Chemical Bonding I: The Covalent Bond
1 Chemical Bonding I: The Covalent Bond Lewis dot symbols (9.1) The ionic bond (9.2) Lattice energy of ionic compounds (9.3) The covalent bond (9.4) Electronegativity (9.5) Writing Lewis structures (9.6) Formal charge and Lewis structure (9.7) The concept of resonance (9.8) Exceptions to the octet rule (9.9) Bond enthalpy (9.10) Chemical Bonding I
Valence electrons of atoms Ionic bonding ◦ Lattice energy ◦ Born-Haber cycle Covalent bonding ◦ Lewis dot structures ◦ Bond lengths ◦ Electronegativity ◦ Resonance ◦ Formal charges ◦ Exceptions to the octet rule Bond Enthalpy 9.1 Lewis dot symbols
Main group elements What do the symbols show? What do the symbols not show?
Figure 9.1, p. 288 9.2 The ionic bond
What is an ionic bond? How have we already discussed ionic bonds? ◦ Definition ◦ Formula ◦ Nomenclature ◦ Formula calculations ◦ Redox (formation of ionic compounds) ◦ Electron configurations ◦ Ionization energy ◦ Electron affinity 9.3 Lattice energy
What is lattice energy? What is the associated reaction for this (use NaCl)? ◦ Can this be measured directly? ◦ Is this a positive or negative energy? How can we think about this in terms of opposite charges? ◦ What is Coulomb’s law and what is the equation for lattice energy? ◦ Is this a positive or negative values? ◦ How does this relate to lattice energy? 9.3 Lattice energy
How is lattice energy affected by ionic charges? How is lattice energy affected by the distance between the ions? What is the Born-Haber cycle? ◦ How does this relate to energy diagrams? ◦ How is this based on Hess’s Law? Born-Haber Cycle Born-Haber Born-Haber Cycle 9.3 Lattice energy Figure 9.2, p. 293 9.3 Lattice energy
What is the thermochemical equation for the formation of NaCl? What is the energy diagram for this? How will these be integrated for the Born-Haber Cycle? ◦ What does the length of the arrow mean? ◦ What does the direction of the arrow mean? Born-Haber Cycle –Born-Haber NaCl 9.3 Lattice energy
What are the thermochemical equations for the: Enthalpy of formation of NaCl(s) Enthalpy of sublimation of Na
Bond dissociation energy of Cl2(g) First ionization energy of Na Electron affinity for Cl
What is the lattice energy (in kJ/mol) of sodium chloride? Born-Haber Cycle –Born-Haber NaCl 9.3 Lattice energy Born-Haber Cycle –Born-Haber NaCl 9.3 Lattice energy
Table 9.1, p. 293 9.3 Lattice energy
Q+ is the charge on the cation Q- is the charge on the anion QQ Ek r is the distance between the ions r k is a constant (that is positive)
Lattice energy Lattice Energy / increases as Q kJ·mol-1 increases and/or as r decreases. MgF2 2957 MgO 3890
Born-Haber Cycle –Born-Haber NaCl LiF 1017 LiCl 828 9.4 The covalent bond
How is the representation different for ionic versus covalent bonds? What are lone pairs of electrons? How does bond length vary? Are multiple bonds longer or shorter than single bonds?
Table 9.2, p. 295 9.5 Electronegativity
What is the definition of electronegativity? ◦ The ability of an atom to attract toward itself the electrons in a chemical bond What is the trend for electronegativity? 9.5 Electronegativity
Figure 9.5, p. 296 9.5 Electronegativity
How does this relate to metallic character? What is a polar covalent bond and how is this modeled? 9.5 Electronegativity
Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms
electron rich electron poor region region e- poor e- rich H F H F + -
Figure 9.4, p. 296 9.5 Electronegativity
How does the difference in electronegativity relate to covalent versus ionic bonding? How does this relate to percent ionic character?
Figure 9.7, p. 297 9.5 Electronegativity
Figure 9.5, p. 296 9.6 Writing Lewis structures
This should be review (can use either a previous method you have learned, the method provided in the lecture activity or the method given in the text, p. 299). 9.7 Formal charge
What is formal charge? ◦ The electrical charge difference between the valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure How does it differ from oxidation state? How does formal charge assist in “placing” a charge on an atom for a polyatomic ion? 9.8 Resonance
What is a resonance structure? ◦ One of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure What is the criteria for a structure to be a resonance structure? 9.9 Exceptions to the octet rule
Must all atoms in molecules or ions obey the octet rule? How can formal charge be useful in determining expanded or reduced octets? Can molecules have an odd number of electrons? 9.10 Bond enthalpy What is bond enthalpy? ◦ The enthalpy change required to break a particular bond in 1 mole of gaseous molecules What is the associated reaction? Is bond enthalpy exothermic or endothermic? Always? How can reactions be considered using bond enthalpies? How do Lewis dot structures help in determining enthalpy of reaction using bond enthalpies? 9.10 Bond enthalpy
Table 9.3, p. 314 9.10 Bond enthalpy
Imagine reaction proceeding by breaking all bonds in the reactants and then using the gaseous atoms to form all the bonds in the products.
H0 = total energy input – total energy released
H0 = BE(reactants) – BE(products) 9.10 Bond enthalpy
endothermic reaction exothermic reaction
Figure 9.8, p. 315 2H2 (g) + O2 (g) 2H2O (g) Bond Enthalpy –Bond Enthalpy Examples
Margin figure, p. 315 9.10 Bond enthalpy
Table 9.3, p. 314 H2 (g) + Cl2 (g) 2HCl (g) Bond Enthalpy –Bond Enthalpy Examples
Margin figure, p. 315 9.10 Bond enthalpy
Table 9.3, p. 314 The lattice energy for magnesium oxide is 3890 kJ∙mol–1. Which chemical equation describes this?
A. Mg(s) + ½O2(g) MgO(s) B. MgO(s) Mg(s) + ½O2(g) C. Mg2+(g) + O2–(g) MgO(s) D. MgO(s) Mg2+(g) + O2–(g)
Which portion of the periodic table contains the elements with the highest electronegativity?
What is the correct Lewis dot structure of NO2?
A. B. C. D. Chapter 9 –Chapter Practice Which thermochemical equation describes the bond enthalpy of hydrogen? o o A 2H(g) H2(g) ∆H < 0 B 2H(g) H2(g) ∆H > 0 o o C H2(g) 2H(g) ∆H < 0 D H2(g) 2H(g) ∆H > 0
– How many resonance structures are there for the formate ion, CO2H ? A. 0 B. 1 C. 2 D. 3
Complete (balance) the chemical equation using structural formulas – you may NOT use any stoichiometric coefficients. Chapter 9 –Chapter Practice What is the change in enthalpy for this reaction using bond enthalpies?
What is the thermochemical equation for the bond enthalpy of chlorine?
All bond enthalpies are EXOTHERMIC / ENDOTHERMIC.