Experiment 1: Freezing Point Depression

Transcription

1 Experiment 1: Freezing Pint Depressin Purpse: The purpse f this lab is t experimentally determine the freezing pint f tw slutins and cmpare the effect f slute type and cncentratin fr each slutin. Intrductin: Slutins are hmgeneus mixtures that cntain tw r mre substances. The majr cmpnent is called the slvent, and the minr cmpnent(s) are called the slute(s). Since the slutin is primarily cmpsed f slvent, the physical prperties f the slutin resemble thse f the pure slvent. Hwever, sme f these physical prperties, called clligative prperties, are independent f the nature f the slute and depend nly upn the cncentratin f slute particles. Examples f clligative prperties include biling pint elevatin, freezing pint depressin, and smtic pressure. Pure water freezes at 0.0 C (273 K), bils at C (373 K). These values are altered by the presence f a slute. Yu are prbably familiar with sme cmmn examples f these effects: Antifreeze is used t lwer the freezing pint and raise the biling pint f yur engine clant (water); and salt is used t melt ice n rad. These effects are expressed quantitatively by the clligative prperty law, which states that the freezing pint and biling pint f a slutin differ frm thse f the pure slvent by amunts that are directly prprtinal t the mlal cncentratin f the slute. In its general frm this equatin is written: ΔT = K m where ΔT is the biling pint elevatin r freezing pint depressin, K is a cnstant that is specific fr each slvent, and m is the mlality f the slutin and is expressed as the number f mles f slute per kilgram f slvent. Sme representative cnstants, biling pints, and freezing pints are given in the table belw. Slvent Freezing Pint ( C) K f ( C/m) Biling Pint ( C) K b ( C/m) CH 3 CO 2 H (acetic acid) C 6 H 6 (benzene) CHCl 3 (chlrfrm) C 2 H 5 OH (ethyl alchl) H 2 O (water) C 10 H 8 (naphthalene) C 6 H 12 (cyclhexane) C 10 H 16 O (camphr) C 4 H 10 O (t butanl) Page 1 f 9

2 The equatins relating the biling pint elevatin and freezing pint depressin t mlality are: ΔT b = K b m i ΔT f = K f m i where ΔT b and ΔT f are the change in biling pint and freezing pint respectively, K b and K f are the biling and freezing cnstants fr a particular slvent, and i is the factr which is used t accunt fr the number f particles a given slute generates in slutin. Substances are usually classified as inic r cvalent, r mre precisely, electrlytes and nn electrlytes. Electrlytes disslve in a slvent t give ins (charged particles) in slutin while nn electrlytes disslve t give mlecules. A mle f nn electrlyte such as ethanl wuld disslve t prduce a mle f ethanl mlecules. Hwever, a mle f sdium chlride (NaCl) wuld disslve t frm tw mles f ins (Na + and Cl ). Because clligative prperties are related t the number f slute particles, we wuld expect a mle f NaCl t have twice the effect as a mle f ethanl. Therefre, (i =1) fr ethanl and ther nn electrlytes, (i =2) fr NaCl, and (i = 5) fr Al 2 (CO 3 ) 3. Prelab Exercise: If 10g f ethanl, C 2 H 5 OH, is disslved in 100 g f pure water, what wuld the biling pint and freezing pint be fr this slutin? First determine the mlality f the slutin: Fr water the K b = C/m, and K f = 1.86 C/m. Calculate T b and T f fr this slutin: Page 2 f 9

3 Methd: In this experiment yu will need t determine the freezing pint depressin fr several slutins. Hwever, the temperature at which a slutin freezes is difficult t determine by direct visual bservatin because f a phenmenn called super cling and als because slidificatin f slutins usually ccurs ver a brad temperature range. T vercme these prblems, temperature time graphs, called cling curves, are used which readily reveal the true freezing temperature. The majr features f a general cling curve are shwn belw: Yu will cnstruct cling curves fr bth the pure slvent (water) and varius slutins. Frm the cling curve yu will extraplate the true freezing temperature. Extraplatin is necessary because as the slutin freezes the slid that is frmed is essentially pure slvent. The remaining slutin becmes mre and mre cncentrated and its freezing pint becmes lwer and lwer. Als, super cling intrduces an ambiguity in the freezing pint and shuld be minimized using thrugh stirring. Materials: Beaker f ice water Beaker f warm water Thermmeter 25 ml Test Tube Parafilm t butyl alchl Benzic acid Ethylene glycl Page 3 f 9

4 Prcedure: Freezing Pint Depressin fr t Butanl: 1. Fill a large test tube apprximately ¼ way with t butanl. 2. Place the test tube, and a thermmeter int the water bath s that the level f the t butanl in the test tube is belw the level f the water in the beaker. Make sure yu can still read the thermmeter. Cver the tp f the tube with a small piece f parafilm 3. Cntinue heating the test tube until all its cntent melts. 4. Remve the water bath and allw the test tube t cl in air. 5. Recrd the temperature f the test tube every 30 secnds while gently agitating the cntents with the thermmeter. 6. Cntinue t recrd the temperature fr 15 minutes, even after the t butanl slidifies. Yu will have t cntinually cl the water bath with ice in rder t freeze the sample. 7. Repeat steps 2 6 t btain data fr a secnd trial. 8. Cnsult the instructr t determine if the trials are successful, r if yu shuld perfrm a third trial. 9. Once the trials are cmpleted, melt the cntents f the test tube and discard int waste cntainer in the hd Freezing Pint Depressin f Other Slutins: 1. Similar t the directins abve, determine the freezing pint f the ther slutins (0.5 m ethylene glycl, 1 m ethylene glycl and 0.5 m benzic acid). 2. Place all used slutin samples in the waste cntainer. Page 4 f 9

5 DATA: Pure t Butanl # Time 1 ST Trial 2 nd Trial Temp ( C) Temp ( C) # Time 1 0:00 1 0:00 2 0:30 2 0:30 3 1:00 3 1:00 4 1:30 4 1:30 5 2:00 5 2:00 6 2:30 6 2:30 7 3:00 7 3:00 8 3:30 8 3:30 9 4:00 9 4: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :00 t Butanl with Ethylene Glycl (0.5 m) 1 ST Trial Temp ( C) 2 nd Trial Temp ( C) Page 5 f 9

6 t Butanl with Ethylene Glycl (1 m) t Butanl with Benzic Acid (0.5 m) # Time 1 ST Trial 2 nd Trial 1 ST Trial 2 nd Trial # Time Temp ( C) Temp ( C) Temp ( C) Temp ( C) 1 0:00 1 0:00 2 0:30 2 0:30 3 1:00 3 1:00 4 1:30 4 1:30 5 2:00 5 2:00 6 2:30 6 2:30 7 3:00 7 3:00 8 3:30 8 3:30 9 4:00 9 4: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :00 Page 6 f 9

7 Calculatins: Graph each trial data and btain the freezing pints fr each slutin: The freezing pint f pure t butanl: Trial 1.. Trial 2.. Average. The freezing pint f the 0.5 m t butanl and ethylene glycl slutin: Trial 1.. Trial 2.. Average. DT f (1) K f (exp.). (shw calculatins belw) The freezing pint f the 1 m t butanl and ethylene glycl slutin: Trial 1.. Trial 2.. Average. DT f (2). K f (exp.). (shw calculatins belw) Page 7 f 9

8 Calculatins (cnt d): The freezing pint f the 0.5 m t butanl and benzic acid slutin: Trial 1.. Trial 2.. Average. DT f (3) K f (exp.). (shw calculatins belw) Average experimental value fr K f. Theretical value fr K f Percent Errr. (Shw calculatins belw) Page 8 f 9

9 Questins: 1. Hw did the freezing pint f pure t butanl cmpare with the freezing pints f the slutins? Supprt yur bservatins with theretical cncepts. 2. Hw did the freezing pints f 0.5 m and 1 m slutins f ethylene glycl cmpare with ne anther? Supprt yur answers with theretical cncepts. 3. Hw did the freezing pints f slutins with different slutes (ethylene glycl and benzic acid) cmpare t ne anther? Supprt yur bservatins with theretical cncepts. Page 9 f 9