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SCH4U – UNIT 1 STRUCTURE AND PROPERTIES
CHAPTER 4 – CHEMICAL BONDING
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Activity With a partner discuss everything you remember about chemical bonding Eg. Types of bonds? why? What happens?
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4.1 Types of Chemical Bonds
What are the two main types of chemical bonds? Ionic: chemical bond between oppositely charged ions Electrostatic attraction Covalent: a chemical bond in which atoms share bonding electrons Bonding Electron Pair: electron pair that is involved in bonding
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Bond type depends on the attraction for electrons of the atoms involved
i.e. electronegativity
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Ionic Compounds
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How do these work? Metal + Non-Metal Metal+ + Non-Metal- Low IE High IE Isoelectronic with noble gases Low EA High EA Opposites attract in no particular direction, considered non-directional Ions cling together in clusters known as crystals
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Get a lattice structure
Lattice energy: energy change when one mole of an ionic substance is formed from its gaseous ions Depends on: Charge on the ions Size of the ions
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Ionic Compounds and Bonding
Properties – WHY? Do not conduct electric current in the solid state Conduct electric current in the liquid state When soluble in water, form good electrolyte Relatively high MP and B Brittle, easily broken under stress
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Covalent Bonds Balance of attractive and repulsive forces
What are the forces acting here?
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Octet Rule Atoms share electrons so that they are surrounded by 8 electorns # bonds = 8 - # valence electorn Example: Carbon, Oxygen, Nitrogen Two covalent bonds = double bond Three covalent bonds = triple bond
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Lewis Structures Atoms and ions are stable if they have a full valence shell Electrons are most stable when they are pair Atoms form chemical bonds to achieve full valence shells of electrons Full valence shell may occur by an exchange or by sharing electrons Sharing – covalent; exchange - ionic
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Polar Covalent Bonds When electrons are shared unevenly in a covalent bond Example: HF, H2O
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Coordinate Covalent Bonds
Both electrons are contributed by one atom Example: NH4+ H3O+ CO N2O NHO3
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Resonance Structures Single bonds are longer than double bonds, which are longer than triple bonds Example: SO3 Resonance Structure: Electron pair is shared over three bond evenly Delocalized electrons
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Less than 8 BeH2 BCl3
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More than 8 Octet rule only applies to the first two periods
After that, can have expanded octets Example: PF5 BrF5 SiF63-
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Practice - Worksheet H2 F2 OF2 O2F2
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Valence Bond Theory and Quantum Mechanics
Covalent bonds occur when orbitals overlap and two electrons occupy the same region of space Example: H2
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HF What are the electron configurations for H and F?
How would the orbitals interact
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H2O What are the electron configurations for H and O?
How would the orbitals interact
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Problem We know from experiments in atomic structure that the bond angle in H2O is 104.5°… not 90° as predicted by valence bond theory True for CH4 (109.5°) and NH3 (107.5°) – VBT always predicts bond angles of 90° So, we need a better theory…
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Hybrization Two problems still exist from Lewis Bonding Theory
Carbon atoms form 4 EQUAL C-H bonds in CH4 (or any other molecule) Not predicted due to electron configuration of C Recall: s orbitals have lower energy than p orbitals, therefore the bond length would be different Existence of double and triple bonds
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Hybridization of Carbon Orbitals
An s electron gets promoted to the empty p-orbital This stabilizes the p- and s- orbitals and gives them all the same energy; Half-filled subshells Called sp3 orbitals (HYBRID ORBITALS) Each sp3 orbital lies at 109.5°
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Additional Hybrid Orbitals – sp - LINEAR
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Additional Hybrid Orbitals – sp2 - PLANAR
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Additional Hybrid Orbitals – sp3 - Tetrahedral
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Double and Triple Bonds
Two types of orbital overlap exist What we have seen so far is one type Sigma bonds: σ-bonds End-on-end overlap of orbitals Pi bonds:π-bonds Sideways overlap of orbitals
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Sigma Bonds Occur in single bonds and account for the FIRST bond in a double or triple bond Examples:
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Pi Bonds Occur when p-orbitals not on the bonding axis (py or pz) overlap with each other
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Making Double Bonds Example: C2H4 Draw a Lewis Structure
What occurs with the C atoms hybridization?
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For double bonds, there must be one σ-bond from overlapping hybrid orbitals and one π-bond from overlapping py or pz orbitals Come from sp2 hybridized orbitals and result in trigonal planar structures
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Making Triple Bonds Triple bonds have one σ-bond and two π-bonds; come from sp-hybridized orbitals, and result in linear structures Central atom has two un-hybridized p-orbitals
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Practice Explain the structure of the following molecules using electron configurations, orbital hybridization and VBT. C2Cl4 C2Cl2 CO2
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VSEPR Theory - Valence Shell Electron Pair Repulsion Theory
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Work through VSEPR Chart
Fun times with molecular structure…
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Practice Problems Use Lewis Theory and VSEPR Theory to predict the structure of the following molecules: Homework/Practice - Worksheet
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Polar Molecules Polar molecules are molecules where the electron charge is not distributed evenly
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Electronegativity and Polar Covalent Bonds
Ionic Bond: ΔEN = >1.7 Electron transfer Polar Covalent Bond: ΔEN = Electrons shared unevenly Pure Covalent Bond: ΔEN = Electrons shared evenly Remember: Think of electrons as electron probabilities, electron cloud density is greater around one atom or another, therefore one gets a slight negative, the other slight positive charge
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Think of the scale as a continuum
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Polar Molecules Cannot exist if there are no polar bonds!
Bond dipole: electronegativity difference of two atoms represented by an arrow pointing from the positive to the negative end (lower to higher EN) Non-polar molecule: either perfectly symmetrical so the bond dipoles cancel out, or when no polar bonds exist Polar molecule: occur when bond dipoles do not cancelout
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Example: Determine the polarity of the following molecules H2O, CCl4, NH3, PCl5 Practice: CH3Cl, BeCl2, SiO2, BrF4 CHF2Cl CH3NH2
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Intermolecular Forces
Forces that exist between molecules Three types: Dipole-Dipole Hydrogen Bonds London Dispersion In order to determine the Intermolecular Forces (IMF), you need to first determine the polarity of the molecule Much weaker than covalent bonds
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Dipole-Dipole Forces Occur in polar molecules
The slightly negative end on one molecule is attracted to the slightly positive end on another molecule Strength depends on the size of the dipole
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London Dispersion Forces
Simultaneous attraction of the electrons in one molecule to the nuclei in the surrounding molecules Increase as the number of electrons and protons in a molecule increase Exist in ALL molecules Weakest Force
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Hydrogen Bonds Attraction between H on one molecule and O, N, or F on another molecule Strongest of the intermolecular forces Found in H2O, NH3, and HF, or whenever there is a –OH, -NH2 in a molecule
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Predicting Strength of IMF
Use pol
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Predicting Boiling Points
Boiling points increase as IMF strength increases Arrange the following molecules in order of increasing boiling points SiH4, SnH4, GeH4, CH4, C3H8, C2H4, C4H10 CH4, CCl3H, CBr3H
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Practice Determine the intermolecular forces that exist in each molecule CCl4, C5H12, CH3CH2OH Which molecules would have the strongest IMF C2H5OH, C2H6, C2H5Cl Explain you answer
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Structure and Properties of Solids
Different types of solids result depending on the type of bonding in the solid These solids have different properties
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Ionic Crystals Crystal Lattice
Properties result from the lattice structure Brittle, high melting/boiling point, conduct electricity when dissolved in water or in liquid form, hard
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Molecular Crystals Arrangement of neutral molecules held together by weak intermolecular forces Properties vary depending on the strength of the IMF
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Covalent Network Solids
Array of covalently bonded atoms, structure is held together by covalent bonds High MP/BP Example: Silicate (SiO)
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Carbon Network Solids Diamond, Graphite, Carbon Nanotubes, Buckminster Fullerenes Explain the difference in properties between graphite and diamond.
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Metallic Crystals Lots of electrons, but low ionization energy means they are loosely held Lots of empty valence orbitals with similar energy, therefore electrons are free to move around Strong, non-directional bonding
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Properties of Metals Property Explanation Shiny, Silvery Flexible
Electrical Conductivity Hard Solids Crystalline
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End of UNIT 1 – STRUCTURE AND PROPERTIES!!
Your unit test is on: October 28 Review package handed out
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