Solutions Unit 6 1
Solutions Homogenous Mixture (Solution) – two or more substances mixed together to have a uniform composition, its components are not distinguishable from one another Heterogenous Mixture Homogenous Mixture (Not a Solution) (A Solution) Ice Water (w/ Ice Cubes) Sea Water Smog Air Oil and Water Gasoline (a mixture of hydrocarbons) 2
Solution Terminology Solvent – the dissolving medium (usually present in the greatest amount) . . . for our purposes is generally water Solute – the substance dissolved in the solvent + = http://www.docbrown.info/page03/3_52states.htm 3
Solution Formation 1. Solvent molecules surrounds solute molecules and “attach” to them . Move of Dissolution: http://www.northland.cc.mn.us/biology/Biology111 1/animations/dissolve.html http://www.elmhurst.edu/~chm/vchembook/171solublesalts.html 4
Solution Formation 2. Attraction between solvent /solute particles is greater than those between the solute particles. Solute particles get pulled apart. Movie of Dissolution w/ Narration: http://www.elmhurst.edu/~chm/vchembook/171solublesalts.html http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/molvie1.swf 5
Solubility The maximum amount of solute a given amount of solute can dissolve is called the solubility . Solubility is often referred to in units of grams of solute per 100 mL of solvent. A solution containing the maximum amount of solute is saturated . Unsaturated means the solution can hold more solute. 6
Factors Affecting Solubility Factors Affecting Solubility: • Type of solute and solvent. • Temperature of liquid solvent – Solids and liquids generally increase solubility with temp. – Gases generally decrease solubility with temp. • Atmpospheric pressure increases solubility of gases 7
Solubility Graphs 1. Which solute is most/least soluble at 25 o C? At 100 o C? 2. Is it possible to dissolve 100 g of sodium nitrate in 100 mL of water at 30 o C? (How much, if any, will not dissolve?) Solid solute that will not dissolve is called a precipitate . 8
Concentration of Solutions I Concentrated Solutions – lots of solute per given amount of solvent Dilute Solutions – little solute per given amount of solvent 9
Concentrations of Solutions II Molarity (M) – a way to express concentration numerically Moles of Solute Molarity (M) Liters of Solution Steps to make a solution: 1. Mass the correct number of moles of solute. 2. Add solid to volumetric flask of correct Volumetric volume. Flask 3. Add part of solvent, dissolve solute, add the rest of solution. 10
Molarity Calculations Moles of Solute Molarity (M) Liters of Solution e.g. What is the molarity of 30.0 g of NaOH (M. Wt. = 40.0 g/mol) dissolved in 0.500 L of solution? 30.0 g NaOH 1 mol NaOH Ans: x 0.750 mol NaOH 1 40.0 g NaOH 0.750 mol NaOH 1.50 M 0.500 L 11
More Molarity Calculations e.g. How would you make 250 mL of a 0.50 M CaCl 2 solution? (M. Wt. = 111 g/mol) 0.250 L 0.50 mol 111 g 1. Find mass of CaCl 2 needed: x x 14 g CaCl 2 1 1L 1 mol 2. Add 14 g CaCl 2 to 250 mL Flask 3. Add ½ the water, dissolve the solid, then add enough water to equal 250 mL of solution. For movies of solution preparation: http://www.mhhe.com/physsci/chemistry/animations/chang_7e_esp/crm3s1_2.swf 12
Mole Highway Calculations with Solutions Finally, the last leg of the mole highway! X Molarity A X Mole Ratio X 1 / Molarity B (Balanced Rxn) Solution Solution Moles Moles Volume A Volume B A B (L) (L) L Solution A moles A moles A moles B moles B L Solution B x x x 1 1 L 1 moles A 1 moles B 13
Mole Highway Calculations with Solutions Use the reaction below to find how many grams of Mg it would take to react with 10.0 mL of 6.0 M HCl? Mg (s) + 2 HCl (aq) → H 2 (g) + MgCl 2 (aq) 0.010 L HCl 6.0 molesHCl 1 mole Mg 24 g Mg x x x 1 1L 2 moles HCl 1 mole Mg Ans. = 0.72 g Mg 14
Mole Highway Calculations with Solutions II Use the reaction below to find the concentration of NaOH if it takes 25.0 mL NaOH to completely react with 30.0 mL of 0.750 M H 2 SO 4 . 2 NaOH + H 2 SO 4 → 2 H 2 O + Na 2 SO 4 0.0300 L H SO 0.750 mol H SO 2 mol NaOH 1 2 4 2 4 x x x 1 mol H 2 SO 1 L H SO 1 0.0250 L NaOH 4 2 4 Ans. = 1.80 L NaOH 15
Electron/Proton Model of Atoms (Review of Ion Formation) 1. Atoms contain (+) charged protons and (-) charged electrons. . . The symbol for an electron is e -1 . 2. Neutral atoms contain equal #s of protons and electrons. 3. Atoms can lose electrons to become (+) charged ions (cations). 4. Atoms can gain electrons to become (-) charged ions (anions) . 16
Electron/Proton Model of Atoms Cation Formation Anion Formation Na +1 from Na O -2 from O Na → Na +1 + e -1 O + 2 e -1 → O -2 Na Loses an electron to become O gains two electrons to become O -2 Na +1 Na +1 O 2- Na O + 2 e -1 + e -1 + 11 + 11 + 8 + 8 - 11 - 10 - 8 - 10 17
Electric Properties of Solutions Soluble ionic solids dissolve in water to form solutions that conduct electricity. . . These are called electrolytic solutions. Ionic solids form mobile ions in solution that can conduct electric current. Molecular substances do not dissolve in water to conduct electricity because they do not form ions. 18
Ionic Vs. Molecular Solids Remember that ionic substances often contain metal cations and nonmetallic anions . Molecular substances are mostly made of nonmetals . Consult your ion sheet to help you identify ions. Decide which substances below are ionic or molecular: a) KNO 3 b) CO 2 c) CuSO 4 d) H 2 O a) ionic b) molecular c) ionic d) molecular 19
The Dissolution Process Soluble ionic substances dissolve in water to give aqueous ions: e.g. → Na +1 (aq) + Cl -1 (aq) NaCl (s) → Ba +2 (aq) + 2 Br -1 (aq) BaBr 2 (s) → 2 Al +3 (aq) + 3 SO 4 -2 (aq) Al 2 (SO 4 ) 3 (s) http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/thermochem/solutionSalt.html 20
Precipitation Reactions Sometimes when two solutions are mixed, a solid (precipitate) is formed. The precipitate is an ionic substance that is insoluble in water. e.g. NaCl (aq) + AgNO 3 (aq) → NaNO 3 (aq) + AgCl (s) Silver Chloride is a precipitate. It forms a solid and sinks to the bottom of the test tube or beaker. 21
Ionic and Net Ionic Equations I Ionic equations represent soluble compounds as ions and insoluble compounds as molecules. Net Ionic equations cancel out spectator ions. Molecular: NaCl (aq) + AgNO 3 (aq) → NaNO 3 (aq) + AgCl (s) Ionic: Na +1 (aq) + Cl -1 (aq) + Ag +1 (aq) + NO 3 -1 (aq ) → Na +1 (aq) + NO 3 -1 (aq) + AgCl (s) Net Ionic: Na +1 (aq) + Cl -1 (aq) + Ag +1 (aq) + NO 3 -1 (aq ) → Na +1 (aq) + NO 3 -1 (aq) + AgCl (s) 22
Ionic and Net Ionic Equations II Write the following molecular equation into ionic and net ionic equations: Pb(CH 3 COO) 2 (aq) + 2 NaOH (aq) → Pb(OH) 2 (s) + 2 NaCH 3 COO (aq) Ionic: (all ions aqueous) Pb 2+ + 2 CH 3 COO -1 + 2 Na +1 + 2 OH -1 → Pb(OH) 2 (s) + 2 Na +1 + 2 CH 3 COO -1 Net Ionic: (all ions aqueous) Pb 2+ + 2 CH 3 COO -1 + 2 Na +1 + 2 OH -1 → Pb(OH) 2 (s) + 2 Na +1 + 2 CH 3 COO -1 23
Solubility of Ionic Compounds A table of solubilities can be used to predict which ionic compounds will not be soluble in water . . . i.e. which compounds will form precipitates. To use the table, identify the anion and cation and check to see if the combination is soluble or insoluble. 24
Solubilities Use the table of solubilities to find if the following are soluble: a) Na 2 CO 3 b) PbSO 3 c) AgCH 3 COO d) Ba(OH) 2 a) Soluble b) Insoluble c) Insoluble d) Soluble 25
Colligative Properties Adding solutes to solvents affects the properties of solvents as follows: Property Effect Vapor Pressure Decreases Boiling Temp Increases Freezing Temp Decreases 26
Vapor Pressure Non volatile solutes lower the vapor pressure, because they occupy reduce the number of solvent molecules on the surface of the solution and sometimes have attractions for solvent molecules. http://www.chem.purdue.edu/gchelp/solutions/colligv.html 27
Boiling Temperature Solutes lower the vapor pressure of solvents, thus raising the boiling temperature. 28
Freezing Temperature Solutes interfere with solvent molecules aligning together to form solids. The freezing temperature decreases. 29
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