1. Chemical Bonds
LACC Chem101

2. Chemical Bonds Attractive, intramolecular forces between nuclei
Maintains a group of atoms together
No definite time
Generally stable
Lowers electrical potential between nuclei
Requires energy to break apart
This may come from even lower potential in a new bond
Three types
Ionic
Covalent
Metallic
LACC Chem101

3. Ionic Bonding Electron transfer
Metal to nonmetal
Results in two ions of opposite charges
No overall charge
Significant difference in electronegativity
Generally higher than 1.6
Atoms arrange geometrically
Crystal lattice
Compound referred to as “salt”
Very strong
Solids at room temp
Poor electrical conduction in solid state
Electrons confined to nuclei
LACC Chem101

4. Ionic Bonds Electron transfer causes opposite charges
Bond is actually electrostatic force:
Most commonly Main Group elements
Cations I & II
Anions VI & VII
Large electronegativity difference allows for transfer
LACC Chem101

5. Calculation of Attractive Energy
Coulomb’s Law is simple to solve for a cation of charge +1 and anion of charge -1 (we use 3.5Å bond length here)
The molar value (in kJ per mole) is:
For comparison, this is approximately the ∆Hrxn for the formation of one mole of NaBr
Not exact, so bond include more potentials than simple electrostatic force
LACC Chem101

6. Lattice Energy
Difference in molar enthalpy between the solid and a gas of widely separated ions
Magnitude depends on charges of ionic species
Low charge means low lattice energy
This cannot be calculated directly
Requires measurement of gaseous ions condensing into solid
Using enthalpy (state function), we can construct several steps for a known equation
Lattice energy is one of these steps!
Born-Haber cycle
Start with pure elements
Atomize pure elements to form gaseous atoms
Ionize the atoms
Allow ions to form a solid
Convert solid back to pure elements
LACC Chem101

7. Born-Haber Cycle for NaCl
Step 1: The transfer of an electron from an Na atom to a Cl atom is not in itself energetically favorable
Requires +147 kJ/mol of energy
Step 2a: -493 kJ of energy is released when oppositely charged ions come together to form ion pairs
Step 2b: Additional energy 293 kJ is released when these ion pairs form the solid crystal
The lattice energy released when 1 mol of Na+ & Cl- ions react to produce NaCl (s) is 786 kJ/mol
The overall process of NaCl formation is energetically favorable, releasing 639 kJ/mol if gaseous Na & Cl atoms are initially used.
Sublimation of Sodium Na(s) → Na(g) ∆H1 = 108 kJ
Dissociation of Chlorine ½Cl2(g) → Cl(g) ∆H2 = 120 kJ
Ionization of Sodium Na(g) → Na+(g) + e-(g) ∆H3 = 496 kJ
Formation of Chloride ion Cl(g) + e-(g) → Cl-(g) ∆H4 = -349 kJ
Formation of Sodium Chloride Na+(g) + Cl-(g) → NaCl(s) ∆H5 = -786 kJ
Total Reaction Na(s) + ½Cl2(g) → NaCl(s) ∆Hof =
LACC Chem101

8. Workshop on Born-Haber Process
Problem #1: Devise and use a Born-Haber cycle to calculate the lattice enthalpy of potassium chloride.
Problem #2: Calculate the lattice enthalpy of calcium chloride.
Problem #3: Calculate the lattice enthalpy of magnesium bromide.
LACC Chem101

9. LACC Chem101

10. Ionic Radius Ionic Radius: measure of the size of the spherical region
(electron probability) around the nucleus of an ion.
Isoelectronic: species having the same electron configuration
Periodic Trend
ionic radius of cations decreases going left to right across a period until the ion becomes an anion (Group V) then there is an abrupt increase in ionic radius
from Group V to Group VII the radius once again decreases.
The ionic radius increases going down a group.
LACC Chem101

11. Metallic Bonds Electron sea model
Each metal atom is effectively a cation
Donated electron(s) to the metal solid as a whole
Electrons not localized
LACC Chem101

12. Covalent Bonds
LACC Chem101

13. Covalent bonding Sharing of electrons Low electronegativity difference
Typically two or more nonmetals
Compound is called a “molecule”
Each component has a formal charge
No individual charges, and no overall charge
Electrons orbitals overlap into a molecular orbitals
Multiple molecular orbitals may form (Ch10)
Electrons localized between the two nuclei
LACC Chem101

14. Electronegativity
Tendency of an atom (or functional group) to attract electrons to itself
Usually given on the Pauling scale, with a maximum value of 4.0
All elements have an electronegativity, so none are zero
Metals have low electronegativities
Tend to lose electrons when ionized
Nonmetals have comparatively higher electronegativities
Tend to gain electrons when ionized
Periodic Trend is increase across a period, and decrease down a group
LACC Chem101

15. Covalent Bonds Aka: “Chemical Bonds” Generally between nonmetals
Electron pairs are shared between nuclei
Valence electron orbitals overlap in a molecular orbital
Highest probability of find shared electron is in between the nuclei
Similar (or same) electronegativities cause electron to be shared, not transferred
Bond Order: Describes number of electron pairs being shared
Typically 1, but can be 2 (double bond) or 3 (triple bond)
LACC Chem101

16. Potential Energy between Atoms
Balance between attractive and repulsive forces of particles
Also consider kinetic energy of electrons
Zero point occurs as the distance (r) approaches infinity
No attraction or repulsion
As distance approaches zero, potential is infinitely high
Nuclei begin to repel
Bond length is where the energy is at a minimum
LACC Chem101

17. Bond Length Distance between nuclei at minimum potential energy
Bond dissociation energy required to break covalent bond
Larger energies required for more stable bonds
Distance decreases as more bonds made (double or triple)
LACC Chem101

18. Bond Energy
Difference between the standard molar enthalpies of a molecule X-Y and the fragments X and Y on their own
This is an approximation
Energy required to overcome attractive force of a bond
Related to bond strength and number of bonds
Bond energy allows us to conclude:
Endothermic
Exothermic
LACC Chem101

19. Bond Energies & Standard Enthalpy
All bond enthalpies listed in your textbook are POSITIVE because energy must be supplied to break a bond.
Bond breaking is always ENDOTHERMIC
Bond formations is always EXOTHERMIC.
Assume that the average bond energy applies regardless of the specific molecular environment
intermolecular interactions are expected to be minimal and hence are NOT taken into account
These calculations are limited to cases where ALL reactants/products are in the gas phase!
LACC Chem101

20. Bond Energy Example CH3CH2I(g) + H2O(g)  CH3CH2OH(g) + HI(g)
Estimate the enthalpy change of the reaction between gaseous iodoethane and water vapor:
CH3CH2I(g) + H2O(g)  CH3CH2OH(g) + HI(g)
Bond Energies (kJ/mol): C-O 360 H-O 463
C-I 238 H-I 299
LACC Chem101

21. Bond Energy Example
Estimate the standard enthalpy of the following reaction: CCl3CHCl2(g) + 2HF(g)  CCl3CHF2(g) + 2HCl(g) Bond Energies (kJ/mol): C-F 485 C-Cl 339 H-F 565 H-Cl 431
LACC Chem101

22. Polarity
Polarity: charge distribution about a chemical bond or a whole molecule
Polarity helps to determine the equal or unequal distribution of an electron between two atoms
Referring to probability of electron being near one or the other nucleus
Nonpolar covalent bond
Electrons shared with equal probability distribution
Polar covalent bond
Electrons shared, but unequal probability distribution between nuclei
LACC Chem101

23. Dipole Moment
Dipole moment: electrical vector indicating direction of electron density in a molecule
Since it is an electrical moment, it has the formula:
SI units are C-m, but most useful for physical chemists is the Debye:
Arrow pointing towards more electronegative atom
Positive “tail” typically drawn in for clarity
We can find ionic character by comparing this to the actual dipole moment
Actual will be much lower since an electron was not transferred
LACC Chem101

24. Ionic Character example
Hydrofluoric acid has an actual dipole moment of 1.82D. Calculate its ionic character.
LACC Chem101

25. Polarity of a Bond
Nonpolar covalent bonds occur between atoms of similar (same) electronegativity
Ex Cl2 SCN-
Polar covalent bonds occur due to a significant difference in dipole moments of atoms
Ex H2O CO2 ICl
LACC Chem101

26. Example: Ammonia Compare electronegativities of each atom
Nitrogen is highest
Not lessened by presence of three hydrogens
Lone pair gives overall polarity
See geometry
LACC Chem101

27. Nonpolar vs Polar vs Ionic Bonds
Note the overall dipole moments on each bond
LACC Chem101

28. LACC Chem101