Chapter 14. Liquids and Solids

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1 Chapter 14 Liquids and Solids

Review Solid - Has a definite (fixed) shape and volume

Gas Has neither fixed shape nor fixed volume (flows).

2 Review Solid - Has a definite (fixed) shape and volume (cannot flow). Liquid - Definite volume but takes the shape of its container (flows). Gas Has neither fixed shape nor fixed volume (flows). H 2 O(s) H 2 O(l) H = ~6kJ/mol H 2 O(l) H 2 O(g) H = ~41kJ/mol This tells us that liquids more closely resemble solids than gases. 14-2

3 Figure 14.1: Representations of the gas, liquid, and solid states. 14-3

4 Figure 14.7: The heating/cooling curve for water heated or cooled at a constant rate. 14-4

5 Figure 14.2: Intermolecular forces exist between molecules. Bonds(intramolecular forces exist within molecules. 14-5

6 Dipole-Dipole Attractions All polar molecules have a dipole moment: one end of the molecule has a partial positive charge and the opposite end has a partial negative charge. When molecules with dipole moments are put together, they orient themselves such that opposite ends attract each other (like ends repel). This is called a dipole-dipole attraction, these are only about 1% as strong as a covalent or ionic bond. 14-6

7 (a) Interaction of two polar molecules. (b) Interaction of many dipoles in a liquid. 14-7

8 Hydrogen Bonding Hydrogen bonding is just a really strong form of dipole-dipole interaction that only occurs between molecules in which H is bound to a highly electronegative atom (N, O, or F only). Keep in mind hydrogen bonding refers to the IMF s, not the actual N-H, O-H, or F-H bond within the molecule. 14-8

9 Figure 14.4: Hydrogen bonding among water molecules. 14-9

10 Hydrogen Bonding Hydrogen bonding has a major effect on certain physical properties such as melting and boiling points. Think of IMF s as molecules being sticky Velcro is sticky; the bigger the pieces are, the stickier they are. The same is true with IMF s; the bigger the dipole moment, the stickier the molecules will be to each other. The stickier they are, the higher the melting and boiling points

11 London Dispersion Forces (LDF s) All molecules, even non-polar, exert forces on each other. Any substance can be cooled to a point where it turns into a liquid or a solid; this means the atoms or molecules have forces between them that are holding them together. These forces are not important in polar molecules where other IMF s are stronger, but are very important in nonpolar molecules

12 Section 14.2: Water and Its Phase Changes Water covers about 70% of the Earth s surface and about 97% of that is located in the oceans. Pure water is a colorless, tasteless substance that at 1 atm freezes to form a solid at 0 C and vaporizes completely to form a gas at 100 C. It is a liquid between 0 and 100 C 14-12

13 Water and Its Phase Changes When liquid water is heated, the molecules speed up and the temperature increases until it hits 100 C (water s normal boiling point at 1 atm). Once it hits 100 C, bubbles start to form and the water begins to boil and change from a liquid to a gas. Once the phase change begins, the temperature remains constant (100 C) until all liquid has been converted to gas. When the phase change is complete, the temperature will begin to rise again

14 Water and Its Phase Changes When liquid water is cooled, the molecules slow down and the temperature decreases until it hits 0 C (water s normal freezing point at 1 atm). Once it hits 0 C, ice crystals start to form and the water begins to freeze and change from a liquid to a solid. Once the phase change begins, the temperature remains constant (0 C) until all liquid has been converted to solid. When the phase change is complete, the temperature will begin to fall again

15 Figure 14.7: The heating/cooling curve for water heated or cooled at a constant rate

16 Water and Its Phase Changes Most compounds are the most dense as solids and least as gases. Water is the exception to the rule: water is more dense as a liquid than as a solid. The density of water varies with temperature even within the same phase. Water is the most dense around 4 C (1.00 g/ml)

17 Section 14.3: Energy Requirements for the Changes of State Remember: phase changes are physical (NOT chemical) changes; no bonds are broken. Only IMF s must be overcome. Also remember the definitions of exothermic and endothermic

18 State Change Energy Requirements Changing from solid liquid gas is an endothermic process (energy in). Particles must be sped up to overcome IMF s. Changing from gas liquid solid is an exothermic process (energy out). Particles must be slowed down to allow IMF s to take hold. The energy required to melt 1 mol of a substance is called the molar heat of fusion; for water ice it is 6.02 kj/mol. To freeze it we need kj/mol

19 State Change Energy Requirements The energy required to change 1 mol of a liquid substance to its vapor is called the molar heat of vaporization; for liquid water it is 40.6 kj/mol. To condense it back to a liquid we need kj/mol. Note that it takes about seven times as much energy to vaporize a mole of water than to melt it. This is because to vaporize it must overcome more in terms of IMF s

20 State Change Energy Requirements There are two types of energy changes: Temperature changes: Q = S m T (note there is no phase change). Phase changes: H fus or H vap (note there is no temperature change). A typical problem might involve changing 25.0 g water at 25 C to steam, 2 steps: 1) Heat from 25 C to 100 C: (4.18 J/g C) (25 g)(75 C) = 7.8 kj 2) Vaporize at 100 C: (40.6 kj/mol)(1.4 mol) = 56 kj = 64 kj total 14-20

21 Figure 14.7: The heating/cooling curve for water heated or cooled at a constant rate

22 Energy Practice Problems 1) Calculate the energy released when 15.5 g of ice freezes at 0 C. The molar heat of fusion of ice is 6.02 kj/mol. 2) Calculate the energy required to vaporize 35.0 g of water at 100 C. The molar heat of vaporization of water is 40.6 kj/mol. 3) Calculate the energy required to heat 22.5 g of liquid water at 0 C and change it to steam at 100 C. S = 4.18 J/g C and H vap is listed above

Chapter 14. Liquids and Solids

Chapter 14. Liquids and Solids Chapter 14 Liquids and Solids Section 14.1 Water and Its Phase Changes Reviewing What We Know Gases Low density Highly compressible Fill container Solids High density Slightly compressible Rigid (keeps