Chemical Bonding I: Basic Concepts

Transcription

1 Chemical Bonding I: Basic Concepts Chapter 9 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 Valence electrons are the outer shell electrons of an atom. The valence electrons are the electrons that particpate in chemical bonding. Group e - configuration # of valence e - 1A ns 1 1 2A ns 2 2 3A ns 2 np 1 3 4A ns 2 np 2 4 5A ns 2 np 3 5 6A ns 2 np 4 6 7A ns 2 np

3 Lewis Dot Symbols for the Representative Elements & Noble Gases 9.1

4 The Ionic Bond Li + F Li + F - 1s 2 2s1s 2 2s 2 2p 5 [He][Ne] 1s 22 2s 2 2p 6 Li Li + + e - e - + F F - Li + + F - Li + F - 9.2

5 Electrostatic (Lattice) Energy Lattice energy (E) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions. E = k Q +Q - r Q + is the charge on the cation Q - is the charge on the anion r is the distance between the ions Lattice energy (E) increases as Q increases and/or as r decreases. cmpd MgF 2 MgO LiF LiCl lattice energy 2957 Q= +2, Q= +2,-2 r F - < r Cl - 9.3

6 Born-Haber Cycle for Determining Lattice Energy o o o DH overall = DH 1 + DH 2 + DH 3 + DH 4 + DH 5 o o o 9.3

7 9.3

8 Chemistry In Action: Sodium Chloride Mining Salt Solar Evaporation for Salt

9 A covalent bond is a chemical bond in which two or more electrons are shared by two atoms. Why should two atoms share electrons? F + F F F 7e - 7e - 8e - 8e - Lewis structure of F 2 single covalent bond lone pairs F F lone pairs lone pairs F F lone pairs single covalent bond 9.4

10 Lewis structure of water single covalent bonds H + O + H H O H or H O H 2e - 8e - 2e - Double bond two atoms share two pairs of electrons O C O or O C O double 8e - 8ebonds - - double bonds Triple bond two atoms share three pairs of electrons N N or N N 8e - 8e - triple bond triple bond 9.4

11 Lengths of Covalent Bonds Bond Lengths Triple bond < Double Bond < Single Bond 9.4

12 9.4

13 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 e - rich H F H F d + d - 9.5

14 Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond. Electron Affinity - measurable, Cl is highest X (g) + e - X - (g) Electronegativity - relative, F is highest 9.5

15 The Electronegativities of Common Elements 9.5

16 Variation of Electronegativity with Atomic Number 9.5

17 Classification of bonds by difference in electronegativity Difference Bond Type 0 Covalent 2 Ionic 0 < and <2 Polar Covalent Increasing difference in electronegativity Covalent share e - Polar Covalent partial transfer of e - Ionic transfer e - 9.5

18 Classify the following bonds as ionic, polar covalent, or covalent: The bond in CsCl; the bond in H 2 S; and the NN bond in H 2 NNH 2. Cs 0.7 Cl = 2.3 Ionic H 2.1 S = 0.4 Polar Covalent N 3.0 N = 0 Covalent 9.5

19 Writing Lewis Structures 1. Draw skeletal structure of compound showing what atoms are bonded to each other. Put least electronegative element in the center. 2. Count total number 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 electrons, form double and triple bonds on central atom as needed. 9.6

20 Write the Lewis structure of nitrogen trifluoride (NF 3 ). Step 1 N is less electronegative than F, put N in center Step 2 Count valence electrons N - 5 (2s 2 2p 3 ) and F - 7 (2s 2 2p 5 ) 5 + (3 x 7) = 26 valence electrons Step 3 Draw single bonds between N and F atoms and complete octets on N and F atoms. Step 4 - Check, are # of e - in structure equal to number of valence e -? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons F N F F 9.6

21 Write the Lewis structure of the carbonate ion (CO 3 2- ). Step 1 C is less electronegative than O, put C in center Step 2 Count valence electrons C - 4 (2s 2 2p 2 ) and O - 6 (2s 2 2p 4 ) -2 charge 2e (3 x 6) + 2 = 24 valence electrons Step 3 Draw single bonds between C and O atoms and complete octet on C and O atoms. Step 4 - Check, are # of e - in structure equal to number of valence e -? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons Step 5 - Too many electrons, form double bond and re-check # of e - O C O O 2 single bonds (2x2) = 4 1 double bond = 4 8 lone pairs (8x2) = 16 Total =

22 Two possible skeletal structures of formaldehyde (CH 2 O) H C O H H H C O An atom s formal charge is the difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure. formal charge on an atom in a Lewis structure = total number of valence electrons in - the free atom total number of nonbonding electrons total number of bonding electrons ( ) The sum of the formal charges of the atoms in a molecule or ion must equal the charge on the molecule or ion. 9.7

23 -1 +1 H C O H C 4 e - O 6 e - 2H 2x1 e - 12 e - 2 single bonds (2x2) = 4 1 double bond = 4 2 lone pairs (2x2) = 4 Total = 12 formal charge on an atom in a Lewis structure = total number of valence electrons in - the free atom total number of nonbonding electrons total number of bonding electrons ( ) formal charge on C formal charge on O = 4-2 -½ x 6 = -1 = 6-2 -½ x 6 =

24 H H 0 0 C O C 4 e - O 6 e - 2H 2x1 e - 12 e - 2 single bonds (2x2) = 4 1 double bond = 4 2 lone pairs (2x2) = 4 Total = 12 formal charge on an atom in a Lewis structure = total number of valence electrons in - the free atom total number of nonbonding electrons total number of bonding electrons ( ) formal charge on C formal charge on O = 4-0 -½ x 8 = 0 = 6-4 -½ x 4 = 0 9.7

25 Formal Charge and Lewis Structures 1. For neutral molecules, a Lewis structure in which there are no formal charges is preferable to one in which formal charges are present. 2. Lewis structures with large formal charges are less plausible than those with small formal charges. 3. Among Lewis structures having similar distributions of formal charges, the most plausible structure is the one in which negative formal charges are placed on the more electronegative atoms. Which is the most likely Lewis structure for CH 2 O? H C O H H H 0 0 C O 9.7

26 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. + - O O O - O + O O What are the resonance structures of the carbonate (CO 32 -) ion? - - O C O O C O - - O C O O O - O - 9.8

27 The Incomplete Octet Exceptions to the Octet Rule BeH 2 Be 2e - 2H 2x1e - 4e - H Be H B 3e - BF 3 3F 3x7e - F B F 24e - F 3 single bonds (3x2) = 6 9 lone pairs (9x2) = 18 Total =

28 Odd-Electron Molecules Exceptions to the Octet Rule NO N 5e - O 6e - 11e - N O The Expanded Octet (central atom with principal quantum number n > 2) SF 6 S 6e - 6F 42e - 48e - F F F S F F F 6 single bonds (6x2) = lone pairs (18x2) = 36 Total =

29 Chemistry In Action: Just Say NO NO 2- (aq) + Fe 2+ (aq) + 2H + (aq) NO (g) + Fe 3+ (aq) + H 2 O (l) N 2 (g) + O 2 (g) 2NO (g)

30 The enthalpy change required to break a particular bond in one mole of gaseous molecules is the bond energy. Bond Energy H 2 (g) H (g) + H (g) DH 0 = kj Cl 2 (g) HCl (g) Cl (g) + Cl (g) DH 0 = kj H (g) + Cl (g) DH 0 = kj O 2 (g) O (g) + O (g) DH 0 = kj O O N 2 (g) N (g) + N (g) DH 0 = kj N N Bond Energies Single bond < Double bond < Triple bond 9.10

31 Average bond energy in polyatomic molecules H 2 O (g) H (g) + OH (g) DH 0 = 502 kj OH (g) H (g) + O (g) DH 0 = 427 kj Average OH bond energy = = 464 kj 9.10

32 Bond Energies (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. DH 0 = total energy input total energy released = SBE(reactants) SBE(products) 9.10

33 H 2 (g) + Cl 2 (g) 2HCl (g) 2H 2 (g) + O 2 (g) 2H 2 O (g) 9.10

34 Use bond energies to calculate the enthalpy change for: H 2 (g) + F 2 (g) 2HF (g) DH 0 = SBE(reactants) SBE(products) Type of bonds broken Number of bonds broken Bond energy (kj/mol) Energy change (kj) H H F F Type of bonds formed Number of bonds formed Bond energy (kj/mol) Energy change (kj) H F DH 0 = x = kj 9.10