Regents Chemistry

1. Regents Chemistry • Properties of Solutions

2. Properties of Solutions • Review - What’s a solution • a solution is a homogeneous mixture of substance in the same physical state • Most chemical reactions take place in solutions • We will learn the nature and properties of solutions and ways to express the concentration of solutions

3. What do Solutions Contain? • Solutions contain atoms, ions or molecules in which one substance spread uniformly throughout a second substance • Ex: Salt water

4. Types of Solutions • Solutions exist in all three states! • A solid may be dissolved in another solid • ex: Brass is a mixture if zinc and copper • A Metal solution is called an alloy • Air is a gaseous solution and can vary depending on the conditions • ex: amount of water vapor varies daily

5. Liquid Solutions • We will mostly focus on solutions containing a liquid • We identify parts of a liquid solution by how it is made • Solute - is the substance that is being dissolved, and it is the substance present in the smaller amount • Solvent - substance that dissolves the solute - most common is water

6. NaCl NaCl(s) Na+(aq) + Cl-(aq) Once the salt and water is stirred and the mixture becomes homogeneous, the dissolved particles will not settle - cannot filter! Liquid solutions are clear but may have color and light will pass through without being dispersed

7. Liquid Solution Summary • 1. Solutions are homogeneous mixtures • 2. Solutions are clear and do not disperse light • 3. Solutions can have a color • 4. Solutions will pass through a filter

8. Solubility Factors • Some things dissolve in solvents and some don’t, so.. • Solubility - is how much of a solute will dissolve in a certain amount of solvent at a certain temperature • Materials with high solubility are said to be soluble • Materials with a low solubility are said to be insoluble

9. Nature of Solute and Solvent • NaCl dissolves in water because its positively and negatively charged ions are attracted to oppositely charged ends of the polar water molecule • The attractive forces between the water molecules and sodium ions are greater than the attractive forces between the sodium and chloride ions • Same goes for the chloride ions and positive end of water molecule

10. Like dissolves Like • Ionic substance dissolve in ionic solvents • Nonpolar substances, such as fats, dissolve in nonpolar solvents • So fats do not dissolve in water! No strong attractive forces between water molecules and fat molecules - must be dissolved in a nonpolar solvent • Why..because the forces are weak and they simply mix together

11. Table summary

12. Effect of Temperature • As temperature increases, most solids become more soluble in water • A few exceptions exist: • Gases react in the opposite manner • As temperature increases, the solubility of all gases in liquids decreases

13. Effect of Pressure • Pressure has little or no effect on the solubility of solid or liquid solutes • Pressure does affect the solubility of gases in liquids • As pressure increases, the solubility of gases in liquids increases • Ex: opening a can of soda - the pressure decreases • CO2 is no longer as soluble at the lowered pressure and escapes as bubbles

14. Regents Chemistry • Solubility Graphs and saturated and unsaturated solutions

15. Solubility Information • Solubility information may be presented in different ways • Table G in your Reference Tables shows the relationship between grams of solute that can be dissolved at various temperatures • Table F in Reference Tables provides some general guidelines about the solubility of ionic substances

16. Using Table G • Shows the maximum number of grams that can be dissolved in 100g H2O at specific temperatures • Most show increasing solubility as temp increases, but a few don’t • these are gaseous NH3, HCl and SO2 • gases decrease in solubility as temp increases

17. Using Table G • Any point that is below the curve of a substance is considered unsaturated • Any point that is on the curve of a substance is considered saturated • Any point that is above the curve of a substance is considered supersaturated

18. Saturation • Unsaturated solutions hold less solute than maximum and no solid should be present • Saturated solutions hold the max amount and any additional will simply stay as a solid • Supersaturated solutions occur when the temperature is reduced but no crystals (solid) form out of solution - any additional solute added will cause crystals to form and solution will return to saturated state

19. Recognizing Degree of Saturation • Because solutions are clear, it is difficult to simply look at a solution and determine whether it is saturated, unsaturated or supersaturated • So how can we tell? • 1. We can compare the number of grams dissolved in a given volume to table G • 2. Add additional solute and see what happens!

20. Using Table F • Contains some guidelines for the solubility of common ionic compounds • YOU HAVE USED THIS TABLE BEFORE! • Explains if a reaction will form

21. Table G Practice Problem • Which substance on table G (solubility curve) is saturated with 20g at 49 C? • How many grams of HCl would have to be added to a 70g in solution to make it saturated at 10 C?

22. Regents Chemistry • Concentrations of Solutions – Molarity

23. What’s Molarity • Let’s first review a mole…video clip • We sometimes refer to solutions as concentrated or dilute…but these are not scientifically precise terms.. • We need to know specific strengths to run reactions.. • This is the purpose of molarity!

24. Molarity • Molarity – is the number of moles of solute in 1 Liter of solution • This tells us the exact “strength” of the solution • We add a specific amount of solute to a specific amount of water..once this is made, the molarity doesn’t change! • The formula is below and on your reference tables Molarity = moles of solute g / mol M = = liters of solution L

25. Solving Basic Molarity Problems • What is the molarity of a solution that contains 4.0 mol of NaOH in 0.50 L of solution? M = mole of solute = 4.0 moles NaOH liters of soln 0.50 L Molarity = 8.0 M

26. Molarity w / no moles given… • What is we are given a gram amount instead of mole amount…can we still solve for molarity? • Yes! Practice Problem What is the molarity of a solution containing 82.0 g of Ca(NO3)3 in 2.0 L of solution? • Convert 82.0 grams to moles by using molar mass • Plug into Molarity equation and solve!

27. Additional Practice Problem • What is the molarity of a solution containing 26.0 g KCl in 750 mL of solution?

28. Rearranging the Equation • We can rearrange the equation to solve for mole amount or liters of solution Example • How many moles of BaSO4 are in a 2.0 M solution originally made with 1.5 L of solution?

29. Regents Chemistry • % by mass, % by volume and ppm

30. Percent by Mass • Common to find labels that list the concentration of ingredients by mass • Percent Mass – is simply the mss of an ingredient divided by the total mass expressed as a percent • Percent mass is essentially the same as percent composition – you have done this in lab!

31. Percent by Mass mass of part X 100% Percent mass = mass of whole What is the percent mass of sodium hydroxide If 2.50 g of NaOH are added to 50.00 g of H2O?

32. Percent by Volume • When two liquids are mixed to form a solution, it is common to express the concentration of the solute as a percent by volume • For example, a solution of isopropyl alcohol contains 70% alcohol by volume Volume of solute X 100% Percent by volume = Volume of solution

33. Practice Problem • What is the percent by volume of alcohol if 50.0 mL of ethanol is dilluted with water to form a total volume of 300 mL?

34. Parts per Million (ppm) • Parts per million is similar to % comp because it compares masses • It’s a ratio between mass of the solute to total mass of the solution • This method of reporting concentrations is useful for extremely dilute solutions when molarity and % mass would be to difficult to interpret

35. ppm • For example • Chlorine is used as a disinfectant in swimming pools. Only about 2g of chlorine per 1,000,000 g of swimming pool water is necessary to keep the pool sanitized Grams of solute x 1,000,000 ppm ppm = Grams of solution

36. Practice Problem • Approximately 0.0043 g of oxygen can be dissolved in 100 mL of water at 20 degrees Celsius. Express this in terms of ppm • (assume 1mL water = 1.0 g water

37. Regents Chemistry • Colligative Properties

38. What are Colligative Properties? • Colligative properties are properties of a substance that are affected by the nature of a solute added to it • In terms of water: • Freezing and boiling points are colligative properties that are affected by the nature of the solute..as we shall see…

39. Molecular vs. Ionic • Molecular substances affect the freezing and boiling points of water different than ionic substances.. • Why?? • Because ionic substance break apart into ions and molecules do not! • Ex: Salt vs. sugar

40. Salt vs. Sugar C12H22O11 (s)  C12H22O11 (aq) Vs. NaCl (s)  Na+ (aq) + Cl- (aq) 1 mole of salt will raise the boiling point and depress the freezing point twice as well as 1 mole of sugar!

41. Vapor Pressure and Boiling Point • When a substance that is normally a liquid enters a vapor phase, it is called a vapor • A liquid normally has molecules that escape its surface • The pressure that these molecules exert in the surrounding atmosphere is called vapor pressure

42. Vapor Pressure • Why do these molecules escape? • Liquids are held together by rather weak intermolecular forces • These forces are called dipole-dipole forces • As temperature increases, these forces become less effective and more molecules escape…thus VP increases!

43. Water is different… • Water is different than most liquids.. • It participates in hydrogen bonding in addition to dipole-dipole interactions.. • Thus it has a high boiling point and requires more energy to break the intermolecular forces.. • This is seen by observing the relationship between molecular weights and vapor pressure

44. Table H • Table H on your reference tables shows us the vapor pressure at various temperatures.. • Notice the boiling point for each liquid • Boiling Point – is when the vapor pressure of a liquid is equal to the atmospheric pressure… • This occurs when we see bubbles!

45. Using Table H • Find the Vapor Pressure of water at 75 degrees Celsius. • Which of the substances has the weakest intermolecular forces? Why? • Which has the strongest intermolecular forces? Why?