CHEMISTRY - UTEXAS 1E CH.5 - INTERMOLECULAR FORCES (IMFS)

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2 CONCEPT: POLARITY Molecules that have sharing of electrons contain a molecular polarity. For these molecules, both and determine the molecular polarity. POLARITY RULES TO BEING NON-POLAR: 1) If central element has NO lone pair(s): a. Central element must be connected to the elements. b. Central element must be electronegative than the surrounding elements. PRACTICE 1: Determine if carbon dioxide, CO2, is polar or nonpolar. 2) If central element has lone pair(s): a. Central element must be connected to the elements. b. Central element must be electronegative than the surrounding elements. c. Use dipole arrows to point to the electronegative element. These dipole arrows must cancel out. d. Dipole arrows extend lone pairs. These lone pair dipole arrows must cancel out. PRACTICE 2: Determine if xenon tetrafluoride, XeF4, is polar or nonpolar. Page 2

3 CONCEPT: POLARITY PT.2 Molecules that have unequal sharing of electrons contain a molecular polarity. Some molecular shapes are seen as perfect and will always lead to a non-polar molecule overall. POLARITY RULES TO BEING NON-POLAR: 1) A perfect shape will be non-polar as long as: a. The central element is connected to the same elements. b. The central element is less electronegative than the surrounding elements. 2 Groups 3 Groups F O C O H C N AX 2 - Linear B F F AX 3 - Trigonal Planar Sn F F AX 2 E 1 - Bent, Angular or V-Shaped 4 Groups Cl N C H Cl H H O Cl Cl H H AX 4 - Tetrahedral AX 3 E 1 - Trigonal Pyramidal AX 2 E 2 - Bent, Angular or V-Shaped 5 Groups Cl Cl P Cl Cl Cl AX 5 - Trigonal Bipyramidal F F S F F AX 4 E 1 - Seesaw F Cl F F AX 3 E 2 - T-Shaped F Xe F AX 2 E 3 - Linear 6 Groups Cl Cl Cl S Cl Cl Cl AX 6 - Octahedral F F S F F F AX 5 E 1 - Square Pyramidal H H Xe H H AX 4 E 2 - Square Planar EXAMPLE 1: Determine if silicon tetrachloride, SiCl4, is polar or nonpolar. EXAMPLE 2: Determine if phosphorus trihydride, PH3, is polar or nonpolar. Page 3

4 PRACTICE: POLARITY PRACTICE 1: Determine if the following compound is polar or nonpolar. a. SiBr 4 2- PRACTICE 2: Determine if the following compound is polar or nonpolar. a. H 2 S PRACTICE 3: Determine if the following compound is polar or nonpolar. a. PCl 2 F 3 PRACTICE 4: Determine if the following compound is polar or nonpolar. a. IF 2 Page 4

5 CONCEPT: INTERMOLECULAR FORCES When looking at a molecular substance such as H 2 O you will discover two types of electrostatic forces at work: forces exist within a molecule and influences the properties of the substance. forces exist between molecules and influence the properties of the substance. is the force that exists between an ion and a polar compound. (Strongest) Ex: is the force that exists when H is directly connected F, O, N. (2 nd Strongest) Ex: is the force that exists when two polar covalent compounds interact. (3 rd Strongest) Ex: is the force that exists when a nonpolar covalent compound interacts with a polar covalent compound. (4 th Strongest) Ex: is the force that exists when two nonpolar covalent compounds interact. (Weakest) Ex: Page 5

6 PRACTICE: INTERMOLECULAR FORCES EXAMPLE: Based on the given compounds, answer each of the following questions: a. CH3CH3 b. KBr c. C6H5OH d. CaS e. Ne a) Which compound will have the lowest boiling point? b) Which compound will have the highest surface tension. c) Which compound will have the highest vapor pressure. PRACTICE 1: The predominant intermolecular force in C6H5NH2 is: a. London Dispersion b. Hydrogen Bonding c. Ion-Dipole d. Dipole-Dipole e. Dipole-induced Dipole PRACTICE 2: The predominant intermolecular force in HBr is: a. London Dispersion b. Hydrogen Bonding c. Ion-Dipole d. Dipole-Dipole e. Dipole-induced Dipole PRACTICE 3: The predominant intermolecular force in ZnBr2 with H2O is: a. London Dispersion b. Hydrogen Bonding c. Ion-Dipole d. Dipole-Dipole e. Dipole-induced Dipole PRACTICE 4: The predominant intermolecular force in Ne with H2O is: a. London Dispersion b. Hydrogen Bonding c. Ion-Dipole d. Dipole-Dipole e. Dipole-induced Dipole Page 6

7 CONCEPT: SOLUBILITY According to the theory of dissolves compounds with the same intermolecular force or polarity will dissolve into each other. EXAMPLE: Identify the intermolecular forces present in both the solute and the solvent, and predict whether a solution will form between the two. a. CCl4 and P4 b. CH3OH and C6H6 c. C6H5CH2NH2 and HF d. IF4 and NH3 PRACTICE: Which of the following statements is/are true? a. Methane will dissolve completely in acetone, CH3COCH3. b. Hydrofluoric acid (HF) will form a heterogeneous mixture with tetrachloride, CCl4. c. Pentane will form a homogeneous mixture with CBr4. d. Methanethiol (CH3SH) is miscible in fluoromethane (CH3F). Page 7

8 CONCEPT: SOLID STATE STRUCTURAL FEATURES Solids are grouped into two broad categories based on the organization of their particles and shapes: solids. Tend to have well-established shapes because their particles occur in an organized pattern. solids. Tend to have poor-established shapes because their particles lack an overall organized pattern. When examining the particles within a crystal you may observe them tightly packed in an organized pattern. The point represents the area within the crystal that has identical surroundings all around. The represents the smallest portion of the crystal that, if reproduced in all three directions, would give the crystal. Page 8

9 CONCEPT: THE SIMPLE CUBIC UNIT CELL There are crystal systems and types of unit cells that naturally occur, but we will only focus on those that follow cubic lattice system. The simple cubic unit cell is made of a cube with one atom at each corner. The edge length is twice the radius of the atoms so. Then unit cell does not contain 8 atoms, but actually only atom because each corner atom is shared by eight other unit cells. The packing efficiency is equal to. The coordination number is equal to. Unit Cell Diagram Lattice Diagram Space Filling Unit Cell Diagram Page 9

10 CONCEPT: THE BODY-CENTERED CUBIC UNIT CELL The body-centered cubic unit cell is made of a cube with one atom at each corner and one atom of the same kind in the center of the cube. The edge length is equal to. The packing efficiency is equal to. The coordination number is equal to. Unit Cell Diagram Lattice Diagram Space Filling Unit Cell Diagram Page 10

11 CONCEPT: THE FACE-CENTERED CUBIC UNIT CELL The face-centered cubic unit cell is made of a cube with one atom at each corner and one atom of the same kind in the center of each cube face. The edge length is equal to. The packing efficiency is equal to. The coordination number is equal to. Unit Cell Diagram Lattice Diagram Space Filling Unit Cell Diagram Page 11

12 PRACTICE: THE CUBIC UNIT CELL (CALCULATIONS 1) EXAMPLE 1: An element crystallizes in a face-centered cubic lattice and it possesses a density of 2.03 g/cm 3. The edge of its unit cell is 4.11 x 10-8 cm. How many atoms are in each unit cell? EXAMPLE 2: Based on Example 1, what is the volume of a unit cell? EXAMPLE 3: Based on Example 1, what is the mass a unit cell? EXAMPLE 4: Based on Example 1, calculate the approximate atomic mass of the element. Page 12

13 PRACTICE: THE CUBIC UNIT CELL (CALCULATIONS 2) EXAMPLE 1: KF has the same type of crystal structure as NaCl. The unit cell of KF has an edge length of 5.39 angstroms. Find the density of KF. EXAMPLE 2: Which one of the following is not a general property of ionic solids? a) Hard and brittle b) Good electrical conductors in the solid state c) Relatively high melting points d) Strongest interparticle attractions are electrostatic e) Positions of ions define the unit cell (lattice) EXAMPLE 3: Lead crystallizes in a face-centered cubic structure. What is the edge length of the unit cell if the atomic radius of gold is 180 pm? a) 185 pm b) 288 pm c) 360 pm d) 509 pm Page 13

14 PRACTICE: THE CUBIC UNIT CELL (CALCULATIONS 3) EXAMPLE 1: Chromium metal crystallizes in a body-centered cubic structure with a unit cell edge length of 2.89 angstroms. The radius of a chromium atom is angstroms. a) 0.91 b) 3.90 c) 2.25 d) 1.25 e) 1.57 EXAMPLE 2: A solid has a very high melting point, great hardness, and poor electrical conduction. This is a(n) solid. Example: Rubies a) covalent network b) metallic and covalent network c) ionic d) metallic e) molecular EXAMPLE 3: Determine the radius of an Ag atom (in pm) if the density of silver is 10.5 g/cm 3. Silver crystallizes in a face centered cubic structure with an edge length of 4r 2. a) 144 pm b) 186 pm c) 83 pm d) 303 pm e) 127 pm Page 14