Unit 3 - Intermolecular Forces Flashcards
Intermolecular forces vs Intramolecular forces
Intermolecular – the attractive forces that exist BETWEEN molecules or particles; weak and particles are relatively far apart
EX: Between different H2O molecules
Intramolecular – the attractive forces that exist WITHIN a molecule; strong and particles are relatively close together
EX: The covalent bond connecting H and O within one H2O molecule
Explain the relationship between boiling point and IMFs
The stronger the attractive forces (IMFs) between the particles, the more energy is required to overcome and break these forces to convert the substance from liquid to gas, resulting in a higher boiling point.
essentially, higher temperature = stronger IMFs
How do London dispersion forces form?
London dispersion forces are interactions/attractive forces between atoms/molecules as a result of temporary or instantaneous dipoles.
One atom/molecule has an instantaneous dipole where the electrons are not distributed evenly, creating a partially positive dipole and a partially negative dipole. This can induce a neighboring atom/molecule to have an uneven distribution in its electron cloud as well.
LDFs occur because of the continuous motion of electrons
Stronger for larger/heavier molecules (which have a larger electron cloud) and for linear molecules (straight hydrocarbons have more surface area to interact with other molecules compared to a highly branched one)
Polarizability
How easily the charge distribution in an electron cloud can be distorted/induced
A larger electron cloud (greater # of electrons) has greater polarizability and can more easily be distorted/induced. This creates stronger London dispersion forces
What are dipole-dipole forces?
Dipole-dipole forces are the interactions/attractive forces between the partially positive end of one polar molecule and the partially negative end of another nearby polar molecule.
*The molecule has to be polar, not the bond
What are hydrogen bonds?
Hydrogen bonds are the interactions/attractive forces between an H atom (bonded to an N, O, or F) on one molecule and the lone pairs of an N, O, or F on another molecule
It is a strong dipole-dipole interaction
What are ion-dipole forces?
Ion-dipole forces are interactions/attractive forces between an ion and a polar molecule
EX: Between solute ions and solvent H2O molecules during the dissolution process in aqueous solution. H2O molecules orient in a certain way so that the opposite charges are attracted.
*increases with higher charge and smaller ionic radius
What is a dipole?
A dipole is an area of either negative or positive charge
Represents how likely electrons from a bond will be found around a specific atom
Negative dipole = shared electrons will spend more time around that atom (higher electronegativity)
Explain the relationship between IMFs and vapor pressure
The stronger the attractive forces (IMFs) between the particles, the more energy is required to overcome and break these forces to convert the substance from liquid to gas, resulting in a higher boiling point. As a result, less liquid particles have enough energy to break free and become gas particles, which creates a lower vapor pressure (less particles = less force exerted)
Essentially, stronger IMFs = lower vapor pressure
However, vapor pressure is not affected by double bonds because during phase changes, it is the IMFs that are being broken, not the chemical bonds within the molecule
Define vapor pressure
Vapor pressure is the force exerted by a gas in dynamic equilibrium with its liquid phase at a given temperature (rate of evaporation = rate of condensation)
Define normal boiling point
Normal boiling point is the temperature at which the vapor pressure of liquid is exactly 1 atm
Boiling occurs when vapor pressure = atmospheric pressure
100 C for water at 1 atm or 760 Torr
Explain the relationship between temperature and vapor pressure
As temperature increases, a greater proportion of the particles have enough kinetic energy to escape the surface of the liquid and become gas particles, which increases the vapor pressure (more particles = more force exerted).
Describe the relationship between boiling point and vapor pressure
Higher boiling point = lower vapor pressure
They are inversely related and connected through the concept of IMFs
Describe differences in particle space for solids, liquids, and gases.
Solids – particles are close to each other. The motion of the particles is limited to vibration within the crystal structure
Liquids – particles are moderately close to each other and are constantly moving + colliding. Properties of liquids (boiling point, vapor pressure, etc) are related to the relative strength of the IMFs.
Gases – particles are far away from each other and are constantly moving + colliding. Gases have no definite shape or volume, minimal to no IMFs. Can be described in terms of amount (moles), pressure, volume, and temperature
Define pressure
Pressure is the force exerted by gas particles colliding with the walls of its container
Define partial pressures of gases
The pressure exerted by each gas (the partial pressure) is independent of the other gases in a mixture.
Total pressure = sum of all the partial pressures
EX: If you take two containers (both with volume 1 L), one with a pressure of 1 atm and the other with a pressure of 2 atm, and mix them in a new container with the same volume (1 L), the partial pressure of each gas will remain the same. So the total pressure will be 1 atm + 2 atm = 3 atm.
Total pressure = vapor pressure of water + pressure of gas
How do you calculate partial pressure?
Partial pressure only depends on mole fraction (same # of moles = same partial pressure)
partial pressure of gas A = [ (# of moles of A) / (# of total moles) ] x total pressure
Maxwell Boltzmann diagram
Shows the # of gas particles that are traveling at each speed in the range
At higher temperatures and lower molar mass, the curve of the speed distributions will be at a greater width + lower peak height
Lower molar mass → higher average velocity to keep average kinetic energy constant (AKE = 1/2 mv^2) → greater range of individual gas particle velocities (wider distribution curve)
Ideal gases
Ideal gases have no volume and no attraction between particles (no IMFs)
Low pressure and high temperature
What kinds of gases deviate from ideal behavior?
Conditions for deviation – high pressure and low temperature → volume and attraction
At high pressures, when gas molecules are packed too tightly together, the volume of molecules becomes significant compared to the volume of the container. Gas molecules also attract one another and stick together. Attractive/repulsive forces can impact the strength of collisions
Gases with stronger IMFs are more likely to deviate because of the attractive forces present between gas particles, which cause fewer collisions with the walls of the container. Thus, the actual pressure is less than the predicted pressure under the ideal gas law.
EX: H2O > CH4
Chromatography
This process separates the components of a solution using differences in intermolecular interactions
Distillation
This process separates the components of a solution using differences in boiling point (which is caused by differences in intermolecular interactions and their effects on vapor pressure).
Explain how distillation works.
- A mixture must have two liquids of different boiling points. EX: Mixture of water (bp = 100 C) and ethanol (bp = 78 C)
- The mixture is slowly heated to a temperature that is between the boiling points of the liquids. The liquid with the lower boiling point will boil first. EX: Heating the mixture to 85 C will cause ethanol to boil while water does not
- The ethanol vapor rises up to the thermometer, which monitors the temperature of the flask to ensure it is kept constant (85 C in this case).
- The vapor then runs through the condensation tube where it cools and condenses back into liquid (due to the continuous stream of cool water flowing through the tube).
- The liquid can then be collected on the other side in the receiving flask as the distillate
Miscibility
The property of two substances to mix, or fully dissolve in each other, at any concentration, forming a homogeneous mixture (a solution)
Usually applied to liquids
Substances with similar intermolecular interactions (nonpolar and nonpolar vs polar and polar) tend to be more miscible in each other
