Presentation on theme: "2013 General Chemistry I * The coverage of the Quiz (Q) * The coverage of the Study Summary (SS) WeekPeriodChapterChemical Principles / Atkins, JonesNote."— Presentation transcript:
2013 General Chemistry I * The coverage of the Quiz (Q) * The coverage of the Study Summary (SS) WeekPeriodChapterChemical Principles / Atkins, JonesNote 1st3/4-3/7 2nd3/11-3/14 3rd3/18-3/21 4th3/25-3/ Ionic Bonds, Lewis Structures and Covalent Bonds SS: Chap 2 (3/25) 5th4/1-4/ VSEPR and Valence-Bond Theory Q: Chap 1 (4/1) 6th4/8-4/ Molecular Orbital Theory SS: Chap 3 (4/8) 7th4/15-4/18 Q: Chap 2 (4/15) 8th4/22-4/25Mid-term Exam(Chapters 1-3) 9th4/29-5/ The Gas Laws, Molecular Motion 10th5/6-5/ Real Gases, Intermolecular ForcesSS: Chap 4 (5/6) 11th5/13-5/ Liquids and Solids SS: Chap 5 (5/13) 12th5/20-5/ Enthalpy Q: Chap 4 and 5 (5/20) 13th5/27-5/ Chemical Change and Entropy SS: Chap 7 and 8 (5/27) 14th6/3-6/ Gibbs Free Energy Q: Chap 7 (6/3) 15th6/10-6/13Additional Class / Review 16th6/17-6/20Final Exam(Chapters 4, 5, 7, 8)
2013 General Chemistry I Chapter 2. CHEMICAL BONDS 2013 General Chemistry I IONIC BONDS COVALENT BONDS 2.1 The Ions That Elements Form 2.2 Lewis Symbols 2.3 The Energetics of Ionic Bond Formation 2.4 Interactions Between Ions 2.5 Lewis Structures 2.6 Lewis Structures of Polyatomic Species 2.7 Resonance 2.8 Formal Charge
2013 General Chemistry I Understanding the bond formation between atoms ⇒⇒⇒ understanding properties and reactivity of materials Designing new materials Chapter 2. CHEMICAL BONDS ( 화학결합 ) Lowering of energy by rearranging valence electrons Ionic bond( 이온결합 ); electron transfer + electrostatic attraction, NaCl Covalent bond( 공유결합 ); sharing electrons, NH 3 Metallic bond( 금속결합 ); cations held by a sea of electrons, copper Lewis structure ( 루이스구조 ) Octet rule ( 옥텟 규칙 ) coordinate covalent bond ( 배위결합 ) Resonance ( 공명 ) Formal charge ( 형식전하 ) Oxidation number ( 산화수 )
2013 General Chemistry I Key IdeasBond formation by lowering of energy Goal Understanding the bond formation between atoms ⇒⇒⇒ Designing new materials Chapter 2. CHEMICAL BONDS Chemical bond is the link between atoms. Lowering of energy by rearranging valence electrons Ionic bond; electron transfer + electrostatic attraction, NaCl Covalent bond; sharing electrons, NH 3 Metallic bond; cations held by a sea of electrons, copper How are we going to apply Q.M. knowledge ?
2013 General Chemistry I IONIC BONDS (Sections ) 55 ionic model: the description of bonding in terms of ions ionic solid: three-dimensional crystalline solid an assembly of cations and anions stacked together in a regular pattern Metal + nonmetal
2013 General Chemistry I 2.1 The Ions That Elements Form 56 Cations: Remove outermost electrons in the order np, ns, (n-1)d Metallic elements in the s block and on the left of the p block in Period 3 (Al) ⇒ lose electrons down to their noble-gas cores; 1s 2 (duplet) or ns 2 np 6 (octet) Li +, Be 2+, Na +, Mg 2+, Al 3+, ··· Metallic elements of the p block in Periods 4 and later ⇒ lose electrons down to their noble-gas cores surrounded by d 10 ; (n – 1)d 10 ns 2 np 6 The delectrons, in most cases, cannot be lost. Ga 3+ ([Ar]3d 10 ), ··· Elements in the d block ⇒ ns-electrons first, then variable number of (n – 1)d-electrons Fe 2+ ([Ar]3d 6 ), Fe 3+ ([Ar]3d 5 ), ···
2013 General Chemistry I 56 Anions: Add electrons until the next noble-gas configuration is reached Many elements in the p and d blocks; variable valence ⇒ Inert-pair effect; p-electrons alone or all their valence p- and s-electrons The order of losing electrons: In +, In 3+, Sn 2+, Sn 4+, ···
2013 General Chemistry I 2.2 Lewis Symbols 58 - valence electrons – depicted as dots; a pair of dots for paired electrons - cations and anions (1916)
2013 General Chemistry I Formulation of ionic compounds 1) Cations by removing all dots from metal atoms: 2) Anions by adding dots to complete the valence shell (octet or duplet): 3) Adjust the number of each element to conserve the total number of dots. 4) Write the charge of each ion: CaCl 2 ; empirical formula
2013 General Chemistry I 2.3 The Energetics of Ionic Bond Formation NaCl ionic crystal formation; electron transfer from Na to Cl and then Coulomb stabilization The energy required for the formation of ionic bonds is supplied largely by the attraction between oppositely charged ions.
2013 General Chemistry I
2.4 Interactions Between Ions 59 -In an ionic solid, each cation is attracted to all the anions to a greater or lesser extent. → a “global” characteristic of the entire crystal i.e. ionic bond is not a bond between two ions ! Lattice energy: the difference in energy between the ions packed together in a solid and the ions widely separated as a gas - strong electrostatic interactions in ionic solids → high melting points and brittleness
2013 General Chemistry I - Coulomb potential energy of the interactions of two individual ions 60 e is the fundamental charge; z 1 and z 2 are the charge numbers of the two ions; r 12 is the distance between the centers of the ions; 0 is the vacuum permittivity. 2.4 Interactions Between Ions
2013 General Chemistry I 61 - In a one-dimensional crystal in which cations and anions alternate along a line, ; A = 2 ln2 or molar potential energy of a three-dimensional crystal The factor A is the Madelung constant, dependent on how the ions are arranged about one another toward right hand side toward left hand side
2013 General Chemistry I Short-range repulsions between ions Total potential energy = E P + E P * - real potential energy of an ionic solid → attractive and repulsive interionic interactions in close range.
2013 General Chemistry I Born-Meyer equation; correction for repulsions to the Madelung constant repulsive effect The coulombic interaction between ions in a solid is large when the ions are small and highly charged.
2013 General Chemistry I COVALENT BONDS "With brilliant insight, and before anyone knew about quantum mechanics or orbitals, Lewis proposed" Chemical bonding in H 2 + by sharing an electron between two protons An electron between the two nuclei exerts an attractive force on the nuclei. Quantum mechanical view of covalent bond
2013 General Chemistry I 2.5 Lewis Structures Covalent bonding; octet (or duplet) by sharing (Lewis, 1916) Nonmetal atoms share electrons to complete their octet; lines (bonding pairs), dots (lone pairs) - octet rule: atoms go as far as possible toward completing their octets - A line (-) represents a shared pair of electrons : a bond.
2013 General Chemistry I 2.6 Lewis Structures of Polyatomic Species 64 - Lewis structure does not portray the 3D shape of a molecule or ion, but simply displays which atoms are bonded together. - bond order: the number of bonds that link a specific pair of atoms. - Each atom completes its octet by sharing pairs of electrons. Methane; CH 4 (why not CH 3 nor CH 5 ?)
2013 General Chemistry I Writing a Lewis structure - terminal atom: bonded to only one other atom central atom: bonded to at least two other atoms 65 - The element with the lowest ionization energy (less greedy) as the central atom electronegativity is a better indicator Ex. HCN - Atoms symmetrically around the central atom; SOS for S 2 O (Exception; NNO) - OH is attached to the central atom in oxoacids; HO–Cl for HClO i.e. H 2 SO (HO) 2 SO 2 - Polyatomic ions; total number of electrons should be adjusted to represent the overall charge
2013 General Chemistry I Toolbox 2.1: How to write the Lewis structure of a polyatomic species Step 1: # of electron pairs = total # of valence electrons / 2 Step 2: Write down the most likely arrangements of atoms. Step 3: One electron pair between each pair of bonded atoms Step 4: Complete the octet (duplet for H) of each atom using the remaining electron pairs. Step 5: A line for each bonding pair Take care of charges. Form multiple bonds if in short of electron pairs.
2013 General Chemistry I
Ex 2.4 Lewis structures for CH 3 COOH (multiple central atoms) # of electron pairs = ( ) / 2 = 12 CH 3 COOH Lewis structures for C 2 O 2 H 4 (multiple central atoms) How many structures can you draw?
2013 General Chemistry I Lewis structures for NO 3 - # of electron pairs = ( ) / 2 = double-headed arrows (↔), indicating a blend of the contributing structures - delocalization: a shared electron pair is distributed over several pairs of atoms and cannot be identified with just one pair of atoms. Identical N–O bond lengths of 124 pm (> 120 pm for N=O, < 140 pm for N–O)
2013 General Chemistry I 69 Benzene, C 6 H 6 - All the carbon-carbon bonds with the same length - Only one 1,2-dichlorobenzen exists.
2013 General Chemistry I 2.8 Formal Charge 70 Formal charge – the charge it would have if the bonding were perfectly covalent in the sense that the atom had exactly a half-share in the bonding electrons V = the number of valence electrons in the free atom L = the number of electrons present on the bonded atom as lone pairs B = the number of bonding electrons on the atom Formal charge indicates the degree of redistribution of electrons relative to free atoms not the real charge of an atom - A Lewis structure in which the formal charges of the individual atoms are closest to zero typically represents the lowest energy arrangement of the atoms and electrons.
2013 General Chemistry I OCO with lower formal charges is more likely for CO 2 than COO. NNO with lower formal charges is more likely for ON 2 than NON. - Formal charge exaggerates the covalent character of bonds by assuming that the electrons are shared equally. - Oxidation number exaggerates the ionic character of bonds. It represents the atoms as ions, and all the electrons in a bond are assigned to the atom with the lower ionization energy (higher electronegativity (2.12)). formal charge oxidation state HCN has lower formal charges than HNC.
2013 General Chemistry I Formal charges differ from oxidation numbers! Neither of them is the true charge. Quantum mechanically, there is no true localized charge on an atom! Ca 2+ is an oxidation state of calcium with the oxidation number of “+2”. Oxidation number is important in following the oxidation-reduction reaction. - octet rule: In covalent bond formation, atoms go as far as possible toward completing their octets by sharing electron pairs. There are many exceptions to the octet rule
2013 General Chemistry I 2.9 Radicals and Biradicals 72 Radicals: species with an unpaired electron, highly reactive Biradicals: molecules with two unpaired electrons 2.10 Expanded Valence Shells Odd number of electrons # of electron pairs = (5 + 2 x 6) / 2 = 8.5 Expanded valence shell (expanded octet); large atoms with empty d-orbitals (Period 3 or later) may accommodate more than 8 electrons. PCl 5 vs. NCl 5
2013 General Chemistry I 72 - hypervalent compound: a compound that contains an atom with more atoms attached to it than is permitted by the octet rule ST 2.10B Linear I 3 – ion 3x7 + 1 = 22 electrons, 11 electron pairs 2 bonds, = 8 lone pairs Remaining one pair into the central I
2013 General Chemistry I Lewis structures for C 2 O 2 H 4 (multiple central atoms) # of electron pairs = ( ) / 2 =
2013 General Chemistry I = 32 valence electrons, 16 electron pairs Ex 2.8 Dominant resonance Lewis structure of SO 4 2– - variable covalence: the ability to form different numbers of covalent bonds most preferred structure number of resonance structures
2013 General Chemistry I THE PERIODICITY OF ATOMIC PROPERTIES 39
2013 General Chemistry I 2.11 The Unusual Structures of Some Group 13/III Compounds 76 - incomplete octet: fewer than eight valence electrons - boron and aluminum - completing octets by a coordinate covalent bond, in which both electrons come from one of the atoms
2013 General Chemistry I 2.11 The Unusual Structures of Some Group 13/III Compounds 76 Possible due to the atomic radius of Al in AlCl 3 larger than that of B in BCl 3. borane
2013 General Chemistry I Chapter 2. CHEMICAL BONDS Ionic bond; electron transfer + electrostatic attraction, NaCl Covalent bond; sharing electrons, NH 3 Formal charge – the charge it would have if the bonding were perfectly covalent in the sense that the atom had exactly a half-share in the bonding electrons Oxidation number– exaggerates the ionic character of bonds. It represents the atoms as ions, and all the electrons in a bond are assigned to the atom with the lower ionization energy (higher electronegativity ). Nonpolar covalent bond; the average charge on each atom is zero.
2013 General Chemistry I 76 - partial charges: the charges on the atoms - polar covalent bond: a bond in which ionic contributions to the resonance result in partial charges lower energy structure - electric dipole: a partial positive charge next to an equal but opposite partial negative charge size of an electric dipole ---- electric dipole moment ( ) Unit: Debye (D) the average charge on each atom is not zero. definition: a dipole between electron and proton separated by 100 pm is 4.80D
2013 General Chemistry I Electronegativity ( ) – Electron-pulling power of an atom when it is part of a molecule (by Linus Pauling in 1932) Correcting the Covalent Model: Electronegativity Cl-H bond: = ~1.1 D : Cl has ~23% of an electron’s charge Rough guide to the charge separation in a bond between two atoms Average based on measured bond energies from a large range of compounds; can be revised Measure of extra stability due to ionic contributions
2013 General Chemistry I 77 - Mulliken scale: = ½(I + E ea ) average of the ionization energy and electron affinity 2.12 Correcting the Covalent Model: Electronegativity Mulliken’s electronegativity scale (1934); properties of an isolated atom Exactly defined
2013 General Chemistry I 77 - rough rules of thumb ionic polar covalent covalent i.e. NaCl or KF : ionic C-O : polar covalent Ca-O : ionic
2013 General Chemistry I
2.13 Correcting the Ionic Model: Polarizability 78 - All ionic bonds have some covalent character. - highly polarizable atoms and ions: readily undergo a large distortion of their electron cloud i.e. large anions and atoms such as I -, Br -, and Cl - - polarizing power: property of ions (and atoms) that cause large distortions of electron clouds - increases with decreasing size and increasing charge of a cation i.e. the small and/or highly charged cations Li +, Be 2+, Mg 2+, and Al 3+
2013 General Chemistry I THE STRENGTHS AND LENGTHS OF COVALENT BONDS Bond Strength Dissociation energy (D): energy required to separate the bonded atoms - The bond breaking is homolytic, which means that each atom retains one of the electrons from the bond. - average dissociation energy for one type of bond found in different molecules i.e. C-H single bond: average strength of bonds in a selection of organic molecules, such as methane (CH 4 ), ethane (C 2 H 6 ), and ethene (C 2 H 4 ) measured by
2013 General Chemistry I 2.15 Variation in Bond Strength 80 Strongest bond between two nonmetal atoms; CO (1062 kJ/mol) Lone pair-lone pair repulsion due to the short F–F distance
2013 General Chemistry I 80
2013 General Chemistry I How can we explain the discrepancies ?
2013 General Chemistry I
ATP(aq) + H 2 O ADP(aq) + H 2 PO 4 – (aq) kJ/mol bond stiffness ( bond strength); resistance to stretching and compressing will be discussed in the Major Technique 1: Infrared (IR) Spectroscopy
2013 General Chemistry I 2.16 Bond Length 82 Bond length: the distance between the centers of two atoms joined by a covalent bond - corresponding to the internuclear distance at the potential energy minimum for the two atoms - affecting the overall size and shape of a molecule evaluated by using spectroscopy or x-ray diffraction methods - Factors influencing bond length
2013 General Chemistry I Box 2.2Bond length measurements quantum mechanical rotational energy with a rotational quantum number J microwave spectroscopy Reproducibility of bond lengths; constant within a few percent in similar arrangements
2013 General Chemistry I
82 Covalent radius: contribution an atom to the length of a covalent bond Bond length: Approximately the sum of the covalent radii of the two atoms - Decreases from left to right (increasing Z eff ) - Increases in going down a group (size of valence shells and better shielding by inner core electrons)
2013 General Chemistry I bond stiffness ( bond strength); resistance to stretching and compressing INFRARED SPECTROSCOPY Infrared radiation: electromagnetic radiation with longer wavelengths (lower frequencies) than red light ~ 1000 nm or ~ 3×10 14 Hz - Molecules by infrared radiation become vibrationally excited.
2013 General Chemistry I 90 - “stretching” mode: the atoms moving closer and away again. “bending” mode: bond angles periodically increase and decrease. Vibrational frequencies - The stiffness of a bond measured by its force constant, k Force = -k × displacement by Hooke’s law - Vibrational frequency,, of a bond between two atoms A and B of mass m A and m B = effective mass (or reduced mass)
2013 General Chemistry I 91 Normal modes of vibration of polyatomic molecules A nonlinear molecule consisting of N atoms → 3N-6 normal modes A linear molecule → 3N-5 normal modes i.e. H 2 O, n = 3 → 3 normal modes CO 2, n = 3 → 4 normal modes
2013 General Chemistry I 91 Actual spectrum Characteristic frequencies of functional groups detectable in a spectrum - fingerprint region: a complex series of absorptions