View
214
Download
0
Tags:
Embed Size (px)
Citation preview
Chemical Bonding I:The Covalent Bond
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 thatparticpate in chemical bonding.
1A 1ns1
2A 2ns2
3A 3ns2np1
4A 4ns2np2
5A 5ns2np3
6A 6ns2np4
7A 7ns2np5
Group # of valence e-e- configuration
4
Li + F Li+ F -
The Ionic Bond
1s22s11s22s22p5 1s2 1s22s22p6
[He] [Ne]
Li Li+ + e-
e- + F F -
F -Li+ + Li+ F -
LiF
Ionic bond: the electrostatic force that holds ions together in an ionic compound.
5
Lattice energy (U) (LE) of an ionic compound is the energy required to break apart the ions in their lattice arrangement into the ions in the gas phase.
For example the energy to take NaCl from its lattice arrangement into the gas phase ions would be represented by NaCl(s) ---> Na+(g) + Cl-(g)
Electrostatic (Lattice) Energy
8
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+
7e- 7e-
F F
8e- 8e-
F F
F F
Lewis structure of F2
lone pairslone pairs
lone pairslone pairs
single covalent bond
single covalent bond
9
Sigma () and Pi Bonds ()
Single bond 1 sigma bond
Double bond 1 sigma bond and 1 pi bond
Triple bond 1 sigma bond and 2 pi bonds
How many and bonds are in the acetic acid (vinegar) molecule CH3COOH?
C
H
H
CH
O
O H bonds = 6 + 1 = 7
bonds = 1
10
8e-
H HO+ + OH H O HHor
2e- 2e-
Lewis structure of water
Double bond – two atoms share two pairs of electrons
single covalent bonds
O C O or O C O
8e- 8e-8e-
double bondsdouble bonds
Triple bond – two atoms share three pairs of electrons
N N8e- 8e-
N N
triple bondtriple bond
or
12
H F FH
Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms
electron richregion
electron poorregion e- riche- poor
+ -
13
Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond.
Electron Affinity - measurable, Cl is highest
Electronegativity - relative, F is highest
X (g) + e- X-(g)
15
Covalent
share e-
Polar Covalent
partial transfer of e-
Ionic
transfer e-
Increasing difference in electronegativity
Classification of bonds by difference in electronegativity
Difference Bond Type
0 Covalent
2 Ionic
0 < and <2 Polar Covalent
16
Classify the following bonds as ionic, polar covalent, or covalent: The bond in CsCl; the bond in H2S; and the NN bond in H2NNH2.
Cs – 0.7 Cl – 3.0 3.0 – 0.7 = 2.3 Ionic
H – 2.1 S – 2.5 2.5 – 2.1 = 0.4 Polar Covalent
N – 3.0 N – 3.0 3.0 – 3.0 = 0 Covalent
17
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.
Writing Lewis Structures
18
Write the Lewis structure of nitrogen trifluoride (NF3).
Step 1 – N is less electronegative than F, put N in center
F N F
F
Step 2 – Count valence electrons N - 5 (2s22p3) and F - 7 (2s22p5)
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
19
Write the Lewis structure of the carbonate ion (CO32-).
Step 1 – C is less electronegative than O, put C in center
O C O
O
Step 2 – Count valence electrons C - 4 (2s22p2) and O - 6 (2s22p4) -2 charge – 2e-
4 + (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-
2 single bonds (2x2) = 41 double bond = 4
8 lone pairs (8x2) = 16Total = 24
20
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+ -
OOO+-
O C O
O
- -O C O
O
-
-
OCO
O
-
-
What are the resonance structures of the carbonate (CO3
2-) ion?
21
Exceptions to the Octet Rule
The Incomplete Octet
H HBeBe – 2e-
2H – 2x1e-
4e-
BeH2
BF3
B – 3e-
3F – 3x7e-
24e-
F B F
F
3 single bonds (3x2) = 69 lone pairs (9x2) = 18
Total = 24
22
Exceptions to the Octet Rule
Odd-Electron Molecules
N – 5e-
O – 6e-
11e-
NO N O
The Expanded Octet (central atom with principal quantum number n > 2)
SF6
S – 6e-
6F – 42e-
48e-
S
F
F
F
FF
F
6 single bonds (6x2) = 1218 lone pairs (18x2) = 36
Total = 48
23Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chemical Bonding IIMolecular Geometry and
Hybridization of Atomic Orbitals
24
Valence shell electron pair repulsion (VSEPR) model:
Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs.
AB2 2 0
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
linear linear
B B
26
AB2 2 0 linear linear
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0trigonal planar
trigonal planar
28
AB2 2 0 linear linear
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0trigonal planar
trigonal planar
AB4 4 0 tetrahedral tetrahedral
30
AB2 2 0 linear linear
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0trigonal planar
trigonal planar
AB4 4 0 tetrahedral tetrahedral
AB5 5 0trigonal
bipyramidaltrigonal
bipyramidal
32
AB2 2 0 linear linear
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0trigonal planar
trigonal planar
AB4 4 0 tetrahedral tetrahedral
AB5 5 0trigonal
bipyramidaltrigonal
bipyramidal
AB6 6 0 octahedraloctahedral
34
Predicting Molecular Geometry1. Draw Lewis structure for molecule.
2. Count number of lone pairs on the central atom and number of atoms bonded to the central atom.
3. Use VSEPR to predict the geometry of the molecule.
What are the molecular geometries of SO2 and SF4?
SO O
AB2E
bent
S
F
F
F F
AB4E
distortedtetrahedron
35
Dipole Moments and Polar Molecules
H F
electron richregion
electron poorregion
Dipole moment is defined as magnitude of charge (Q) times distance (r) between the charges. u = (Q)(r) Q charge in coulombs (C) r distance between charges in meters (m)
37
Which of the following molecules have a dipole moment? H2O, CO2, SO2, and CF4
O HH
dipole momentpolar molecule
SO
O
CO O
no dipole momentnonpolar molecule
dipole momentpolar molecule
C
F
FFF
no dipole momentnonpolar molecule
40
Bond Enthalpy Bond Length
H2
F2
436.4 kJ/mol
150.6 kJ/mol
74 pm
142 pm
Valence bond theory – bonds are formed by sharing of e- from overlapping atomic orbitals.
Overlap Of
2 1s
2 2p
How does Lewis theory explain the bonds in H2 and F2?
Sharing of two electrons between the two atoms.
42
Valence Bond Theory and NH3
N – 1s22s22p3
3 H – 1s1
If the bonds form from overlap of 3 2p orbitals on nitrogen with the 1s orbital on each hydrogen atom, what would the molecular geometry of NH3 be?
If use the3 2p orbitalspredict 90o
Actual H-N-Hbond angle is
107.3o
43
Hybridization – mixing of two or more atomic orbitals to form a new set of hybrid orbitals.
1. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals.
2. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process.
3. Covalent bonds are formed by:
a. Overlap of hybrid orbitals with atomic orbitals
b. Overlap of hybrid orbitals with other hybrid orbitals
48
# of Lone Pairs+
# of Bonded Atoms Hybridization Examples
2
3
4
5
6
sp
sp2
sp3
sp3d
sp3d2
BeCl2
BF3
CH4, NH3, H2O
PCl5
SF6
How do I predict the hybridization of the central atom?
1. Draw the Lewis structure of the molecule.
2. Count the number of lone pairs AND the number of atoms bonded to the central atom
Stoichiometry
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
50
By definition: 1 atom 12C “weighs” 12 amu
On this scale
1H = 1.008 amu
16O = 16.00 amu
Atomic mass is the mass of an atom in atomic mass units (amu)
Micro Worldatoms & molecules
Macro Worldgrams
51
The average atomic mass is the weighted
average of all of the naturally occurring
isotopes of the element.
52
Naturally occurring lithium is:
7.42% 6Li (6.015 amu)
92.58% 7Li (7.016 amu)
(7.42 x 6.015) + (92.58 x 7.016)100
= 6.941 amu
Average atomic mass of lithium:
54
The mole (mol) is the amount of a substance that contains as many elementary entities as there are atoms in exactly 12.00 grams of 12C
1 mol = NA = 6.0221367 x 1023
Avogadro’s number (NA)
Dozen = 12
Pair = 2
The Mole (mol): A unit to count numbers of particles
55
Molar mass is the mass of 1 mole of in gramseggsshoes
marblesatoms
1 mole 12C atoms = 6.022 x 1023 atoms = 12.00 g
1 12C atom = 12.00 amu
1 mole 12C atoms = 12.00 g 12C
1 mole lithium atoms = 6.941 g of Li
For any element
atomic mass (amu) = molar mass (grams)
57
x 6.022 x 1023 atoms K1 mol K
=
How many atoms are in 0.551 g of potassium (K) ?
1 mol K = 39.10 g K
1 mol K = 6.022 x 1023 atoms K
0.551 g K 1 mol K39.10 g K
x
8.49 x 1021 atoms K
58
Molecular mass (or molecular weight) is the sum ofthe atomic masses (in amu) in a molecule.
1S 32.07 amu
2O + 2 x 16.00 amu SO2 64.07 amu
For any molecule
molecular mass (amu) = molar mass (grams)
1 molecule SO2 = 64.07 amu
1 mole SO2 = 64.07 g SO2
SO2
59
How many H atoms are in 72.5 g of C3H8O ?
1 mol C3H8O = (3 x 12) + (8 x 1) + 16 = 60 g C3H8O
1 mol H = 6.022 x 1023 atoms H
5.82 x 1024 atoms H
1 mol C3H8O molecules = 8 mol H atoms
72.5 g C3H8O1 mol C3H8O
60 g C3H8Ox
8 mol H atoms
1 mol C3H8Ox
6.022 x 1023 H atoms
1 mol H atomsx =
60
Formula mass is the sum of the atomic masses (in amu) in a formula unit of an ionic compound.
1Na 22.99 amu
1Cl + 35.45 amuNaCl 58.44 amu
For any ionic compound
formula mass (amu) = molar mass (grams)
1 formula unit NaCl = 58.44 amu
1 mole NaCl = 58.44 g NaCl
NaCl
61
What is the formula mass of Ca3(PO4)2 ?
1 formula unit of Ca3(PO4)2
3 Ca 3 x 40.08
2 P 2 x 30.97
8 O + 8 x 16.00310.18 amu
62
Percent composition of an element in a compound =
n x molar mass of elementmolar mass of compound
x 100%
n is the number of moles of the element in 1 mole of the compound
C2H6O
%C =2 x (12.01 g)
46.07 gx 100% = 52.14%
%H =6 x (1.008 g)
46.07 gx 100% = 13.13%
%O =1 x (16.00 g)
46.07 gx 100% = 34.73%
52.14% + 13.13% + 34.73% = 100.00%
63
Percent Composition and Empirical Formulas
Determine the empirical formula of a compound that has the following percent composition by mass: K 24.75, Mn 34.77, O 40.51 percent.
nK = 24.75 g K x = 0.6330 mol K1 mol K
39.10 g K
nMn = 34.77 g Mn x = 0.6329 mol Mn1 mol Mn
54.94 g Mn
nO = 40.51 g O x = 2.532 mol O1 mol O
16.00 g O
64
Percent Composition and Empirical Formulas
K : ~~ 1.00.63300.6329
Mn : 0.63290.6329
= 1.0
O : ~~ 4.02.532
0.6329
nK = 0.6330, nMn = 0.6329, nO = 2.532
KMnO4
65
g CO2 mol CO2 mol C g C
g H2O mol H2O mol H g H
g of O = g of sample – (g of C + g of H)
Combust 11.5 g ethanol
Collect 22.0 g CO2 and 13.5 g H2O
6.0 g C = 0.5 mol C
1.5 g H = 1.5 mol H
4.0 g O = 0.25 mol O
Empirical formula C0.5H1.5O0.25
Divide by smallest subscript (0.25)
Empirical formula C2H6O
Combusted 11.5 g ethanol
Q. What is the Empirical formula of Ethanol
66
3 ways of representing the reaction of H2 with O2 to form H2O
A process in which one or more substances is changed into one or more new substances is a chemical reaction
A chemical equation uses chemical symbols to show what happens during a chemical reaction
reactants products
67
How to “Read” Chemical Equations
2 Mg + O2 2 MgO
2 atoms Mg + 1 molecule O2 makes 2 formula units MgO
2 moles Mg + 1 mole O2 makes 2 moles MgO
48.6 grams Mg + 32.0 grams O2 makes 80.6 g MgO
NOT
2 grams Mg + 1 gram O2 makes 2 g MgO
68
Balancing Chemical Equations
1. Write the correct formula(s) for the reactants on the left side and the correct formula(s) for the product(s) on the right side of the equation.
Ethane reacts with oxygen to form carbon dioxide and water
C2H6 + O2 CO2 + H2O
2. Change the numbers in front of the formulas (coefficients) to make the number of atoms of each element the same on both sides of the equation. Do not change the subscripts.
2C2H6 NOT C4H12
69
Balancing Chemical Equations
3. Start by balancing those elements that appear in only one reactant and one product.
C2H6 + O2 CO2 + H2O start with C or H but not O
2 carbonon left
1 carbonon right
multiply CO2 by 2
C2H6 + O2 2CO2 + H2O
6 hydrogenon left
2 hydrogenon right
multiply H2O by 3
C2H6 + O2 2CO2 + 3H2O
70
Balancing Chemical Equations
4. Balance those elements that appear in two or more reactants or products.
2 oxygenon left
4 oxygen(2x2)
C2H6 + O2 2CO2 + 3H2O
+ 3 oxygen(3x1)
multiply O2 by 72
= 7 oxygenon right
C2H6 + O2 2CO2 + 3H2O72
remove fractionmultiply both sides by 2
2C2H6 + 7O2 4CO2 + 6H2O
71
Balancing Chemical Equations
5. Check to make sure that you have the same number of each type of atom on both sides of the equation.
2C2H6 + 7O2 4CO2 + 6H2O
Reactants Products
4 C12 H14 O
4 C12 H14 O
4 C (2 x 2) 4 C
12 H (2 x 6) 12 H (6 x 2)
14 O (7 x 2) 14 O (4 x 2 + 6)
72
1. Write balanced chemical equation
2. Convert quantities of known substances into moles
3. Use coefficients in balanced equation to calculate the number of moles of the sought quantity
4. Convert moles of sought quantity into desired units
Amounts of Reactants and Products
73
Methanol burns in air according to the equation
2CH3OH + 3O2 2CO2 + 4H2O
If 209 g of methanol are used up in the combustion, what mass of water is produced?
grams CH3OH moles CH3OH moles H2O grams H2O
molar massCH3OH
coefficientschemical equation
molar massH2O
209 g CH3OH1 mol CH3OH
32.0 g CH3OHx
4 mol H2O
2 mol CH3OHx
18.0 g H2O
1 mol H2Ox =
235 g H2O
74
Limiting Reagent:
2NO + O2 2NO2
NO is the limiting reagent
O2 is the excess reagent
Reactant used up first in the reaction.
75
In one process, 124 g of Al are reacted with 601 g of Fe2O3
2Al + Fe2O3 Al2O3 + 2Fe
Calculate the mass of Al2O3 formed.
g Al mol Al mol Fe2O3 needed g Fe2O3 needed
OR
g Fe2O3 mol Fe2O3 mol Al needed g Al needed
124 g Al1 mol Al
27.0 g Alx
1 mol Fe2O3
2 mol Alx
160. g Fe2O3
1 mol Fe2O3
x = 367 g Fe2O3
Start with 124 g Al need 367 g Fe2O3
Have more Fe2O3 (601 g) so Al is limiting reagent
76
Use limiting reagent (Al) to calculate amount of product thatcan be formed.
g Al mol Al mol Al2O3 g Al2O3
124 g Al1 mol Al
27.0 g Alx
1 mol Al2O3
2 mol Alx
102. g Al2O3
1 mol Al2O3
x = 234 g Al2O3
2Al + Fe2O3 Al2O3 + 2Fe
At this point, all the Al is consumed and Fe2O3 remains in excess.
77
Theoretical Yield is the amount of product that wouldresult if all the limiting reagent reacted.
Actual Yield is the amount of product actually obtainedfrom a reaction.
% Yield = Actual Yield
Theoretical Yieldx 100%
Reaction Yield
78
1. What is the character of the bond in carbon monoxide?
a. ionicb. polar covalentc. non-polar covalentd. coordinate covalent
79
2. Which of the following is the bet explanation of the phenomenon of hydrogen bonding?a. Hydrogen has a strong affinity for holding onto
valence electronsb. Hydrogen can only hold two valence electronsc. Electron negative atoms disproportionately
carry shared pairs when bonded to hydrogend. Hydrogen bonds have ionic character
80
3. Which of the following has a formula weight between 74 and 75 grams per mole?a. KClb. C4H10Oc. (LiCl)2
d. BF3
81
4. In the reactoin shown, if 39.03 g of Na2S are reacted with 113.3 g of AgNO3, how much, if any, of either reagent will be left over once the reaction has gone to completion?
Na2S+ 2 AgNO3Ag2S + 2 NaNO3
a. 41.37 g AgNO3 b. 13.00 g Na2Sc. 26.00 g Na2Sd. 74.27 g AgNO3
83
Sulfur dioxide oxidizes in the presence of O2 gas as per the reaction
2 SO2 + O2 2 SO3
Approximately how many grams of sulfur trioxide are produced by the complete oxidation of 1 mole of sulfur dioxide?
a. 1gb. 2gc. 80gd. 160 g
85
When an electron moves from a 2p to a 3s orbital, the atom containng that electron:
a. becomes a new isotopeb. becomes a new elementc. absorbs energyd. releases energy
87
5. One way to test for the presence of iron in solution is by adding potassium thiocyanate to the solution. The product when this reagent reacts with iron is FeSCN2+, which creates a dark red color in solution via the following net ionic equation.
Fe 3+ (aq) + SCN-FeSCN2+
How many grams of iron sulfate would be needed to produce 2 moles of
a. 400 gb. 800 gc. 200 gd. 500 g