SECTION 10.5 Water Water commonly exists in all three physical states on Earth, where it is by far the most abundant liquid. It covers nearly three-quarters of Earth s surface. Water is an essential component of life; from 70% to 90% of the mass of living things is water. Chemical reactions that are necessary for life take place in water, and often involve water as a reactant or a product. The structure of a water molecule gives water the unique properties that make it such an important molecule. Hydrogen bond The properties of water in all phases are determined by its structure. Water molecules consist of two atoms of hydrogen linked to one atom of oxygen by polar-covalent bonds. A molecule of water is bent, with its two bonds forming an angle of about 105. O H H 105 Different molecules of water are linked by hydrogen bonding. The number of linked molecules decreases with increasing temperature because hydrogen bonds have difficulty forming between molecules with greater kinetic energies. Usually, four to eight molecules of water are linked in a group, as shown at the right. This ability of water molecules to form groups prevents molecules from escaping to become gas particles. Water would be a gas at room temperature without this ability. The diagram on the next page shows water molecules in the solid state. These molecules form an orderly, hexagonal arrangement in ice crystals. The large empty spaces between molecules in this pattern explain why solid water has the unusual property of being less dense than its liquid form. Liquid water In a group of liquid water molecules, hydrogen and oxygen are bonded within each molecule, and different molecules are held together by hydrogen bonds. READING CHECK 1. Order the three forms of water from least dense to densest. 332 CHAPTER 10
Density of Water The diagrams of water molecules to the right and on the preceding page represent water in the liquid state and the solid state at 0 C. Liquid water has fewer and more disorderly hydrogen bonds than ice. When energy is added to ice, and the crystal structure breaks down, the water molecules can actually crowd closer together in the liquid state. This is why water is more dense than ice. As liquid water is warmed from 0 C, its particles pack closer together until the temperature of 3.98 C is reached. At temperatures above 3.98 C, the kinetic energy of the water molecules moving around in the liquid keeps the molecules from being packed so closely close together. This unusual property of water helps protect organisms that live in water. Most liquids freeze from the bottom up. Water freezes from the top down, because the surface water is cooler than the deeper water. In addition, the ice stays on the surface of a lake or pond because it is less dense than water and acts as an insulator. This effect makes it difficult for a large body of water to freeze solid. Ice Hydrogen bond Ice contains the same types of bonding as liquid water. However, the structure of the hydrogen bonding is more rigid than in liquid water. The molar enthalpy of water determines many of its physical characteristics. At room temperature, liquid water is transparent, odorless, tasteless, and nearly colorless. Any observed odor or taste is a result of dissolved substances in the water. The density of water is 0.999 84 g/ cm 3, while the density of ice is 0.917 g/ cm 3. Water has a relatively high boiling point. A large amount of kinetic energy is necessary for the water molecules to completely overcome the hydrogen bonding. At atmospheric pressure, ice s molar enthalpy of fusion is 6.009 kj/mol. That value is relatively large compared to other solids. Water also has a relatively high molar enthalpy of vaporization, 40.79 kj/mol. These high values both result from the strong attractive forces in hydrogen bonds. The high molar enthalpy of vaporization makes steam (vaporized water) ideal for household heating systems. Steam can store a great deal of energy as heat. When the steam condenses in a radiator, it releases this energy. READING CHECK 2. Why is a relatively large amount of energy required to turn liquid water into water vapor? States of Matter 333
SAMPLE PROBLEM How much energy is absorbed when 47.0 g of ice melt at a temperature of 0 C and a pressure of 1 atm? SOLUTION 1 ANALYZE Determine what information is given and unknown. Given: mass of H 2 O(s) = 47.0 g molar enthalpy of fusion = 6.009 kj/mol Unknown: energy absorbed when ice melts 2 PLAN Determine how to find the value of the unknown. First, convert the mass of water to moles. Then use the molar enthalpy of fusion as a conversion factor. 3 SOLVE Find the value of the unknown using the given information. 47.0 g H 2 O = 47.0 g H 2 O 1 mol H 2 O 18.02 g H 2 = 2.61 mol H 2 O Energy absorbed = 2.61 mol H 2 O 6.009 kj H 2 O 1 mol H 2 = 15.7 kj 4 CHECK YOUR WORK Check to see if the answers make sense. A mass of 47 g of water is about 3 mol and 3 6 = 18. The answer has the right units and is close to the estimate. PRACTICE A. What mass of steam is required to release 4.97 10 5 kj of energy on condensation? Moles of steam required = 4.97 10 5 kj 1 mol H 2 O 40.79 kj H 2 = Mass of steam required = mol H 2 O 18.02 g H 2 O 1 mol H 2 = 334 CHAPTER 10
SECTION 10.5 REVIEW REVIEW 1. Why is the water molecule polar? 2. How is the structure of water responsible for its unique characteristics? 3. Describe the arrangement of molecules in liquid water and ice. 4. Why does ice float? Why is this phenomenon important? 5. Why is ice less dense than liquid water? 6. Is more energy required to melt one gram of ice at 0 C or to boil one gram of water at 100 C? How do you know? Critical Thinking 7. RELATING IDEAS Why is exposure to steam dangerous? States of Matter 335
Math Tutor Calculating Using Enthalpies of Fusion When one mole of a liquid freezes to a solid, a certain amount of energy is released. The attractive forces between particles pull the disorderly particles of the liquid into a more orderly crystalline solid. When the solid melts to a liquid, the solid must absorb the same amount of energy in order to separate the particles of the crystal and overcome the attractive forces opposing separation. The energy used to melt or freeze one mole of a substance at its melting point is called its molar enthalpy of fusion, H f. Problem-Solving TIPS The enthalpy of fusion of a substance can be given as either joules per gram or kilojoules per mole. Molar enthalpy of fusion (kilojoules per mole) is the value that is most commonly used in calculations. The enthalpy of fusion is the energy absorbed or given off as heat when a substance melts or freezes at its melting point. No net change in temperature occurs as the change in state occurs. SAMPLE Determine the quantity of energy that will be needed to melt 2.50 1 0 5 kg of iron at its melting point, 1536 C. The Δ H f of iron is 13.807 kj/mol. The number of moles of a substance that is equal to a given mass of a substance can be determined from the following equation. moles of a substance = mass of substance/molar mass of substance The energy as heat absorbed by a substance as it is going through a phase change from a solid to a liquid is energy absorbed = H f moles of a substance The first equation can be substituted into the second equation to give the energy absorbed in terms of the given information. The given information can then be used to solve the problem. energy absorbed = H f mass of substance molar mass of substance energy absorbed = 13.807 kj 2.50 1 0 8 g Fe 1 mol 55.847 g Fe/mol = 6.18 1 0 7 kj Practice Problems: Chapter Review practice problems 16 18 336 CHAPTER 10