3/5/2014. iclicker Participation Question: A. MgS < AlP < NaCl B. MgS < NaCl < AlP C. NaCl < AlP < MgS D. NaCl < MgS < AlP

Transcription

1 Today: Ionic Bonding vs. Covalent Bonding Strengths of Covalent Bonds: Bond Energy Diagrams Bond Polarities: Nonpolar Covalent vs. Polar Covalent vs. Ionic Electronegativity Differences Dipole Moments Molecular Polarities: Molecular Symmetry QUIZ 2 Next Week in Lab: Covering Lectures 7-10 See study guide on Canvas Next Monday: Read Chp. 10, Sections , pp iclicker Participation Question: Predicting Melting Points from Structure Rank the following in order of INCREASING melting point: Magnesium sulfide (MgS), Aluminum Phosphide (AlP), & Sodium Chloride (NaCl). A. MgS < AlP < NaCl B. MgS < NaCl < AlP C. NaCl < AlP < MgS D. NaCl < MgS < AlP NaCl: 801 o C MgS: 2226 o C AlP: 2530 o C E. Not enough information provided DIRECTION: How do periodic properties & electron configurations influence physical properties like melting point, boiling point, surface tension, viscosity, and elasticity? 1

2 Ionic Compounds versus Molecular Covalent Compounds Substance Melting Point ( o C) Boiling Point ( o C) Phase at Room Temperature HCl GAS HF GAS/LIQUID C 8 H LIQUID NaCl SOLID B 4 C SOLID Ionic Compounds: High melting points & high boiling points Molecular Covalent Compounds: Low melting points & low boiling points Ionic vs. Covalent Bonding Ionic Bonds: A transfer of electrons makes one atom negatively charged (an anion) & one atom positively charged (a cation) But why do different combinations of elements produce different types of bonds? Covalent Bonds: Two atoms sharing electrons with a mutual attraction to the negative charge holding the nuclei together Electronegativity: Measuring an element s pull on electrons Ionic Bonds form between elements with a large difference in electronegativities: typically when METALS combine with NON-METALS Covalent Bonds occur between elements with similar electronegativities: when NON-METALS bond with other NON-METALS Table of Electronegativities A table of electronegativities will always be provided on exams if needed 2

3 Bond Energy Diagram: Quantifying the strength of a covalent bond Bond Dissociation Energies: Quantifying the strength of a covalent bond Shorter Bonds = Stronger Bonds Higher Bond Order = Stronger Bonds iclicker Participation Question: Understanding Bond Energy Diagrams The three curves below represent the bonds for C F, C O & C=O. Which curve would correspond to the C O bond? A. BLACK B. BLUE C. RED D. NONE of these E. Not enough information provided 3

4 A Continuum of Bonding In reality, few bonds are purely ionic or purely covalent The electronegativity difference ( EN) between two atoms will allow us determine whether two atoms in a bond will display more ionic properties or more covalent properties. Bond Polarity Electronegativity Differences generate POLES of charge separation Bond Dipole Moment Measuring the degree of CHARGE SEPARATION δ: Lower case delta symbols indicate a build up of partial charge at an atom NOT FULL IONIZED, though. δ+ δ- Bond Dipole Moment: A vector quantifying the degree of charge separation across a bond. Points in the direction of partial negative charge. 4

5 A Continuum of Bond Polarities: Electrostatic Potential Energy Maps: Tracking electron distribution to understand how molecules react and interact with each other iclicker Participation Question: Bond Polarities from Electronegativities Which bond below would be the most polarized with the largest dipole moment? A. C N B. C H C. N O D. N F E. O H 5

6 Bond Polarity influences Molecular Polarity: and molecular polarity influences how molecules interact with one another POLAR MOLECULES: 1. Contain POLAR bonds 2. Have ASYMMETRIC SHAPES that cause the bond dipole moments to add together NONPOLAR MOLECULE POLAR MOLECULE Without Polar Bonds a Molecule CANNOT be Polar: Nonpolar bonds generate nonpolar molecules ΔEN: = First consider bond polarities: If all bonds are NONPOLAR, then the molecule is also NONPOLAR. ONLY if polar bonds are present, MIGHTthe molecule be polar. Sometimes even Polar Bonds DO NOT Guarantee a Polar Molecule: If a molecule is completely symmetric, the dipole moments cancel out and the molecule overall is NONPOLAR Even though B F bonds are strongly POLAR, overall BF 3 is NONPOLAR because it has a trigonal planar molecular geometry ΔEN: = First consider bond polarities. If polar bonds are present, the molecule MIGHT be polar. 2. Then consider MOLECULAR GEOMETRY. If the molecule is symmetric, it CANNOT be polar. 6

7 Molecular SHAPE influences Molecular POLARITY Molecular Geometry Influences Molecular Polarity: Symmetric Molecules are NEVER polar, even if they have polar bonds Asymmetric Molecules WITH POLAR BONDS pull electrons unevenly across the structure, this makes a POLARIZED molecule REMEMBER: Even if a molecule is asymmetric, it CANNOT be polar if it only has NONPOLAR bonds. Symmetric vs. Asymmetric Molecules: Symmetric Molecules are NEVER polar, even if they have polar bonds Asymmetric Molecules WITH POLAR BONDS pull electrons unevenly across the structure, this makes a POLARIZED molecule 7

8 Is Sulfur Dioxide POLAR or NONPOLAR? ΔEN: = First write the Lewis dot structure. 2. Use a table of electronegativities to calculate ΔEN for all bonds present. If ΔEN is less than or equal to 0.4, the bonds are NONPOLAR. STOP HERE. The molecule must also be NONPOLAR. If ΔEN is greater than 0.4, polar bonds are present & the molecule MIGHT be polar (depending on molecular shape). 3. Next, use the dot structure to count the number of electron regions & determine the MOLECULAR GEOMETRY. If the molecule is SYMMETRIC, it is NONPOLAR. If the molecule is ASYMMETRIC, it is POLAR. iclicker Participation Question: Comparing Relative Molecular Polarities Which molecule below is the MOST POLAR? A. CH 4 B. CHF 3 C. CF 4 D. CCl 4 E. They are all NONPOLAR Is Carbon Tetrachloride POLAR or NONPOLAR? Tetrahedral ΔEN: = First write the Lewis dot structure. 2. Use a table of electronegativities to calculate ΔEN for all bonds present. If ΔEN is less than or equal to 0.4, the bonds are NONPOLAR. STOP HERE. The molecule must also be NONPOLAR. If ΔEN is greater than 0.4, polar bonds are present & the molecule MIGHT be polar (depending on molecular shape). 3. Next, use the dot structure to count the number of electron regions & determine the MOLECULAR GEOMETRY. If the molecule is ASYMMETRIC, it is POLAR. If the molecule is SYMMETRIC, it is NONPOLAR. 8

9 Predicting Reactions with Polarity Applying Coulomb s Law to understand how molecules interact and react 9