Chapter 9 Chemical Bonding I: Lewis Theory

Why Do Atoms Bond? processes are spontaneous if they result in a system with lower potential energy chemical bonds form because they lower the potential energy between the charged particles that compose atoms the potential energy between charged particles is directly proportional to the product of the charges the potential energy between charged particles is inversely proportional to the distance between the charges 2 The attraction between opposite-charged particles increases as the particles get closer together. Bringing them closer lowers the potential energy of the system.

Potential Energy Between Charged Particles  0 is a constant ◦ = 8.85 x C 2 /J∙m for charges with the same sign, E potential is + and the magnitude gets less positive as the particles get farther apart for charges with the opposite signs, E potential is  and the magnitude gets more negative as the particles get closer together remember: the more negative the potential energy, the more stable the system becomes 3

Bonding a chemical bond forms when the potential energy of the bonded atoms is less than the potential energy of the separate atoms have to consider following interactions: ◦ nucleus-to-nucleus repulsion ◦ electron-to-electron repulsion ◦ nucleus-to-electron attraction 4

Ionic Bonds when metals bond to nonmetals, some electrons from the metal atoms are transferred to the nonmetal atoms ◦ metals have low ionization energy, relatively easy to remove an electron from ◦ nonmetals have high electron affinities, relatively good to add electrons to 5

Covalent Bonds nonmetals have relatively high ionization energies, so it is difficult to remove electrons from them when nonmetals bond together, it is better in terms of potential energy for the atoms to share valence electrons ◦ potential energy lowest when the electrons are between the nuclei shared electrons hold the atoms together by attracting nuclei of both atoms 6

Lewis Theory and Ionic Bonding Lewis symbols can be used to represent the transfer of electrons from metal atom to nonmetal atom, resulting in ions that are attracted to each other and therefore bond ◦ electrons are transferred until the metal loses all its valence electrons and the nonmetal has an octet ◦ numbers of atoms are adjusted so the electron transfer comes out even 7 +

Energetics of Ionic Bond Formation the ionization energy of the metal is endothermic ◦ Na(s) → Na + (g) + 1 e ─  H° = +603 kJ/mol the electron affinity of the nonmetal is exothermic ◦ ½Cl 2 (g) + 1 e ─ → Cl ─ (g)  H° = ─ 227 kJ/mol generally, the ionization energy of the metal is larger than the electron affinity of the nonmetal, therefore the formation of the ionic compound should be endothermic but the heat of formation of most ionic compounds is exothermic and generally large; Why? ◦ Na(s) + ½Cl 2 (g) → NaCl(s)  H° f = -410 kJ/mol 8

Ionic Bonds electrostatic attraction is nondirectional!! ◦ no direct anion-cation pair no ionic molecule ◦ chemical formula is an empirical formula, simply giving the ratio of ions based on charge balance ions arranged in a pattern called a crystal lattice ◦ every cation surrounded by anions; and every anion surrounded by cations ◦ maximizes attractions between + and - ions 9

Lattice Energy the lattice energy is the energy released when the solid crystal forms from separate ions in the gas state ◦ always exothermic ◦ hard to measure directly, but can be calculated from knowledge of other processes lattice energy depends directly on size of charges and inversely on distance between ions 10

Born-Haber Cycle for NaCl 11 ΔH o f = kJ/mol ΔH o f =107.7 kJ/mol ΔH o f = kJ/mol ΔH o f = kJ/mol E a = -349 kJ/mol

Born-Haber Cycle method for determining the lattice energy of an ionic substance by using other reactions ◦ use Hess’s Law to add up heats of other processes  H° f (salt) =  H° f (metal atoms, g) +  H° f (nonmetal atoms, g) +  H° f (cations, g) +  H° f (anions, g) +  H° f (crystal lattice) ◦ H° f (crystal lattice) = Lattice Energy ◦ metal atoms (g)  cations (g),  H° f = ionization energy  don’t forget to add together all the ionization energies to get to the desired cation  M 2+ = 1 st IE + 2 nd IE ◦ nonmetal atoms (g)  anions (g),  H° f = electron affinity 12

Trends in Lattice Energy Ion Size the force of attraction between charged particles is inversely proportional to the distance between them larger ions mean the center of positive charge (nucleus of the cation) is farther away from negative charge (electrons of the anion) ◦ larger ion = weaker attraction = smaller lattice energy 13 Metal Chloride Lattice Energy (kJ/mol) LiCl-834 NaCl-787 KCl-701 CsCl-657

Trends in Lattice Energy Ion Charge the force of attraction between oppositely charged particles is directly proportional to the product of the charges larger charge means the ions are more strongly attracted ◦ larger charge = stronger attraction = larger lattice energy of the two factors, ion charge generally more important 14 Lattice Energy = -910 kJ/mol Lattice Energy = kJ/mol

Ionic Bonding Model vs. Reality ionic compounds have high melting points and boiling points ◦ MP generally > 300°C ◦ all ionic compounds are solids at room temperature because the attractions between ions are strong, breaking down the crystal requires a lot of energy ◦ the stronger the attraction (larger the lattice energy), the higher the melting point ionic solids are brittle and hard the position of the ion in the crystal is critical to establishing maximum attractive forces – displacing the ions from their positions results in like charges close to each other and the repulsive forces take over 15

Ionic Bonding Model vs. Reality molecular compounds do not conduct electricity in the liquid state molecular acids conduct electricity when dissolved in water, but not in the solid state in molecular solids, there are no charged particles around to allow the material to conduct when dissolved in water, molecular acids are ionized, and have the ability to move through the structure and therefore conduct electricity

Ionic BondingModel vs. Reality molecular compounds do not conduct electricity in the liquid state molecular acids conduct electricity when dissolved in water, but not in the solid state in molecular solids, there are no charged particles around to allow the material to conduct when dissolved in water, molecular acids are ionized, and have the ability to move through the structure and therefore conduct electricity 17

Ionic BondingModel vs. Reality ionic compounds conduct electricity in the liquid state or when dissolved in water, but not in the solid state to conduct electricity, a material must have charged particles that are able to flow through the material in the ionic solid, the charged particles are locked in position and cannot move around to conduct in the liquid state, or when dissolved in water, the ions have the ability to move through the structure and therefore conduct electricity some molecular solids are brittle and hard, but many are soft and waxy the kind and strength of the intermolecular attractions varies based on many factors the covalent bonds are not broken, however, the polarity of the bonds has influence on these attractive forces 18

Bond Energies chemical reactions involve breaking bonds in reactant molecules and making new bond to create the products the  H° reaction can be calculated by comparing the cost of breaking old bonds to the profit from making new bonds the amount of energy it takes to break one mole of a bond in a compound is called the bond energy ◦ in the gas state ◦ homolytically – each atom gets ½ bonding electrons 19

Using Bond Energies to Estimate  H° rxn the actual bond energy depends on the surrounding atoms and other factors we often use average bond energies to estimate the  H rxn ◦ works best when all reactants and products in gas state bond breaking is endothermic,  H(breaking) = + bond making is exothermic,  H(making) = −  H rxn = ∑ (  H(bonds broken)) + ∑ (  H(bonds formed)) 20

Trends in Bond Energies the more electrons two atoms share, the stronger the covalent bond ◦ C≡C (837 kJ) > C=C (611 kJ) > C−C (347 kJ) ◦ C≡N (891 kJ) > C=N (615 kJ) > C−N (305 kJ) the shorter the covalent bond, the stronger the bond ◦ Br−F (237 kJ) > Br−Cl (218 kJ) > Br−Br (193 kJ) ◦ bonds get weaker down the column 21

Estimate the Enthalpy of the Following Reaction 22

Examples Use bond energies to estimate the enthalpy of reaction for the combustion of methane: CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2H 2 O(l)

Bond Lengths the distance between the nuclei of bonded atoms is called the bond length because the actual bond length depends on the other atoms around the bond we often use the average bond length ◦ averaged for similar bonds from many compounds 24

Trends in Bond Lengths the more electrons two atoms share, the shorter the covalent bond ◦ C≡C (120 pm) < C=C (134 pm) < C−C (154 pm) ◦ C≡N (116 pm) < C=N (128 pm) < C−N (147 pm) decreases from left to right across period ◦ C−C (154 pm) > C−N (147 pm) > C−O (143 pm) increases down the column ◦ F−F (144 pm) > Cl−Cl (198 pm) > Br−Br (228 pm) in general, as bonds get longer, they also get weaker 25

Bond Lengths 26

Metallic Bonds low ionization energy of metals allows them to lose electrons easily the simplest theory of metallic bonding involves the metals atoms releasing their valence electrons to be shared by all to atoms/ions in the metal ◦ an organization of metal cation islands in a sea of electrons ◦ electrons delocalized throughout the metal structure bonding results from attraction of cation for the delocalized electrons 27

Metallic BondingModel vs. Reality metallic solids conduct electricity because the free electrons are mobile, it allows the electrons to move through the metallic crystal and conduct electricity as temperature increases, electrical conductivity decreases heating causes the metal ions to vibrate faster, making it harder for electrons to make their way through the crystal metallic solids conduct heat the movement of the small, light electrons through the solid can transfer kinetic energy quicker than larger particles metallic solids reflect light the mobile electrons on the surface absorb the outside light and then emit it at the same frequency 28

Metallic Bonding Model vs. Reality metallic solids are malleable and ductile because the free electrons are mobile, the direction of the attractive force between the metal cation and free electrons is adjustable this allows the position of the metal cation islands to move around in the sea of electrons without breaking the attractions and the crystal structure 29

Metallic Bonding Model vs. Reality metals generally have high melting points and boiling points ◦ all but Hg are solids at room temperature the attractions of the metal cations for the free electrons is strong and hard to overcome melting points generally increase to right across period the charge on the metal cation increases across the period, causing stronger attractions melting points generally decrease down column the cations get larger down the column, resulting in a larger distance from the nucleus to the free electrons 30