Prentice Hall Chemistry (c) 2005

1. Prentice Hall Chemistry(c) 2005 Section Assessment Answers Chapter 12 By Daniel R. Barnes Init: 12/17/2008 WATER

2. 1. What causes the high surface tension and low vapor pressure of water? Water molecules are quite strongly attracted to each other by “hydrogen bonds”. Why is this so? 15.1 Section Assessment The oxygen atom in a water molecule is slightly negative and the hydrogen atoms are slightly positive. Why is this so? Oxygen has a much higher electronegativity than hydrogen, so when O & H share electrons in a covalent bond, they don’t share them evenly. The oxygen atom gets more than a 50% of the shared electrons, making it negative, and the hydrogen gets less than a 50% of the electrons, making it positive.

3. 2. How would you describe the structure of ice? “Ice has a honeycomb-like structure of hydrogen-bonded water molecules.” In other words, ice is a crystal held together by intermolecular forces. 3. What effect does a surfactant have on the surface tension of water? 15.1 Section Assessment Surfactants interfere with hydrogen bonding, which weakens surface tension. Soap acts as a surfactant, allowing thin-skinned bubbles to last longer than in pure water. In regular water, the molecules are so strongly attracted to each other that bubbles quickly turn into droplets. In soap water, bubbles don’t turn into tight little droplets as quickly, so soap bubbles and suds can last a long time.

4. 4. What are two factors that determine how spherical a drop of a liquid will be? “The surface tension of a liquid tends to hold a drop of liquid in a spherical shape; gravity tends to flatten the drop.” 5. The molecules in water (H2O) and methane (CH4) have similar masses, but methane changes from a gas to a liquid at -161oC. Water becomes a gas at 100oC. What could account for the difference? 15.1 Section Assessment Water molecules experience hydrogen bonding, but methane molecules are not very attracted to each other. This is because . . . Oxygen atoms in water are negative and hydrogen atoms in water are positive, whereas in methane all atoms are pretty neutral. This is because . . . Oxygen and hydrogen have very different electronegativities, but carbon and hydrogen are almost equal.

5. 8. In the formation of a solution, how does the solvent differ from the solute? The solvent dissolves the solute. Typically, the solvent always remains in its original state of matter, but the solute will typically assume the state of matter of the solvent. For instance, when salt dissolves in water, the salt turns from a solid to a liquid, so it’s the solute. The water starts as a liquid and remains as a liquid, so it’s the solvent. 15.2 Section Assessment When two materials in the same state mix, then since neither one is changing to the other’s state, the tie is broken by declaring the more abundant chemical to be the “solvent”. For instance, nitrogen and oxygen are both gases, and when they mix, they become a gaseous solution. Since neither changes state in order to mix, nitrogen is considered the solvent, but only because air is mostly (78%) nitrogen, not because it changes less than the oxygen.

6. 9. Describe what happens to the solute and the solvent when an ionic compound dissolves in water. Individual ions break away from the ionic crystal, partly because of their own kinetic energy, but also because they are attracted to the oppositely-charged regions of solvent (water, usually) molecules bumping into them. (Breaking away from the crystal is easier when the crystal is hot, since the ions in a hot crystal are jiggling faster than ions in a cold crystal. Also, a hot solvent’s molecules will wander to and from the crystal faster, so a hot solvent dissolves a crystal faster than a cold solvent does.) 15.2 Section Assessment Dissolved ions, quickly become surrounded by solvent molecules and wander around the solvent randomly. The solid has now become, in effect, a liquid.

7. 10. Why are all ionic compounds electrolytes? All ionic compounds are made of positive and negative ions. When an ionic compound is melted or dissolved, these ions break free from each other and wander randomly. No longer stuck to each other, oppositely-charged ions are able to move in different directions when a voltage is applied. Thus, ionic materials in the liquid state can conduct electric current, and, therefore, are considered to be “electrolytes”. 15.2 Section Assessment

8. 11. How do you write the formula for a hydrate? “In writing the formula of a hydrate, a dot is used to connect the formula of the compound with the number of water molecules per formula unit” EXAMPLE: Copper sulfate, CuSO4, is a whitish solid in its “anhydrous” (waterless) state. However, copper sulfate can bond easily with water molecules to form copper sulfate pentahydrate, which is a deep blue in color. 15.2 Section Assessment The formula of copper sulfate pentahydrate is: CuSO4 5H2O

9. 12. Which of the following substances dissolve to a significant extent in water? Explain your answer in terms of polarity. C & H have electronegativities too close to each other to have polar covalent bonds. Therefore, no +’s or –’s on a CH4 molecule. CH4 is nonpolar. insoluble a. CH4 K and Cl are from opposite ends of the periodic table, and, therefore, have very different electronegativities. Cl is so much higher that it steals an electron from K, making the two atoms into K+ and Cl-. KCl is an ionic compound. b. KCl soluble 15.2 Section Assessment He is a noble gas. He atoms travel alone, unbonded, and, therefore, neutral, and, therefore, nonpolar. insoluble c. He

10. 12. Which of the following substances dissolve to a significant extent in water? Explain your answer in terms of polarity. soluble MgSO4 is an ionic material, composed of Mg2+ and SO42- ions. d. MgSO4 soluble Sucrose (table sugar) molecules contain polar covalent bonds between oxygen and hydrogen atoms. The oxygen atoms (the electron hogs) are negative, and the hydrogen atoms are positive. e. sucrose 15.2 Section Assessment soluble NaHCO3 is an ionic material, composed of Na+ and HCO3- ions. f. NaHCO3

11. 13. Identify the solvent and solute in vinegar, a dilute aqueous solution of acetic acid. An “aqueous” solution is a “watery” solution . . . Water is known as “the universal solvent” because it can dissolve so many things, including acetic acid. In vinegar, water is the solvent and acetic acid (CH3COOH) is the solute. Incidentally, the fact that vinegar is “dilute” means that . . . 15.2 Section Assessment . . . vinegar has a low concentration of acetic acid. Dilute aqueous solutions, by definition, are made mostly of water.

12. 14. Distinguish between efflorescent and hygroscopic substances. Efflorescent compounds lose water to the air. Hygroscopic substances take water vapor from the air. Extremely hygroscopic compuonds are called “deliquescent”, and can become aqueous solutions rather than just hard, hydrated crystals. 15.2 Section Assessment

13. 15. Calculate the percent by mass of water in magnesium sulfate heptahydrate, MgSO4*7H2O. This calculation requires a specially-enhanced molar mass calculation . . . 7H2O: MgSO4*7H2O: 7 x 16.00 O = 112.0 1 Mg = 24.31 x 24.31 = 14.14 14 x 1.01 H 1 = 32.07 S x 32.07 15.2 Section Assessment g/mol 126.1 11 x 16.00 O = 176.0 = 14.14 14 x 1.01 H g/mol 246.5 water 126.1 g/mol = % water = = 0.5116 = 51.16% 246.5 g/mol whole thing