8.5 ELECTRONEGATIVITY AND POLARITY

SUBTOPICS 1. Electronegativity 2. Bond types based on the difference in electronegativity 3. Polarity – in particular of water 4. Solubility 5. Properties of molecular compounds based on the strength of bonding 6. Physical and chemical changes and molecular bonding

1. REVIEW: WHAT IS ELECTRONEGATIVITY?

The trends in electronegativity are: left to right across a period: down a group: Metals have a LOW HIGH electronegativity. Nonmetals have a LOW HIGH electronegativity.

TRUE FALSE The largest of the noble gases can sometimes bond with highly electronegative elements like

2. BOND TYPES BASED ON DIFFERENCES IN ELECTRONEGATIVITY Up till this point, if I give you the formula for a compound and ask you whether it is ionic or molecular – how do you know?

The character and type of bond can be predicted using the ________________________________ between the two elements that are bonded together. A chemical bond is always completely ionic or covalent TRUE OR FALSE??

If you are only given 2 choices for types of bonds (i.e., ionic or covalent) the cutoff is

You may be given 3 or 4 choices for types of bonds. Again, the type is determined by the difference in

charges on atoms Na F E.N. = difference in E.N. = _______

charges on atoms F F E.N. = difference in E.N. = _______

charges on atoms C F E.N. = difference in E.N. = ______

Electronegativity Difference Type of Bond Textbook’s version : 4 choices Version we’ll use: 3 choices

3. POLARITY (in particular of water)

In chemistry, we will describe something (like a bond or a molecule) as POLAR when it has ___________________________ ___________________________

What happens when you bring a charged object near to a stream of water? Try it. Explain your results.

Carbon-chlorine bonds are polar covalent. Why is CCl 4 a nonpolar molecule?

WATER is one of the single most important compounds to life as we know it…Let’s look more closely at it…

Draw a water molecule in its correct shape. Write in the electronegativities of each atom. Label the type of bond and the type of charges found on each atom. Is water a polar or nonpolar molecule?

Water is a molecular compound, but it is unusual because: * water molecules are highly POLAR NONPOLAR * Its bonds, O-H bonds, are quite NONPOLAR COVALENT POLAR COVALENT MOSTLY IONIC * The intermolecular bonding in water is _____________ ___________________ which is the strongest (most polar) of the intermolecular bonds.

4. SOLUBILITY

We’ll do a demonstration to observe solubility. It’s called: Why is styrofoam bad for the environment?

Here are the structures of styrofoam and acetone.

Salt dissolves in _____________ but not in _____________. Styrofoam dissolves in __________ but not in _____________. Why??

Sodium chloride is a very POLAR NONPOLAR substance. Water is a very POLAR NONPOLAR substance.

Most of the bonds in styrofoam and acetone are C-C and C-H. BONDDIFFERENCE IN ELECTRO- NEGATIVITY BOND TYPE C - C C - H

The rule of solubility is _____________________________ Polar compounds dissolve in _________________ solvents, and nonpolar compounds dissolve in ____________________ solvents.

POLARITY IN TERMS OF SOLUBILITY For small molecules, e.g., ones with only one central atom, the polarity of the overall molecule is determined by whether or not the centers of positive and negative charge coincide. Polarity in terms of solubility for larger molecules is more a function of how polar the bonds are within the molecules.

The more nonpolar covalent bonds in a molecule, the less soluble it is in water. For example: SOLUBILITIES OF SOME ALCOHOLS IN H 2 O ALCOHOLSSOLUBILITY IN H 2 O (mol/100 g H 2 O) CH 3 OH (methanol) ∞ CH 3 CH 2 OH (ethanol) ∞ CH 3 CH 2 CH 2 OH (propanol) ∞ CH 3 CH 2 CH 2 CH 2 OH (butanol) 0.11 CH 3 CH 2 CH 2 CH 2 CH 2 OH (pentanol) CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 OH (hexanol)

You know that “oil and water DO DON’T mix.” Why? What does this tell you about oil?

Why are oil spills so devastating to the environment?

5. DIFFERENCES IN PROPERTIES BASED ON BONDING

DIFFERENCES IN PROPERTIES are a result of differences in ____________________________________ For example – why are bones hard, but butter is soft?

Ionic bonds are STRONG WEAK Covalent bonds are STRONG WEAK Intermolecular bonds are STRONG WEAK

The properties of molecular compounds are related to the relatively WEAK STRONG intermolecular bonds holding the molecules together. This results in the relatively HIGH LOW melting and boiling points of molecular substances versus ionic ones.

If a substance is a: SOLID LIQUID GAS at room temperature, it is most likely IONIC MOLECULAR If a substance is a soft solid at room temperature, it is most likely IONIC MOLECULAR

Intermolecular bonds are also known as Give examples of each of the 3 types. (definitions in reading guide) 1) London Dispersion Forces 2) Dipole-dipole forces 3) Hydrogen bonding

COVALENT NETWORK SOLIDS

Covalent network solids

6. PHYSICAL VS CHEMICAL CHANGES FOR MOLECULAR COMPOUNDS. Remember from back in chapter 3- What is the difference between a physical and a chemical change?

An example of a physical change is BOILING WATER. Which type of molecular bonds are broken in a physical change?

In a chemical change, which type of molecular bonding is broken?

Final question: When Dorothy threw water on the Wicked Witch of the West, the Witch screamed: “I’m melting! I’m melting!” was her use of the word ‘melting’ scientifically correct?

Also, what can you deduce about the properties of the Wicked Witch, based on this incident?