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Chapter 8

Chapter 8. Solutions, Acids, and Bases. 8.1 Formation of Solutions. DISSOLVING Recall that a solution is a homogeneous mixture of two or more substances. Solute - substance whose particles are dissolved in a substance. Solvent -The substance in which the solute dissolves .

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Chapter 8

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  1. Chapter 8 Solutions, Acids, and Bases

  2. 8.1 Formation of Solutions • DISSOLVING • Recall that a solution is a homogeneous mixture of two or more substances. • Solute- substance whose particles are dissolved in a substance. • Solvent-The substance in which the solute dissolves. • These can be solids, liquids, or gases. Most familiar is liquids.

  3. 3 WAYS TO FORM SOLUTIONS • Substances can dissolve in liquids in three ways: dissociation, dispersion, and ionization • DISSOCIATION OF IONIC COMPOUNDS • The particles that hold solutes to each other and solvents to each other must be overcome. Ex: NaCl is an ionic compound. Water is a polar molecule, and is attracted to the salt. The sodium and chlorine ions are pulled into solution one by one. • Dissociation-The process in which an ionic compound separates into ions as it dissolves. When an ionic compound dissolves in water, the charged ends of water molecules surround the oppositely charged ions.

  4. 3 WAYS TO FORM SOLUTIONS • DISPERSION OF MOLECULAR COMPOUNDS • Ex: Sugar and water are both polar molecules, so they attract one another. When the attractions between them are great enough to overcome the attractions holding the sugar molecule to the surface of the crystal, The sugar molecule breaks free, and is pulled into solution by the water molecules. • Dispersion-Breaking into small pieces that spread throughout the water. Saliva dissolves the sugar in hard candy by dispersion. As water molecules collide with sugar crystals, attractions develop between the water molecules and sugar molecules at the surface of the solid.

  5. 3 WAYS TO FORM SOLUTIONS • IONIZATION OF MOLECULAR COMPOUNDS • When HCl dissolves in water the hydrogen atom gets pulled away from chlorine and forms a hydronium ion. • Ionization-Neutral molecules gain or lose electrons. This is different from the previous two because it is a chemical change. The solution that results contains new substances.

  6. Properties of Liquid Solutions • Only three physical properties of a solution will differ from those of its solute and solvent. They are conductivity, freezing point, and boiling point. • CONDUCTIVITY • When NaCl is dissolved in water, the ion are free to move. This will conduct an electric current. NaCl not dissolved will not conduct a current.

  7. Properties of Liquid Solutions • FREEZING POINT AND MELTING POINT • MgCl2 is used in winter to melt ice and snow. It dissociates into its ions. The structure of ice is disrupted by the ions and cannot freeze. • Coolant is added to cars to raise the boiling point and prevent the car from overheating. The presence of solute particles affects how a solvent freezes. A Pure water freezes in a hexagonal pattern. B In water “salted” with MgCl2, the dissociated Mg2+ and Cl− ions disrupt the formation of ice crystals.

  8. HEAT OF SOLUTION • During the formation of a solution, energy is either released or absorbed. Like chemical reactions, this is described as endothermic or exothermic. • In order for a solution to form, bonds must be broken, and new attractions formed. • Breaking attractions requires energy • The formation of attractions releases energy • The difference in these energies is known as the heat of solution.

  9. FACTORS AFFECTING RATES OF DISSOLVING • Like rates of chemical reactions, rates of dissolving depend on the frequency and energy of collisions that occur between very small particles. • Factors that affect the rate of dissolving include surface area, stirring, and temperature.

  10. Surface Area • When a sugar cube dissolves in water, the dissolving takes place at the surfaces of the cube. • The greater the surface area of a solid solute, the more frequent the collisions are between solute and solvent particles. • More collisions result in a faster rate of dissolving

  11. Stirring • Stirring moves dissolved particles away from the surface of the solid, and allows for more collisions between solute and solvent particles. • Stirring will make it dissolve faster. The rate of dissolving can be increased by reducing the particle size of the solute, by stirring, and by heating the solvent.

  12. Temperature • Increasing the temperature of a solvent causes its particles to move faster, on average. • As a result, both the number of collisions and the energy of these collisions with solute particles increase. • The solute goes into solution more quickly. • Increase in temp. will speed up the rate of dissolving.

  13. 8.2 Solubility & Concentration • Solubility- The maximum amount of a solute that dissolves in a given amount of solvent at a constant temperature • Solubilities are usually expressed in grams of solute per 100 grams of solvent at a specified temperature At a given temperature, different solutes have different solubilities in water.

  14. 3 Ways Solutions are Classified • Solutions are described as saturated, unsaturated, or supersaturated, depending on the amount of solute in solution. • Saturated Solutions - contains as much solute as the solvent can hold at a given temperature • The solvent is “filled” with solute. If you add more solute, it will not dissolve.

  15. 3 Ways Solutions are Classified • Unsaturated Solution - A solution that has less than the maximum amount of solute that can be dissolved • Supersaturated Solution is one that contains more solute than it can normally hold at a given temperature. • Supersaturated solutions are very unstable. If even a tiny crystal of the solute falls into a supersaturated solution, the extra solute can rapidly deposit out of solution • If you heat a solvent up, more solute may dissolve. Then you then carefully cool the solvent back without jarring it, you may be able to keep the extra solute in solution. In the photo sequence, a single crystal of sodium acetate, NaC2H3O2, is added to a supersaturated solution of sodium acetate in water. The excess solute rapidly crystallizes out of the solution.

  16. Factors Affecting Solubility • Three factors that affect the solubility of a solute are the polarity of the solvent, temperature, and pressure Generally, a solute is more likely to dissolve in a “like” solvent than an “unlike” solvent.

  17. Polar and Nonpolar Solvents • Oil does not dissolve in water because oil molecules are nonpolar and water molecules are polar. • guideline for predicting solubility is “like dissolves like.” Soaps and detergents are used to remove grease and oil stains. Soap molecules form attractions to both polar water molecules and nonpolar oil molecules. As the water flows away, it carries the oil with it.

  18. Temperature • In general, the solubility of solids increases as the solvent temperature increases • When a glass of cold water warms up to room temperature, bubbles form on the inside of the glass. These bubbles are gases that were dissolved in the water. They come out of the solution as the water temperature rises. • Unlike most solids, gases usually become less soluble as the temperature of the solvent increases.

  19. Pressure • Increasing the pressure on a gas increases its solubility in a liquid. • The pressure of CO2 in a sealed 12-ounce can of soda at room temperature can be two to three times atmospheric pressure

  20. Concentration of Solutions • Concentration of a solution-the amount of solute dissolved in a specified amount of solution • Concentration of solution is usually expressed as % by volume or % by mass. • Percent by volume = volume of solute x 100 volume of solution • Useful when dealing with liquids • Percent by mass = mass if solute x 100 mass of solution • Useful when solute is a solid

  21. 8.3 Properties of Acids and Bases • Identifying Acids • Acid- a compound that produces hydronium ions (H3O+) when dissolved in water.

  22. Properties of Acids • Sour Taste • Lemons, grapefruits, limes, and oranges have citric acid • Vinegar has acetic acid • Spoiled dairy products have butyric acid • NEVER TEST AN ACID BY TASTING IT

  23. Properties of Acids • Reactivity with Metals • Acids and metals undergo a single replacement reaction • Color Changes in indicators • Indicator – any substance that changes color in the presence of an acid or base. • Ex: Blue Litmus paper turns red

  24. Identifying Bases • Base- a compound that produces hydroxide ions (OH-) when dissolved in water.

  25. Properties of Bases • Bitter Taste • Unsweetened (baking) chocolate • Cough syrup • Slippery Feel • Wet soap • Wet rocks • Color Changes in Indicators • Red Litmus paper turns blue in a base • Phenolphthalein turns red • Hydrangeas turn blue in acid soil, pink in basic soil

  26. Neutralization and Salts • The reaction between an acid and a base is called neutralization • Negative ions in an acid combine with the positive ions in a base to produce an ionic compound called a salt.

  27. The pH Scale • pH scale – a number scale from 0 to 14 used to describe the concentration of hydronium ions in a solution. • A low pH tells you that the concentration of H+ ions is high. (acid) • A high pH tells you that the concentration of H+ ions is low. (base) • pH lower than 7 = acidic (0-6) • pH higher than 7 = basic (8-14) • pH of 7 = neutral

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