Unit 11: Chapters 15 and 16

Transcription

1 Unit 11: Chapters 15 and 16 Water and Solution Chemistry What makes Water Special? Extensive Hydrogen Bonding!! Unusually... high surface tension low vapor pressure high specific heat capacity high molar heat values high temperature and wide liquid state lower density solid than liquid (max at 4 0 C) δ - δ + H O H 2 Surface Tension Cohesive Force at surface that results from uneven intermolecular forces holding substance together. 3

2 Types of mixtures Type Particle size Will it settle? Tyndall effect? Homo or Hetero Can it be filtered Example out? Solutions 1 nm no no Homo no Koolaid Colloids nm no yes borderline no Milk Suspensions > 100 nm yes yes hetero yes Mud puddle Colloids Tyndall Effect: Scattering of Light as it passes through a colloid. Brownian Motion: Chaotic Motion of colloid particles 5 Components of a solution Solvent: Does the dissolving Water: The universal solvent Greater amount in solution Solute: Gets dissolved Forms: Solid, Liquid, Gas Lesser amount in solution

3 Types of Solutions Solute Solvent Solution Gaseous Solutions gas gas liquid gas air (nitrogen, oxygen, argon gases) humid air (water vapor in air) Liquid Solutions gas liquid solid liquid liquid liquid carbonated drinks (CO 2 in water) vinegar (CH 3 COOH in water) salt water (NaCl in water) Solid Solutions liquid solid solid solid dental amalgam (Hg in Ag) sterling silver (Cu in Ag) 7 How a solute dissolves Solvation (disassociation): the separation of individual ions/molecules from others. Hydration: Water surrounding and stabilizing individual particles Hydrates hygroscopic compounds: Compounds that are able to remove water from air to make hydrates Desiccants: hygroscopic substances used as drying agents. (Silica gel) Deliquescent compounds: hygroscopic substances that absorb so much water they actually dissolve themselves and form concentrated solutions. Effloresce: the loss of water from a hydrate. Anhydrate: hydrate once all water has been removed (anhydrous: without water) 9

4 Like Dissolves Like Polar/Ionic: Since both have effective polarity they can typically dissolve into each other. Check solubility table for ionic!! Nonpolar: has no polarity so polar substance will not dissolve them. Only nonpolar substances can dissolve other nonpolar substances Until later this unit!!!! Electrical Conduction Electrolytes: Substance that will conduct electricity when dissolved in water Strong: Substance completely ionizes in water: high conduction Weak: Substance partially ionizes in water: medium conduction Non: Substance does not ionize in water: no conduction Conduction Activity Fill a 50 ml beaker with distill water. Test its conductivity to establish a baseline. Add a scoop of dextrose Test its conductivity. Rinse and clean beaker. Repeat above steps but use Epson Salt. Now try Gatorade! On front bench are test samples. Test directly without diluting. DO NOT DRINK! 12

5 Dissolution Rate Three factors that affect rate of dissolution. Surface area: the more surface area of the solute that is exposed the faster it will dissolve Agitation: physically stirring, swirling or shaking will dissolve a solute faster Temperature: as temperature increases the rate of dissolution will also increase. All are based on increasing the number of collisions between solute and solvent. How does each increase the collisions? Saturation Saturated: The max amount of solute that will dissolve in a given amount of solvent at a given temperature. Qualitative value. Unsaturated: Less than the max. Supersaturated: Special case when a solution is saturated at high temperatures. Then cooled slowly allowing the solute to remain in solution. Only certain solutes can do this. Must not disturb the solution or change temps too fast Holds in energy also Determining Saturation Add a few crystals of solute to unknown solution. If they dissolve: unsaturated sit on bottom or do not dissolve: Saturated Cause a large amount of particles to fall out of solution: supersaturated

6 How much solute can you dissolve into a solvent? Solubility: The ability of a solute to dissolve in a solvent Represented as a numerical value There is a large range of solubility's for different compounds but we group compounds into one or the other for reference. Soluble: gas or solid solute is able to dissolve in solvent Insoluble: gas or solid solute is not able to dissolve in solvent Miscible: liquid solute able to dissolve in solvent Immiscible: liquid solute not able to dissolve in solvent Temperature dependent for all solutes. (Gas solutes also dependent on pressure) How Pressure change affects Solubility? Pressure ONLY affects gas solutes! As the pressure increases above a solution the solubility of the gas solute also increases Henry s Law: How Temperature change affects Solubility Gas solute: As temperature increases solubility decreases. Inverse relationship

7 How Temperature change affects Solubility Solid/liquid solute: As temperature increases solubility also increases. Direct relationship Exception??? Calculating Concentration of a solution Molarity: concentration of solute in solution moles of solute molarity ( M ) = liters of solution mol L But we can t measure mols in lab so we have to be able to convert mols to grams to be able to make solutions. Also, in lab we measure in ml so make sure to convert from liters. Concentration Practice 1 What is the Molarity of a KCl solution that contains 85.0 g KCl and a volume of ml? Concentration Practice 2 You want to make ml of a.25 M solution of Mg(NO 3 ) 2. How many grams of Mg(NO 3 ) 2 do you need?

8 Molarity Quick Lab For lab tomorrow you need to calculate the correct mass in grams needed to prepare the correct concentration of a solution you will be preparing. We will be preparing two different solutions. There concentrations and total volumes are below. Blue cap: ml of M copper (II) sulfate pentahydrate Yellow cap: ml of M cobalt (II) chloride hexahydrate Make solution using volumetric process and DISTILLED water. We will need to label it properly and store it for a future lab using these solutions. So, precision and accuracy is a must! Determining unknown Concentrations Beer s Law states the amount of light a translucent solution absorbs is proportional to the concentration of the solution. If we know the light absorption of standardized concentrations we can then graph this data and use the graph to determine the concentration of an unknown sample. 23 What is dilution? Dilution Reducing the concentration of a solution by adding solvent.

9 Dilution Formula M 1 V 1 = M 2 V 2 M 1 : molarity of concentrated solution V 1 : volume of concentrated solution M 2 : molarity of dilute solution V 2 : volume of dilute solution Dilution Practice 1 Concentrated HCl is 12.0M. You want to make 250 ml of 3.0M HCl. What volume of 12.0M HCl do you need? How much water do you add to make the 250 ml solution? Dilution Practice 2 You dissolve 269 g CuCl 2 in water to make liters of solution. What is the molarity of this stock solution? How many liters of 0.25 M CuCl 2 solution can you make from this stock solution? Calculating solutions by percent volume solute percent by volume = volume solution i100 mass solute (g) percent by mass = volume solution (ml) i100 When giving values by percent make sure to distinguish if it is by volume (v/v) or mass (m/v) 27

10 Practice You dissolve 25.0 g of NaCl into 125 ml of solution. What is the percent composition by mass? Hydrogen peroxide is sold as a 3.0% solution by volume. How many ml of H2O2 are in a ml bottle of solution? 28 molality ratio of solute to solvent is needed. Not affected by temp changes. colligative properties (next topic) molality (m) molality (m) = moles solute kg solvent 29 practice How many grams of benzene (C6H6) must be dissolved in 500. g of acetone to make a 1.25 molal solution? 30

11 Colligative Properties Defined: Properties dependent on the ratio of solute to solvent particles in solution Nonvolatile solutes only! nonvolatile: little tendency to escape solution. typically solids Vapor Pressure As you increase the amount of nonvolatile solute particles in solution the vapor pressure will decrease Lower vapor pressure = less ability to vaporize Colligative properties Boiling point elevation: needs more energy to boil Requires higher temps Freezing point depression: ISU Animation Phase Diagram Animation needs to reduce energy to freeze Requires lower temps Calculating b.p. elevation and f.p. depression f.p. ΔT f = K f im b.p. ΔT b = K b im molality is based on effective molality of solute particles. 1 mol sugar = 1 mol solute 1 mol NaCl = 2 mol solute 1 mol K3PO4 = 4 mol solute 33

12 Practice What is the boiling point of a 1.75 molal sugar solution with ethanol as your solvent? (kb ethanol = 1.19 o C/molal) (b.p. of ethanol = C) What is the freezing point of a 1.50 molal CaCl2 aqueous solution? (kf water = 1.86 o C/molal) What is the molality of a NaCl aqueous solution that has a freezing point of C? (kf water= C/molal) 34 Colligative Properties Antifreeze and Coolant???????? emulsifier: food product that links non-polar molecules with polar surfactant: non-food product that allows a nonpolar solute to dissolve in a polar solvent