1. LEQ: How does a hydrogen bond form?
Reading 2.2, 32.1
Activator: water use
Think of all the ways in which you have used water during the last week. Create a list.
Is there an alternative to water that you could use instead?
Key terms – hydrogen bond, polar

2. KEY CONCEPT life on Earth and its origin is studied using chemistry.

3. Bonding review Recall:
Covalent bonds are strong, stable bonds that involve electron sharing between participating atoms
Ionic bonds form between elements when one element, the cation, donates an electron to another atom (exposing a positive charge in the nucleus) and another element, the anion, which has accepted an electron from another atom gaining a negative charge. They two atoms do not need to exchange electrons, but simply have opposite full integer charges. These bonds are much weaker than covalent bonds and are typically shorter in duration, especially when forming in solution.

4. Water’s unique properties allow life to exist on Earth.
Water is polar:
slightly charged regions.
Nonpolar molecules do not O H _ +
H-bond Movie
Atoms that share electrons do not always share them equally. Oxygen is notorious for strongly attracting electrons in covalent bonds toward its nucleus and gaining fractional, negative charges in addition to the paired electrons oxygen already has. This property is called electronegativity.
Consider water (above): Hydrogen’s electrons move toward the oxygen atom exposing the remaining proton in each respective atom causing a fractional, positive charge to form.
Since both ends of the covalent bond now have fractional, opposite charges the molecule is now polar and its ends are attracted to oppositely charged regions of other chemicals
A hydrogen bond forms between slightly positive hydrogen atoms and slightly negative atoms.

5. Note: the electrons XX have moved in relation to the original atoms that were sharing them in a covalent bond (middle).

6. What makes a covalent bond become polar?
Summary:
What makes a covalent bond become polar?
Explain how a hydrogen bond forms.
Answer: a strong electron acceptor like oxygen or nitrogen pulling on electrons in the covalent bond.
Answer: hydrogen bonds form between the oppositely charged ends of polar molecules

7. KEY CONCEPT Water’s unique properties allow life to exist on Earth.

8. Hydrogen bonds are responsible for unique properties:
Moderating temp.
stays cool, hot
stores energy
“Stickiness”
surface tension
cohesion
adhesion
Floatation
ice formation
Versatile solvent

9. Share your responses with your group
Possible sentences:
Pretend that you are writing to a person who has little or no scientific expertise:
Explain to them the significance of water to life on Earth using three vocabulary terms or less
Clarify any term you use by providing an example, analogy, etc.
Share your responses with your group

10. Solvents dissolve other substances. Solutes dissolve in a solvent.
Any compound with a partial or full electric charge can dissolve in water.
Solvents dissolve other substances.
Solutes dissolve in a solvent.
Liquid, gases, or solids
A solution is formed when one substance dissolves in another
solution

11. Solvent rules: “Like dissolves like.”
Polar solvents dissolve polar solutes-hydrophilic (left).
Nonpolar solvents dissolve nonpolar solutes.
Polar substances and nonpolar substances generally remain separate-hydrophobic (right).

12. Some compounds release or accept protons in solution.
An acid donates a hydrogen ion when it dissolves in water.
pH measures conc. of H+ in sol’n
high H+ concentration, pH less than 7
more acidic
stomach acid pH between 1 and 3

13. A base removes hydrogen ions from a solution.
lowers H+ concentration
pH greater than 7
bile pH between 8 and 9
more basic

14. A neutral solution has a pH of 7.0 ~ physiological pH of cells
pure water pH 7

15. Summary:
How do polar molecules form hydrogen bonds?
What determines whether a compound will dissolve in water?
Compare and contrast acids and bases
How do polar molecules differ from nonpolar molecules?