SOLUTIONS Definitions solvent bulk material used to dissolve substance solute material dissolved in solvent

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1 1 SLUTINS Definitions solvent bulk material used to dissolve substance solute material dissolved in solvent Examples 1) salt water solvent water solute salt 2) dish water solvent water solute dish soap 3) engine coolant solvent ethylene glycol solute water Types of solutions Gas in Gas Gas in Liquid Liquid in Liquid Solid in Liquid Solid in Solid xygen in Nitrogen arbon dioxide in Water (soda pop) il in Gasoline (two-stroke engine fuel) Salt water, Nail polish in nail polish remover Brass (Zinc in opper) Miscibility Miscible - when a pair of liquids or a pair of solids mix together in all proportions - water and ethanol (grain alcohol) are miscible. - copper and tin are miscible. (and make bronze) Immiscible - when pairs of liquids do not mix together at all - water and oil are immiscible. SLUBILITY - Amount of solute that can be dissolved into a standard amount of solvent Example: At 25, the solubilities of the following compounds are ompound Solubility Nal 5.47 M Sugar 6.00 M Mg() M M M moles of Nal (217 g) can be dissolved in 1 L of solution moles of 2 (1.1 g) can be dissolved in 1 L of solution.

2 2 Some definitions For solid solutes, the process of putting of solute into solution is dissolution or solvation. For solid solutes, the process of a solute coming out of solution is crystallization. Saturated Solution - A solution where no more solute can be put into solution. - Dissolution and crystallization are happening at the same time and at the same rate. Supersaturated Solution - Unstable solution with an excess of solute dissolved. - A small disturbance causes supersaturated solution to crystallize into solid solute and saturated solution. Factors in Solution Formation 1. Intermolecular Forces When attractions between solute molecules and solvent molecules are strong, solutions are able to form. Solute molecules are surrounded by solvent molecules. (solvent cage) + Like dissolves like Molecular solutions involving only molecular substances (no ions) Polar substances dissolve polar substances. - water dissolves acetone, ethanol, etc - primary intermolecular forces involved: hydrogen bonding and dipole-dipole forces Nonpolar substances dissolve nonpolar substances. - gasoline dissolves oil - hexane dissolves benzene - primary intermolecular force involved: dispersion (London) forces

3 3 Ionic (electrolytic) solutions - ionic compounds (salts) dissolve in a polar solvent. - ions in salt dissociate to become solvated ions - primary intermolecular force involved: ion-dipole - ionic compounds dissolve in other solvents other than water such as methanol and ethanol. Like dissolves like also implies that oil and water don t mix. - Water molecules are more attracted to each other than to oil molecules. 2. Enthalpy constant pressure heat (chemical heat) When solutes are dissolved, usually heat is evolved. - i.e., enthalpy of solvation is often exothermic. - Ex: al2 in water - enthalpy of solvation can be positive. - Ex: Most ammonium salts (N4N3) usually have positive enthalpies of solution - instant cold packs. Enthalpy of solvation can be broken into three parts. 1. Energy needed to separate solute molecules. - breaking of ionic or intermolecular bonds. - always positive 2. Energy needed to separate solvent molecules. - breaking of intermolecular bonds - always positive 3. Energy released when solute molecules mix with solvent molecules. - formation of intermolecular bonds - always negative Total enthalpy of solvation may be positive or negative. (usually negative) solvation = solute + solvent + mix Enthalpy of hydration is always negative. hydration = solvent( ) + mix For ionic solutions, enthalpy of hydration depends on the charge density of the ion. Q V charge density Q charge V volume

4 4 Smaller ions have smaller volumes thus larger charge densities and larger enthalpies of hydration. Ion Ionic Radius (pm) hydration (kj/mol) Li Na K Mg a Sr F l Br Larger enthalpies of hydration favor solution formation. 3. Entropy - Entropy is disorder - A system with more disorder has more entropy. - A messy bedroom has more entropy than a clean bedroom. - A pyramid of stacked oranges has less entropy than a pile of oranges. - More precisely, a system of low entropy has a low number of arrangements. igh entropy systems have lots of possible arrangements. **Systems naturally tend to change from order to disorder. (Entropy of the universe always increases.)** Increase of entropy is a contributing factor as to why molecular solutions form. onsider dissolving sugar in water - Sugar crystal is very ordered. (low entropy) - Sugar molecules in water are much less ordered. (higher entropy) onsider blue dye in water - Molecules in a drop of blue dye are relatively ordered. - Entropy increases as dye spreads throughout solution. - Dye molecules will never return to a single drop. An increase in entropy can compensate for a positive enthalpy of solvation. Ionic solutions may have negative entropy changes since the solvent may become more organized around the ions. (If the entropy change is negative, the enthalpy will be very exothermic.)

5 5 TEMPERATURE AND PRESSURE EFFETS N SLUBILITY Temperature - Most often higher temperature increases solubility of solid in liquids. - Enthalpy and entropy are both important. - Solubility of sugar is substantially increased in boiling water (as in making rock candy). - Most often higher temperature decreases solubility of gas in liquids. - Increased motion of solvent knocks-out gas molecules. - Thermal pollution: eated river water from industrial plants decreases solubility of 2, harming aquatic life). External Gas Pressure - high external air pressures increase solubility of gas in liquid - enry s Law - The solubility of a gas in a liquid is proportional to the external partial pressure. s k P s solubility P pressure k enry s law constant - important for applications in deep sea diving, anesthesia, beverage carbonation, etc F F l Example: Isoflurane, 32F5l, is an inhalation anesthetic used in human and veterinary surgeries. If the enry s law constant for isoflurane in F blood is M/atm at 35 and the isoflurane partial pressure is 55 torr, calculate the solubility of isoflurane in blood at 35. F F M 1atm s 55 torr M atm 760 torr NENTRATIN AND NENTRATIN UNITS oncentration is how much solute is dissolved in a set amount of solvent. Mass percent mass of component Definition: Mass % 100 % total mass Example: The components of an aspirin tablet (acetylsalicylic acid, caffeine, etc ) can be separated using thin layer chromatography (TL). To use TL, the aspirin must be dissolved in a solvent such as acetone. What is the mass percent of aspirin when 5.02 g of aspirin is dissolved in 231 g of acetone?

6 6 Parts per million (billion) mass of component Definition: ppm 10 total mass ppb mass of component total mass 10 Example: The EPA standard for lead(ii) ion in drinking water is 15 ppb. What is the ppb of Pb 2+ If 2.11 mg of the mineral cerussite (Pb3) is dissolved in 200 kg (1 drum) of water, is the water safe to drink? 9 6 mass 207.2g mol mass 267.2g mol 2 Pb %Pb Pb3 2.11mg 1g 1kg 200 kg 1000 mg 1000g ppb Thus according to the EPA, the water in the drum is safe to drink. Mole Fraction Definition: X A moles of component A total moles Example: Dry cleaning solvents are often nonpolar solvents such as perchloroethylene (perc) 2l4 that can readily dissolve grease. What is the mole fraction of grease, 2042, when 8.9 g of grease is dissolved in 84.0 g of 2l4?

7 7 Molarity (upper case M) moles of solute Definition: Molarity M liters of solution mol L Molality (lower case m) moles of solute Definition: Molality m kg of solvent mol kg - molality is important as a unit of concentration since it is temperature independent. - volume changes with temperature - mass does not change with temperature - for dilute aqueous solutions only, molality is approximately the same as molarity. onversions Mass % X ppm molarity density molality oncentrations are intensive quantities. (They are independent of the amount of solution.) Therefore, when doing concentration conversions, the problem solver is free to choose how much solution with which he wants to work. A good tip to begin a conversion is to consider the denominator of the starting unit and choose a convenient amount.

8 8 Example: heeses such as Limburger or amembert are washed with a brine solution while ripening. Assume that such a solution is 10.4% Nal by mass and that its density is 1.14 g/ml. a) alculate the molality of the solution. Assume 100 g of solution. mass of Nal 10.4 g mass of g 10.4 g = 89.6 g b) alculate the molarity of the solution. Example: oncentrated aqueous ammonia is 14.4 M with a density of g/ml. a) alculate the mass percent of ammonia Assume 1 L of solution. alculate mass of solute (ammonia) alculate mass of solution Mass percent of ammonia is b) alculate the mass percent of water.

9 9 c) alculate the molality Assume 1 L of solution. alculate moles of solute (ammonia) alculate mass of solvent msolvent = msolution msolute msolvent = 900 g 245 g = 655g = kg Molality is mol c 22. 0m kg LLIGATIVE PRPERTIES - Properties that depend only on the number of solute particles and the specific solvent. - olligative properties are independent of the specific nature of the solute. - 4 properties to be examined 1.) Vapor pressure lowering (Raoult s Law) 2.) Boiling point elevation 3.) Freezing point depression 4.) smosis The Special ase of Ionic Solutions Because colligative properties are independent of the specific nature of the solute, no distinction is made between a molecule in solution and an ion in solution. Thus, when taking into account the colligative properties of ionic solutions, the dissociation of the ionic compound must be considered M 0.12 M 0.12 M 0.24 M Nal Na l 0.45M 0.90 M 0.45M 1.35M K 2 2S 4 K S4 particles particles

10 10 VAPR PRESSURE LWERING Vapor Pressure the partial pressure of vapor above its liquid ompound Vapor Pressure (25 ) water atm (23.8 Torr) gasoline atm (80.46 Torr) acetone atm (124.4 Torr) mercury 2.2 x 10-6 atm ( Torr) Every liquid has some fraction of its molecules with enough kinetic energy to break the intermolecular forces binding it to the liquid s surface. Raoult s Law The vapor pressure of solvent is proportional to the mole fraction of the solvent. P x P 0 A A A xa mole fraction PA partial pressure 0 P A partial pressure of pure solvent - Vapor pressure over solution is an equilibrium between evaporation and condensation. - Adding nonvolatile component takes up space of surface of solution. Na + l - l - Na + Na + l - pure water salt water - fewer water molecules have opportunity to escape from surface in salt water - overall fewer molecules go into air; therefore, salt water has lower vapor pressure than pure water

11 11 Example: The vapor pressure of methanol at 27 is torr. alculate the vapor pressure above a 1.1 m al2 in methanol solution. First convert 1.1 molal to mole fraction Assume 1 kg of solvent. alculate moles of solute particles alculate moles of solvent Mole fraction of solvent is Use Raoult s Law BILING PINT ELEVATIN - Addition of solute lowers vapor pressure; therefore, raises boiling point. - hange in boiling point is proportional to concentration of solute molecules or ions. Tb = Kb m m molality - Each solvent has its own Kb. - hange in boiling point temperature is independent of specific solute. It depends only number of solute particles.

12 12 Example: Ethylene glycol, 262, is also known as antifreeze. A 50/50 mixture by volume with water is used as engine coolant. Assuming that the ethylene glycol is nonvolatile, what is the boiling point of the 50/50 mixture? Tb(2) = Kb(2) = /m d(2) = 1.00 g/ml, d(262) = 1.11 g/ml. We need to find the molality of the mixture. Assume 50 ml of 2 and 50 ml of 262 alculate the moles of ethylene glycol. alculate the kilograms of water. alculate the molality and the boiling point elevation. Note: The cooling system in a car quickly becomes pressurized as the engine heats. The high pressure makes the actual boiling point considerably higher. FREEZING PINT DEPRESSIN - Addition of solute disrupts formation of molecular (or ionic) lattice. - hange in freezing point is proportional to concentration of solute particles. Tf = Kf m - Each solvent has its own Kf. - hange in freezing point temperature is independent of specific solute. It depends only number of solute particles. Example: Putting salt on roads makes salt water, which has lower freezing point. Salt only works when temperature is above 0 F. (0 F is freezing point of saturated salt solution.)

13 13 SMSIS Semipermeable Membrane - membrane which allows only certain substances to flow through - in biology, cell membranes allow transport of water but not ions, proteins, biomolecules, etc smosis occurs when solvent flows across membrane from high concentration to low concentration. **areful! We are considering solvent concentration, not solute concentration.** l Na salt water (low water conc.) 2 pure water (high water conc.) smotic Pressure - - The desire of solvent to flow through membrane creates pressure. smosis salt water pure water not as salty pure water water height in column shows that pure water is pushing on it demonstrating osmotic pressure - - smotic pressure is proportional to concentration of solute particles. n Latm V nrt RT crt R V mol K c concentration (molarity) of solute particles Example: Seawater has an average salinity of 35 ppt or 0.6 M Nal. What is the osmotic pressure of seawater against a semipermeable membrane with pure water at 25?

14 14 Reverse smosis Desalination (removing salt from water) can be done using the process known as reverse osmosis. Salt water is placed on one side of a semipermeable membrane and an external pressure is applied to overcome the osmotic pressure against the salt water. Using osmosis to measure molar mass. smotic pressure is a common way to measure the molar masses of large molecules such as DNA, proteins, polymers, etc With the right information, the osmotic pressure equation can be used to calculate number of moles of solute molecules. Then the mass of the molecules is divided by the number of moles to yield the molar mass. Example: 4.02 mg of human insulin is dissolved in 10.7 ml of water. The osmotic pressure of the solution is 1.25 torr at 37. alculate the molar mass of the protein. alculate concentration of solute alculate moles of solute alculate molar mass

15 15 LLIDS Distinction between colloids and solutions - colloidal particles (including solvent cage) range in size from 5 nm to 1 m. - solution particles (including solvent cage) are less than 5 nm (typically 1 nm in size). - colloidal particles are dispersed; whereas, solution particles are dissolved. - colloids reflect a light beam whereas solutions do not. (Tyndall effect) - colloids can be coagulated whereas solutions can be precipitated. Soaps and Detergents - Soaps have a natural source from fats - Fats undergo saponification to make soap when mixed with a strong base (such as Na) Na 3 3 Na + 22 Products are sodium laurate (a soap) and glycerine. - Detergents are synthetic and often involve a sulfate group. - Soap and detergent molecules have a dual nature. They have a polar head and a nonpolar tail. - Polar head is dissolved by water. - Nonpolar tail dissolves oil, grease, etc sodium dodecyl sulfate (SDS) 3 S - Na + Nonpolar tail Polar head

16 16 - Individual soap molecules congregate around a grease particle to form a micelle. grease - Soap also lowers the surface tension of water by interrupting the hydrogen bonding network of water. - Soap micelles in water are not dissolved in water but are dispersed as a colloid. Emulsions - Emulsions are colloids where a liquid is dispersed in another liquid. - Examples of emulsions 1. Milk 2. Butter 3. Mayonnaise 4. Salad dressings - Formation of emulsions is often aided with the addition of an emulsifying agent. - In mayonnaise, the oil is dispersed in water with help of egg protein that is used as an emulsifying agent.

17 17 IDEAL AND NNIDEAL SLUTINS Solutions that follow Raoult s Law are labeled as ideal solutions. Implicit in Raoult s Law are the following assumptions for an ideal solution. - The solute and solvent molecules have the same size. - The solute solvent interactions are the same as the solvent solvent interactions. Graphically Raoult s Law can be illustrated as - Note that the partial pressure is directly proportional to the mole fraction. - Note also that when the mole fraction equals one, the partial pressure of the gas above the solution is partial pressure of the pure solvent. P A P A 0 0 X A 1 Now consider the possibility that the solute solvent interactions are stronger than the solvent solvent interactions. - The solvent molecules pull on the solute molecules making the solvent less likely to go into the vapor phase. - This nonideal solution is labeled as having a negative deviation from ideality. P A P A 0 0 X A 1 Now consider the possibility that the solute solvent interactions are weaker than the solvent solvent interactions. - The solvent molecules have less pull on the solute molecules making the solvent more likely to go into the vapor phase. - This nonideal solution is labeled as having a positive deviation from ideality. P A P A 0 0 X A 1