States of Matter and Intermolecular Forces Chapter 11.2-11.3

COMPARING SOLIDS, LIQUIDS, AND GASES The densities of solids and liquids are usually much higher than gases The densities and molar volumes of solids and liquids are more similar than they are to the gas state This is because molecules in liquid water are close to each other (nearly touching) while in it's gaseous state, molecules are separated by large distances Water displays atypical behavior by having a less dense solid than the liquid state This is due to the unique crystal structure formed in ice causing the molecules to move slightly apart The main difference between liquids and solids comes from freedom of movement Even though liquid molecules are close, thermal energy overcomes the attractions between them, allowing them to move more freely

COMPARING SOLIDS, LIQUIDS, AND GASES In solids, however, atoms or molecules are essentially locked in place, only vibrating back and forth about a fixed point Liquids have the ability to assume the shape of their containers because atoms or molecules in liquids are free to flow and move The same could be said to for gases, but to a more extreme degree Solids have a fixed shape because they lack the ability to move Like solids, liquids have a definite volume due to the close proximity of atoms or molecules Solids can be crystalline (where atoms or molecules are arranged in a well-ordered three dimensional array) or amorphous (where atoms or molecule that have no consistent order)

STATE CHANGES STATE CHANGES The states of substances can be transformed by adjusting temperature, pressure, or both For example, liquid water is changed into ice by lowering the temperature and ice is melted by raising the temperature A way to change gaseous water into liquid is by increasing pressure Increasing pressure results in a more dense substance (gas to liquid to solid) Example, when you buy a tank of propane to use as fuel, the inside of the tank contains pressurized liquid propane. When you unscrew the tank, you lower the pressure in the tank for a moment. This causes a state change from a liquid to now a gas This is a more efficient way to store propane because as a liquid it occupies less space

INTERMOLECULAR FORCES Intermolecular forces - the forces that hold together a substance Because of this, IMFs determine whether a substance is solid, liquid or gaseous at room temperature Moderate to to strong IMFs tend to result in liquids and solids (high melting and boiling points) Weak IMFs typically result in gases (low melting and boiling points) Intermolecular forces originate from the interactions between charges (both temporary and partial) on molecules (or atoms and ions) Consistent with Coulomb’s law, molecules with partial or temporary charges are attracted to each other because their potential energy decreases as they get closer Because of the smaller magnitude of charge, IMFs are significantly less strong than ionic or covalent bonds

DISPERSION FORCE Dispersion Force (Also known as London Force) is the IMF present in all molecules and atoms They result from slight changes in electron distribution within molecules or atoms This uneven electron distribution exists at any single moment This brief even distribution is referred to as a instantaneous dipole or temporary dipole The magnitude of the dispersion force depends on how easily electrons can move (polarize) into a temporary dipole. Larger electron cloud = stronger dispersion force If all other variables are kept constant, higher molar mass usually results in the stronger dispersion force and in turn a higher boiling point Keep in mind: shape matters! Long molecules with large surfaces for bonding will have higher boiling points

Which halogen has the highest boiling point? Cl2 Br2 I2

DIPOLE-DIPOLE FORCE The dipole-dipole force is present in all polar molecules Polar molecules have electron rich regions (partially negative) and electron poor areas (partially positive) resulting in a permanent dipole that interact with other polar molecules Because of this, polar molecules have higher boiling points than nonpolar molecules with similar molar mass Miscibility: the ability to mix without separating into two states As a general rule, polar liquids are miscible with other polar molecules and nonpolar molecule are miscible with nonpolar molecules This is why (polar) water does not mix with (nonpolar) oil

HYDROGEN BONDING Hydrogen bonding occurs when polar molecules containing hydrogen bond directly with atoms with large electronegativity Most importantly fluorine, oxygen, or nitrogen Examples: HF, NH3, and H20 A hydrogen bond can be thought of as a super dipole-dipole force making it quite strong The strength of hydrogen bonds are derived from the large electronegativity difference between hydrogen and any of these other electronegative atoms Also, because all of these atoms are quite small, they can get very close together Hydrogen bonds should not be confused with chemical bonds Chemical bonds = bonds between two (or more) atoms within a molecule Hydrogen bonds = bonds between two (or more) molecules

ION-DIPOLE FORCE The ion-dipole force occurs when an ionic compound is mixed with a polar compound This is particularly important as it relates to aqueous solutions of ionic compounds Example: When sodium chloride is mixed with water, the sodium and chloride ions interact with the water molecules via ion-dipole forces. The positive sodium ions interact with the negative poles of water molecules, while the negative chloride ions interact with the positive poles of the water molecules. Ion dipole forces are the strongest of the types of IMFs and are responsible for the ability of ionic substances to form solutions with water

Determine the kinds of intermolecular forces that are present in each element or compound. HCl H20 Br2 He

HCl H20 Br2 He Dispersion, dipole-dipole Dispersion, dipole-dipole, hydrogen bonding Dispersion