Chapter 14

Polarity

Alkanes

• Are a simple group of hydrocarbons having the general formula CnH2n+2

Pentane (C5H12)

Hexane (C6H14)

How well do they stick together?

• hexane

• table salt

• methane

table salt > hexane > methanegreatest attractive weakest

attractive

forces forces

How well do they stick together?

• hexane (liquid)

• table salt (solid)

• methane (gas)

table salt > hexane > methanegreatest attractive weakest

attractive

forces forces

Table salt (NaCl)

Methane and Hexane are Molecular

They stick together through Van der Waals forces (not true bonds)

Larger molecules have greater Van der Waals forces

Boiling Point = –161.7°C

Boiling Point = 68.7°C

Rank the molecules from least to greatest Van der Waals forces?

• Hexane (C6H14)

• Methane (CH4)

• Hydrogen (H2)

• Water (H2O)

A A a a a a a a a a a a a

a a a a a a

Did you get this order?

Hydrogen(H2) < Water(H2O) < Methane(CH4) < Hexane(C6H14)

A A a a a a a a a a a a a

a a a a a a

It’s WrongHydrogen(H2) < Water(H2O) < Methane(CH4) < Hexane(C6H14)

A A a a a a a a a a a a a

a a a a a a

The state of matter determines the attractive force?

• Methane (CH4) and Hydrogen (H2) are gases Water (H2O) and Hexane (C6H14) are liquids

A A a a a a a a a a a a a

a a a a a a

The state of matter determines the attractive force?? H2 < CH4 < H2O < C6H14 ?

gases liquidsA A a a a a a a a a a a a

a a a a a a

Boiling points can confirm your answers.

Boiling Points

• Hydrogen has a boiling point -252.87 ºC.

• Methane has a of −161 °C

Boiling Points

68.7°C 100°C

Rank the molecules from least to greatest Van der Waals forces?

Hydrogen (H2) <Methane (CH4) < Hexane (C6H14) < Water (H2O)

A A a a a a a a a a a a a

a a a a a a

What is the shape of a water molecule?

A water molecule is bent. It therefore lacks symmetry.

This makes a water molecule have different ends. This makes a water

molecule _____.

This makes a water molecule have different ends. This makes a water

molecule polar.

A water molecule is polar because it has oppositely charged ends.

A

A

A

A

A

A

A

A

A

A

A

A

A

A polar molecule is called a dipoleA

A

A

A

A

A

A

A

A

A

A

A

A

Polar molecules act like little magnets. The oppositely charged ends attract.

Van der Waals Forces are based on:

1. Size

2. Polarity

Lucky for us water is polar

• Hydrogen has a boiling point -252.87 ºC.

• Methane has a of −161 °C

Dipole Moment

• The dipole moment is a measurement of a molecules polarity.

• Example:

H2O and CO are both dipoles however water has a greater dipole moment.

Polarity Demo: Hexane vs. Water

Homework

• Chapter 14 Worksheet