Unit 3 - Intermolecular Forces

Question 1

Intermolecular forces vs Intramolecular forces

Answer 1

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

Question 2

Explain the relationship between boiling point and IMFs

Answer 2

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

Question 3

How do London dispersion forces form?

Answer 3

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)

Question 4

Polarizability

Answer 4

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

Question 5

What are dipole-dipole forces?

Answer 5

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

Question 6

What are hydrogen bonds?

Answer 6

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

Question 7

What are ion-dipole forces?

Answer 7

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

Question 8

What is a dipole?

Answer 8

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)

Question 9

Explain the relationship between IMFs and vapor pressure

Answer 9

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

Question 10

Define vapor pressure

Answer 10

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)

Question 11

Define normal boiling point

Answer 11

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

Question 12

Explain the relationship between temperature and vapor pressure

Answer 12

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).

Question 13

Describe the relationship between boiling point and vapor pressure

Answer 13

Higher boiling point = lower vapor pressure

They are inversely related and connected through the concept of IMFs

Question 14

Describe differences in particle space for solids, liquids, and gases.

Answer 14

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

Question 15

Define pressure

Answer 15

Pressure is the force exerted by gas particles colliding with the walls of its container

Question 16

Define partial pressures of gases

Answer 16

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

Question 17

How do you calculate partial pressure?

Answer 17

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

Question 18

Maxwell Boltzmann diagram

Answer 18

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)

Question 19

Ideal gases

Answer 19

Ideal gases have no volume and no attraction between particles (no IMFs)

Low pressure and high temperature

Question 20

What kinds of gases deviate from ideal behavior?

Answer 20

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

Question 21

Chromatography

Answer 21

This process separates the components of a solution using differences in intermolecular interactions

Question 22

Distillation

Answer 22

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).

Question 23

Explain how distillation works.

Answer 23

1. A mixture must have two liquids of different boiling points. EX: Mixture of water (bp = 100 C) and ethanol (bp = 78 C)
2. 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
3. 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).
4. 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).
5. The liquid can then be collected on the other side in the receiving flask as the distillate

Question 24

Miscibility

Answer 24

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

Question 25

How can you tell what an unknown dye is out of several options of known dye based on paper chromatography?

Answer 25

Compare the Rf value (retention factor) of the unknown dye with all the known dye options. You can estimate this by estimating the distance traveled by the sample in proportion to the solvent front in order to get an approximate fraction/decimal. Same Rf value corresponds to the same dye.

Don’t rely on the position of the spot on the paper to determine which option matches because the solvent front could have been different

Question 26

Explain the relationship between a region of the

electromagnetic spectrum and the types of molecular/electronic transitions associated with it

Answer 26

1. Microwave radiation → used to detect molecular rotation
2. Infrared radiation → used to detect molecular vibrations
3. Ultraviolet/visible radiation → used to detect electronic energy

Question 27

How can you use electromagnetic radiation to study particles?

Answer 27

The smaller the size of the particles that need to be studied, the higher the frequency of the radiation used

1. For studying electrons, ultraviolet radiation is used
2. For studying bonds (shared pairs of electrons), infrared radiation is used
3. For studying entire molecules, microwave radiation is used

Question 28

Spectrophotometer

Answer 28

Spectrophotometer: by emitting light at a particular wavelength (such as ultraviolet), it can measure the amount of light the solution absorbs and thus its concentration over time

If a solution changes color (and therefore wavelength) as the reaction progresses, the amount of light absorbed will also change

Colorimeter: a spectrophotometer that can only emit light at specific frequencies

Question 29

What’s a possible experimental error that could have resulted in a lower absorbance?

Answer 29

Distilled water in the cuvette

This would decrease the concentration of the ion and since concentration and absorbance have a directly proportional relationship, this would in return result in a lower absorbance value.

Question 30

If you get a Maxwell distribution curve for a reaction at 120 C, what is the relative change you will see in the curve with the same reaction at 30 C?

Answer 30

The curve will have a peak that is above and to the left of the 120 C curve. The curve in the region beyond the activation energy (represented by a vertical line) is also below the corresponding 120 C curve

Question 31

Induced dipole

Answer 31

Polar molecule induces a dipole in a nonpolar molecule

Question 32

If the dye moves high up the chromatography paper, what can be concluded about the dye?

Answer 32

It has a weaker attraction for the stationary phase (paper) than it has for the mobile phase (solvent). A greater distance traveled means that the dye has a similar polarity to the solvent (and thus is very attracted) because like dissolves like (polar and polar vs nonpolar and nonpolar).

Question 33

pH meter

Answer 33

Used to measure the pH of an aqueous solution.

From this, you can find pOH (14 - pH), [H+] (10^-pH), or [OH-] (10^-pOH).