11.1 Intermolecular forces

Question 1

Intermolecular Forces: Between different molecules
* We don’t use intermolecular forces to talk about ionic compounds: All the ions are held together by ionic bonds
* we use it to talk about covalently bound moelcules
* network covalents also arent held togethe by intermolecular forces

When you have covalent bonds, thats talking about whats binding the actual molecule together.
* However, whats holding all these molecules together (like molecule to molecule) are the intermolecular forces, and these are much weaker binds than the covalent bonds

Question 2

Intermolecular forces = way weaker than an ionic or covalent bond
* Like 25-100 times weaker

So as a result of the intermolcular forces being so weak compared to ionic, the covalent compound structures tend to be held together much less strongly than an ionic compound or a network covalent
* Not talking about the actual covalent bonds in the molecules, but the intermolecular forces that hold covalent molecules togther are weak, meaning covlanet molecular sturcutres dont hold together that well compared to ionic compounds and network covalent compounds

Question 3

Intermolcular forces:
* Hydrogen bonding - strongest
* Dipole-Dipole Forces - middle
* London Disperson Forces -weakest
* Ion Dipole forces

NOTE: these rules are true for molecules that are similar in size. However, if the molecules differ in size it changes

Question 4

Dipole-Diple Forces
* Only present when theres polar molecules that create partial charges

NOTE: hydrogen bonding is like a super strong dipole-dipole

Question 5

So this is showing a dipole dipole fore

Basically since theres a large electronegativity difference between the H and Cl it creates a partial charge (Cl is negative because its holding that electron closer most of the time)
* So the covalent bond between hydrogen and chlorine is strong, however the intermolecule force between the partial negative charge on the Cl and the partial positive charge on the H is weaker
* It makes since that this dipole-dipole intermolecular force is weaker than an ionic bond, because ionic bonds are full charges, attracting to each other, while these are only partial charges interacting with one another.

The bigger the electronegativity delta between the two covalently bound atoms, the bigger the partial charges = stronger intermolecule dipole dipole force because were now working w/ bigger partial charges (appraoching 1, but never actually quite at 1)

Question 6

Hydrogen Bonding:
* Like a super strong Dipole-Dipole force

Has to have hydrogen interacting w/ N,O,F
* it needs to be bound to these
* it has to be bound to these atoms because theyre very small and very electronegative
* so N and Cl are basically tied for their electronegativity, however, because N is smaller it works with hydrogen bonding

Must be either
* F-H, O-H, N-H somewhere in the structure

Question 7

Water does hydrogen bonding

So again, we classify it as hydrogen bonding instead of dipole-dipole, because this intermolecular force is so much stronger than any of the other dipole-dipole forces even though its the same idea

That lone pair is what interacts, but for it to be a hydrogen bond it has to be a Hydrogen bound to an N,O,F. And that hydrogen has to be interacting with another N,O,F bound to a hydrogen

hydrogen bond acceptor: atom w/ F/O/N w/ lone pairs = hydrogen bond acceptor

Hydrogen bond donor: The molecular that has the hydrogen thats involved in the interaction thats bound to a N/O/F = hydrogen bond donor

So in water below you can see every oxygen has two lone pairs, which means that it could act as a hydrogen bond acceptor twice

Every water molecular has two hydrogens bound to an oxygen, meaning it can act as a hydrogen bond donor up to 2 times as well

So every water molecular has the chance to be interacting with up to 4 other water molecules

Question 8

NOTE: the molecular below would not be capable of hydrogen bonding. The hydrogen and oxygen need to be directly bound to get the delta in electrongeativity. We don’t see that here

Question 9

Why N-O-F do hydrogen bonding, but Cl (which has the same electronegativity has N) doesnt:

Hydrogen bonding happens when two things are true
1) High electronegativity difference between H and the atoms its bound to
2) Very small atomic radius of the electronegative atom (N, O, F only) - Cl is much bigger which is why it isnt involved in hydrogen bonding

Cl has a larger atomic radii, meaning its electron cloud is spread out over a larger volume
* So the negative chrage is more diffuse - not tightly packed in one spot.

Question 10

Lond disperson forces:
* electrons rotate around the atom, meaning sometimes some places are more negative than in other places

When the atom/molecule interacts with another molecule that has its electrons rotated in the opposite orientation one will be a little negative and the other will be a little bit positive, and we will have an interaction

Then the electrons continue rotating and the attraction is gone

So they have a temporary dipole, but not a perm one

So by one having its electrons in one orientation, it can make the molecule near it put its electrons in the opposuite orietantion makign a temporarry attractive force
* can also be called an induced dipole

Question 11

All molecules have london disperson forces because they all have electrons

London disperson forces dependent on 2 things
1) Size - bigger molecules = more electrons = greater chance of having london disperson forces
* This is the more important one
2) Surface area
* more surface over which these forces interact

Bigger molecules = more london disperson forces
* all molecules have these

Size = a function of molecular weight
* if its like a difference of more than 20% in molecular weight then size is a deciding factor in which ones make stronger bonds

Question 12

Ion dipole forces:
* typically stronger than hydrogen bonding
* these are between a polar molecule w/ a dipole and an ion - doesnt even seem like an actual intermolecualr force

These always have a mixture because they’re between an ion and something w/ a partial charge
* so they could never be a pure substance

The smaller the ions the greater the strength of the bond

Also affected by charge strengths
* the more positive the cation, the bigger the interaction
* the most negativiely charged anion = bigger interaction

below showing an ionic compound dissolved in water

However, note sometimes hydrogen bonding can be stronger than these, but not typically.

Question 13

A question might come up asking about pure substances. Hydrogen bonding is the strongest intermolecular force in pure substances.
* ion dipole forces not considered a pure susbstance (meaning all of the same compound) because they include an ion and something thats polar (not an ion), meaning they can’t be a pure substance

Question 14

Boiling point: going from liquid to gas
* basically you’re going from lots of intermolecular forces, to basically no intermolecular forces
* the stronger these intermolecular forces are the more energy it takes to break them. = higher boiling point

Greater intercoluar forces = higher boiling point
* so we might get a question that asks “what has the highest boiling point” and we’d have to find the ones w/ the strongest intermolecular forces and that would be our answer
* it would have the highest melting point as well, because you’re still having to break some of those intermolecular forces. However w/ the boiling point you’re basically breaking all of those intermolecular forces.

Question 15

Enthalpy of vaporization: Amount of heat at that it takes to break those intermolecular forces to go from liquid to gas
* This is going to be higher for something w/ greater intermolecular force strength

Increased viscosity w/ increased intermolecular forces
* this is something thats thicker and moves slower (think honey vs water)
* greater intermolecular forces = more friction between the layers = more they all want to flow together = more viscus

Question 16

Greater intermolecular forces = greatest surface tension
* this is basically the force to puncture the water

Question 17

higher intermolecular forces = lower vapor pressure

vapor pressure = what fraction of the molecules in a liquid have jumped into the gas phase
* so if those intermolecular forces are lower, they need less KE to jump out of the liquid phase and more into the gas phase

Less sticky molecules = easier to jump out.

higher intermolecular force = lower vapor pressure

Question 18

Question 19

as you heat a substance up, you’re going to be getting more vapor, because you’re giving more average KE to the molecules.
* a greater percentage will have enough to get up into the vapor phase

Below you can see the more you heat something up the more vapor pressure you have

if you want to boil water, the vapor pressure has to = its external pressure (1 atm pushing down on it) so at 100C thats when the molecules get enough KE to = ATM pressing down on them and overcome it to jump into the vapor phase

so vaporization is lower at higher altitiudes because theres less than an ATM pushing down on it. So you basically don’t need as much heat to get it into the vaporization phase

When vapor pressure reaches the external pressure (1 ATM) thats when the substance boils

Question 20

Which has the highest intermolcular forces?

Answer 20

Well they’re all hydro carbons, meaning they’re all non polar bonds (because carbon has a low electornegativity and so does H)
* so none are dipole-dipole / hydrogen bonding

So that leaves us London disperson, and the last one if the biggest, meaning it has the propinceity for have the largest london disperson force
* so that last one has the highes inermolecular forces

Question 21

Which one has the greatest intermolcular forces

Answer 21

The last one has hydrogen bonding = biggest intermolecular force

draw these out, the first one has both H and F but they arent bound

Question 22

which one has the strongest intermolecular forces

Answer 22

The last one because the carbon F bound is polar, meaning it has dipole-dipole

however, you need to check molecular weights. They’re all similar so the dipole-dipole is the strongest. However, if one was way bigger it would be the strongest.

Question 23

Which one has the strongest intermolecular forces

I like this example.

Answer 23

The last one

While CH3F is polar, meaning it has dipole dipole

However CH3CH2CH3 is much bigger and the london forces are super strong. Meaning it has th ebiggest intermolecular forces, even though it doesnt have dipole-dipole forces

Question 24

So say one molecule is like 15% bigger and doesnt have dipole-dipole while the smaller one does. Well the london one thats bigger is still stronger

however, it needs to be like 30% bigger to overcome a smaller molecular w/ hydrogen bonding (because this is a much stronger intermolecular force)

Question 25

Answer 25

London and Dipole-dipole

Question 26

Answer 26

its going to be the biggest one with the highest molecular weight.

Question 27

Answer 27

Its all of them. Because with water theres already a hydrogen bond, basically all it needs is to have a N-O-F in it now, doesnt need to be bound to oxygen To be capable of hydrogen bonding as a pure sybstance a molecule must have an F-H, O-H, or N-H bond in its sutruecutre. The equaturement is different when mixed w/ water. Water can already act as a hydrogen bond donor (has the hydrogen atoms bonded to oxygen). All that is needed from the compound mixing with water is a hydrogen bon acceptor (F, O, Or n with at least one lone pair of nonbonding electrons avilable). As all of these molecules contain either oxygen or nitrogen they would all be capable of hydrogen bonding when mixed with water * so they need a lone pair, need to be O, N or F. * Don't need to be bound to hydrogen

Question 28

Answer 28

Compound A has a higher boiling point because it has greater london disperson forces (meaning greater intermolecular forces = will take more KE to rip them apart and send it into the vapor phase)

Question 29

Answer 29

While HBr is more polar (because Br has a super high electronegativity and H is low) HI will actually have increased intermolcular forces because its a much bigger molecular, even though the dipole-dipole isnt as strong, meaning that its boiling point will be higher (take more E to rip apart the intermolecular forces) D

Question 30

Answer 30

Niether will have any dipole because hydrogen and carbon have low electronegativity They have the same # of carbon and hydrogen's = same london CH3CH2CH2CH2CH3 has a larger surface area = higher boiling point because that makes it have greater london Basically london is impacted by surface area and molecular weight * because molecular weight was the same i next drew out the molecules and saw this molecular was less compared = more surface = more room for these london forces to actually occur

Question 31

Answer 31

So put the one with the highest boiling point first Highest boiling point = most intermolecular forces realistically I should've gotten the molecular weights first, realized they were far apart and didnt actually need to draw everything out. Because if there weight differes by a lot the heavy one is going to have the most intermolecualr forces, even if the other has hydrogen/dipole bonding

Question 32

at boiling point the vapor pressure is = extenral pressure * meaning it has enough KE to escape the pressure of 1ATm water boils at 100C and 1ATm is the force down on it

Question 33

Answer 33

740 The boiling point of a compound is the point at wich the vapor pressure is equal to the external pressure. As a substance is heated its vapor pressure increases until this point is reached. Water boils at 100C at atmospheric pressure (760 torr). At a slightly lower temperatuer 97C the vapor pressure would be slightly lower than 760 torr and 740 torr is the only answer in this range * basically at a higher point the water needs less KE to go into the vapor state because theres less pressure on it

Question 34

Answer 34

2 only becuase as temperature increases, the KE increases, which will send it to the vapor state vapor pressure = a measure of the tendency of a material to change into the gaseous state

Question 35

Answer 35

Viscosity increases as intermolecular forces increase. I can see the bottom one is easily the biggest. So that one will have the most intermolecular forces, meaning it has the most viscoity

Question 36

I like this question

Answer 36

Surface tension increases as intermolcular forces increase * this is essnetialy the ability to poke a hole in the water. Im going to start by taking all the mases, then draw out the ones that are close they're all close so im going to have to draw them The reason 2 was correct was because it did hydrogen bonding