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 Three Molecules + e- + e- + e- When one molecule has a temporary imbalance in electrons + e- δ- δ+ + e- + e- + e- δ- δ+ + e- δ- δ+ + e- the other molecules respond to the imbalance + e- + e- δ- δ+ + e- δ- δ+ + e- + e- + e- δ- δ+

London Dispersion – Practice Which has stronger London dispersion forces, F2 or Br2? 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! F Br

London Dispersion – Practice Which has stronger London dispersion forces, CF2O or SiF2O? 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! F O C F O Si

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 One end of a molecule is permanently positive and one end is permanently negative Opposites attract δ+ δ - δ+ δ -

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: 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 S O

Dipole-Dipole – Practice Which intermolecular force(s) are in SiS2? Draw the Lewis structure: 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! S Si

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 - δ + O H The negative electron pairs are attracted to the positive hydrogen. δ +

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 CH3OH HNO The non-bonding pairs of electrons on the N, O, or F are strongly attracted to the H on another molecule. O H O H H C O H C O H O N H O N

Hydrogen Bonding – Practice Which intermolecular force(s) are in NH3? Draw the Lewis Structure 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 H N

Hydrogen Bonding – Practice Which intermolecular forces(s) are in PH3? Draw the Lewis Structure 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 P H

Hydrogen Bonding – Practice Which intermolecular forces(s) are in NH4+1? Draw the Lewis Structure 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? ] [ N H +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 C 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 C H O Is the molecule polar or nonpolar? Polar = London Dispersion and Dipole-Dipole So this molecule has all three intermolecular forces: London, 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