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Chapter 15

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  1. Chapter 15 Solutions

  2. 15.1 What are solutions? A. Characteristics of Solutions • 1. Solutions are homogeneous mixtures containing two or more substances called the solute and the solvent. • The solute is the substance that dissolves. • The solvent is the dissolving medium.

  3. The solute is the substance that dissolves. • The solvent is the dissolving medium.

  4. Solutions may exist as solids, liquids, or gases • Ex: air, brass, salt water • But most solutions are liquid

  5. Solutions Air: gas in gas Seawater: solids in water Soft drinks: gas in liquid

  6. Solutions Brass: solid in solid Fog: Liquid in gas

  7. Solution Terms • Soluble- dissolves in a solvent • Insoluble- does not dissolve in a solvent

  8. Miscibility, the ability of liquids to mix. Think of it as “mixibility”! • Oil and water are immiscible. They won’t mix. That’s why you have to mix oil and vinegar before you pour it in a salad. • Immiscible- one is insoluble in the other and do not mix.

  9. Miscible -two liquids that are soluble in each other and will mix. • Water and alcohol are miscible. • Likewise, chocolate syrup and milk are miscible.

  10. Dissolution in Aqueous Solutions • Dissolution- process of surrounding solute particles with solvent particles to form a solution.

  11. Water has polarity: a positive and a negative region.

  12. When a solute such as salt, is added to a solvent such as water, the solvent particles surround it.

  13. The solvent particles pull the solute particles apart .

  14. The ions are completely surrounded by the water molecules. Dissolution - the solute particles are surrounded with solvent particles forming a solution.

  15. Show phet video


  17. “Like dissolves Like” Determines whether solvation will occur in a specific solvent. • Determine the polarity of the particles. • Polar solutes dissolve in Polar solvents • Nonpolar solutes dissolve in nonpolar solvents.

  18. A crystal of iodine (I2) is in water on the left. And in carbon tetrachloride on the right. The purple iodine crystal remains un-dissolved in the water but has dissolved in the carbon tetrachloride. Water is polar, Iodine is not. So what is Carbon tetrachloride, polar or non-polar?

  19. Aqueous solutions of ionic compounds • Water molecules are dipoles with positive and negative ends and Ionic compounds have positive and negative ions. BOTH POLAR “like” • So Ionic compounds dissolve easily in water.

  20. Insolubility • Insolubility occurs because attractive forces among the ions are so strong they can not be overcome by the water molecules. • Precipitates form. • If no dissolution, no solution is formed.

  21. Aqueous solutions of molecular compounds • If a molecular compound is polar it will dissolve in water. • Ex: sucrose • Oil does not dissolve in water because it is nonpolar. “not like”

  22. Factors that affect rate of Solvation How can we dissolve sugar faster in our iced tea? 1. Agitating the mixture. Stir the sugar. This increases the contact with unsaturated tea.

  23. Increasing Rate of Dissolving How can we dissolve sugar faster in our iced tea? 2. Increase the surface area. Use smaller crystals like powdered sugar instead of lumps. This increases the surface area of the sugar in the tea. Powdered sugar

  24. Increasing Rate of Dissolving How can we dissolve sugar faster in our iced tea? 3. Temperature. Add the sugar to the hot tea before you add the ice.

  25. Solubility • Solubility – the maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature and pressure.

  26. Crystallize – as the number of solvated particles increases, colliding solute particles rejoin the crystal.

  27. Types of Solubility • Saturated Solution- contains the maximum amount of dissolved solute for a given amount of solvent at a specific temperature and pressure. • over all amount of dissolved solute remains constant.

  28. Unsaturated Solution- contains less dissolved solute for a given temperature and pressure than a saturated solution. • more solute can be dissolved

  29. Supersaturated solution – contains more dissolved solute than a saturated solution at the same temperature. • Formed from a saturated solution at a high temperature and then cool slowly. • Unstable – crystallize quickly

  30. Factors that Affect Solubility Temperature and Pressure Temperature - many substances are more soluble at higher temperatures than at lower temperatures • Solids generally dissolve faster in hot solutions than in cold solutions. • Gases dissolve faster in cold solutions than in warm solutions.

  31. Temperature & Solubility If we raise the temperature from 0C to 80C the solubility of NaCl (table salt) increases by only 2g/100 ml of H20. However, KNO3 (potassium nitrate) increases by 150g.

  32. Temperature and Solubilityin Gases The trend for solubility in gases is different. As the temperature increases the solubility decreases. Open a cold can of soda and a warm can of soda. Which will have the greatest rush of gas? Will more oxygen be dissolved in a cold stream or a warm stream?

  33. Solubility Rules

  34. All common compounds of Group I and ammonium ions are soluble. • All nitrates, acetates, and chlorates are soluble. • All binary compounds of the halogens (other than F) with metals are soluble, except those of Ag, Hg(I), and Pb. Pb halides are soluble in hot water.) • All sulfates are soluble, except those of barium, strontium, calcium, lead, silver, and mercury (I). The latter three are slightly soluble. • Except for rule 1, carbonates, hydroxides, oxides, silicates, and phosphates are insoluble. • Sulfides are insoluble except for calcium, barium, strontium, magnesium, sodium, potassium, and ammonium.

  35. Salt (NaCl) is very soluble in water. 350 g/liter. However, if water evaporates, there will be too much salt for the water to hold in solution. The salt begins to form crystals. A lake near Death Valley is supersaturated with salt causing the salt to crystallize out.

  36. Solubility increases as External Pressure increases Pressure and Solubility

  37. Pressure and Solubility Ex: soda; as can is opened, pressure decreases, causing CO2 to come out of the solution • Solids are not generally affected by pressure. • Gases dissolve faster under pressure.

  38. Pressure and Solubility Scuba divers have a threat with dissolved nitrogen in the blood. During a deep descent there is increased pressure and more nitrogen can be dissolved in the blood. As the diver surfaces the nitrogen can form bubbles just like your can of soda. This is called “bends” and can be very painful and cause paralysis or death. Divers must be put into pressure chambers and slowly decrease pressure.

  39. Electrolytes and Colligative Properties • Electrolytes form a solution that conducts electric current. • Strong – produce many ions in solution Ex: Sodium chloride • Weak – produce only a few ions in solution • Ex: NaCl – 2 ions MgCl2 – 3 ions

  40. Conductivity Testing Relative amounts of salts can be detected in water using a “conductivity meter”. The meter sends an electric current through the solution. The strength of the current is proportional to the concentration of dissolved electrolytes

  41. Nonelectrolytes in Aqueous Solution • Nonelectrolyte- molecular compound that dissolve in solvents but do not ionize, and so do not conduct an electrical current • Ex: sucrose

  42. 15.2 Solution Concentration • Concentration- a measure of how much solute is dissolved in a specific amount of solvent or solution

  43. Expressing Concentration • Commonly used quantitative expressions for concentration are: • Percent by Mass • Percent by Volume • Molarity • Molality

  44. Molarity • Molarity- number of moles of solute dissolved per liter of solution • Unit is M, which stands for molar

  45. Molarity Equation Molarity (M) = Moles of Solute Liters of Solution

  46. Diluting Solutions • Increasing solvent decreases concentration, so: V1 M1= V2 M2

  47. Examples • A 100.5 ml intravenous (IV) solution contains 5.10 grams of glucose (C6H12O6). What is the molarity of this solution? Steps for Solving • Change all grams to moles • Change all liquid units to liters • Molarity = Moles of solute Liters of solution

  48. Grams to moles 5.10 grams glucose (given) 180.16 g glucose (molar mass of glucose) = 0.0283 mols glucose Change all liquid units to liters 100.5 ml 1000 ml = 0.1005 Liters Calculate molarity 0.0283 (mols) 0.1005 (Liters) = 0.282M

  49. Dilution Examples Steps • Write formula : V1M1= V2M2 • List all the information you have • Solve for unknown • Check your units!