HOW THE WORLD WORKS, ACCORDING TO CHEMISTRY

ELECTRONEGATIVITY AND POLARITY

1. REVIEW: ELECTRONEGATIVITY is…. “the attraction an atom has for _________________ within a chemical _________________.”

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.

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 tell? The type of bond can also be predicted using the ______________________________________________ between the two elements.

You may be given 3 or 4 choices for types of bonds. In this case, the type of bond must be determined by calculating 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. = ______

DIFFERENCE IN ELECTRO- NEGATIVITY TYPE OF BOND TYPE OF CHARGE Your textbook’s version has 4 choices The version we’ll use has 3 choices

3. POLARITY OF SMALL MOLECULES

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 are in the same place. Example: water vs carbon dioxide…

DEMONSTRATION: WATER VS HEXANE What is a hydrocarbon? A hydrocarbon is a molecule that contains only 2 particular elements. These 2 elements are ____________________ and _____________________)

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

4. SOLUBILITY is ”the ability of a substance to _____________ in another substance”

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. They are made almost entirely of the elements _______________ and _____________

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 TYPECHARGE C - C C - H

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

For larger molecules, polarity (in terms of solubility) is more a function of how polar the majority of the bonds are within the molecules.

ALCOHOLS 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) 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)

OH stands for hydroxide only when it is in an ______________ compound. In a molecular compound, OH represents an ______________________ group. The next slide give the: SOLUBILITY IN H 2 O (amount of alcohol per 100 g H 2 O)

The solubility of these compounds in water INCREASES DECREASES as the number of C-C and C-H bonds increases. C-C and C-H bonds are POLAR NONPOLAR covalent bonds O-H is a highly POLAR NONPOLAR covalent bond, so it contributes to increased solubility in _____________________. The higher the percentage of nonpolar bonds in the compound, the LESS MORE soluble the compound is in water.

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?

4. DIFFERENCES IN PROPERTIES BASED ON BONDING

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

1. Ionic bonds are STRONG WEAK 2. Covalent bonds are STRONG WEAK 3. Intermolecular bonds are STRONG WEAK 4 The properties of molecular compounds are related to the relatively STRONG WEAK intermolecular bonds holding the molecules together. 5. This results in the relatively HIGH LOW melting and boiling points of molecular substances versus ionic ones.

If a substance is a 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

5. PHYSICAL VS CHEMICAL CHANGES FOR MOLECULAR COMPOUNDS. In a physical change, the substance DOES DOESN’T change to a different substance In a chemical change, the substance DOES DOESN’T change to a different substance

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

In a chemical change (that involves molecules) which type of molecular bonding is broken and/or formed?

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?