Intermolecular Attractions and the properties of liquids and Solids Chapter 12

Learning Objective: To understand why the physical properties of liquids and solids depend so heavily on their chemical composition while the properties of gases don’t 12.1 Gases, liquids, and solids and intermolecular distances

3 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

Converting gas  liquid or solid Molecules must get closer together Cool or compress Converting liquid or solid  gas Requires molecules to move farther apart Heat or reduce pressure 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 Intermolecular Attractions

5 Intermolecular Forces Physical state of molecule depends on Average kinetic energy of particles Recall KE  T ave Intermolecular Forces Energy of Inter-particle attraction Physical properties of gases, liquids and solids determined by How tightly molecules are packed together Strength of attractions between molecules

12.2 Types of intermolecular Forces Learning Objectives: To learn nature and relative strength of the principal kinds of intermolecular attractions To understand the factors that influence the strengths of intermolecular attractions To be able to use a Lewis structure, bond polarities and predicted molecular geometry to anticipate the nature of the intermolecular attractions that exist in liquid and solid states of a substance.

Intermolecular vs. Intra-Molecular Forces Intramolecular forces Covalent bonds within molecule Strong  H bond (HCl) = 431 kJ/mol Intermolecular forces Attraction forces between molecules Weak  H vaporization (HCl) = 16 kJ/mol Covalent Bond (strong) Intermolecular attraction (weak)

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) Generally, intermolecular forces are much weaker than intramolecular forces. “Measure” of intermolecular force boiling point melting point DH vap DH fus DH sub

9 Electronegativity Review Electronegativity: Measure of attractive force that one atom in a covalent bond has for electrons of the bond

10 Bond Dipoles Two atoms with different electronegativity values share electrons unequally Electron density is uneven Higher charge concentration around more electronegative atom Bond dipoles Indicated with delta (δ) notation Indicates partial charge has arisen

Net Dipoles Symmetrical molecules Even if they have polar bonds Are non-polar because bond dipoles cancel Asymmetrical molecules Are polar because bond dipoles do not cancel These molecules have permanent, net dipoles Molecular dipoles Cause molecules to interact Decreased distance between molecules increases amount of interaction

12 Intermolecular Forces When substance melts or boils Intermolecular forces are broken Not covalent bonds Responsible for non-ideal behavior of gases Responsible for existence of condensed states of matter Responsible for bulk properties of matter Boiling points and melting points Reflect strength of intermolecular forces

13 Three Important Types of Intermolecular Forces 1.London dispersion forces 2.Dipole-dipole forces Hydrogen bonds 3.Ion-dipole forces Ion-induced dipole forces

London Forces When atoms near one another, their valence electrons interact Repulsion causes electron clouds in each to distort and polarize Instantaneous dipoles result from this distortion Effect enhanced with increased volume of electron cloud size Effect diminished by increased distance between particles and compact arrangement of atoms

15 London Forces Instantaneous dipole- induced dipole attractions London Forces Dispersion forces Operate between all molecules Neutral or net charged Nonpolar or polar

16 London Dispersion Forces Ease with which dipole moments can be induced and thus London Forces depend on 1.Polarizability of electron cloud 2.Points of attraction Number atoms Molecular shape (compact or elongated )

Polarizability  Plarizability: ease with which the electron cloud can be distorted  Larger molecules often more polarizable  less tightly held electrons

18 Table 12.1 Boiling Points of Halogens and Noble Gases Larger molecules have stronger London forces and thus higher boiling points.

19 Number of Atoms in Molecule 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 CH 4 –161.5C 5 H C2H6C2H6 –88.6C 6 H C3H8C3H8 –42.1:: C 4 H 10 –0.5C 22 H

Hexane, C 6 H 14 BP 68.7 °C  More sites (marked with *) along its chain where attraction to other molecules can occur 20 How Intermolecular Forces Determine Physical Properties Propane, C 3 H 8 BP –42.1 °C

21 Molecular Shape Increased surface area available for contact = increased London forces London dispersion forces between spherical molecules are lower than chain-like molecules More compact molecules Hydrogen atoms not as free to interact with hydrogen atoms on other molecules Less compact molecules Hydrogen atoms have more chance to interact with hydrogen atoms on other molecules

22 Physical Origin of Shape Effect Small area for interaction Larger area for interaction More compact – lower BPLess compact – higher BP

23 Dipole-Dipole Attractions Occur only between polar molecules Possess dipole moments Molecules need to be close together Polar molecules tend to align their partial charges Positive to negative As dipole moment increases, intermolecular force increases +   + + 

24 Dipole-Dipole Attractions Tumbling molecules Mixture of attractive and repulsive dipole-dipole forces Attractions (- +) are maintained longer than repulsions(- -) Get net attraction ~1–4% of covalent bond

25 Dipole-Dipole Attractions Interactions between net dipoles in polar molecules About 1–4% as strong as a covalent bond Decrease as molecular distance increases Dipole-dipole forces increase with increasing polarity

26 Hydrogen Bonds Special type of dipole-dipole Interaction Very strong dipole-dipole attraction ~10% of a covalent bond Occurs between H and highly electronegative atom (O, N, or F) H—F, H—O, and H—N bonds very polar Electrons are drawn away from H, so high partial charges H only has one electron, so  + H presents almost bare proton  – X almost full –1 charge Element’s small size, means high charge density Positive end of one can get very close to negative end of another

27 Examples of Hydrogen Bonding

28 Hydrogen Bonding in Water Responsible for expansion of water as it freezes Hydrogen bonding produces strong attractions in liquid Hydrogen bonding (dotted lines) between water molecules in ice form tetrahedral configuration water crystallizes into an open hexagonal form.

Example: List all intermolecular forces for CH 3 CH 2 OH. A. Hydrogen-bonds B. Hydrogen-bonds, dipole-dipole attractions, London dispersion forces C. Dipole-dipole attractions D. London dispersion forces E. London dispersion forces, dipole-dipole attractions 29

In the liquid state, which species has the strongest intermolecular forces, CH 4, Cl 2, O 2 or HF? A. CH 4 B. Cl 2 C. O 2 D. HF 30 Example:

31 Ion-Dipole Attractions Attractions between ion and charged end of polar molecules 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

32 Ex. Ion-Dipole Attractions: AlCl 3 ·6H 2 O  Positive charge of Al 3+ ion attracts partial negative charges  – on O of water molecules  Ion-dipole attractions hold water molecules to metal ion in hydrate  Water molecules are found at vertices of octahedron around aluminum ion Attractions between ion and polar molecules

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

34 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

35 Summary of Intermolecular Attractions 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