Intermolecular Forces

Intermolecular Forces “If we do not hang together, we will surely hang separately” Benjamin Franklin

Any substance can be solid, liquid, or gas • Depends on temperature and pressure • Some force holds particles together; loosely in a liquid, rigidly in a solid. • The stronger the force, the higher the melting and boiling points.

Ex: Fe melts at 1809k and boils at 3135 k, while Cl2 melts at 172k and boils at 239k. • Why the big difference? Iron atoms stick together better than chlorine molecules do.

In a substance: • All of the particles are identical. • Their tendency to hang together (intermolecular forces) depends upon the nature of that type of particle.

Your possibilities are: (weakest to strongest) A. Nonpolar molecules, or single, non-metallic atoms B. Polar molecules (without then with H in a polar bond) C. Ions D. Metallic atoms E. Atoms making a network of covalent bonds.

Dispersion forces. • In a substance composed of single, non-metallic atoms, or non-polar molecules, there really isn’t much holding them together… • …but there is something. Anything can be liquefied.

Picture this: • Two argon atoms. Completely non-polar. Due to the uncertainty in the position of the electrons, just for an instant, the valence electrons in one of them are slightly off center. One side of one atom has a slight (-) charge and the other a slight (+) charge

The argon atom with the charge imbalance affects the one next to it. The (-) charged side repels the electrons in the other atom—just for an instant—and induces a—slight—charge imbalance in the second atom. Then—just for an instant—there is a—slight—attraction of the random (-) charge and the (+) charge it induced

just for an instant slightly off center slight charges random • Doesn’t sound like much, does it? • It isn’t—but it’s all there is, and that’s enough to liquefy argon when you cool it to 87k.

These are called London dispersion forces, named for the author Jack London, who wrote “White Fang” • Actually, it’s not. It was first described by Fritz London, who was nowhere near so cool.

Dipole interactions • Next! • If the substance is polar it has a permanent dipole. The (+) and (-) ends are built into the molecule. They always attract each other.

Polar molecules also have dispersion forces, but the dipole interactions are stronger. The dispersion forces are important only when you are comparing the melting and boiling points of two polar substances. If they have the same dipole interactions, then you look at the dispersion forces to see which is stronger.

When are dispersion forces stronger? • A substance that has a lot of electrons in each particle, and one that has longer particles is more polarizable • --It’s more likely that the electrons could be found slightly off center. • A more polarizable substance has stronger dispersion forces

Hydrogen Bonding • There is one extreme form of dipole interaction. It occurs when the less electronegative atom loses partial custody of its last electron—when that atom is hydrogen. • Hydrogen bonding is the strongest dipole interaction, strong enough to merit its own category

Station Break • The previous announcement refers to the weak interactions of particles. • The following announcement refers to strong interactions • And now we return to our regular programming.

Ionic Bonds • In an ionic compound, each formula unit is made up of (+) and (-) ions. They attract each other. They also attract the oppositely charged ions in other formula units. The whole substance is held together with ionic bonds within and in between all of the formula units.

The ionic bonds are stronger than the dipole interactions since they are full (+) and (-) charges, instead of partial (+) and (-) charges.

Metallic Bonds. • Metals consist of an array of positive metal ions floating in a sea of shared valence electrons. These shared electrons are responsible for most of the obvious metallic properties. The attraction of the positive metal ions for the shared electrons is the “intermolecular” force between the atoms

There is a large overlap between the strengths of ionic and metallic bonds– look at melting points— • Hg (metallic) MP 234 K vs • MgO (ionic) MP 3070 K (which has stronger IM forces?) • W (metallic) MP 3680 K vs • KOH (ionic) MP 678 K

Network Solids. • What if the intermolecular forces were as strong as intramolecular forces? (Oh, yeah– like in ionic compounds or metallic solids….) • Yeah, but what if they were stronger intramolecular forces, like covalent bonds?

The network solids show intramolecular covalent bonds extending to intermolecular forces—making the entire solid one immense “molecule” • Examples include diamond and silicate minerals

Quiz Q: What holds water together?

Quiz Q: What holds water together? A: It’s a trick question! There are two answers, both important.

A(1) • Polar covalent bonds between the hydrogen and oxygen atoms hold the atoms together as water molecules !

! • A(2) • Hydrogen bonding, due to the hydrogen being the less electronegative atom of a polar covalent bond, holds water molecules together as a liquid or a solid. • Please notice that the first answer leads to the second.

For Mixtures: • The intermolecular forces in mixtures affect their physical properties… but • …the particles in a mixture are not all identical.

Different types of particles interact to make new intermolecular forces. • Specifically: Ions and polar molecules induce dipoles in nonpolar molecules. • These make new weak interactions—none are strong enough to affect the top end.

From weakest to strongest (?) • (Dispersion forces) • Dipole-induced dipole forces • Ion-Induced dipole • (Dipole Interactions) • (Hydrogen bonding) • Ion-dipole interactions • (ionic bonds) • (metallic bonds) • (covalent bonds)

2005 7. Use principles of atomic structure, bonding, and/or intermolecular forces to respond to each of the following. Your responses must include specific information about all substances referred to in each question. a) At a pressure of 1 atm, the boiling point of NH3 (1) is 240k whereas the boiling point of NF3 (l) is 144k i. Identify the intermolecular force(s) in each substance ii. Account for the difference in the boiling points of the substances

2005 7. Use principles of atomic structure, bonding, and/or intermolecular forces to respond to each of the following. Your responses must include specific information about all substances referred to in each question. b) The melting point of KCl (s) is 776 oC whereas the melting point of NaCl (s) is 801oC i. Identify the type of bonding in each substance ii. Account for the difference in the melting points of the substances

2005 Form B 8. Use principles of atomic structure, bonding, and/or intermolecular forces to respond to each of the following. Your responses must include specific information about all substances referred to in each question. c) (Draw the…) complete structural formulas for propane and methanoic acid…( and identify the) types of intermolecular forces that occur in each substance d) Use the principles if intermolecular attractive forces to explain why methanoic acid has a higher boiling point than propane

2006 6. Answer each of the following in terms of principles of molecular behavior and chemical concepts. a. The structures for glucose, C6H12O6 and cyclohexane, C6H12, are shown below.

2006 6. Answer each of the following in terms of principles of molecular behavior and chemical concepts. a. The structures for glucose, C6H12O6 and cyclohexane, C6H12, are shown below. Identify the type(s) of intermolecular attractive forces in i. Pure glucose ii. Pure cyclohexane

2006 • Answer each of the following in terms of principles of molecular behavior and chemical concepts. b. Glucose is soluble in water, but cyclohexane is not soluble in water, explain

2006 c. Consider the two processes represented below. Process 1: H2O (l)H2O (g) DHo=44.0 kJ/mol Process 2: H2O(l)H2(g)+½O2(g) DHo=286 kJ/mol For each of the two processes, identify the type(s) of intermolecular or intramolecular attractive forces that must be overcome for the process to occur

2006 c. Consider the two processes represented below. Process 1: H2O (l)H2O (g) DHo=44.0 kJ/mol Process 2: H2O(l)H2(g)+½O2(g) DHo=286 kJ/mol Indicate whether you agree or disagree with the statement below. Support your answer with a short explanation When water boils, H2O molecules break apart to form hydrogen molecules and oxygen molecules.

Intermolecular Forces