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Chemical Bonding I:The Covalent Bond

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?

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?

9.3 Lattice energyBo

rn-H

aber

Cyc

le

Born

-Hab

er C

ycle

9.3 Lattice energy

Figure 9.2, p. 293

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?

9.3 Lattice energyBo

rn-H

aber

Cyc

le –

NaC

l

What are the thermochemical equations for the:Enthalpy of formation of NaCl(s)Enthalpy of sublimation of NaBond dissociation energy of Cl2(g)First ionization energy of NaElectron affinity for Cl

What is the lattice energy (in kJ/mol) of sodium chloride?

9.3 Lattice energyBo

rn-H

aber

Cyc

le –

NaC

l

9.3 Lattice energyBo

rn-H

aber

Cyc

le –

NaC

l

9.3 Lattice energy

Table 9.1, p. 293

Lattice energy increases as Q increases and/or as r decreases.

Q+ is the charge on the cationQ- is the charge on the anionr is the distance between the ions

Lattice Energy / kJ·mol-1

MgF2 2957MgO 3890LiF 1017LiCl 828

Q QE k

r

9.3 Lattice energyBo

rn-H

aber

Cyc

le –

NaC

l

k is a constant (that is positive)

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?

H F FH

Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms

electron richregionelectron poor

region e- riche- poor

+ -

9.5 Electronegativity

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

H0 = total energy input – total energy released

H0 = BE(reactants) – BE(products)

Imagine reaction proceeding by breaking all bonds in the reactants and then using the gaseous atoms to form all the bonds in the products.

9.10 Bond enthalpy

endothermic reaction exothermic reaction

9.10 Bond enthalpy

Figure 9.8, p. 315

2H2 (g) + O2 (g) 2H2O (g)

Bond

Ent

halp

y –

Exam

ples

Margin figure, p. 315

9.10 Bond enthalpy

Table 9.3, p. 314

H2 (g) + Cl2 (g) 2HCl (g)

Bond

Ent

halp

y –

Exam

ples

Margin figure, p. 315

9.10 Bond enthalpy

Table 9.3, p. 314

Which portion of the periodic table contains the elements with the highest electronegativity?

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)

What is the correct Lewis dot structure of NO2?

A. B. C. D.

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Which thermochemical equation describes the bond enthalpy of hydrogen?

A 2H(g)H2(g) ∆Ho < 0 B 2H(g)H2(g) ∆Ho > 0C H2(g)2H(g) ∆Ho < 0 D H2(g)2H(g) ∆Ho > 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.

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.

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