Lesson 7- Intermolecular Forces

Question 1

Intramolecular forces

Answer 1

forces which physically keep substances together (e.g., ionic and molecular bonds).
Determines the chemical behaviour of a substance

Question 2

intermolecular force

Answer 2

an attraction between molecules

Question 3

TRUE OR FALSE: Ionic compounds do HAVE have intermolecular forces

Answer 3

Ionic compounds do NOT have intermolecular forces because they do not form molecules!

Question 4

What are ionic crystals held by?

Answer 4

electrostatic ionic bonds, which are much stronger than intermolecular forces!

Question 5

Why are intermolecular forces imporatnt?

Answer 5

properties of molecular compounds cannot be explained simply by covalent bonds

Question 6

What would happen if there was no such thing as intermolecular bonds and covalent wre the only things at work?

Answer 6

If covalent bonds were the only forces at work, most molecular compounds would be gases, as there would be no attraction between the molecules strong enough to order the molecules into solids or liquids!!

Question 7

TRUE OR FALSE: The strength of intermolecular forces determines physical properties of molecular compounds

Answer 7

As the intermolecular forces between molecules in a molecular compound increase, the compound’s melting point, boiling point, and surface tension also increase.

Question 8

What are our intermolecular forces:

Answer 8

Dipole-Dipole Force
Ion-Dipole Force
London Dispersion Force

Question 9

Dipole-Dipole Force

Answer 9

Neighbouring polar molecules align themselves so that oppositely charged dipoles are directed toward each other

dipole-dipole forces are only about 1% as strong as ionic or covalent bonds

Question 10

What is the special dipole-dipole force?

Answer 10

Hydrogen Bonds
A dipole-dipole force is classified as a hydrogen bond if at least one of the dipoles involved arises from an H-F, H-O, or H-N bond (FON)

Question 11

TRUE OR FALSE: Hydrogen bonds are much stronger than regular dipole-dipole forces

Answer 11

TRUE

Question 12

2. Ion-Dipole Force

Answer 12

two types of ion-dipole interactions:

between a polar molecule and a cation,

between a polar molecule and an anion

-The strength of this force depends on the charge and size of the ion, and on the magnitude of the dipole

The charges on cations are generally more concentrated, because cations are usually smaller than anions, so a cation interacts more strongly with dipoles than does an anion that has a charge of the same magnitude

Question 13

London Dispersion Force

Answer 13

• All molecules have this force as e- are in constant motion

-All molecules spontaneously form temporary dipoles due to the random motion of their electrons

-Since e- are always moving in the molecule, 1 molecule will have temporary dipole–> making this an “instance dipole” whicch is tempory to the molecule it is close to

The process “disperses” through the substance, creating many temporary dipoles that attract one another

Question 14

Two factors affect the magnitude of LD Forces:

Answer 14

• # of e- in a molecule –> The more electrons, the greater the chances of forming a temporary dipole, and so the stronger the LD forces

-The shape of the molecule. The more linear the molecule, the greater the surface area of contact between adjacent molecules, and so the stronger the LD forces

Question 15

TRUE OR FALSE: For a pure entity, the stronger its intermolecular forces, the higher its melting point and boiling point

Answer 15

TRUE but why????

In liquids and (to a greater extent) gases, entities are able to freely move past one another
To achieve this, entities must have sufficient kinetic energy (as measured by its temperature) to overcome its intermolecular forces
For entities with strong intermolecular forces, this sufficient level of kinetic energy is higher – that is, they melt and boil at higher temperatures

Question 16

TRUE OR FALSE: As we go up groups the boiling point is increasing?

Answer 16

True! Why?

• As we go across periods boiling point because theres mnore electrons meaniing more LDF which meand boiling point is increasing which makes the molecule harder to brwak apart

Question 17

To dissolve in water, an entity must be capable of being….

Answer 17

hydrated by water

Question 18

Hydration can only occur if the water molecules are

Answer 18

sufficiently attracted to the entity

Since water has a net dipole, the entity must have a net dipole or a full charge to achieve sufficient attraction to the water molecules

Question 19

TRUE OR FALSE:Water is one of the only compounds that is less dense as a solid than a liquid

Answer 19

TRUE!

As liquid water freezes, the molecules lose kinetic energy, and slow down
Hydrogen bonds start to arrange the molecules into a crystalline structure
This arrangement contains more space between the molecules than does the more random arrangement of liquid water molecules

Question 20

Why does Water have unusually high surface tension

Answer 20

The hydrogen bonds in water cause water molecules to hold strongly to each other, pulling together and resisting being broken apart.

This causes water to take a shape that has the smallest possible surface area, forming beads or droplets.

Other liquids don’t do this!