Intermolecular Forces Forces Between Molecules

Why are intermolecular forces important? They determine the phase of a substance at room temperature. (Competition with kinetic energy.)

Solids Strong Intermolecular Forces

Gases Weak Intermolecular Forces

3 Types of Intermolecular Forces Dispersion or Van der Waals Dipole-dipole Hydrogen Bonds

Strongest Intermolecular Force Hydrogen Bonds

When do Hydrogen bonds occur? Between molecules containing F, O, N bonded to a H atom

Strongest hydrogen bonds? Between molecules containing F-H bonds

Weakest hydrogen bonds? Between molecules containing N-H bonds

What effect does H-bonding have on the properties of the substance? Hydrogen bonding leads to substantial increases in the expected boiling point.

Weakest Intermolecular Force Dispersion or Van der Waals forces

When do dispersion forces occur? Occur between all molecules. Most important between nonpolar molecules

Nonpolar No Poles! The molecule is symmetric! Cannot tell 1 end from the other.

2 Easy categories of Nonpolar Molecules Monatomic Gases (Column 18) (kickballs) & Diatomic Elements (H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 ) (footballs)

2 More Easy categories of Nonpolar Molecules Hydrocarbons (C x H y ) & Small symmetric molecules (CO 2, CS 2, CF 4, CBr 4, etc.)

What are the diatomic elements? H 2, N 2, O 2, F 2, Br 2, Cl 2, & I 2

What can you say about dispersion forces as a function of molecular size? The larger the molecules the stronger the dispersion forces!

When do dipole-dipole forces occur? Between polar molecules. Permanent separation of charge. 1 side  - (electron rich) vs. 1 side  + (electron poor)

Polar Has Poles! The molecule is NOT symmetric! Can tell 1 end from the other.

Vapor Gas phase of a substance that is normally a liquid at room temperature.

Vapor Pressure The partial pressure of a vapor above its liquid.

What does vapor pressure depend on? (Hint: see Table H) Vapor pressure depends on the temperature of the liquid.

What happens to the vapor pressure as the temperature of the liquid is increased? The vapor pressure always increases with temperature.

Does the vapor pressure depend on the amount of liquid present? No. A thimbleful is as good as an ocean.

What are some differences between evaporation & boiling? Evaporation occurs at all temperatures. Boiling occurs at a definite temperature. Evaporation occurs at the surface. Boiling occurs throughout the liquid.

Heat of fusion Amount of energy required to convert 1 gram of a pure solid to the liquid phase at its melting point.

Heat of vaporization Amount of energy required to convert 1 gram of a pure liquid to the gas phase at its boiling point.

What happens to the boiling point as the intermolecular forces increase? The stronger the intermolecular forces, the higher the boiling point.

What happens to the melting point as the intermolecular forces increase? The stronger the intermolecular forces, the higher the melting point.

What happens to the heat of fusion as the intermolecular forces increase? The stronger the intermolecular forces, the higher the heat of fusion.

What happens to the heat of vaporization as the intermolecular forces increase? The stronger the intermolecular forces, the higher the heat of vaporization.

What happens to the evaporation rate as the intermolecular forces increase? The stronger the intermolecular forces, the lower the evaporation rate.

What happens to the vapor pressure as the intermolecular forces increase? The stronger the intermolecular forces, the lower the vapor pressure.

Pressure, atm Temperature,  C

Relationship between IMF and physical properties? IMFBPMPHfHf HvHv Ev.RtV.P. IMFBPMPHfHf HvHv EvRtVP

Boiling Point Temperature at which the vapor pressure of a liquid is equal to the external pressure.

Normal Boiling Point Temperature at which the vapor pressure of a liquid is equal to 1 atm or kPa or 760 torr.

What happens to the boiling point when the external pressure is reduced? The boiling point is reduced.

Sublimation Solid to gas

Deposition Gas to solid

Vaporization Liquid to Gas

Condensation Gas to liquid

Fusion Solid to liquid Or Melting

Freezing Liquid to solid Crystallization Solidification

Which phase changes are endothermic? Solid to liquid Liquid to Gas Solid to Gas GLSGLS

Which phase changes are exothermic? Gas to Liquid Liquid to Solid Gas to Solid G L S

Given 3 substances at room temperature: 1 solid, 1 liquid, & 1 gas, which has the strongest intermolecular forces? Solid > Liquid > Gas

Describe the mp & bp of gases. Low!

What happens to the temperature as heat is added to a substance at its melting or boiling point? Nothing, until the phase change is complete. The heat energy is going into the potential energy of the system.

HFHF HFHF Hydrogen Bonding!

Ne Van der Waals or dispersion forces

H-Cl Dipole-dipole forces

H-N H-F Hydrogen Bonding H H - - O H H

Time Temperature I IIIIIIVV Solid Solid & Liquid Liquid Liquid & Gas Gas K.E.  K.E.↔ P.E.↔ P.E.  Melt pt. Boil pt.

Equation for a pure substance in a single phase being warmed up or cooled down. Q = mC  T Q = energy in Joules m = mass in grams C = specific heat in J/g   T = temperature change

Equation for a substance melting or freezing at the freezing point. Q = mH f Q = energy in Joules m = mass in grams H f = heat of fusion

Equation for a substance boiling or condensing at the boiling point. Q = mH v Q = energy in Joules m = mass in grams H v = heat of vaporization

Temperature of the ice-water equilibrium. 0  C or 273K Freezing point Melting point

Temperature of the steam-water equilibrium. 100  C or 373K Boiling point Condensation point