Gen Chem Experiment 3: Fractional Crystallization

difference between precipitation and crystallization

both terms refer to formation of solid from a supersaturated soln. term precipitation used when formation of the solid happens rapidly and a finely divided solid (precipitate) is produced. precipitate doesn't appear crystalline to the naked eye, actually consists of tiny crystals term crystallization used when formation of the solid happens more slowly allowing larger crystals to form

fractional crystallization

important method to purify substances and separate mixtures based on differences in solubility.

calculations

in high-temp oven, waters of hydration in CuSO4•5H2O are partially removed. new hydrate has formula CuSO4•XH2O, where X is a positive number smaller than 5. how do you find X? mass of sample before heating in high-temp oven = mass of copper(II) sulfate pentahydrate= mCuSO4•5H2O mass of sample after heating in high-temp oven= mass of new hydrate = mCuSO4•XH2O using both masses and molar masses of CuSO4 and H2O, calculate following: 1. mass of water removed in high-temp oven, mH2O removed 2. moles of water removed, nH2O removed 3. moles of copper(II) sulfate pentahydrate, nCuSO4•5H2O 4. moles of copper(II) sulfate, nCuSO4 5. initial moles of water in copper (II) pentahydrate, nH2O initial 6. moles of water remaining after heating sample in high-temp oven, nH2O remaining 7. X= nH2O remaining/nCuSO4

purpose

in parts I and II, mixture of copper (II) sulfate pentahydrate, CuSO4•5H2O, and salicylic acid, C7H6O3, separated using fractional crystallization. in part III, recovered CuSO4•5H2O heated at 130ºC to form new hydrate, CuSO4•XH2O, X is determined

procedure part I: recovery of salicylic acid

1. obtain test tube w/ unknown mixture 2. transfer mixture to tared 150 mL beaker and weight it. it should be 1.5g. record mass accurately 3. add 50 mL of DI water and 10 drops of 3M sulfuric acid (H2SO4). acid is added to prevent any rxns between the copper(II) sulfate and the salicylic acid 4. place beaker on hot plate and heat mixture under constant stirring until all of solid material is dissolved. avoid boiling the soln 5. remove beaker from hot plate (use hot hands or mittens), cover w/ watch glass (concave side up) and allow to cool down to r.t. 6. place beaker in an ice-water bath and allow the salicylic acid to crystallize out of soln (~10 min). notice rapid cooling by placing hot beaker in ice-water bath will result in formation of very small crystals that may absorb impurities from soln 7. set up vacuum filtration apparatus. place a pre-weighed filter paper into the Buchner funnel and start the aspirator. wet filter paper. this causes paper to adhere to the funnel and keeps materials from passing under the paper during filtration. press funnel gently onto flask to improve vacuum 8. slowly pour contents of the beaker to center of the funnel. use no more than 10 mL of DI water to rinse beaker 9. when crystals appear dry and funnel stopped dripping, carefully remove rubber hose from the flask before closing the faucet. if water pressure in laboratory line drops suddenly, pressure in the filter flask may become less than that in the aspirator resulting in water to be drawn into the flask 10. transfer filtrate to clean 150-mL beaker 11. reassemble vacuum filtration apparatus, start aspirator and allow crystals to remain exposed to vacuum for 10 min in order to dry them 12. turn off aspirator and transfer filter paper w/ crystals to pre-weighed watch glass and let crystals air dry for a while. obtain mass of air-dried crystals

procedure part II: recovery of copper (II) sulfate pentahydrate

1. place beaker w/ filtrate on a hot plate. add some boiling chips (usually made of porous material that provides nucleation sites for boiling process allowing a liquid to boil calmly w/out superheating and bumping) and a stirring rod for controlled boiling. reduce volume of the soln by boiling to about 20 mL 2. carefully remove beaker from heat and remove boiling chips 3. slowly add 20 mL of ethanol while constantly stirring 4. cool beaker to r.t. and place it in ice-water bath to complete the crystallization (~10 min) 5. as in part I, filter soln. this time, use no more than 10 mL of ethanol to transfer the crystals. allow the crystals to remain exposed to vacuum for 10 min in order to dry them 6. using a permanent marker (no tape!) write lab bench number on a watch glass, weigh it, and place filter paper w/ crystals on it 7. dry crystals in "low-temp oven" at 90ºC for 15 min. allow sample to cool down before weighing it. although products of your fractional crystallization may appear pure, analysis would show that salicylic acid is contaminated by trace amounts of CuSO4•5H2O and vice versa. repeating dissolution and crystallization procedure would result in purer products

procedure part III: partial removal of waters of hydration

1. place watch glass w/ dried CuSO4•5H2O crystals in the "high-temp oven" at 130ºC for 15 min. at that temperature, approx 4/5 water molecules can be removed. a higher temp would be required to remove the last water molecule. understand difference between moisture and water of hydration. water of hydration is incorporated in the crystalline framework in a particular stoichiometric ratio (it's part of the compound), while moisture is due to water molecules absorbed to the surface of a substance 2. allow sample to cool down before weighing it