Unit 13

Question 1

Intermolecular interactions

Answer 1

-an attractive force that arises between the positive components (or protons) of one molecule and the negative components (or electrons) of another molecule
-for two or more molecules to interact, the molecules must come into contact with one another

Question 2

Ideal gas law

Answer 2

-equation of state of a hypothetical ideal gas, approximation of the behavior of many gases under many conditions
-PV = nRT

Question 3

Variables in ideal gas law

Answer 3

-PV = nRT
-P= pressure of the gas (atm)
-V= volume of the gas (L)
-n= number of moles of gas
-T= temperature of the gas (K)
-R= gas constant (0.08206 L atm/mol K)

Question 4

What can pressure be increased by?

Answer 4

-increase in number of collisions (leads to increase in n)
-increase in the momentum of colliding gas particles (causes increase in KE of gas and increase in temperature, (KE of gas correlates to temperature))

Question 5

Pressure for mixture of ideal gases

Answer 5

P(total)= (nA + nB + nC)/V
or
P(A) + P(B) + P(C)

Question 6

mole fraction of a gas in an ideal gas mixture

Answer 6

• (n(A))/(n(total))

Question 7

Low pressure

Answer 7

-P(actual)= P(ideal)
-ideal gas behavior
-gas molecules occupy a negligible volume compared to the volume of their container
-gas molecules have elastic collisions
-atoms/molecules, on average, are very large distances apart
-interactions are negligible

Question 8

elastic collision

Answer 8

a collision in which there is no net loss in kinetic energy in the system as a result of the collision

Question 9

medium pressure

Answer 9

-P(actual)< P(ideal)
-non-ideal behavior
-gas molecules are mutually attractive and cluster together
-average distance between atoms/molecules is small enough that attractive interactions dominate
-when clusters of molecules form, number of collisions decrease
-ATTRACTIONS DOMINATE

Question 10

high pressure

Answer 10

-P(actual)> P(ideal)
-non-ideal behavior
-the volume occupied by the gas is no longer negligible and the molecules repel each other
-average distance between atoms/molecules is very small
-REPULSIONS DOMINATE

Question 11

circumstances in which we compare low to medium to high pressure

Answer 11

-V and T are fixed
-n is varied (increases) to see effect on P

Question 12

Ideal behavior

Answer 12

-occurs when molecules collide and behave as billiard balls
-collisions are described as “elastic”
-collide and transfer momentum (total momentum is conserved)
-have negligible volume
-no intermolecular forces between them (no attraction/repulsion) and no interactions after collision
-only collide elastically with each other and the container walls, meaning they move randomly with no energy loss during collisions
-all gases display ideal behavior over some range of temperature and pressure (usually high T, low P)

Question 13

Real behavior

Answer 13

-deviates from ideal gas behavior
-particles have a finite volume
-particles experience intermolecular attractive/repulsive forces
-occurs usually at low T, high P

Question 14

Dalton’s Law of Partial Pressure

Answer 14

-the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the gases in the mixture
-each individual gas exerts a partial pressure that contributes to the total pressure
-can be used to quantify the pressure of the entire mixture (Ideal Gas Law can quantify partial pressure of each component)

Question 15

Vanderwaals equation

Answer 15

-extends the ideal gas law to include the non-zero size of gas molecules and the interactions between them (real behavior)
-P= ((nRT)/(V-nb)) - ((an^2)/(v^2))
-a: positive constat, related to attractive interactions (large a, lower P)
-b: positive constant, related to repulsive interactions (large b, higher P)

Question 16

how medium/high pressure correlates to vanderwaals equation

Answer 16

-high pressure: more repulsive forces=higher b, lower P
-medium pressure: more attractive force=higher a, lower P
-low pressure: equal a and b, medium P

Question 17

molecular dipole

Answer 17

-the separation of charge within a molecule
-one side of the molecule has a partial positive charge and the other side has a partial negative charge
-arises from unequal sharing of electrons between atoms due to differences in electronegativity
-measure of molecule’s overall polarity
-sum of all bond dipoles

Question 18

bond dipole

Answer 18

-the separation of partial positive and negative charges within a covalent bond
-occurs when electrons are shared unevenly between two atoms due to differences in their electronegativity
-M>0, molecule is polar
-A bond dipole always points from the less electronegative atom towards the more electronegative atom

Question 19

Bond dipole vs molecule dipole

Answer 19

-A “bond dipole” refers to the polarity of a single chemical bond between two atoms within a molecule, determined by the difference in their electronegativity
-a “molecular dipole” represents the overall polarity of the entire molecule, calculated by summing up the individual bond dipoles considering the molecule’s geometry
-a molecular dipole considers the combined effect of all the bond dipoles in a molecule, taking into account their spatial arrangement

Question 20

How to determine if a bond is polar or nonpolar

Answer 20

1. draw all bond dipoles in correct geometry
2. sum dipole vectors head-to-tail
   -if bond dipoles point same direction, permanent molecular dipole exists
   -if bond dipoles point different directions, no permanent molecular dipole exists (often occurs in symmetrical molecules where bond dipoles cancel one another)

Question 21

permanent vs temporary molecular dipole

Answer 21

-A permanent molecular dipole refers to a molecule that has a consistent separation of charge due to unequal sharing of electrons between atoms with different electronegativities meaning it always has a positive and negative end
-a temporary/induced dipole is a fleeting separation of charge that occurs due to random fluctuations in electron distribution within a molecule, only present for a brief moment in time
-a non-polar molecule can temporarily become polar due to these fluctuations

Question 22

Polarizability

Answer 22

-the ability of electron density in the atom/molecule to be distorted when interacting with a nearby atom/molecule/charge
-for any type of intermolecular interaction, the interaction is made stronger when the molecules involved are more polarizable

Question 23

Factors that affect polarizability

Answer 23

1. molecular size (larger molecules are more polarizable (more points of interaction))
2. heavier atoms (Cl2 vs I2, I2 is more polarizable due to Z)
3. σ vs π electrons (molecules with π electrons are more polarizable)
4. surface area (extended molecules are more polarizable (have larger surface area) then compact molecules)

Question 24

Hydrocarbons

Answer 24

-neutral molecules that contain only C and H
-assume they are nonpolar unless otherwise told
-more extended molecular chains are more polarizable and have stronger id-id interactions

Question 25

Boiling points (Tbp)

Answer 25

-temperature at which the vapor pressure of a liquid equals the external pressure
-boiling points give us info about the relative strengths of intermolecular interactions
-the higher the boiling point, the more energy is required to break intermolecular interactions (not covalent bonds)
-higher the boiling point, the more energy required to break intermolecular interactions and therefore stronger the intermolecular itneractions
-at temperatures above boiling point, kinetic energies of molecules are high enough to overcome dissociation energy and separate from other molecules

Question 26

Dissociation Energy (Do)

Answer 26

-the amount of energy needed to break a specific chemical bond in a molecule
-A higher dissociation energy generally corresponds to a higher boiling point because a higher dissociation energy indicates stronger bonds within a molecule, meaning more energy is needed to separate the molecules and cause them to vaporize

Question 27

Types of intermolecular interactions

Answer 27

-dipole-dipole interactions (d-d)
-dipole induced itnereactions (d-id)
-instantaneous dipole-induced dipole (id-id)
-hydrogen bonding (H-bond)

Question 28

Dipole-Dipole Interactions

Answer 28

-approximate strength= 2 kJ/mol
-occurs between 2 polar molecules
-strongest interactions typically occur with head-to-tail alignment of molecular dipoles
-ex. HCl-HCl

Question 29

Dipole-Induced Interactions

Answer 29

-approximate strength= <2 kJ/mol
-occurs between a polar and nonpolar molecule
-occurs when 2 molecules closely approach one another
-permanent dipole of polar molecule influences electron density of nonpolar molecule, causing nonpolar’s electron density to adopt induced dipole in direction of permanent dipole
-ex. HCl-I₂

Question 30

Instantaneous Dipole-Induced Dipole

Answer 30

-approximate strength= «2 kJ/mol
-sometimes called ‘London Dispersion forces’ or ‘Vanderwaals forces’
-occurs between 2 nonpolar species (atoms/molecules)
-random fluctuation in electron density results in an instantaneous dipole
ex. Xe-Xe

Question 31

Hydrogen bonding

Answer 31

-approximate strength= 20 kJ/mol
-special class of d-d interactions
-interaction between an acceptor bond dipole and a donor dipole (can assume donor dipole is a lone pair dipole)
-occurs when element is O, N, or F (due to high electronegativity, small size, and lone pairs)
-H-bonds are usually very strong because H is really small, allowing for very close approach of interacting acceptor and donor dipole
-strongest interaction aligns lone pair dipole and bond dipole head to tail with 180 angle
-high electronegativity of N, O, and F cause H electrons to migrate towards these elements giving H a slightly positive charge. This causes H to be attracted to the negativity of lone pairs on different molecules.

Question 32

Intermolecular Interactions ranked by strength

Answer 32

1. Hydrogen bonding (20 kJ/mol)
2. Dipole-Dipole (2 kJ/mol)
3. Dipole-Induced Dipole (<2 kJ/mol)
4. Instantaneous dipole-Induced dipole (dispersion) («2 kJ/mol)

Question 33

How to tell what molecule has higher boiling point

Answer 33

-decide intermolecular interaction strength, polarity, and size/surface area
-polar, H-bonding, or large surface area have higher boiling point
-compounds with larger molar mass have higher boiling points (increased polarizability of heavier compounds)

Question 34

Binding Energy

Answer 34

-amount of energy needed to separate two molecules/atoms

Question 35

Low temperatures

Answer 35

-intermolecular forces dominate
-low kinetic energies permit molecules to be bound together