Chemical Bonding Flashcards
Lattice Enthalpy and its factors
- Energy released when one mole of ionic compound is formed by gaseous ions (-ve value) OR energy required to break one mole of ionic compound into its gaseous ions (+ve).
- Lattice enthalpy depends on charge and size. (between them charge dominates.)
- Size - smaller the size, more the lattice enthalpy - NaCl > KCl.
- More the charge, more the lattice enthalpy - MgO > CaCl2 > NaCl
Formal Charge in a Lewis Structure
[Valance electrons - Lone pairs of electrons] - no. of bonds
Inadequacies of the Octet Rule + Examples
- Fails to explain the stability of molecules having incomplete octet. ex: LiCl, BeH2, BeCl3, BeCl2, BF3, AlCl3 (central atom have incomplete octet)
- ” “ of expanded octet. ex: PCl5, SF6, IF7, H2SO4, ClF3 and co ordination compounds.
- ” “ of odd electron molecules ex: NO (nitric oxide) and NO2 (nitrogen dioxide).
- Based on inertness of noble gases but some noble gases like Xe and Kr form compounds like XeF2, KrF2 and XeOF2.
- Does not explain the shape of molecules.
- Does not explain the energy changes during bond formation.
True or False : Ionization is always endothermic.
True
Electrovalent bond will be easily formed between
Elements having low ionization enthalpy with elements having a high negative value of electron gain enthalpy.
Bond Enthalpy and it’s factors
Def: Amount of energy required to break one mole of bonds between two atoms in the gaseous state.
More the bond enthalpy, stronger is the bond.
Bond enthalpy depends on 1) size of atoms and 2) multiplicity of bonds.
1) smaller the size, stronger the bond.
2) multiplicity inc, bond enthalpy inc.
(Triple > double > single but triple is not 3* single)
In case of polyatomic molecules average bond enthalpy is used.
Bond length and it’s factors
Def: Equilibrium distance between the nuclei of two bonded atoms in a molecule.
1. Covalent radii - Half the distance between two similar atoms joined by a covalent bond.
2. Van der waal’s radii - Half the distance between two similar atoms in separate molecules in a solid.
Vanderwaals > Covalent
Factors 1) size 2) multiplicity
1) size - greater the size, more the bond length
2) multiplicity - greater the multiplicity, less the bond length - single > double > triple (pi bond decreases bond length)
Bond Order and its factors
No. of shared pair of electrons
ex: H2 is one, O2 is two and N2 is three.
For isoelectronic species - no. of electrons + bond order is the same. ex: O2 ^2- and F2 have the B.O 1 and electrons 18. N2, CO and NO+ have B.O 3 and electrons 14.
As bond order increases, bond enthalpy increases and bond length decreases.
Preference order for recognizing resonance stability (to solve which of these resonance structures is most stable type of q)
- More covalent bonds- more stable.
- No charge - more stable
- With charge - -ve charge on more electro-ve element
- Less separation of opposite charges
- Least stable - two similar charges on adjacent atoms
Partial Ionic Character of Covalent bond (Non Polar and Polar covalent bonds)
- Non polar covalent bond - covalent bond between two atoms of the same element - same electronegativities - electron cloud is equally distributed - pure covalent bond. eg: H2, Cl2
- Polar covalent bond - covalent bond between diff elements having diff electronegativities - more electronegative element pulls electron cloud towards itself - electron cloud gets distorted - more electro -ve gets partial -ve charge and less electronegative gets partial +ve charge. This leads to partial ionic character. eg: HF (F- partial -ve and H - partial +ve)
Also leads to dipole
More electro-ve difference - more ionic character.
Difference of 1.7 in electronegativities produces 50% ionic character in covalent bond.
Dipole Moment
Due to diff in electronegativity of a polar covalent bond dipole is produced.
Dipole moment is the product of the magnitude of the charge and the distance between the positive and negative charges.
Dipole moment (μ) = charge (Q) * distance of separation (r)
Expressed in Debye units.
1 D = 3.33564×10−30 C m
Dipole moment is a vector quantity and in polyatomic molecules spatial arrangement is also considered.
Which out of NH3 and NF3 has higher dipole moment and why?
NH3 has the higher dipole moment. Even though the electronegativity diff is greater in NF3, the dipole of the lone pair of N are oriented in the same direction as the N-H dipole in NH3, whereas they are oriented opposite to the N-F dipole in NF3 which why NH3 has higher dipole moment.
How decide if a molecule is polar or non polar?
- AB - homonuclear (H2) - non polar but heteronuclear (HF) - polar.
- AB2 - linear (CO2) - non polar but bent (H20) is polar.
- AB3- trigonal planar (BF3) - non polar but trigonal pyramidal (NH3) is polar.
- AB4 - tetrahedral is usually non polar (CCl4) but if all bonds are not the same (CHCl3) it can be polar.
Polar or Non polar ?
HX (X is halogen i.e. HF, HCl, HI, HBr)
Polar
Polar or Non Polar ?
H2
Non polar
Polar or Non Polar ?
H2O
Polar
Polar or Non Polar ?
H2S
Polar
Polar or Non Polar ?
BeF2
Non polar
Polar or Non Polar ?
NH3
Polar
