Liquids and Solids

What is the primary difference between intramolecular and intermolecular forces?

Intramolecular forces are bonds within a molecule that holds atoms together

Intermolecular forces are weaker attractions between separate molecules

What are the three types of intermolecular forces?

Hydrogen bonding
Dipole-dipole forces
Dispersion forces

What are the three types of intramolecular forces?

Covalent
Ionic

Metallic

What do intermolecular forces determine?

Physical properties like boiling point, melting point, viscosity, vapor pressure, and surface tension

Intermolecular forces are _____ than intramolecular forces like covalent or ionic bonds.

Weaker

What is the strongest intermolecular force found in neutral atoms?

Hydrogen bonding

To form a hydrogen bond, hydrogen must be covalently bonded to which three specific elements?

Nitrogen, Oxygen, or Fluorine

Why does oxygen create a strong partial positive charge on hydrogen in a molecule?

Oxygen is highly electronegative and strongly pulls electron density away from the hydrogen atoms.

Among neutral molecules, what is the strongest type of intermolecular force?

Hydrogen bonding.

What are dipole-dipole forces?

Forces between molecules that have permanent net dipoles

The positive end of one polar molecular is attracted to the negative end of another

What do dipole dipole forces apply to?

Only to polar, nonionic/covalent molecules

What are dispersion forces?

Weak attractive forces all atoms and molecules experience.

Why do dispersion forces occur?

Because electrons are in constant motion this causes temporary dipoles when electron density concentrates on one side of a molecule

What is the only type of intermolecular force that exists in nonpolar, covalent molecules?

Dispersion forces (London dispersion forces).

Rank the three main intermolecular forces from strongest to weakest.

Hydrogen > dipole-dipole > london dispersion forces

How do particle size and shape affect dispersion forces?

Larger particles with more electrons form stronger temporary dipoles, increasing dispersion forces

Between a straight-chain molecule and a branched molecule of the same molar mass, which will have stronger dispersion forces?

The straight-chain molecule, because it can pack together more efficiently.

Why do branched isomers generally have lower boiling points than their straight-chain counterparts?

Branching decreases packing efficiency, which weakens London dispersion forces.

Which of the following is NOT an intermolecular force: hydrogen bonding, dipole-dipole interactions, ionic bonding, dispersion forces, or van der Waals forces? Why?

Ionic bonding. Intermolecular forces are between two different molecules; ionic bonding holds ions together within a compound

What does it mean to have strong intermolecular forces?

Molecules stick together more tightly and require more energy to separate

What does it mean for a liquid to vaporize?

Liquid boils

What happens when a liquid boils?

Its molecules separate and enter the gas phase

What does it mean if the molecules are held together by strong intermolecular forces when it comes to boiling point?

It means more heat is needed to overcome these forces and so you’ll have a higher boiling point

So if a question asks you which molecule has the highest boiling point, what is it looking for?

Identify the molecule with the stronger intermolecular force

So if a question asks you which molecule has the lowest boiling point, what is it looking for?

Identify the molecule with the weakest intermolecular force

What does viscosity measure?

A liquids resistance to flow

When it comes to viscosity molecules with stronger intermolecular forces…

Bind together more tightly and so they have higher viscosities

So if a question asks you which molecule will have a higher viscosity, what are you looking for?

The molecule that has the highest intermolecular force

What is surface tension?

When a liquid’s surface is able to resist disruption

Which type of intermolecular force would exhibit high surface tension?

Strong intermolecular forces

What is the relationship between intermolecular force strength and surface tension?

Stronger intermolecular forces result in higher surface tension

What is vapor pressure?

Pressure exerted by vapor produced through evaporation from liquids and solids at equilibrium

When does boiling happen?

When a liquid’s vapor pressure becomes equal to atmospheric pressure

How does increasing the strength of intermolecular forces affect the vapor pressure of a liquid?

Vapor pressure will decrease

Because molecules are less likely to escape into the gas phase.

At what specific point is the boiling point of a liquid reached?

When the liquid's vapor pressure equals the surrounding atmospheric pressure.

Molecules with stronger intermolecular forces will vaporize…

Less readily

This causes a lower vapor pressure

What are the four types of solids?

• Metallic solics

• Ionic solids

• Covalent network solids

• Molecules solids

What are metallic solids?

When metals are held together by a sea of electrons moving freely between each atom

What are the properties of metallic solids?

• Conduct electricity

• Hard

What are ionic solids?

Held together by strong ionic bonds between metals and nonmetals

What are the properties of ionic solids?

• DON’T conduct electricity as solids

• DO conduct electricity when molten or in aqueous solution

What are covalent network soldis?

Nonmetals held together by networks of covalent bonds

What are the properties of covalent network solids?

• Very hard

• Used to make semiconductors

What are molecular solids?

Nonmetals held together by intermolecular forces

What are the properties of molecular solids?

• Low melting point

• Do NOT conduct electricity

What are unit cells?

The smallest, repeating 3D structural unit of a solid

What do unit cells form?

Lattice structures

Unit cell with only 1 atom in the entire cell

Simple cubic unit cell

Unit cell with 2 atoms per unit cell

Body centered cubic unit cell

Unit cell with 4 atoms per cell

Face centered cubic unit cell

List the endothermic phase changes.

• Fusion (melting)

• Vaporization

• Sublimation

What is fusion?

Solid ⟢ liquid

What is vaporization?

Liquid ⟢ gas

What is sublimation?

Solid ⟢ gas

What does it mean to be endothermic?

The phase change REQUIRES heat

What is the ΔH and ΔS of an endothermic reaction?

They are both positive
ΔH > 0

ΔS > 0

List the exothermic phase changes.

• Crystallization

• Condensation

• Deposition

What is crystallization?

Liquid ⟢ solid

What is condensation?

Gas ⟢ liquid

What is deposition?

Gas ⟢ solid

What does it mean to be exothermic

The phase change RELEASE heat

What is the ΔH and ΔS of an exothermic reaction?

They’re both negative

ΔH < 0

ΔS < 0

Which requires more energy; evaporation (liquid to gas) or fusion/melting (solid to liquid)?

Evaporation needs more energy then melting

Which requires more energy; condesation or crystallization?

Condesation releases more heat energy into the surroundings than crystallization

Why is this the case?

• Melting: In a solid, particles are held tightly in place. When you melt it, you only need to give them enough energy to move around more freely.

• Evaporation: In a liquid, particles are already moving around, but they are still attracted to each other. To become a gas, a particle has to get enough energy to completely escape those attractions.

• Because escaping those attractions takes much more energy than simply loosening the particles, evaporation requires more energy than melting

What is the relationship between vapor pressure and boiling point?

Vapor pressure is inversely related to boiling point

What is the tea between atmospheric pressure at higher altitudes?

At higher altitudes there is less atmospheric pressure (this is b/c there are fewer air molecules bove and around to press down on the liquid)

Due to this, at higher altitudes, vapor pressure can…

Exceed atmospheric pressure more easily

How does boiling differ between high altitudes and sea level?

Liquids boil at lower temperatures than they do at sea level

On a typical phase diagram, where does the solid region occur?

At low temperature and high pressure

On a typical phase diagram, where does the gas region occur?

At high temperature and low pressure

On a typical phase diagram, the liquid region lies…

At moderate temperatures and pressures

What is #1/#3?

Critical point

What is #2?

Liquid phase

What is #4?

Solid phase

What is #5?

Triple point

What is #6?

Gas phase

What is number 7?

Temperature

What is number 8?

Pressure

What is the critical point on a phase diagram?

The temperature and pressure where liquids and gases no longer exist as distinct phases

Beyone the critical point, what happens to liquids, and gases?

They merge into a supercritial fluid

What is the triple point on a phase diagram?

The point at which all three phases exist in equilibrium

What is the densest phase between solids, liquids, and gases?

Solids

Why are solids more dense than the other phases?

Becuase their particles are packed together the tighest

How does water differ in terms of their most dense phase?

Water is denser in the liquid phase than the solid phase

Why is water more dense in the liquid phase?

In liquid water, molecules are packed closely due to hydrogen bonding

So how does water’s phase diagram look?

It’s slope is negative

What does it mean for the slope to be negative?

Applying pressure can convert ice into liquid water rather than vice versa

What is molar heat of vaporization?

The amount of energy required to convert one mole of a substance from liquid to gas at boiling point