Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Section 9.1 Solutes, Solvents and Water 9.1 Solutes, Solvents and Water Definition In Chapter 2 we talked about solutions and you have worked with them extensively in the laboratory. This chapter will look at solutions more closely and develop some of their properties. A true solution is homogenous on the molecular level. That means there are no clumps bigger than a molecule. An example solution Sixteen grams of copper sulfate (CuSO4) in 100 mL makes a deep blue solution when every particle is dissolved. This solution is 1 molar because there is 1 mole of copper sulfate per liter of solution. Solvent and solute A solution always has a solvent and at least one solute. The solvent is the substance that makes up the biggest percentage of the mixture, or is liquid. The copper sulfate solution has water as the solvent. The solutes are the other substances in the solution. Copper sulfate is the solute in the example. Dissolving When the solute particles are evenly distributed throughout the solvent, we say that the solute has dissolved. The copper sulfate starts as a solid powder. Water is added and the mixture is carefully stirred until all the solid powder has dissolved. Once the copper sulfate has dissolved, the solution is transparent again. All true solutions are transparent Why solutions are transparent True solutions are transparent because all the particles are single molecules. Milk is not a true solution because each of the tiny fat particles in milk are thousands, or tens of thousands, of molecules. Particles this size are large enough to block light and scatter it to the side. The scattering of light is the reason milk is opaque. solvent - the substance that makes up the biggest percentage of the mixture, or is liquid. solute - any substance in a solution other than the solvent. dissolved - when molecules of solute are completely separated from each other and dispersed into a solution. 262 A NATURAL APPROACH TO CHEMISTRY The water molecule Water is critical to life Your body is about 60% water by weight. In 1 hour of exercise, you may lose as much as a half-gallon of water sweating and breathing! You also lose small amounts of dissolved salts. If the lost water and dissolved salts are not replaced, your body stops working. The unique physical and chemical properties of water explain why it is so important. Water is a polar molecule The water molecule is polar. That means the electric charge is unevenly distributed in a water molecule. The two shared electrons from the hydrogen atoms are attracted to the oxygen atom. This makes the oxygen side of the molecule slightly negative and the hydrogen side slightly positive. The diagram on the right shows the surface of the molecule shaded red where it is more positive and blue where it is more negative. Hydrogen bonding Water molecules tend to stick to each because of the polarity. Positive hydrogen atoms from one water molecule are attracted to the negative oxygen atoms in another molecule. This attraction is not as strong as a chemical bond. BUT, it is strong enough that we give it its own name: hydrogen bonding. Hydrogen bonding explains some curious properties of water. Why ice floats When water freezes, it becomes LESS dense! This is unusual, most materials are more dense as solids than as liquids. Ice is less dense than liquid water because hydrogen bonds force water molecules to align in a crystal structure where molecules are farther apart than they are in a liquid. polar - a molecule is polar when there is a charge separation that makes one side (or end) of the molecule more positive or negative than the other side (or end). hydrogen bond - an intermolecular bond that forms between a hydrogen atom in one molecule and the negatively charged portion of another molecule (or another part of the same molecule). A NATURAL APPROACH TO CHEMISTRY 263 Section 9.1 Solutes, Solvents and Water The importance of hydrogen bonding Comparing water and methane Consider two molecules with similar molecular masses. Methane (CH4) has a molecular mass of 16 g/mole and water (H2O) has a molecular mass of 18 g/mole. Methane boils at -161°C, so at room temperature, methane is a gas. Water has a similar mass, but boils at an astounding +100°C! This is far hotter than almost any other low molecularweight hydrogen compound. What is the reason? The answer has to do with hydrogen bonding and the shape of the two molecules. Hydrogen bonding causes water’s high boiling point Due to its symmetrical shape, a methane molecule is non-polar! Hydrogen bonding does not occur between neighboring methane molecules. Without hydrogen bonding, it is easier for a methane molecule to escape its neighbors and become a gas. By comparison, water has strong hydrogen bonding between neighboring molecules. The hydrogen bonding is the reason for water’s unusually high boiling point. The high boiling point of water is of critical importance to life on earth. Life needs liquid water. If water was a gas at room temperature, it would be impossible for life to exist as we know it. Making solid steel float! Gently place a pin on the surface of water and the surface of alcohol. The pin sinks immediately in the alcohol. The pin floats on the water! Pins are made of steel, which is dense enough to sink in water. Why can a pin float on water but not on alcohol? Surface tension The answer is a property called surface tension. Surface tension is a force that pulls molecules together along a liquid surface. The high surface tension of water is a direct consequence of hydrogen bonding. The pin floats on water because it has to separate water molecules on the surface to break through and sink. The hydrogen bonds between water molecules are held tightly enough that a gently placed pin will float. The fact that water drops pull themselves into spheres is also caused by surface tension. Alcohol has a lower surface tension, because it has fewer hydrogen bonds than water and the pin easily drops through the surface to sink. surface tension - a force created by intermolecular attraction in liquids, such as hydrogen bonding in water. Surface tension acts to pull a liquid surface into the smallest possible area, for example, pulling a droplet of water into a sphere. 264 A NATURAL APPROACH TO CHEMISTRY Water as a solvent The universal solvent Water is often called the “universal solvent.” While water doesn’t dissolve everything, it does dissolve many substances such as salts and sugars. Water is a good solvent for two reasons: 1. 2. How water dissolves salt A water molecule is small and light; Water molecules are polar. The polar molecules of water are especially good at dissolving ionic compounds like salt (NaCl). When salt is mixed with water, the polar water molecules are attracted to sodium and chlorine ions in a solid salt crystal. When they hit, they transfer some energy which causes the ions in the salt crystal to separate. The negative side of the water molecules are attracted to the Na+ ions and the positive ends are attracted to the Cl- ions. Once a Na+ or Clion has broken off the crystal, it instantly attracts the oppositely charged sides of nearby water molecules. This process is called hydration. The hydrated sodium and chlorine ions stay in solution because of their accompanying polar water molecules. Water dissolves many molecular compounds Glucose is a covalent molecule, but it dissolves easily in water. The glucose molecule has areas of positive and negative charge (red and blue shading). When glucose is mixed with water, the charged areas attract many water molecules. Molecules of glucose are not dissociated into ions, but remain together (and neutral) in solution. Hydration - the process of molecules, with any charge separation, to collect water molecules around them. While not “chemically bonded”, a hydrated molecule does hold fairly tightly to its “private collection” of water molecules. A NATURAL APPROACH TO CHEMISTRY 265 Section 9.1 Solutes, Solvents and Water Distilled water vs. tap water Distilled water is used to make solutions When making solutions with water in the chemistry lab we often use distilled water. Distilled water is water that has been purified so that it contains no dissolved substances. The distillation process heats water to it’s boiling point of 100oC. The steam that forms is collected in a separate clean container. Any ions and solid contaminants are left behind as “residue” in the original container. Distilled water is sometimes incorrectly referred to as deionized water however, this is incorrect. Deionized water is water that has had its ions removed using a specific filtration process. Deionized water is sometimes used instead of distilled water, because it is cheaper to produce and, for most purposes, yields sufficiently pure water. In contrast, tap water often contains dissolved salts and minerals from the ground water supply and even from the pipes that carry it from the reservoir. Many minerals, such as Fe2+, Ca2+ and Mg2+, that may be present in water are important for our health. Other sources of ions in tap water come from water supplies that are treated with chloride ions (Cl-) for purification purposes. Chloride ions kill bacteria that may be present in the water supply. Fluoride (F- ion) is also added at 4 mg/L to help with dental health. Evidence? How do we know that tap water contains ions and distilled water does not? Why do we use distilled water? When making solutions in the laboratory we need to know there are no other chemicals in the solution, besides the ones that we add. Contaminating ions and minerals can interfere with the chemical reaction we are trying to perform. For this reason we use distilled or deionized water when we make our solutions in the chemistry laboratory. Distilled water - water purified by heating water to steam and condensing the steam into a clean or sterilized container. Deionized water - water without ions, purified using filtration methods. Tap water - drinking water from the faucet or “tap.” 266 A NATURAL APPROACH TO CHEMISTRY Reactions in aqueous solutions Aqueous means dissolved in water A solution with water as the solvent is called an aqueous solution. Aqueous solutions are so important that chemists consider being dissolved in water to be almost a fourth state of matter! In writing reactions, we use the symbols (s), (l), (g) and (aq) to show what state of matter the reactants and products are in. (s) indicates a solid (l) indicates a liquid (g) indicates a gas (aq) indicates a substance dissolved in water A good example is the familiar baking soda and vinegar reaction. All four states of matter appear in this reaction. Reading the symbols (s), (g), (l) and (aq) The (s) tells us the baking soda is a solid in the reactants and the (aq) tells us the acetic acid (vinegar) is an aqueous solution. On the product side of the reaction, the first (aq) tells us the sodium acetate is dissolved in the water that was part of the vinegar. The water (l) is a liquid. The carbon dioxide (g) is a gas. The gas is what makes the bubbles. If you are not careful, any product that is a gas quickly escapes, taking its mass with it! If the CO2 is allowed to escape, we would expect to measure less mass in the products of this reaction compared to the reactants. You can learn a lot about a reaction by knowing how to read these special symbols! Aqueous solution - An aqueous solution is any solution where the solvent is water. Molecules, atoms, or ions in aqueous solutions are identified by the symbol (aq) after a chemical formula. For example CO32-(aq) is how you would write a carbonate ion (CO32-) that is dissolved in water. A NATURAL APPROACH TO CHEMISTRY 267 Section 9.1 Solutes, Solvents and Water The importance of aqueous solutions Reactions in solids In order for a chemical reaction to occur, molecules must touch each other. This seems obvious, but think about what it means. In a solid, molecules cannot easily move around. Chemical reactions DO occur in solids, but they are slow, like changing limestone into marble under heat and pressure in the earth. This takes thousands of years. Reactions in gases In a gas, molecules move around quickly. Reactions (like combustion) can happen quickly in a gas, but there are very few chemicals that are gases at room temperature. Also, gases are not very dense. One mL of a gas (like air) contains a thousand times fewer molecules than the same volume of a liquid, like water. Reactions in liquids Liquids combine the advantages of both high density and mobility. In a liquid there are lots of molecules, close together, and they can move around and reach each other. Relatively few chemicals are liquid at room temperature either. Sugar is solid, salt is solid, and virtually all proteins and carbohydrates are also solids at room temperature. Why life requires water Life involves reactions of thousands of complex chemicals. This can only happen in aqueous solutions! Only in a solution can the molecules in living things circulate and reach each other to have reactions. For example, a sequence of reactions called the Krebs Cycle occurs in virtually all living things on Earth. In one step of this reaction, malic acid reacts with a chemical called NAD+ to produce oxaloacetic acid and NADH. This is part of how energy is extracted from glucose in every cell of your body. 268 A NATURAL APPROACH TO CHEMISTRY Other solvents Non-polar solvents Water may be the most important solvent to living things, but lots of other solvents exist, and you will probably use them from time to time in your life. For example, artists thin oil-based paints with odorless mineral spirits. Mineral spirits is the common name for hexane (C6H14), a linear molecule derived from oil. Hexane is non-polar, and good at dissolving other non-polar solutes. Why oil does not dissolve in water Oil does not dissolve in water because water is a polar molecule and oil molecules are nonpolar. However, oil dissolves easily in mineral spirits. In general, polar solvents dissolve polar solutes, like water dissolving salt. Nonpolar solvents dissolve non-polar solutes, such as mineral spirits dissolving oil. In general, like dissolves like Polar solvents dissolve polar solutes Non-polar solvents dissolve non-polar solutes “Solvent” means different things in everyday use In every-day conversation the word “solvent” usually means organic liquids such as mineral spirits, acetone (nail polish remover), turpentine, alcohol, and similar chemicals. Many of these chemicals are flammable, and noxious to breathe. In chemistry, the term “solvent” means something that dissolves something else. The common meaning and the chemistry meaning are similar, so don’t get confused! Water is a solvent in chemistry, the most important one to living things. TABLE 9.1. Some common solvents other than water Name Formula Typical uses Hazards Mineral spirits C6H14 paint thinner flammable, toxic Methanol CH3OH solvent, fuel flammable, toxic Acetone CH3COCH3 Tetrachloroethylene A NATURAL APPROACH TO CHEMISTRY C2Cl4 nail polish remover flammable, toxic, carcinogen dry cleaning chemical flammable, toxic, carcinogen 269