, BF3 is AB3 type molecule in which central atom boron(B) has 3 electron pairs and all electron pairs are bonded pairs ,no lone pair on Centra atom, hence arrangement of electron pairs is trigonal planar. And molecular geometry is trigonal planar. Like the figure below.

Ammonia (NH3 ) belongs to AB4 type molecule which usually attain tetrahedral geometry but here in ammonia, central atom( nitrogen ) has a total 4 electron pairs, 3 are bonded pairs and 1 is the lone pair. Due to the presence of the lone pair at one corner of tetrahedron ammonia has trigonal pyramidal geometry .

Question:

Bond Angle in NF3 ( 102) is less than NH3 (107.5). even both are AB4 type with nitrogen as central atom? Justify

Answer:

In NF3, The strong polarity of the N-F bond pulls the lone pair of N atom closer to its nucleus which in turn exerts stronger repulsion over the bonding electrons. Thus the angle further shrinks to 102. Moreover the bond pairs in N-F bonds are more close to F atoms than N atoms. The increased distance in these bond pairs make their repulsion less operative.

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