Ch 12(3), 13(1) Intermolecular forces PDF

Ch. 12(3) Types of Intermolecular Forces Chemistry The Molecular Nature of Matter and Change Ninth Edition Martin S. Silberberg and Patricia G. Amateis ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted...

Ch. 12(3) Types of Intermolecular Forces Chemistry The Molecular Nature of Matter and Change Ninth Edition Martin S. Silberberg and Patricia G. Amateis ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw-Hill Education. Molecular Attractive Forces Intramolecular forces or chemical bonding are found within each molecule. The chemical behavior of each phase of matter is the same because the same constituent particle is present in each case. – H2O molecules are present whether the substance is in the solid, liquid, or gas phase. Intermolecular or nonbonding forces are found between molecules. The physical behavior of each phase of matter is different because the strength of these forces differs from state to state. ©McGraw-Hill Education. 12.3 The Nature of Intermolecular Forces Intermolecular forces arise from the attraction between molecules with partial charges, or between ions and molecules. Intermolecular forces are relatively weak compared to bonding forces because they involve fractional charges (< 1) that are farther apart. Intermolecular forces were first postulated in 1873 by J.D. van der Waals, Dutch physicist. ©McGraw-Hill Education. Johannes D. van der Waals (1837 – 1923) Nobel Prize in Physics (1910) Van der Waals Equation of State for a Real Gas The van der Waals equation adjusts the ideal gas law to take into account: – the real volume of the gas particles and – the effect of inter-particle attractions. Van der Waals equation for n moles of a real gas 𝑛2 𝑎 𝑃 + 2 𝑉 − 𝑛𝑏 = 𝑛𝑅𝑇 𝑉 The term a/V2 was introduced as correction of pressure due to intermolecular forces of attraction. The constant b relates to particle volume. ©McGraw-Hill Education. Covalent and Van Der Waals Radii The van der Waals distance is the average distance between two nonbonded atoms in adjacent molecules in a solid. The van der Waals radius is one-half the closest distance between the nuclei of two nonbonded atoms. The VDW radius is always larger than the covalent radius. ©McGraw-Hill Education. Periodic Trends In Covalent And Van Der Waals Radii ©McGraw-Hill Education. Comparison of Bonding Forces Basis of Attraction Energy (kJ/mol) Example Ionic Cation–anion 400–4000 NaCl Covalent Nuclei– shared e−pair 150–1100 H─H Metallic Cations– delocalized electrons 75–1000 Fe Force ©McGraw-Hill Education. Model Comparison of Non-Bonding Forces Basis of Attraction Energy (kJ/mol) Ion–dipole Ion charge– dipole charge 40–600 H bond Polar bond to H-dipole charge (high EN of N, O, F) 10–40 Dipole–dipole Dipole charges 5–25 l─Cl⋯l─Cl Ion–induced dipole Ion charge– polarizable e−cl 3–15 oud Fe2+⋯O2 Dipole– induced dipole Dipole charge– polarizable e−cl 2–10 oud H─Cl⋯Cl─Cl Dispersion (London) Polarizable e−clouds F─F⋯F─F Force ©McGraw-Hill Education. Model 0.05–40 Example Ion-Dipole Forces: dissolved ions in water Figure 4.2 NaCl(s) Solvation: surrounding with solvent molecules ©McGraw-Hill Education. aq = aqua Na+(aq) + hydrated Na Cl (aq) hydrated Cl- Electron Distribution in Molecules of H2 and H2O Water is a polar molecule, meaning that it has an uneven distribution of electron density around it. The density is higher around O and diminished around H. At submolecular level, water has a negative pole at O and a positive pole half-way between the 2 H atoms. Because of this electric inhomogeneity, water is able to attract both positive ions and negative ions. ©McGraw-Hill Education. Polar Molecules and Dipole-Dipole Forces Figure 12.13 ©McGraw-Hill Education. Dipole-Dipole forces Only polar molecules exhibit dipole-dipole forces. The larger the molecular dipole moment, the stronger the attraction. For molecules with similar molar masses: the stronger the dipole-dipole forces, the higher the boiling point. Dipole-dipole force (potential) is weaker than the ion-dipole force. Dipole-dipole force is NOT the dominant attraction force between molecules with molecular dipole moment. The dominant force is the dispersion (London) force (covered later). ©McGraw-Hill Education. Dipole Moment and Boiling Point Figure 12.14 These compounds have similar molar masses. The boiling point increases with the value of dipole moment. ©McGraw-Hill Education. The Hydrogen Bond The Hydrogen bond is a special, stronger dipole-dipole force. Hydrogen bonding is possible for molecules that have a hydrogen atom covalently bonded to a small, highly electronegative atom with lone electron pairs, specifically N, O, or F. A hydrogen bond only forms with these 3 elements. An intermolecular hydrogen bond is the attraction between the H atom of one molecule and a lone pair of the N, O, or F atom of another molecule. ©McGraw-Hill Education. Hydrogen Bonding and Boiling Point Figure 12.15 ©McGraw-Hill Education. Sample Problem 12.4 – Problem and Plan Drawing Hydrogen Bonds Between Molecules of a Substance 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) CH3OH (c) PLAN: If the molecule does not contain N, O, or F, it cannot form H bonds. If it contains any of these atoms covalently bonded to H, we draw two molecules in the -B:⋯H-Apattern. ©McGraw-Hill Education. Sample Problem 12. - Solution SOLUTION: (a) No N, O, or F, so no H bonds can form. (b) The H covalently bonded to the O in one molecule forms an H bond to the lone pair on the O of an adjacent molecule: (c) Two of these molecules can form one H bond between an H bonded to N and the O, or they can form two such H bonds: ©McGraw-Hill Education. Polarizability and Induced Dipoles A nearby electric field can induce a distortion in the electron cloud of an atom, ion, or molecule. – For a nonpolar molecule, this induces a temporary dipole moment. – For a polar molecule, the field enhances the existing dipole moment. The polarizability of a particle is the ease with which its electron cloud is distorted. https://www.youtube.com/watch?v=MosUfb61kro https://www.youtube.com/watch?v=pIy0iMVrkB0 ©McGraw-Hill Education. Trends in Polarizability Smaller particles are less polarizable than larger ones because their electrons are held more tightly. Polarizability increases down a group because atomic size increases and larger electron clouds distort more easily. Polarizability decreases across a period because of decreasing atomic radii. Cations are smaller than their parent atoms and less polarizable. Anions are larger than their parent atoms and more polarizable. ©McGraw-Hill Education. Dispersion Forces Among Nonpolar Particles When atoms are farther apart than their van der Waals radius, they do not influence one other. When atoms are close together, the instantaneous dipole in one atom induces a dipole in the other. The process occurs throughout the sample. ©McGraw-Hill Education. Dispersion (London) Forces Dispersion forces or London forces arise when an instantaneous dipole in one particle induces a dipole in another, resulting in an attraction between them. Dispersion forces exist between all particles, increasing the energy of attraction in all matter. Dispersion forces tend to be the significant attraction force. – In HCl, attraction due to London forces represents 85%. Only 15% is due to dipole-dipole forces. – In H2O, H-bonding accounts for 75% of the attraction. Dispersion forces are stronger for more polarizable particles. – In general, larger particles experience stronger dispersion forces than smaller ones. ©McGraw-Hill Education. Molar Mass and Trends In Boiling Point Dispersion forces are stronger for larger, more polarizable particles. Polarizability correlates closely with molar mass for similar particles. Molar mass may be used as proxy for polarizability and dispersion forces in qualitative comparisons. Figure 12.17 ©McGraw-Hill Education. Molecular Shape, Intermolecular Contact, and Boiling Point Figure 12.18 Linear molecules have larger boiling point than the branched molecules. ©McGraw-Hill Education. Determining the Intermolecular Forces In a Sample Figure 12.19 ©McGraw-Hill Education. Sample Problem 12.5 – Problem and Plan 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 PLAN: We examine the formulas and structures for key differences between members of the pair: Are ions present? Are molecules polar or nonpolar? Is N, O, or F bonded to H? Do the molecules have different masses or shapes? ©McGraw-Hill Education. Sample Problem 12.5 – Solution (a) and (b) SOLUTION: (a) MgCl2 consists of Mg2+ and Cl− ions held together by ionic bonding forces; PCl3 ,with a trigonal pyramidal geometry, consists of polar molecules, so intermolecular dipoledipole forces are present. The forces in MgCl2 are stronger, so it should have a higher boiling point. (b) CH3NH2 and CH3F both consist of polar molecules of about the same molar mass. CH3NH2 has N─H bonds, so it can form H bonds. CH3F contains a C─F bond but no H─F bond, so dipole-dipole forces occur but not H bonds. Therefore, CH3NH2 should have the higher boiling point. ©McGraw-Hill Education. Sample Problem 12.5 – Solution (c) and (d) SOLUTION: (c) CH3OH and CH3CH2OH molecules both contain an O─H bond, so they can form H bonds.CH3CH2OH has an additional ─CH2─ group and thus a larger molar mass, which correlates with stronger dispersion forces; therefore, it should have a higher boiling point. (d) Hexane and 2,2-dimethylbutane are nonpolar molecules of the same molar mass but different molecular shapes. Cylindrical hexane molecules make more intermolecular contact than more compact 2,2dimethylbutane molecules do, so hexane should have stronger dispersion forces and a higher boiling point. ©McGraw-Hill Education. Problem 12.5A,B – Follow-up 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) CH3Br or CH3F (b) CH3CH2CH2CH2OH or CH3CH2OCH3 (c) C2H6 or C3H8 (d) CH3CHO or CH3CH2OH (e) SO2 or CO2 ©McGraw-Hill Education. Ch. 13(1) Liquid Solutions: Types of Intermolecular Forces, Solubility Rules Chemistry The Molecular Nature of Matter and Change Ninth Edition Martin S. Silberberg and Patricia G. Amateis ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw-Hill Education. 4.1. Refresher on Solutions A solution is a homogeneous mixture of two or more compounds: uniform composition and physical properties at molecular scale in any and all directions. a solute is any of the compounds present in quantity smaller than 50%. the solvent is the substance present in the largest quantity. The physical state of the solvent usually determines the physical state of the solution. A liquid solvent typically generates a liquid solution – the focus in this chapter. Liquid solutions are by far the most common medium to study reactions: easy to use, reactants mix quickly at molecular scale. What types of intermolecular forces generate dissolving and mixing of compounds? ©McGraw-Hill Education. Solutions and Solubility The solubility (S) of a solute is the maximum amount that dissolves in a fixed quantity of solvent at a given temperature. ▪ Solubilities of different solutes in water: S(NaCl/H2O) = 36.0 g / 100 g at 25°C, S(AgCl/H2O) = 0.00019 g / 100g at 25°C ▪ Solubilities in different solvents: S(NaCl/CH3CH2OH) = 0.065 g / 100 g at 25°C ▪ Solubility also depends on the temperature. Solute and solvent that are soluble in each other in any proportion are miscible; liquids that do not dissolve in each other are immiscible. “Like-dissolves-like” rule of solubility: Substances that exhibit similar types of intermolecular forces dissolve in each other. ©McGraw-Hill Education. Types of Intermolecular Forces in Solutions ©McGraw-Hill Education. Electron Distribution in H2O Water is a polar molecule, meaning that it has an uneven distribution of electron density around it. The density is higher around O and diminished around H. At submolecular level, water has a negative pole at O and a positive pole half-way between the 2 H atoms: a molecular dipole moment. Because of this electric inhomogeneity, water is able to attract both positive ions and negative ions (see next slide). ©McGraw-Hill Education. An Ionic Compound Dissolving in Water: from solid ionic crystal to hydrated individual ions Figure 4.2 NaCl(s) Solvation: surrounding with solvent molecules ©McGraw-Hill Education. aq = aqua Na+(aq) + hydrated Na Cl (aq) hydrated Cl- Hydration Shells Around an Na+ Ion Figure 13.2. Types of intermolecular forces: ion-dipole between Na+ and H2O, H-bonding and dipole-dipole between H2O. ©McGraw-Hill Education. Dissolving 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. – Ionic compounds are typically soluble in water. Exceptions: precipitates. – Ionic compounds are insoluble in non-polar compounds (e.g., hydrocarbons) – Oil (non-polar) is insoluble in water (polar, hydrogen-bonding) – Oil is soluble in hydrocarbons. ©McGraw-Hill Education. Dual Polarity and Effects on Solubility Alcohols are organic compounds that have dual polarity. The general formula for an alcohol is CH3(CH2)nOH. 1) The –OH group of an alcohol is polar. It interacts with water through H bonds and, with hexane through weak dipole-induced dipole forces. 2) The hydrocarbon portion, CH3(CH2)n is nonpolar. It interacts through weak dipole-induced dipole forces with water. And through dispersion forces with hexane. ©McGraw-Hill Education. Solubility (mol alcohol/1000 g solvent) of a Series of Alcohols in Water and in Hexane Solubility in Water Solubility in Hexane 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 ∞ CH3(CH2)5OH (1-hexanol) 0.058 ∞ Alcohol ©McGraw-Hill Education. Model Like Dissolves Like: Solubility of Methanol in Water In an alcohol: – the H-bonding within solute and within water is replaced by H-bonding between solute and solvent. – The extended dispersion forces between the alkyl groups (CnH2n+1), however, are not properly replaced by dispersion forces of similar magnitude with water. Thus, only smaller alcohols are miscible with water. ©McGraw-Hill Education. Sample Problem 13.1: Problem and Plan Predicting Relative Solubilities PROBLEM: Predict which solvent will dissolve more of the given solute. (a) Sodium chloride in methanol (CH3OH) or in 1-propanol (CH3CH2CH2OH). (b) Ethylene glycol (HOCH2CH2OH) in hexane (CH3CH2CH2CH2CH2CH3) or in water. (c) Diethyl ether (CH3CH2OCH2CH3) in water or in ethanol (CH3CH2OH). PLAN: We examine the formulas of solute and solvent to determine the forces in and between solute and solvent. A solute is more soluble in a solvent whose intermolecular forces are similar to, and therefore can replace, its own. ©McGraw-Hill Education. Sample Problem 13.1: Solution SOLUTION: (a) Methanol. NaCl is ionic, so it dissolves in polar solvents through ion-dipole forces. Both methanol and 1-propanol have a polar –OH group, but the hydrocarbon portion of each alcohol interacts only weakly with the ions and 1-propanol has a longer hydrocarbon portion than methanol. (b) Water. An ethylene glycol molecule has two –OH groups, so these molecules interact with each other through H bonding. H bonds formed with H2O can replace the H bonds between solute molecules better than the dipole-induced dipole forces that form with hexane. (c) Ethanol. Diethyl ether molecules interact through dipole-dipole and dispersion forces. They can form H bonds to H2O or to ethanol. However, ethanol can also interact with the ether effectively through dispersion forces because it has a hydrocarbon chain. ©McGraw-Hill Education. Problem 13.1A: Follow-up Predicting Relative Solubilities PROBLEM: Predict which solute will dissolve more in the given solvent. (a) 1-butanol (CH3CH2CH2CH2OH) or 1,4-butanediol (HOCH2CH2CH2CH2OH) in water. (b) Chloroform (CHCl3) or carbon tetrachloride (CCl4) in water. ©McGraw-Hill Education. Problem 13.1B: Follow-up Predicting Relative Solubilities PROBLEM: Predict which solvent will dissolve more of the given solute. (a) Chloromethane (CH3Cl) in chloroform (CHCl3) or methanol (CH3OH). (b) 1-pentanol (CH3CH2CH2CH2CH2OH) in water or in hexane (C6H14). ©McGraw-Hill Education.