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 > methane
greatest attractive
forces
weakest attractive
forces
How well do they stick together?
• hexane (liquid)
• table salt (solid)
• methane (gas)
table salt > hexane > methane
greatest attractive
forces
weakest attractive
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)
AA a a a a a a a a a a a
aaaaaa
Did you get this order?
Hydrogen(H2) < Water(H2O) < Methane(CH4) < Hexane(C6H14)
AA a a a a a a a a a a a
aaaaaa
It’s Wrong
Hydrogen(H2) < Water(H2O) < Methane(CH4) < Hexane(C6H14)
AA a a a a a a a a a a a
aaaaaa
The state of matter determines
the attractive force?
• Methane (CH4) and Hydrogen (H2) are gases
Water (H2O) and Hexane (C6H14) are liquids
AA a a a a a a a a a a a
aaaaaa
The state of matter determines
the attractive force?
? H2 < CH4 < H2O < C6H14 ?
gases
liquids
AA a a a a a a a a a a a
aaaaaa
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)
AA a a a a a a a a a a a
aaaaaa
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 dipole
A
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 (due tomorrow)
• Study Guide Chapters 12 – 14 (due next
week?)