Properties of Solutions - Chapter 11 - Chemistry Lecture PDF

Chapter 11 Lecture I Properties of Solutions Dr. Kenesha Wilson  1. Types of Solutions 2. Factors Affecting Solubility Chapt 3. Energies of Solution Formation er 4. Solution Composition Outlin (concentration unit) e 5. Colligative...

Chapter 11 Lecture I Properties of Solutions Dr. Kenesha Wilson  1. Types of Solutions 2. Factors Affecting Solubility Chapt 3. Energies of Solution Formation er 4. Solution Composition Outlin (concentration unit) e 5. Colligative Properties Vapor Pressure Lowering Boiling Point Elevation Freezing Point Depression Osmostic Pressure  Solutions Types of Solutions Factors Affecting Solubility Energies of Solution Formation  Definitions A solution is a homogeneous mixture A solute is dissolved in a solvent. solute is the substance being dissolved solvent is the liquid in which the solute is dissolved an aqueous solution has water as solvent  Concept Check Coke lists as its ingredients as: “carbonated water, high fructose corn syrup and/or sucrose, caramel color, phosphoric acid, natural flavors, caffeine”. What is the solvent? What are the solutes? IPC-Solutions-Borders  Dissolution vs Reaction dry Ni(s) + HCl(aq) NiCl2(aq) + H2(g) NiCl2(s) Dissolution is a physical change—you can get back the original solute by evaporating the solvent. If you can’t, the substance didn’t dissolve, it IPC-Solutions-Borders reacted.  Solubility Solubility is a measure of how much of a substance (the solute) can dissolve in a given amount of another substance (the solvent) to form a homogeneous solution at a specific temperature and pressure. It is typically expressed as the maximum amount of solute that can dissolve in a given quantity of solvent. Unit of solubility is grams of solute per 100 mL of solvent under the given conditions. g/100mL  Sparingly soluble: < 0.1 g /100 mL Solubili Slightly ty in water soluble: 0.1 – 1 g /100 mL Soluble: > 1 g /100 mL  Solubility of some common compounds Table salt (NaCl): 35.7g/100 mL water (25 oC) Table sugar (sucrose): 211g /100 mL water (25 oC) Aspirin (acetylsalicylic acid): 0.33g/100 mL water (25 oC) CO2 : 0.145 g/ 100 mL water (25 oC) Various Types of Solutions  Saturated Solution A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature. A saturated solution represents an equilibrium: the rate of dissolving is equal to the rate of crystallization. The salt continues to dissolve but crystallizes at the same rate so that there “appears” to be nothing happening.  Unsaturated Solution An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature  Supersaturated Solution A solution of a chemical compound in a liquid will become supersaturated when the temperature of the saturated solution is changed. In most cases , solubility decreases with decreasing temperature; in such cases, the excess of solute will rapidly separate from the solution as crystals or an amorphous powder. In some cases (e.g. sodium acetate, CH₃COONa) , crystals do not form quickly, and the solution remains supersaturated after cooling due to a thermodynamic barrier to the formation of a crystal in a liquid medium. Supersaturated Solutions The recrystallization of the excess dissolved solute in a supersaturated solution can be started by the addition of a tiny crystal of solute, called a seed crystal. The seed crystal provides a site on which the excess dissolved crystals can begin to grow.  Solubility Terms Concept Check What term is given to a Which of the following is a substance that can dissolve typical unit of solubility? in a particular liquid? a) g/dm3 a) Solvable b) g/100 cm3 b) Soluble c) mol/dm3 c) Insoluble In a solution, what term is What term is given to the given to the solid that has liquid in a solution? dissolved? a) Solvent a) Solvable b) Reagent b) Soluble c) Solute c) Insoluble  Miscible vs. Immiscible These terms describe the ability of a liquid to form a solution in a given solvent.  Miscible vs. Immiscible Alcohol is Oil is miscible in immiscible water. in water.  Soluble vs. Insoluble These terms describe the ability of a solid to form a solution in a given solvent. Salt is highly CuCl2 is CuCl2 is Chalk is soluble in insoluble in soluble in insoluble in water. water. water. toluene.  Concept Check Define the following terms: o Soluble o Insoluble o Miscible o Immiscible  Solutions How does a solid dissolve into a liquid? What ‘drives’ the dissolution process? What are the energetics of dissolution? Dissolution of NaCl in H2O Video  The Energies of Solution Formation What factors affect solubility? Generally, the rule of solubility is like dissolves like. That is a polar solvent is used to dissolve a polar or ionic solute. A non-polar solvent is used to dissolve a nonpolar solute. To simplify the discussion on how solutions are formed, it will be assumed that the formation of a liquid solution takes place in three distinct steps.  The Energies of Solution Formation The three steps are: Separating the solute into its individual components (expanding the solute). Overcoming intermolecular forces in the solvent to make room for the solute (expanding the solvent). Allowing the solute and solvent to interact to form the solution.  Enthalpy Enthalpy is a measure of the heat energy involved in a chemical reaction when the process occurs at constant pressure. It represents the energy change as heat is absorbed or released during the reaction. Symbol: Enthalpy is represented by the letter H. Units: joules (J) or kilojoules (kJ). Change in Enthalpy (ΔH): When a reaction releases heat, ΔH is negative (exothermic reaction). When a reaction absorbs heat, ΔH is positive (endothermic reaction). Steps in the Dissolvi ng Process  Steps in the Dissolving Process Steps 1 (∆H1) and 2 (∆H2) require energy, since forces must be overcome to expand the solute and solvent. Step 3 (∆H3) usually releases energy. Steps 1 and 2 are endothermic, and step 3 is often exothermic. 26  Energy Changes for Solution Formation Separation of Solute must overcome intermolecular forces or ion-ion attractions in solute requires energy, ENDOTHERMIC ( + DH) Separation of Solvent must overcome intermolecular forces of solvent particles requires energy, ENDOTHERMIC (+ DH) Interaction of Solute & Solvent attractive bonds form between solute particles and solvent particles “Solvation” or “Hydration” (where water = solvent) releases energy, EXOTHERMIC (- DH) DHsoln = DH1 + DH2 + DH3  Steps in the Dissolving Process (a) ∆Hsoln has a negative (b) ∆Hsoln has a positive sign (the process is sign (the process is exothermic) if step 3 endothermic) if steps 1 releases more energy than and 2 require more energy that required by steps 1 and 2. than is released in step 3. If the energy changes for steps 1 and 2 equal that for step 3, then ∆Hsoln is zero.  How to calculate ∆Hsoln DHsoln = DH1 + DH2 + DH3 Consider the formation of NaCl (aq) DH1 = 786 kJ/mol (DHhyd = DH2 + DH3) = -783 kJ/mol The enthalpy (heat) of hydration (DHhyd) combines the terms DH2 (for expanding the solvent) and DH3 (for solvent–solute interactions). DHsoln = 786 kJ/mol + (-783 KJ/mol) = 3 kJ/mol Enthalpy Changes During Dissolution DHsoln = DH1 + DH2 + DH3 The enthalpy of solution, DHsoln, can be either positive or negative. DHsoln (MgSO4)= -91.2 kJ/mol (exothermic) DHsoln (NH4NO3)= 26.4 kJ/mol (endothermic) Intermolecular Forces and Dissolution Two substances with similar intermolecular forces are likely to be soluble in each other. The stronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve Intermolecular forces = H-bonds; dipole-dipole; dispersion Ions in water also have ion-dipole forces. Examples of intermolecular forces  Examples of intermolecular forces Factors Affecting Solubility 1. Nature of solute and solvent Like Dissolves Like 2. Temperature & Solubility Solids Gases 3. Pressure and Gas Solubility Nature of Solute and Solvent Chemists use the axiom “like dissolves like”:  Polar substances tend to dissolve in polar solvents.  Ex. C2H5OH in H2O Nature of Solute and Solvent Non-polar substances tend to dissolve in non- polar solvents.  Ex. oil in C6H6 Ionic compounds are more soluble in polar solvents  Ex. NaCl in H2O or NH3 (l)  Solid Solubility Vs. Temperature Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature  Temperature & the Solubility of Gases The solubility of gases DECREASES at higher temperatures  Concept Check 1. What relationship exists between solubility and temperature for most of the substances shown? 2. Which compound's solubility does not increase with an increase in temperature? 3. Approximately how many grams of NaNO3 will dissolve in 100 g water at 10 °C?______ 4. How many grams of KCl will dissolve in 1 liter (1000 mL) of H2O at 50 °C? 5. 100 g of water are used to make a saturated solution of KCl at 15 °C. How many more grams of KCl could be dissolved if the temperature were raised to 78 °C? Socrative activity  Pressure and Gas Solubility The solubility of liquids and solids does not change appreciably with pressure. However, the solubility of a gas in a liquid is directly proportional to its pressure. Increasing pressure above solution forces more gas to dissolve.  Henry’s Law The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry’s law). C= kP c is the concentration (M) of the dissolved gas P is the pressure of the gas over the solution k is a constant (mol/L atm) that depends only on temperature  Henry’s Law low P high P low c high c  Henry’s Law At pressure of few C = solubility of the gas (mole/L) atmosphere or less, k = Henry’s Law Constant solubility of gas solute P = partial pressure of gas follows Henry Law. The amount of solute gas dissolved in Henry’s Law Constants solution is directly (25°C), k proportional to the N2 6.1×10−4M/atm amount of pressure above the solution. O2 1.3×10−3M/atm C=kP CO2 3.1×10−2 M/atm  Solubility & Temp. Concept Check What will happen to a hot saturated solution of What happens to the solubility of a gas as copper(II) sulphate as the temperature the pressure increases? decreases? a) Solubility increases a) The solvent will evaporate b) Solubility decreases b) Crystals will form c) Solubility stays the same c) It will turn colourless Which has more carbon dioxide dissolved in it? For most gases, what happens to the a) Warm lemonade solubility as the temperature increases? b) Cool lemonade a) Solubility increases c) Cold lemonade b) Solubility decreases c) Solubility stays the same Why is it important to quote solubility at a given temperature? For most solid solutes, what happens to the a) Some substances only dissolve at low temperatures solubility as the temperature increases? b) Some substances only dissolve at high a) Solubility increases temperatures c) Solubility depends on temperature b) Solubility decreases c) Solubility stays the same Nearpod review  Sample Exercise Calculate the concentration of CO2 in a soft drink that is bottled with a partial pressure of CO2 of 4.0 atm over the liquid at 25°C. The Henry’s law constant for CO2 in water at this temperature is 3.1  10–2 mol/L-atm. Solution Analyze: We are given the partial pressure of CO2, and the Henry’s law constant, k, and asked to calculate the concentration of CO2 in the solution. Plan: With the information given, we can use Henry’s law to calculate the solubility. Answer  Blood gases and deep-sea diving Solubility of gases increases as pressure increases. Divers who use compressed gases must be concerned about the solubility of the gases in their blood At depth, the blood contains higher concentrations of dissolved gases Ascension, if too rapid, will cause the blood to fizz similar to 7-UP when opened! This is called decompression sickness, or “the bends”, which is painful and can be fatal because the bubbles affect things like nerve impulses  Solubility of Common Compounds Solubility NO32- salts Na+, K+, NH4+ salts Soluble Compounds Except for those Cl , Br , I - - - containing: Ag+, Hg22+, Pb2+ Except for this SO42- salts containing: Ba2+, Pb2+, Ca2+ Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 218  Solubility of Common Compounds Insoluble S2-, CO32-, PO43- salts Except for those OH- salts containing: Na+, K+, Ca2+ Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 218 End of class Questions?