University:
Kentucky State University
Course:
CHE 101 | General Chemistry I
Academic year:

2024
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228

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67
Author:

ViatDithDuada3f3

State of Matter and Intermolecular Forces CHEM-102 1 Intermolecular Forces • Important differences between gases, solids, and liquids: – Gases • Expand to fill their container – Liquids • Retain volume, but not shape – Solids • Retain volume and shape CHEM-102 2 As T decreases, kinetic energy of molecules decreases At certain T, molecules don’t have enough energy to break away from one another’s attraction A change of state or phase transition CHEM-102 ©2017 Cengage Learning. 3 Phase Changes Transformations from one phase to another Least Order → occur when heat is added or removed from a substance CHEM-102 Greatest Order 4 Melting is the change of a solid to the liquid state. Freezing is the change of a liquid to the solid state. Vaporization is the change of a solid to or a liquid to the vapor. Sublimation is the change of a solid directly to the vapor. Condensation is the change of a gas to either the liquid or the solid state. CHEM-102 5 A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a welldefined boundary. 2 Phases Solid phase - ice Liquid phase - water CHEM-102 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6 Intermolecular Forces Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule. Intermolecular vs Intramolecular • 41 kJ to vaporize 1 mole of water (inter) • 930 kJ to break all O-H bonds in 1 mole of water (intra) “Measure” of intermolecular force boiling point Generally, intermolecular forces are much weaker than intramolecular forces. melting point DHvap DHfus DHsub CHEM-102 7 Important Types of Intermolecular Forces ➢ ➢ ➢ CHEM-102 Dipole-dipole forces - Hydrogen bonds Ion-dipole forces - Ion-induced dipole forces London (dispersion) forces 8 Electronegativity Increase CHEM-102 9 Covalent Bond (strong) Cl H Intermolecular attraction (weak) Cl H Solid and liquid HCl CHEM-102 ©2017 Cengage Learning 11 The term van der Waals forces is a general term that includes both dipole-dipole forces and London forces. Polar molecules exhibit dipole-dipole forces that result in alignment of the molecules. This phenomenon is illustrated below for solid and liquid HCl. CHEM-102 12 When substance melts or boils ▪ Intermolecular forces are broken ▪ Not covalent bonds ➢ Responsible for existence of condensed states of matter ➢ Responsible for bulk properties of matter ➢ Boiling points and melting points ▪ Two atoms with different electronegativity values share electrons unequally ▪ Electron density is uneven ▪ Higher charge concentration around more electronegative atom ▪ Bond dipoles H F ▪ Indicated with delta (δ) notation ▪ Indicates partial charge has arisen + − CHEM-102 13 Important Types of Intermolecular Forces ➢ ➢ ➢ CHEM-102 Dipole-dipole forces - Hydrogen bonds Ion-dipole forces - Ion-induced dipole forces London (dispersion) forces 14 Intermolecular Forces Dipole-Dipole Forces Attractive forces between only polar molecules As dipole moment increases, intermolecular force increases CHEM-102 15 Ion-Dipole Forces Attractive forces between an ion and a polar molecule Attractions can be quite strong as ions have full charges (a) Negative ends of water dipoles surround cation (b) Positive ends of water dipoles surround anion CHEM-102 16 Interaction Between Water and Cations in solution Example: Ion-Dipole Attractions: AlCl3·6H2O Positive charge of Al3+ ion attracts partial negative charges – on O of water molecules CHEM-102 17 Dispersion Forces Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules ion-induced dipole interaction dipole-induced dipole interaction CHEM-102 18 Ion-Induced Dipole Attractions • Attractions between ion and dipole it induces on neighboring molecules – Depends on • Ion charge and • Polarizability of its neighbor – Attractions can be quite strong as ion charge is constant, unlike instantaneous dipoles of ordinary London forces – E.g., I– and Benzene CHEM-102 19 London forces are the weak attractive forces between molecules resulting from the small, instantaneous dipoles that occur because of the varying positions of the electrons during their motion about the nuclei. ➢ When atoms near one another, their valence electrons interact. ➢ Repulsion causes electron clouds in each to distort and polarize. CHEM-102 20 The instantaneous dipole of the neon atom induces an instantaneous dipole in adjacent atoms, resulting in an attractive force between them. At some instant, there are more electrons on one side of a neon atom than on the other. CHEM-102 21 Induced Dipoles Interacting With Each Other London Forces depend on 1. Polarizability of electron cloud 2. Points of attraction ▪ Number atoms ▪ Molecular shape (compact or elongated) CHEM-102 22 ➢ Larger molecules have stronger London forces and thus higher boiling points. • London forces depend on number atoms in molecule • Boiling point of hydrocarbons demonstrates this trend Formula BP at 1 atm, C Formula BP at 1 atm, C CH4 –161.5 C5H12 36.1 C2H6 –88.6 C6H14 68.7 : : C3H8 –42.1 C4H10 –0.5 C22H46 327 CHEM-102 23 Dispersion Forces Continued Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted. Polarizability increases with: • greater number of electrons • more diffuse electron cloud Dispersion forces usually increase with molar mass. CHEM-102 24 Intermolecular Forces Hydrogen Bond The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A H…B or A H…A A & B are N, O, or F HCOOH and water CHEM-102 25 Why is the hydrogen bond considered a “special” dipole-dipole interaction? Decreasing molar mass Decreasing boiling point CHEM-102 26 These compounds have unusually low vapor pressures and unusually high boiling points. They have –OH group - Hydrogen bond CHEM-102 27 Example 1: What type(s) of intermolecular forces exist between the following pairs? (a)HBr and H2S (b)Cl2 and CBr4 (c)I2 and (d)NH3 and C6H6 CHEM-102 28 Answers: a) Dipole-dipole forces b) Both are non-polar → only dispersion forces c) Nonpolar with ion: Ion-induced dipole and dispersion forces d) Polar with nonpolar : Dipole-induced dipole and dispersion forces CHEM-102 29 Example 2: Same chemical formula, C5H12. The most compact compound has the weakest intermolecular forces and the lowest heat of vaporization. The stronger the intermolecular forces → the higher the heat of vaporization. CHEM-102 30 Example 3: Which of the following can form hydrogen bonds with water? CH3OCH3 CH4 F− HCOOH Na+ No electronegative elements (F, O, or N) in either CH4 or Na+ CHEM-102 31 1) Which of the following is the weakest of all intermolecular forces? A. Hydrogen-bonds B. London dispersion forces C. Dipole-dipole attractions D. Ion-dipole attractions E. Ion-Ion attractions 2) In the liquid state, which species has the strongest intermolecular forces, CH4, Cl2, O2 or HF? A. CH4 B. Cl2 C. O2 D. HF CHEM-102 32 3) What is the strongest type of intermolecular force in CHCl3? A. Ion-dipole B. dipole-dipole C. Dispersion D. Hydrogen bonding 4) Which of the following has dispersion forces as its strongest intermolecular force? A. CH4 B. CO2 C. O2 D. All of the above CHEM-102 33 Example 4: What type(s) of intermolecular forces exist between each of the following molecules? a) SF6 f) SO2 b) PCl3 c) LiCl d) HBr e) CO2 Example 5: Identify the intermolecular forces that expected for each of the following substances: a. Methane b. Trichloromethane CHCl3 c. Butanol CH3CH2CH2CH2OH a. Nonpolar molecule - London forces b. Unsymmetrical bond - London forces, dipole-dipole forces c. H attached to O - London forces, dipole-dipole forces, hydrogen bonding CHEM-102 34 Example 7: (a) Identify the intermolecular attractions present in the following substances, and (b) Select the substance with the highest boiling point: CH3CH3, CH3OH, and CH3CH2OH. Answer: (a) CH3CH3 has only dispersion forces, whereas the other two substances have both dispersion forces and hydrogen bonds; (b) CH3CH2OH CHEM-102 35 Using Intermolecular Forces • Often can predict physical properties (like BP, MP and many others) by comparing strengths of intermolecular attractions Strongest – Ion-Dipole – Hydrogen Bonding – Dipole-Dipole Weakest – London Forces • Larger, longer, and therefore heavier molecules often have stronger intermolecular forces • Smaller, more 2weaker intermolecular forces CHEM-102 36 Summary of Intermolecular Attractions Dipole-dipole – Occur between neutral molecules with permanent dipoles – About 1–4% of covalent bond; Mid range in terms of intermolecular forces Hydrogen bonding • Special type of dipole-dipole interaction • Occur when molecules contain N—H, H—F and O—H bonds • About 10% of a covalent bond London dispersion ▪ Present in all substances; Weakest intermolecular force ▪ Weak, but can add up to large net attractions Ion-dipole ▪ Occur when ions interact with polar molecules ▪ Strongest intermolecular attraction Ion-induced dipole ▪ Occur when ion induces dipole on neighboring particle ▪ Depend on ion charge and polarizability of its neighbor CHEM-102 37 Summary Ion-dipole > H-Bond > dipole-dipole > ion-Induced dipole > dipoleInduced dipole > Induced-Induced dipole CHEM-102 38 Properties of Liquids •Surface Tension • Adhesion •Cohesion ➢ All of properties in the table, depend on intermolecular forces, which are related to molecular structure. CHEM-102 39 Properties of Liquids Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. Strong intermolecular forces High surface tension Surface tension is the energy required to increase the surface area of a liquid by a unit amount. The values are given in J/m2. Surface tension is the property that allows insects to walk on the surface of water or a pin to float. CHEM-102 40 ➢ Surface tension is also related to capillary rise. when a very-small-diameter glass tube (a capillary) is placed in water, the water rises. ➢ The water is attracted to the glass, so a thin film moves up the surface of the glass. This action expands the surface area, so the water column rises to counteract it. The process then repeats itself. ✓ Capillary rise, due to the attraction of water and glass. ✓ Hydrogen bonds between the water molecules and the glass are illustrated. ✓ The final water level in the capillary is a balance between the force of gravity and the surface tension of water CHEM-102 41 Properties of Liquids Cohesion is the intermolecular attraction between like molecules such as water molecules themselves. Adhesion is an attraction between unlike molecules such as water and the sides of a glass tube. Adhesion The water is attracted to the glass; the result is concave (‫)مقعر‬ CHEM-102 Cohesion Hg is not attracted to the glass; the result is convex (‫)محدب‬ 42 Properties of Liquids Viscosity is a measure of a fluid’s resistance to flow. Strong intermolecular forces High viscosity ➢ The higher the viscosity the greater the resistance to flow Varies inversely with temperature; viscosity decreases when temperature increases CHEM-102 glycerol water 43 3-D Structure of Water Water is a Unique Substance Maximum Density 40C Density of Water Ice is less dense than water CHEM-102 44 ▪ The stronger the intermolecular forces, the lower the vapor pressure at any given temperature. ▪ The stronger the intermolecular forces, the higher the boiling point. ▪ The stronger the intermolecular forces, the higher the surface tension. ▪ Viscosity depends in part on intermolecular forces: The stronger the intermolecular forces, the more viscous the liquid. CHEM-102 45 Phase Changes Transformations from one phase to another Least Order → occur when heat is added or removed from a substance CHEM-102 Greatest Order 46 Solid-Liquid Equilibrium H2O (s) H2O (l) The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium. CHEM-102 47 Molar heat of fusion (DHfus) is the energy required to melt 1 mole of a solid substance at its freezing point. CHEM-102 48 Solid-Gas Equilibrium H2O (s) H2O (g) Molar heat of sublimation (DHsub) is the energy required to sublime 1 mole of a solid. DHsub = DHfus + DHvap ( Hess’s Law) CHEM-102 49 Liquid-Vapor Equilibrium The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation H2O (l) H2O (g) Dynamic Equilibrium Rate of Rate of = evaporation condensation CHEM-102 50 Measurement of Vapor Pressure Before Evaporation CHEM-102 At Equilibrium 51 ➢ The vapor pressure depends on the temperature. Note that as temperature increases, vapor pressure increases. ➢ Liquids and solids with relatively high vapor pressure at normal temperatures are considered volatile. CHEM-102 52 The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure. The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm. CHEM-102 53 ➢ When the pressure on the liquid increases, as is the case with a pressure cooker, the boiling point increases. ➢ Conversely, when the pressure on the liquid decreases, as is the case at high altitude, the boiling point decreases. The freezing point is the temperature at which a pure liquid changes to a crystalline solid (or freezes). The melting point is the temperature at which a crystalline solid changes to a liquid (or melts). CHEM-102 54 Effect of Temperature and Intermolecular Forces on Vapor Pressure ➢ Diethyl ether with London Force ➢ Water with Hydrogen bond ➢ Mercury with very strong electrostatic bond CHEM-102 55 Molar heat of vaporization (DHvap) is the energy required to vaporize 1 mole of a liquid at its boiling point. Clausius-Clapeyron Equation DHvap ln P = +C RT P = (equilibrium) vapor pressure T = temperature (K) R = gas constant (8.314 J/K•mol) Vapor Pressure Versus Temperature CHEM-102 56 Alternate Forms of the Clausius-Clapeyron Equation At two temperatures: or CHEM-102 57 Example 9: • Diethyl ether is a volatile, highly flammable organic liquid that is used mainly as a solvent. • The vapor pressure of diethyl ether is 401 mmHg at 18°C. Calculate its vapor pressure at 32°C. DHvap = 26.0 kJ/mol. CHEM-102 58 DHvap = 26.0 kJ/mol P2 = 656 mmHg CHEM-102 59 Example 10: Estimate the vapor pressure of water at 85°C. Note that the normal boiling point of water is 100°C and that its heat of vaporization is 40.7 kJ/mol. P2   40.7  103 J/mol  1 1  ln  − =   760 mmHg 8.31 J/(K  mol) 373 K 358 K     = (4898 K)  (–1.123  10 –4 /K) = –0.550 P2 = antiln(–0.550) 760 mmHg CHEM-102 60 Therefore, the antilogarithm is e–0.550 = 0.577 Thus, P2 = 0.577 760 mmHg P2 = 0.577  760 mmHg = 439 mmHg CHEM-102 61 Example 12: Determine the enthalpy of vaporization, in kJ/mol, for benzene, using the following vapor pressure data. T1 = 60.6 °C; P1 = 400. mmHg T2 = 80.1 °C; P2 = 760. mmHg A. 32.2 kJ/mol B. 14.0 kJ/mol C. –32.4 kJ/mol D. 0.32 kJ/mol E. –14.0 kJ/mol CHEM-102 62 The phase diagram for water. • AB line, which shows the equilibrium between solid and liquid (melting point), slants to the left. This is because the density of the liquid is greater than the density of the solid. • AC= vapor pressure curve for liquid water (Boiling Point) ➢ In a phase diagram, phases are separated by curves that represent equilibrium between those phases. ➢ The triple point is the point where all three phases coexist in equilibrium. CHEM-102 63 Phase Diagram: It summarizes the conditions at which a substance exists as a solid, liquid, or gas. Phase Diagram of Water CHEM-102 64 The critical temperature (Tc) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure. The critical pressure (Pc) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature. CHEM-102 65 Phase Diagram of Carbon Dioxide ▪ undergoes sublimation when warmed at 1 atm. ▪ The triple point is at -57 oC and 5.1 atm. The soiled sublimes if warmed at any pressure below 5.1 atm At 1 atm CO2 (s) CO2 (g) CHEM-102 66 Effect of Increase in Pressure on the Melting Point of Ice and the Boiling Point of Water CHEM-102 67

State of Matter and Intermolecular Forces

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