Unit 1bsc physics nEP semester 1 .pptx

BUnit 1bsc physics nEP semester 1 .pptx physics nep bsc

1. MOLECULAR POLARITY AND WEAK CHEMICAL FORCES

2. Formal Charge is defined as the excess or lack of electrons on an atom in the molecule in comparison to the free atom. These formal charges affect the chemical reactions of such species. Formal charge on an species is calculated from its Lewis structure as follows: Average electrons around an atom in a given species is calculated using relations: Average electrons= ½ (number of shared electrons)+ Number of non-bonded electrons Formal charge =valence electron-average electron Formal charge

3. Methane Average electron on carbon= ½ 8 +0 =4 Valence electron on carbon= 4 Formal charge = 0 Water Average electron on oxygen= ½ 4 +4 =6 Valence electron on oxygen= 6 Formal charge = 0 Ammonium ion Average electron on nitrogen= ½ 8 +0 =4 Valence electron on nitrogen= 5 Formal charge = 1

4. Polarization – The phenomenon by which the symmetrical shape of the anion gets deformed by approach of the cation is called polarization. Polarising power-The ability of the cation to polarize an anion is called polarizing power. Polarisability - The tendency of an anion to get polarized by a cation is called polarisabiility .

5. FAJAN’S RULE The cation should have high positive charge on it. The cation should be small in size The cation should have ns2 p6 d10 configuration in valence shell The anion should have high negative charge on it The anion should be large in size

6. IMPACT OF FAJAN’S RULE MELTING POINT: Melting point of compound α ionic character α 1 Covalent character EX. Melting point of lithium halide decreases from fluoride to iodide because of increasing size of halide ion and consequently covalent nature. 2. HARDNESS: The hardness of ionic compound decreases with increase in degree of polymerization. 3. SOLUBILITY: The solubility of ionic compound in polar solvents decreases with increase in covalent character i.e. polarization. EX. AgF is soluble in water, AgCl is soluble in hot water, Ag Br is partially soluble while AgI is insoluble in water.

7. Applications of Fajan’s rule Halides having cations with different positive charge on them Na+ < Mg2+ < Al3+ Halides having same cation with different positive charge SnCl2 – ionic, PbCl2 ionic SnCl4 – covalent melting point 246 ˚C PbCl2 covalent, MP - 15 ˚C 3. Halides with cations of different element but having same positive charge Among LiCl , NaCl , KCl , RbCl , CsCl ,: LiCl is covalent in nature with MP 613 ˚C because of small size of Li + and so more polarizing power giving covalent character to LiCl 4. Comparative study of halides of coinage metals with similar type alkyl halides CuCl , NaCl CuCl = Cu+ 2,8,18 NaCl = Na+ 2,8 covalent insoluble in water ionic soluble in water MP 430 ˚C MP 800 ˚C

8. POLARIZING POWER AND POLARIZABILITY OF IONS: FAJAN’S RULES With the help of this rule it is possible to know the conditions under which electrovalency passes into covalency. When two oppositely charged ions approach each other closely, the cation attracts the electron atmosphere of anion and simultaneously repels the anion nucleus. This results in the deformation, destortion or polarization of the anion. Similarly, the anion also polarizes the cation but the effect will be negligible or less pronounced because of smaller size of cation.

9. The net result of this ion polarization is that there occurs some sharing of electrons between the two ions, i.e. the bond becomes partly covalent in character. The polarization or deformation of ions depends upon a number of factors knows as Fajan’s rules.

10. Large charge on the ions Small size of cation Large size of anions A cation with non-noble gas configuration Nature of solvent SOME CASES BASED ON FAJAN’S RULES CuO or CuS LiCl or NaCl NaCl or AgCl AlCl 3 or AlF 3 SnCl 2 or SnCl 4

11. Difference between polar and non-polar covalent bonds S. No. Polar covalent bond Non-polar covalent bond 1. The bond is formed by mutual sharing of one or more electron pairs between two unlike atoms e.g., H- Cl The bond is formed by mutual sharing of one or more electron pairs between two like atoms e.g., Cl-Cl 2. The shared pair of electrons remain near to more electronegative atom. The shared pair of electrons remain in the middle of the two combining atoms. 3. The atoms involved in bond formation contain partial positive and negative charges H δ + - Cl δ - The atoms involved do not have any charges. Cl-Cl 4. The bond breaks in water to give ions. The bond does not break in the polar solvent like water.

12. QUESTION 1: H2O molecule has high dipole moment, explain. H2O molecule has high dipole moment due to its angular structure. It has been found that the dipole moment of each O-H bond is 1.60D and the bond angle is 104.5˚. The resultant dipole moment μ comes out to be 1.84 D form:

13. QUESTION 3:The dipole moment of NaCl molecule is 8.5 D and its bond length is 2.36 A˚. Calculate the percentage ionic character in this molecule. Dipole moment expected for complete ionic character Percentage ionic character

14. QUESTION 4:Find the percentage ionic character of H-F bond. (Dipole moment of H-F is 1.92 D, bond length H-F +0.92 A˚). Dipole moment of expected for complete ionic character Percentage ionic character

15. Q1. BeCl2 is more covalent than MgCl2, explain. BeCl2 will be more covalent than MgCl2. This is because in these compounds anion (Cl-) is the Same but Be2+ ion has smaller size than Mg2+ ion. Thus, Be2+ ion will polarize Cl- ion more than Mg2+ ion. Q2. Oxides are more covalent than fluorides, explain. Oxides are more covalent than fluorides. This is because the size of O 2- ion is greater than F- ion. The cation, therefore, polarize O2- ion more than F-ions. As a result oxides will be more covalent than fluorides.

16. Q3. Cu2Cl2 is more covalent than NaCl, explain. Cu2Cl2 is more covalent than NaCl . In these compounds, Cu+ ion has non-noble gas type of electronic configuration, whereas Na+ ion has noble gas type of electronic configuration. Cu+ = 2,8,18; Na+ = 2,8 The d electron of Cu+ ion screen the nuclear charge less effectively than s and p electron of Na+. As a result, Cu+ ion behaves as if it has a greater charge. On account of this. Cu+ ions will polarize Cl - ions more strongly than Na+ ions.

17. Q4. Lithium salts are slightly covalent whereas sodium salts are ionic, explain. Lithium salts are slightly covalent whereas sodium salts are ionic. This is because Li+ ion has smaller size than Na+ ion. Thus, Li+ ion will polarize anion more than Na+ ion. As a result, in the case of lithium salts, some sharing of electrons between two oppositely charged ions takes place i.e. the bond becomes partly covalent.

18. Q5. Covalent character decreases from BeCl2 to RaCl2, explain. The covalent character decreases from BeCl2 to RaCl2. This is because in all of these compounds, anion is the same and cationic size increases from Be2+ to Ra2+. Thus Be2+ ion will polarize the Cl - more than other cations of the ⅡA group. As a result, the covalent character decreases from BeCl2 to RaCl2. Q6. Which compound is more covalent and why? FeCl2 and FeCl3 FeCl3 will be more covalent than FeCl2. This is because in FeCl3, the Fe3+ ion has smaller size and greater charge than Fe2+ ion in FeCl2. As a result Fe3+ ion will polarize the Cl - ion more strongly than Fe2+ ion.

19. Q7. Which compound is more covalent and why? NaCl or KCl NaCl will be more covalent than KCl . This is because in these compounds, anion is the same but Na+ ion has smaller size than K+ ion. Thus, Na+ ion will polarize Cl - ion more than K+ ions. Q8. Arrange the following in the given order of their covalent character. NaCl , AlCl3, MgCl2 in the increasing order LiCl , LiBr , LiI in the increasing order AgF , AgBr , AgCl in the decreasing order Sol. NaCl < MgCl2 < AlCl3 LiCl < LiBr < LiI AgBr > AgCl > AgF

20. TYPES OF HYDROGEN BONDING INTERMOLECULAR HYDROGEN BONDING: When H bond is formed between two molecules of a compounds. 2. INTRAMOLECULAR HYDROGEN BONDING: When H bond is formed within the same molecule

21. Q1. H2O is a liquid but H2S is a gas explain? Water is a liquid whereas H2S is a gas. This is due to the fact that on account of high electronegativity and small atomic size, oxygen enters into H bond formation very readily. Thus, the hydride of oxygen i.e. H2O forms an associated molecule due to hydrogen bonding and is therefore is a liquid. On the other hand, S on account of low electronegativity, has a very little tendency of H bonding, Thus, the hydride of S does not form an associated molecule and is therefore is a gas.

22. Q2. Show that which types of bonding occurs in salicylic acid and in ammonia. Q3. The density of solid ice is less than that of liquid water . In ice, the H-bonding between water molecule is more extensive than in liquid water. In ice, there is tetrahedral arrangement of water molecule with respect to one another as shown in the following figure:

23. Q4. Why do NH3, H2O and HF have abnormally high boiling points? NH3, H2O and HF have abnormally high boiling points. This is because N, O and F are highly electronegative and have small atomic sizes. Hence, they form associated molecules easily due to H bonding and a sufficient amount of energy is required for breaking these H bonds.

24. Lattice Energy When large number of cations and anions are brought nearer to each other to form an ionic lattice, ions arrange themselves in a regular pattern to attain a closed packed type structure. Energy is released and system gets stabilized and ionic crystal is formed. Thus the lattice energy (U) of an ionic crystal is defined as the amount of energy released when cations and anions in gaseous state are brought from infinite distance to their respective lattice sites in crystal to form one mole of an ionic crystal. Lattice energy is represented as U/ H lat and is denoted with a negative sign.

25. Thus the formation of one mole of ionic crystal AB from its ions A+ and B-, as follows- A+ (g) + B- (g) -U A+ B- (s) Greater the magnitude of the lattice energy, greater would be stability of ionic crystal. A+ B- (s) +U A+ (g) + B- (g) Lattice energies of ionic crystals are quite high due to strong electrostatic attraction between oppositely charged ions present in crystals.

26. FACTORS AFFECTING LATTICE ENERGY Charge on cations and anions Uni -univalent ionic crystal < Uni -bivalent ionic crystal < bi-bivalent ionic crystal 2. Inter ionic distance Force of attraction is inversely proportional to the interionic distance between the ions, lattice energy decrease with increase of size of ions i.e., inter-nuclear distance.

27. LATTICE ENERGY OF SULPHATE, CHLORIDES, NITRATES Size of bivalent cation of alkaline earth metals increase on moving down the group. Be2+ < Mg2+ < Ca2+ < Sr2+ < Ba2+ , lattice energy of their sulphate , chlorides, nitrates etc. should decrease in the same manner but their lattice energy have been found almost similar. The reason for this similarity in their lattice energy may be due to the fact that all these anions are very large in size.

28. LATTICE ENERGY AND PROPERTIES OF IONIC CRYSTALS Solubility of ionic solids in various solvents: When an ionic solid goes in solution, its crystal lattice gets ruptured and ions move apart in solution. The energy need for it comes from solvation energy which generates as a result of strong interaction between solvent and ions. If solvent is water, the process is said to be hydration of ions and energy released as hydration energy.

29. 2. Stability of ionic solids: Stability of ionic solid directly depends on amount of lattice energy. Stability of ionic solid α Lattice energy Thus ionic solids with high lattice energy will Have high MP and BP, high enthalpy of sublimation and high enthalpy of formation.