Chemistry Test Part 2 Review

3 Types of Molecules

Simple Drawing

Ionic (salt: NaCl, CaCl2)

Polar (water, sugar)

Non-polar (baby oil, paraffin

wax)

+ −

+ −

Properties:

•Form crystals (hard/brittle)

•High melting point

•Soluble in water (why?)

•Conduct electricity in solution (why?)

Remember: Water is a Polar Molecule

_

It’s attracted to both positive AND negative charges

+

What makes water polar??

•Unequal sharing of the electrons creates slight positive/negative dipoles

•Other molecules, such as Ethane, are nonpolar, having neither a positive nor a negative side.

•Can Ethane dissolve in water?

•NO! Remember “like dissolves like”!

The polar water molecules are attracted to the + and – ions in the crystal lattice

−

−

−

What does “Solvation” mean?

Solvation is where the charged ends (dipoles, + -) of water molecules interact with the ions in a crystal lattice (like NaCl). The result is a solution of dissolved sodium chloride.

−

−

−

When dissolved, the Na+ and Cl- ions are surrounded by water molecules and float freely in the solution.

• It’s the movement of ions in a liquid that allows a solution to conduct an electric current (see example below)

• Pure mineral oil does not conduct electricity.

• Why? Because like pure water, mineral oil does not contain + or – ions.

+ Probe – Probe

_

_

_+

++

+

_+

Ionic solid ex. NaCl

Oil is nonpolar

1.5 volts

=

Remember:Free moving Ions = Conductivity

No Ions, No conductivity in a solution!

Why doesn’t wax dissolve in water?

Dipole vs No Dipole (Non-Polar)

WAX is Non-polar Covalent

+-

- +

+-

-

+

+- +

+-

-+ - +

-+

-+

+

-

Water and alcohol, both polar molecules, mix together.

-

+

-

Baby Oil is non-polar so it does not mix with either water or alcohol. Being less dense, it floats on top.

+ -

Remember this??

WAVE TOY

+ -

+-

-+

“Like Dissolves Like”= Polar dissolves polar

Water-based red dye added

Baby Oil

Would this molecule dissolve in water?

-

Negatively charged area due to O’s attracting e- from both H & C

+ Positively charged due to O’s hogging H’s e-

Methanol CH3OH

Yes, because methanol has dipoles so water will be attracted to it and it will be attracted to water molecules.

Polar covalent bond Polar

covalent bond

Nonpolar Covalent bonds

What characteristic properties do Non-polar substances have?

Properties:

•Soft

•Low melting point

• NOT Soluble in water (why?)

•Do NOT Conduct electricity in solution (why?)

What characteristic properties do Polar substances have?

Properties:

•Crystals (solid)

•Medium melting point

• Soluble in water (why?)

•Do NOT Conduct electricity in solution (why?)

What else is important to know about water?

Why does water expand when it freezes?

Because of the hydrogen bonds between water molecules. When water freezes, H bonds move and push the water into a more regular or “lattice” shape, causing it to expand (increased volume = less density). This is why ice floats!

Notice anything?

Why does water have a high boiling point?

Again, because of the hydrogen bonds between water molecules. Water must be heated to a relatively high point before the motion of the molecules is great enough to break the hydrogen bonds. At this point the liquid water turns to a gas (steam)

What about Metals?

What is a Metallic Bond?

Think of a metal atom – or more specifically, the nucleus of a metal atom as a rice krispy puff.

When you make rice krispy treats, you mix the rice krispies with melted marshmallows. This is the “glue” that bonds the rice krispies together.

In metals, it’s the freely-moving outer electrons that bond the metal nuclei together!

See the connection??

These loosely-held, outer electrons (valence e-) surrounding the metal atom nuclei are the “glue” that holds the metal atoms tightly together.

Why are metals such good conductors of electricity?

Because metals hold on to their valence (outermost) electrons very loosely. It takes only a small amount of force (voltage) to start a “leap frog” chain reaction, with electrons jumping from one atom to the next –all in the same direction.

This is how electricity flows through a copper wire. It’s the movement of electrons!

Direction of current

Voltage

Compare/Contrast Metals and Non-metals

Compare and Contrast Metals and Non-metals.

Metals (Left side of Periodic Table)

• Few outer (valence) e- : (1 to 3)

• Share e- with other metals (metallic bonds)

• Lose e- to non-metals (form ionic bonds;compounds like salt: NaCl)

Non-metals (Right side of Periodic Table)

• More valence e-, high Electronegativity

• Share e- with other non-metals (covalent bonds)

• Steal e- from metals (form ionic bonds).

Electronegativity

Electronegativity: What is it? Electronegativity: A measure of an atom’s “hunger” for

electrons, or how much energy it has to attract electrons

• Fluorine (electronegativity = 4.0) has the greatest hunger for an electron. Neon, with 8 valence e-, has no electronegativity (already has a full outer shell).

The Cookie Monster would have a lot of Electronegativity!

Me like electrons!!

Highest

Generally, electronegativity increases as you move left to right and from bottom to top (except for He, Ne, & Ar in last column)

The difference in electronegativity between two atoms will determine the type of bonding that occurs:

0.5 – 1.6< 0.5 >1.6

Electronegativity Difference = Tug of War

Equal Strength = Non-Polar Covalent

Sharing e-

One Stronger = Polar Covalent

Not Equal Strength = Polar Covalent Sharing e- but one is being an e- HOG

Stealing e- = Ionic

Why share e- if you’re strong enough to steal?