Chem unit 7 review

The study guide should be completed in time to ask questions. In addition to the questions below, I would also recommend reviewing our warm ups, notes, labs, HWs, etc. 

Vocab

aqueous electrolyte nonelectrolyte solute dissolving solvent solution surface tension homogeneous heterogeneous

concentration dilute solution immiscible solubility miscible molarity saturated unsaturated super saturated

Chapter 15 Review Questions

1a. Draw a water molecule, indicating the regions that have a partial negative charge and those that have a partial positive charge.

1b. After looking at your model above, explain why ice floats in liquid water. Be specific, the answer is not “ice floats in water because it is less dense than water.” 

Ice floats in water because when it is in the solid form, ice has a hexagonal lattice structure due to water’s shape and its polarity/H bonds. The extra space created by the hexagonal lattice causes the density of water to decrease when it freezes. 

2. Define ‘polar molecule’.  Describe how polarity contributes to water’s surface tension, low vapor pressure, and high boiling point.

When a molecule is polar, it means that due to an uneven sharing of electrons between its atoms, partial charges, or dipole moments, exist on the molecule. Water’s polarity causes it to have hydrogen bonds, the strongest IMF. 

Surface tension: Due to having very strong IMFs, when water is in the liquid state water likes to ‘stick to itself’, causing a high surface tension.

3a. Define solvation, solute, solvent, and solution.

Solvation - When an ionic compound is pulled apart into its ions by its solvent.

Solute - The substance that gets dissolved.

Solvent - The substance that does the dissolving.

Solution - A homogeneous mixture.

3b. Fill in the following table accordingly.

4. Draw a diagram showing the solvation process.  Use NaCl as the solute and water as the solvent. 

Can do on board

This is also a good example:

5. Why is it easier for compounds like NaCl or NH₃ to dissolve in water than one like C₈H₁₈?

“Like dissolves like”

If the solute and solvent have similar polarities, the solute will easily dissolve. Since water is polar, it will readily dissolve polar and ionic solutes (you can think of ionic compounds as “very very polar”, the sharing of electrons is so uneven that it results in ions instead of partial charges). 

NaCl is ionic, and NH₃ is polar, so they will dissolve well in water. 

C₈H₁₈ is non polar so it won’t dissolve in water.

6a. What is the difference between an electrolyte and a nonelectrolyte?

An electrolyte is a substance that will separate into its ions when placed in water. This results in cations and anions floating around in solution which allows electricity to flow. 

A non electrolyte doesn’t dissociate into its ions when placed in an aqueous solution and does not conduct electricity. 

6b. Give an example of each, and draw a diagram that represents how each would be expected to dissolve in water.

(can draw diagrams on board)

A strong electrolyte example would be NaCl. When placed in solution 100% of the NaCl gets separated into Na⁺ and Cl^- ions. 

A non electrolyte would be something like sugar, which doesn’t dissociate when in an aqueous solution.

7. Define saturated, unsaturated, and supersaturated.  

Saturated - When a solvent has dissolved as much solute as it can at a given temperature.

Unsaturated - When a solvent can still dissolve more solute at a given temperature. 

Supersaturated - When a solvent is forced to hold more solute than it typically can at a given temperature.

8. What is a supersaturated solution?  How is it made?

When a solvent contains more dissolved solute than it should at a given temperature. You can make a supersaturated solution by taking a saturated solution, add more solute, heat it, then it cools down without disturbing it. The result will be a clear, supersaturated solution. 

When you provide a location for crystals to start growing in a supersaturated solution, the excess solute then comes out of the solution as seen here. 

9. Using the solubility curves below, answer the following Q’s.

21. Which of the following is the most dilute solution? 

1. 1 L solution containing 58.44g of NaCl → This is a 1 M solution

2. 3 L solution containing 4 moles of NaCl → This is a 1.33 M solution

3. 100.0 g of NaCl dissolved in water, then brought u  p to a volume of 1250mL → This is a 1.36 M

22. Describe what the phrase “like dissolves like” means.