Chemical Bonding I: The Covalent Bond

Transcription

1 1 Chemical Bonding I: The Covalent Bond Chapter 9 Copyright The McGrawill Companies, Inc. Permission required for reproduction or display. Valence are the outer shell of an atom. The valence are the that particpate in chemical bonding. Group e configuration # of valence e 1A ns 1 1 A ns 3A ns np 1 3 4A ns np 4 5A ns np 3 5 6A ns np 4 6 7A ns np Lewis Dot Symbols for the Representative Elements & Noble Gases 3

2 4 Li The Ionic Bond Ionic bond: the electrostatic force that holds ions together in an ionic compound. Li + Li + 1s s1s s p 5 1s 1s s p 6 [e] [Ne] Li Li + + e e + Li + + Li E = k Q +Q r Lattice energy increases as Q increases and/or as r decreases. Electrostatic (Lattice) Energy Lattice energy (U) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions. E is the potential energy Q + is the charge on the cation Q is the charge on the anion r is the distance between the ions Compound Mg Mg Li LiCl Lattice Energy (kj/mol) 957 Q: +, Q: +, r < r Cl 5 6 Bornaber Cycle for Determining Lattice Energy D overall = D 1 + D + D 3 + D 4 + D 5 6

3 7 7 8 A covalent bond is a chemical bond in which two or more are shared by two atoms. Why should two atoms share? + 7e 7e Lewis structure of 8e 8e single covalent bond lone pairs lone pairs lone pairs lone pairs single covalent bond 8 9 Lewis structure of water + + or e 8e e single covalent bonds Double bond two atoms share two pairs of C or C 8e 8e 8e double bonds double bonds Triple bond two atoms share three pairs of N N or N N 8e 8e triple bond triple bond 9

4 10 Lengths of Covalent Bonds Bond Lengths Triple bond < Double Bond < Single Bond Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms electron poor region electron rich region e poor d + d e rich 11 1 Electronegativity is the ability of an atom to attract toward itself the in a chemical bond. Electron Affinity measurable, Cl is highest X (g) + e X (g) Electronegativity relative, is highest 1

5 13 The Electronegativities of Common Elements Variation of Electronegativity with Atomic Number Classification of bonds by difference in electronegativity Difference Bond Type 0 Covalent Ionic 0 < and < Polar Covalent Increasing difference in electronegativity Covalent Polar Covalent Ionic share e partial transfer of e transfer e 15

6 16 Classify the following bonds as ionic, polar covalent, or covalent: The bond in CsCl; the bond in S; and the NN bond in NN. Cs 0.7 Cl =.3 Ionic.1 S = 0.4 Polar Covalent N 3.0 N = 0 Covalent Writing Lewis Structures 1. Draw skeletal structure of compound showing what atoms are bonded to each other. Put least electronegative element in the center.. Count of valence e. Add 1 for each negative charge. Subtract 1 for each positive charge. 3. Complete an octet for all atoms except hydrogen 4. If structure contains too many, form double and triple bonds on central atom as needed Write the Lewis structure of nitrogen trifluoride (N 3 ). Step 1 N is less electronegative than, put N in center Step Count valence N 5 (s p 3 ) and 7 (s p 5 ) 5 + (3 x 7) = 6 valence Step 3 Draw single bonds between N and atoms and complete octets on N and atoms. Step 4 Check, are # of e in structure equal to number of valence e? 3 single bonds (3x) + 10 lone pairs (10x) = 6 valence N 18

7 19 Write the Lewis structure of the carbonate ion (C 3 ). Step 1 C is less electronegative than, put C in center Step Count valence C 4 (s p ) and 6 (s p 4 ) charge e C 4 + (3 x 6) + = 4 valence Step 3 Draw single bonds between C and atoms and complete octet on C and atoms. Step 4 Check, are # of e in structure equal to number of valence e? 3 single bonds (3x) + 10 lone pairs (10x) = 6 valence Step 5 Too many, form double bond and recheck # of e single bonds (x) = 4 1 double bond = 4 8 lone pairs (8x) = 16 Total = Two possible skeletal structures of formaldehyde (C ) C An atom s is the difference between the number of valence in an isolated atom and the number of assigned to that atom in a Lewis structure. on an atom in a Lewis structure = of valence in the free atom of nonbonding C 1 ( ) of bonding The sum of the s of the atoms in a molecule or ion must equal the charge on the molecule or ion C on an atom in a Lewis structure = on C on of valence in the free atom C 4 e 6 e x1 e 1 e = 4 ½ x 6 = 1 = 6 ½ x 6 = +1 of nonbonding single bonds (x) = 4 1 double bond = 4 lone pairs (x) = 4 Total = 1 1 ( ) of bonding 1

8 on an atom in a Lewis structure 0 0 C = on C on of valence in the free atom C 4 e 6 e x1 e 1 e = 4 0 ½ x 8 = 0 = 6 4 ½ x 4 = 0 of nonbonding single bonds (x) = 4 1 double bond = 4 lone pairs (x) = 4 Total = 1 1 ( ) of bonding 3 ormal Charge and Lewis Structures 1. or neutral molecules, a Lewis structure in which there are no s is preferable to one in which s are present.. Lewis structures with large s are less plausible than those with small s. 3. Among Lewis structures having similar distributions of s, the most plausible structure is the one in which negative s are placed on the more electronegative atoms. Which is the most likely Lewis structure for C? 1 +1 C 0 0 C 3 4 A resonance structure is one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure. + What are the resonance structures of the carbonate (C 3 ) ion? + C C C 4

9 5 The Incomplete ctet Exceptions to the ctet Rule Be Be e x1e 4e Be B 3e B 3 3 3x7e B 4e 3 single bonds (3x) = 6 9 lone pairs (9x) = 18 Total = ddelectron Molecules Exceptions to the ctet Rule N 5e N 6e N 11e The Expanded ctet (central atom with principal quantum number n > ) S 6 S 6e 6 4e 48e S 6 single bonds (6x) = 1 18 lone pairs (18x) = 36 Total = The enthalpy change required to break a particular bond in one mole of gaseous molecules is the bond enthalpy. Bond Enthalpy (g) (g) + (g) D = kj Cl (g) Cl (g) + Cl (g) D = 4.7 kj Cl (g) (g) + Cl (g) D = kj (g) (g) + (g) D = kj N (g) N (g) + N (g) D = kj N N Bond Enthalpies Single bond < Double bond < Triple bond 7

10 8 Average bond enthapy in polyatomic molecules (g) (g) + (g) D = 50 kj (g) (g) + (g) D = 47 kj Average bond enthalpy = = 464 kj 8 9 Bond Enthalpies (BE) and Enthalpy changes in reactions Imagine reaction proceeding by breaking all bonds in the reactants and then using the gaseous atoms to form all the bonds in the products. D = total energy input total energy released = SBE(reactants) SBE(products) endothermic exothermic 9 30 (g) + (g) (g) 30

11 31 Use bond enthalpies to calculate the enthalpy change for: (g) + (g) (g) D = SBE(reactants) SBE(products) Type of bonds broken Number of bonds broken Bond enthalpy (kj/mol) Enthalpy change (kj/mol) Type of bonds formed Number of bonds formed Bond enthalpy (kj/mol) Enthalpy change (kj/mol) D = x 568. = kj/mol 31 3 C9 W Questions and Problems Pages , 9.18, 9.0, 9.34, 9.38, 9.4, 9.44, 9.50, 9.6 3