Chem 122 Fall 2024 PreLecture Notes PDF

Chem 122 General College Chemistry – ”Matter and Energy” at UIC Chapter 12 Section 3 Intermolecular Forces Fall 2024 Dr. Rita Hatfield...

Chem 122 General College Chemistry – ”Matter and Energy” at UIC Chapter 12 Section 3 Intermolecular Forces Fall 2024 Dr. Rita Hatfield Reminders Discussion Worksheet 15 posted. Chem 122 Final Exam is Week 14 HW due tonight. Tuesday, Dec. 10th from 6-8 PM SB Ch 12/13 due Fri., Dec. 6th. Week 15 HW due Fri., Dec. 6th. 1  Chapter 12: Intermolecular Forces: Liquids, Solids, and Phase Changes 12.1 An Overview of Physical States and Phase Changes 12.2 Quantitative Aspects of Phase Changes 12.3 Types of Intermolecular Forces 12.4 Properties of the Liquid State  The Nature of Intermolecular Forces Ch 12 S 3 Intermolecular forces arise from the attraction between ← stronger covalent molecules with partial charges bond or between ions and molecules. Intermolecular forces are relatively Bonding forces > IMF weak compared to bonding forces because They involve smaller charges that are farther apart. |||| Weaker “H-bond” between 2 water molecules  Intermolecular Attractions The strength of the attractions between the particles of a substance determines its state. At room temperature, moderate to strong attractive forces result in materials being solids or liquids. The stronger the attractive forces are, the higher the boiling point (and melting point) of the substance will be (More on that later!) Comparison of Bonding Forces (intramolecular force) Bonds within the substance Basis of Energy Force Model Attraction (kJ/mol) Example NaCl Ionic Cation–anion 400–4000 (one formula unit) Nuclei– H─H Covalent 150–1100 shared e−pair (one molecule) Cations– Metallic delocalized 75–1000 Fe electrons Notice the energy associated with bonding forces is much greater than that of IMFs (see next slide for IMF values) Comparison of Non-Bonding Forces (Intermolecular forces- IMFs) Notice two substances interact. Force Model Basis of Attraction Energy (kJ/mol) Example Ion–dipole Ion charge–dipole charge 40–600 Overall strongest Polar bond to H-dipole H bond charge (high EN - N, O, F) 10–40 Note kJ/mol values Dipole–dipole Dipole charges 5–25 l─Cl⋯l─Cl are much Lower for IMFs Ion–induced Ion charge– 3–15 Fe2+⋯O2 dipole polarizable e−cloud than bonds. Dipole– Dipole charge– induced polarizable e−cloud 2–10 H─Cl⋯Cl─Cl dipole Overall Dispersion Polarizable e−clouds 0.05–40 F─F⋯F─F weakest (London)  Dispersion (London) Forces attractive forces that are caused primarily by the temporary polarization in the molecules due to unequal electron distribution. All substances will have dispersion forces. Acknowledgement: Dr. Montano Factors Affecting Strength of Induced Dipole Polarizability of the particles Volume of the electron cloud Larger molar mass = more electrons = larger electron cloud = increased polarizability = stronger attractions Shape of the molecule More surface-to-surface contact = larger induced dipole = stronger attraction Effect of Molecular Size on Size of Dispersion Force Effect of Molecular Shape on Size of Dispersion Force  Dipole-dipole Forces attractive forces that are caused permanent polarization in the molecules due to unequal electron distribution caused by their structure net vector https://en.wikipedia.org/wiki/Formaldehyde#/ Polar molecule media/File:Formaldehyde-2D.svg  Hydrogen-bonding Forces A special type of dipole-dipole forces that result when H is bonded to a small, electronegative atom with lone pairs - specifically F, O, N. It can H-bond with an F, O, and N F/O/N H F/O/N CH3OH  PROBLEM: Which of these substances exhibits H bonding? Draw examples of the H bonds between two molecules of each substance that does. (a) C2H6 (b) A little information is included on (1) vapor pressure and (2) boiling points are so that you can understand the boiling point trends with strength of intermolecular forces.  What factors affect vapor pressure (VP)? Vapor pressure is the pressure exerted by the vapor (gas) when in equilibrium with the liquid phase. Two factors affect VP (1) a change in temperature (2) a change in the gas itself (the type and/or strength of intermolecular forces (IMF). (1) As temperature ↑, the fraction of molecules with enough energy to enter the vapor phase ↑, and the VP ↑. (2) The weaker the IMF, the more easily particles enter the vapor phase, and the VP ↑.  Vapor Pressure and Boiling Point The boiling point of a liquid is the temperature at which the vapor pressure = the external pressure. The normal boiling point of a substance is observed at standard atmospheric pressure (760 torr). As the external pressure on a liquid increases, the boiling point increases. Summarizing VP Trends in Graph – 3 different molecules Notice the normal boiling points for these three different substances! VP ↓ as the strength IMF ↑ IMF trend: H2 O > CH3 CH2 OH > CH3 CH2 −O − CH2 CH3 Normal BP trends follow IMF trends  Effect of Dipole–Dipole Attraction on Boiling Points Ethane 30.03 g/mol 185 K Formaldehyde 30.07 g/mol 254 K  Dipole Moment and Boiling Point 450 400 350 300 Boiling Point (K) 250 200 150 100 propane dimethyl ether acetaldehyde acetonitrile Similar molar 50 44.09 g/mol 46.07 g/mol 44.05 g/mol 41.05 g/mol mass substances. 0 How does the 8 130 269 392 dipole moment affect boiling point? Dipole Moment (D) *Figures of molecules from wikipedia Hydrogen Bonding and Boiling Point This graph represents the boiling point for purse substances as shown. Physical Properties and Forces Between Particles Substance Boiling Vapor Surface Viscosity Major (Formula) Point Pressure Tension (N s/m2) @ Force(s) (oC) (atm) (J/m2) @ 20oC 20oC Diethyl ether Dipole-dipole; CH3CH2OCH2CH3 34.6 0.7 1.7x10-2 0.240×10−3 dispersion Ethanol CH3CH2OH 78.4 0.08 2.3×10−2 1.20×10−3 H bonding Butanol H bonding; CH3CH2CH2CH2OH 117.7 0.006 2.5×10−2 2.95×10−3 dispersion  Identifying the Types of Intermolecular Forces PROBLEM: For each substance, identify the key bonding and/or intermolecular force(s), and predict which substance of the pair has the higher boiling point: (a) MgCl2 or PCl3 (b) CH3NH2 or CH3F (c) CH3OH or CH3CH2OH (d) Hexane (CH3CH2CH2CH2CH2CH3) or 2,2-dimethylbutane (a) MgCl2 ionic → higher boiling point PCl3 dipole-dipole (b) CH3NH2 H-bonding → higher boiling point CH3F dipole-dipole (c) CH3OH H-bonding CH3CH2OH H-bonding → Longer and higher MM → higher b.p. (d) hexane dispersion forces → Greater intermolecular contact → higher b.p 2,2-dimethyl dispersion forces butane Chapter 13 - The Properties of Mixtures: Solutions and Colloids 13.1 Types of Solutions: Intermolecular Forces and Solubility  Solutions Chapter 13 Section 1 A solution is a homogeneous mixture of two or more substances and exists as a single phase. A solution consists of a solute and a solvent. NaCl crystals + water The solute is the substance present in a smaller mole quantity and the solvent is the substance present in a larger mole quantity. NaCl mixed with water The solute is dissolved in the solvent. A saturated solution has the maximum amount of solute dissolved in solvent under given conditions. NaCl crystals dissolving in water  Ion-dipole Forces Intermolecular forces that result when an ion and a nearby polar molecule (ones with dipole-dipole forces) attract each other. This will occur when you have two different substances! Notice how the + water molecules surround each ion. + - + - + - + - - + - + - + - - + + - + - + - + - soluble ionic compound  Types of Intermolecular Forces in Solutions What happens when you put a particle (molecule, ions, etc) in a solvent? Strongest Weakest  Solutions and Solubility – Terms to know The solubility (S) of a solute is the maximum amount that dissolves in a fixed quantity of solvent at a given temperature. Solvent and solute that are soluble in any proportion are Salt dissolves in water to form a salt solution. “miscible”. Substances that exhibit similar types of IMF – ion dipole intermolecular force dissolve in each other. “Like dissolves in like.” Image taken from Burdge and Driessen, Intro chemistry, Atoms First Approach  Solutions and Intermolecular Forces When a solution forms: Solute-solute attractions and solvent-solvent attractions are replaced by solute-solvent attractions. This can only occur if the forces within the solute and solvent are similar to the forces that replace them. Ions – dipole attractions between water interacting with Na+ and Cl- compete with and overcome attractions between Na+ and Cl-.  Dual Polarity and Effects on Solubility Alcohols are organic compounds that have dual polarity. The general formula for an alcohol is CH3(CH2)nOH. The –OH group of an alcohol is polar. It interacts with water through H bonds and, with hexane through weak dipole-induced dipole forces. The hydrocarbon portion is nonpolar. It interacts through weak dipole-induced dipole forces with water. And through dispersion forces with hexane.  “Like Dissolves Like”: Solubility of Methanol in Water In a small alcohol such as methanol: The H bonds within solute and within solvent are replaced by H bonding between solute and solvent. Smaller alcohols are miscible with water.  Solubility (mol alcohol/1000 g solvent) of a Series of Alcohols in Water and in Hexane Solubility in Solubility in Alcohol Model Water Hexane Least CH3 groups CH3OH (methanol) ∞ 1.2 CH3CH2OH (ethanol) ∞ ∞ CH3(CH2)2OH (1-propanol) ∞ ∞ CH3(CH2)3OH (1-butanol) 1.1 ∞ CH3(CH2)4OH (1-pentanol) 0.30 ∞ Most CH3 groups CH3(CH2)5OH (1-hexanol) 0.058 ∞  Examples of IMFs in Biological Macromolecules – Important but not on the exam. 13.2 A Membrane The Double Helix of DNA Phospholipid Wikipedia Image Amino Acids can be Classified by Their Side Chains