3B5 Effects of Bonding, Structure, and Interparticle Interactions

Question 1

How do intermolecular forces affect physical properties of substances?

Answer 1

They influence:

• Melting points
• Boiling points
• Freezing points
• Viscosity
• Surface tension
• Solubility

Question 2

Define:

boiling point

Answer 2

The temperature at which a liquid’s vapor pressure equals the external pressure, causing it to transition into a gas.

The boiling point depends on atmospheric pressure; it decreases at higher altitudes due to lower pressure.

Question 3

What factor primarily determines boiling point?

Answer 3

The strength of intermolecular forces.

Stronger forces, such as hydrogen bonds, require more energy to overcome, leading to higher boiling points.

Question 4

Which has stronger intermolecular forces:

covalent or hydrogen

Answer 4

Covalent bonds

Covalent bonds involve shared electrons between atoms, making them stronger than intermolecular hydrogen bonds.

Question 5

Define:

melting point

Answer 5

The temperature at which a substance transitions from solid to liquid.

Melting point is influenced by the lattice energy in crystalline solids and the strength of intermolecular forces.

Question 6

Fill in the blank:

A molecule with stronger intermolecular forces will have a _______ melting point.

Answer 6

higher

Substances like ionic solids have high melting points due to strong ionic interactions.

Question 7

Define:

vapor pressure

Answer 7

The pressure exerted by a vapor in equilibrium with its liquid or solid phase.

Question 8

Which factors affect vapor pressure?

Answer 8

• Temperature
• Intermolecular forces

Vapor pressure increases with temperature as more molecules gain energy to escape into the gas phase.

Weaker intermolecular forces make it easier for molecules to vaporize.

Question 9

True or False:

Ionic compounds are soluble in water.

Answer 9

True

The solubility depends on:

• The compound’s lattice energy and the hydration energy provided by water.
• The ion’s size and charge density; smaller ions tend to dissolve more easily.

Question 10

What type of solvent dissolves polar solutes?

Answer 10

Polar solvents.

Water is a common polar solvent; it dissolves salts and polar organic compounds like alcohols.

Question 11

Which bond type contributes most to water’s high boiling point?

Answer 11

Hydrogen bonds

These bonds occur between water molecules, leading to high boiling and melting points compared to similar-sized molecules.

Question 12

What is the relationship between molecular weight and boiling point?

Answer 12

Generally, higher molecular weight increases boiling point, assuming similar intermolecular forces.

Larger molecules have more electrons, leading to stronger London dispersion forces.

Larger electron clouds are more polarizable, increasing temporary dipole interactions.

Question 13

Fill in the blank:

Molecules with _______ intermolecular forces have higher equilibrium vapor pressures.

Answer 13

weaker

Substances like ether have high vapor pressures due to weak intermolecular forces.

Question 14

Fill in the blank:

Compounds with _______ intermolecular forces evaporate faster.

Answer 14

weaker

Volatile compounds, like acetone, evaporate quickly due to weak intermolecular forces.

Question 15

True or False:

Smaller ions generally have lower melting and boiling points.

Answer 15

False

Smaller ions allow stronger electrostatic interactions due to higher charge density.

Question 16

Why do ionic solids have high melting points?

Answer 16

Due to the strong electrostatic forces between ions in the lattice.

For example, NaCl has a high melting point due to strong ionic interactions.

Question 17

Which has a higher boiling point:

NaCl or H₂O

Answer 17

NaCl

NaCl has ionic bonds which are much stronger than the hydrogen bonds in water.

Question 18

Fill in the blank:

Nonpolar molecules dissolve best in _______ solvents.

Answer 18

nonpolar

Solvents like hexane dissolve nonpolar compounds, such as oils.

Question 19

Why is ethanol soluble in water?

Answer 19

Due to hydrogen bonding between ethanol and water molecules.

Ethanol’s O-H group allows it to form strong interactions with water.

Question 20

What type of compound is likely to have the lowest melting point?

Answer 20

Nonpolar molecular compounds with weak London dispersion forces.

Examples include simple hydrocarbons like methane and ethane.

Question 21

What does ‘like dissolves like’ mean?

Answer 21

• Polar solvents dissolve polar solutes.
• Nonpolar solvents dissolve nonpolar solutes.

This principle explains the immiscibility of oil and water.

Question 22

Which has a higher boiling point:

water (H₂O) or methane (CH₄)

Answer 22

Water (H₂O)

Water has stronger hydrogen bonding that requires more energy to overcome.

Methane, being nonpolar, exhibits only weak London dispersion forces.

Question 23

Which has a higher boiling point:

HF or HCl

Answer 23

HF

HF has stronger hydrogen bonding.

HCl exhibits dipole-dipole interactions but lacks hydrogen bonding.

Question 24

Which has a higher boiling point:

H₂S or H₂O

Answer 24

H₂O

H₂S only exhibits dipole-dipole interactions, which are weaker than hydrogen bonds in H₂O.

Question 25

Why do **metals** generally have **high melting points**?

Answer 25

Due to **strong metallic bonds** formed by a sea of delocalized electrons. ## Footnote Metals like tungsten have very high melting points due to their bonding strength.

Question 26

# True or False: Increasing pressure **increases** boiling point.

Answer 26

True ## Footnote A higher external pressure requires more energy to match vapor pressure.

Question 27

Why does **ice float** on water?

Answer 27

Ice has a **lower density** due to hydrogen-bonded open lattice structure. ## Footnote This structure increases the volume of ice, making it less dense than liquid water.

Question 28

What property of water allows it to **dissolve ionic compounds**?

Answer 28

Water's **polarity** and **ability to form hydration shells** around ions. ## Footnote Water’s partial charges interact with cations and anions, stabilizing them in solution.

Question 29

Why does water have a **high surface tension**?

Answer 29

Due to **hydrogen bonding** that causes water molecules to stick together at the surface.

Question 30

Which force is responsible for water's **high heat of vaporization**?

Answer 30

Hydrogen bonding. ## Footnote Hydrogen bonds require significant energy to break.

Question 31

Why are noble gases able to **liquefy at low temperatures**?

Answer 31

Due to **London dispersion forces** between their atoms. ## Footnote These are the only forces acting between noble gas atoms.

Question 32

Why do **alcohols** have **higher boiling points than alkanes** of similar molecular weight?

Answer 32

Alcohols exhibit hydrogen bonding, whereas **alkanes rely only on weaker London forces**. ## Footnote Hydrogen bonding requires more energy to break.

Question 33

What is the **role of hydrogen bonds** in molecular interactions?

Answer 33

They cause **close packing of molecules** in a substance. ## Footnote This is particularly evident in the behavior of liquids and solids.

Question 34

Why are hydrogen bonds **important in biological systems**?

Answer 34

* They stabilize structures like DNA and proteins. * They contribute to the properties of water, a life essential. ## Footnote Hydrogen bonding is crucial for maintaining the double helix structure of DNA.