Intermolecular Forces of Attraction Learning Targets: Determine the intermolecular forces of a substance Use intermolecular forces to predict physical properties such as viscosity, surface tension, melting point, boiling point, and state of matter at room temperature Intermolecular Forces of Attraction definition: a force of attraction between molecules of the same type Intermolecular Forces of Attraction • Bonds are INTRAmolecular forces because they chemically combine one atom to another atom • INTERmolecular forces do not chemically combine anything. But they do help one molecule “stick” to another Intermolecular Forces of Attraction • Three intermolecular forces – London Dispersion – Dipole-Dipole – Hydrogen Bonding • One strong force (not technically an IMF, but an actual bond!) – Ionic London Dispersion • Weakest of the weak intermolecular forces • Caused by random and temporary movement of electrons • Short-lived, temporary force • More electrons = greater attractions London Dispersion - A Short Visual Molecule 1 e- δ+ + + Molecule 2 e- eδe- ee- δ+ + e- + e- e- + e- Molecule 3 e- + e- e- + e- e- + e- e- + e- e- + e- δδ+ + δ+ + e- + ee- δ- ee- δ- e- δ+ δ+ + + ee- δ- ee- δ- • Three Molecules • When one molecule has a temporary imbalance in electrons • the other molecules respond to the imbalance London Dispersion – Practice • Which has stronger London dispersion forces, F2 or Br2? F F Br Br – Which molecule has more total electrons? • Notice these molecules have the same number of valence electrons. • Go to the periodic table and look how many total electrons they have… F2 has 9+9=19 while Br2 has 35+35=70. • Br2 has more total electrons, so its London dispersion force is greater! London Dispersion – Practice • Which has stronger London dispersion forces, CF2O or F SiF2O? F F C O F Si O – Which molecule has more total electrons? • Even though they have the same number of valence electrons, SiF2O has more total electrons, so its London dispersion force is greater! Dipole-Dipole • Still a very weak force, but stronger than London dispersion • Molecules must be permanently polar with one positive and one negative side • The positive side of one molecule attracts the negative of another molecule Dipole-Dipole - A Short Visual δ+ δ- δ+ • One end of a molecule is permanently δpositive and one end is permanently negative • Opposites attract Dipole-Dipole - Another Short Visual • A quick visual of what this might look like… • Notice there is not a regular pattern of the molecules - the attraction is not strong enough to hold the molecules in a fixed position. Dipole-Dipole – Practice • Which intermolecular force(s) are in SO2? – Draw the Lewis structure: O S O • What is the class, type, and shape? – Class: 3, Type: CA2E, Shape: bent • What is the polarity? – Polar (the shape is polar, based on the CA2E shape) • What IMF’s will this molecule have? – London dispersion AND Dipole Dipole-Dipole – Practice • Which intermolecular force(s) are in SiS2? – Draw the Lewis structure: S Si S • What is the class, type, and shape? – Class: 2, Type: CA2, Shape: linear • What is the polarity? – Nonpolar (all attached atoms are the same) • What IMF’s will this molecule have? – London dispersion only! Hydrogen Bonding • A weak force of attraction, but strongest compared to the previous two (London dispersion and dipole-dipole) • Occurs when the most electronegative elements – N, O, and F – are directly bonded to H. • High electronegative elements attract electrons so strongly that the molecule becomes very polar Hydrogen Bonding • The non-bonding pairs of electrons on the N, O, or F are strongly attracted to the highly positive H on another molecule. • A “quarter bond” or “half bond” is formed between the H and the pair of electrons, resulting in a much stronger force than the previous forces. • Requirements (both are necessary!) – H bonded to N, O, F – Non-bonded pair of electrons on the N, O, or F Hydrogen Bonding – A Short Visual δ- • The very polar molecule has a very negative end and a very positive end. O H δ+ H δO H δ+ H • The negative electron pairs are attracted to the positive hydrogen. Hydrogen Bonding – Another Visual • Here is an example of hydrogen bonding at work! • Notice how the positive hydrogens are attracted to the negative electron pairs on the oxygen! • This is how ice forms. Hydrogen Bonding – Another Visual • Examples: – H2O H H O O H H H – CH3OH H H H O C O C H H H H H – HNO H N N O O • The non-bonding pairs of electrons on the N, O, or F are strongly attracted to the H on another molecule. Hydrogen Bonding – Practice • Which intermolecular force(s) H are in NH3? • Draw the Lewis Structure H N H • What is the class, type, and shape? – class: 4, type: CA3E, shape: pyramidal • What is the polarity? – Polar (since all attached atoms are the same, the pyramidal shape is polar) • Is H attached to N, O, or F? – yes • What IMF’s will this molecule have? – London dispersion, Dipole, AND Hydrogen bonding Hydrogen Bonding – Practice – Which intermolecular forces(s) are in PH3? H • Draw the Lewis Structure H P H • What is the class, type, and shape? – class: 4, type: CA3E, shape: pyramidal • What is the polarity? – Polar (since all attached atoms are the same, the CA3E shape is polar) • Is H attached to N, O, or F? – No, so no hydrogen bonding exists • What IMF’s will this molecule have? – London dispersion and Dipole Hydrogen Bonding – Practice – Which intermolecular forces(s) are in NH4+1? H [ ] • Draw the Lewis H N H Structure H • What is the class, type, and shape? – class: 4, type: CA4, shape: tetrahedral • What is the polarity? – NONE!!! Ions have no polarity. • What IMF’s will this molecule have? – NONE!!! Why would a + molecule be attracted to another + molecule? +1 Ionic • This is an intra molecular force, NOT an intermolecular force of attraction! • Actual bonds are formed between molecules forming a crystal lattice (or network) of atoms. • Attractive forces between positive and negative ions are as strong between molecules as within molecules. Ionic – A Short Visual • Na+ and Cl- ions are attracted to each other because opposite charges attract! • A network of positive/negative alternating charges begins to form. Putting it all together! • How do you determine which intermolecular forces a compound has? – First, determine whether the compound is made of ionic or covalent bonds. • Ionic compounds will have ionic attractions. • Covalent compounds will have some sort of Intermolecular force. Putting it all together! • What forces does Li3N have? – Is the compound made of ionic or covalent bonds? • Lithium is a metal • Nitrogen is a nonmetal • So the compound has ionic attractions. Putting it all together! • All covalent molecules have London dispersion forces. – Every covalent molecule has electrons moving around. – London dispersion forces are made when those electrons move. Putting it all together! • To determine if the molecule has Dipole-Dipole or Hydrogen Bonding, draw the Lewis dot structure. – A polar molecule will have Dipole-Dipole, in addition to its London dispersion forces. Putting it all together! • If the molecule is polar and has Dipole-Dipole forces, determine whether there is a bond between H–O, H–N, and/or H–F. • This will mean that the molecule also has Hydrogen Bonding. Putting it all together! • What forces does CH4 have? – Is the compound made of ionic or covalent bonds? • Covalent – Draw the Lewis Dot Structure H H C H H – Is the molecule polar or nonpolar? • Nonpolar = London Dispersion only Putting it all together! • What forces does CH3OH have? – Is the compound made of ionic or covalent bonds? • Covalent – Draw the Lewis Dot Structure H H C O H H – or nonpolar? – Is Sothe thismolecule moleculepolar has all three • Polar = Londonforces: Dispersion and intermolecular London, Dipole-Dipole Dipole, and Hydrogen – Is there a H bonded to O, N, or F? • Yes = Hydrogen Bonding Physical Properties • Intermolecular forces help determine many physical properties of compounds. – Viscosity • The measure of a fluid’s resistance to flow • Honey is more viscous than water Physical Properties • Intermolecular forces help determine many physical properties of compounds. – Surface Tension • The measure of how well molecules can attract each other at the surface of a liquid • Water bugs can crawl across the surface of ponds because water has a high surface tension Physical Properties • Intermolecular forces help determine many physical properties of compounds. – Melting point • The temperature at which a solid turns into a liquid Physical Properties • Intermolecular forces help determine many physical properties of compounds. – Boiling point • The temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid – simply put, the temperature at which a liquid changes into a gas Physical Properties • Intermolecular forces help determine many physical properties of compounds. – State of matter • Whether the compound exists as a gas, liquid, or solid at room temperature State of Matter (cont.) • From Weakest to Strongest: – London Dispersion (tend to be gases or sometimes liquids) – Dipole (tend to be gases or often as liquids) – Hydrogen Bonding (tend to be liquids or sometimes solids) • Strongest Force (not an IMF): – Ionic (are all solids) Physical Properties • Intermolecular forces help determine many physical properties of compounds. – Higher intermolecular forces cause greater viscosity, greater surface tension, and higher melting and boiling points