Liquids and Solids

1. Liquids and Solids Use the Force Luke!

2. Learning objectives • Describe origins of surface tension and meniscus • Describe different types of cohesive force • Identify type of cohesive force based on molecular formula • Describe origin of hydrogen bonding • Explain unique properties of water • Determine concentrations of solutions using the common concentration scales

3. Liquids and solids exist because of forces • Cohesive forces are attractive forces between molecules of the same substance • Adhesive forces are attractive forces between molecules of different substances

4. Surface tension results from cohesive forces • Molecules on surface are drawn inwards • Tendency to form sphere • Surface behaves like a shrink-wrap film • Liquids form into spherical drops • Denser objects can “float” on the surface tension

5. Consequences of surface tension: What will these hands ne’er be clean? • Cleaning requires wetting • Surface tension prevents wetting • Beading on a waxed car • Detergent acts as go-between

6. Cohesive forces and meniscus • Adhesive forces pull H2O molecules to maximize coverage • Cohesive forces between H2O molecules drag liquid up • Gravity pushes liquid down

7. Solid: strong interactions • Fixed shape • Not compressible • Rigid • Dense

8. Solids • Atoms in solids rotate and vibrate but do not translate • Melting occurs when the translational energy of the atoms is sufficient to break free of the lattice • Usually this is a very well defined point • With amorphous solids it can be smeared out – softening of fats

9. Liquid: medium interactions • Liquid • Not rigid • Assumes shape of container • Not compressible • Dense

10. Vapour pressure and boiling • Molecules do not all have the same energy • Evaporation: High energy molecules escape the liquid – vapour pressure • When vapour pressure = atmospheric pressure boiling occurs – all liquid becomes gas • Sublimation is direct transition of solid to gas (dry ice)

11. The Four Forces of the Apocalypse

12. Dispersion force • Arises from fluctuations in electron clouds in atoms • Only force present in nonpolar molecules • Increases with size of atoms/molecules

13. Dipole-dipole force • Usually stronger than dispersion forces • Present in polar molecules • Polar substances higher boiling point than nonpolar substances

14. Polar or nonpolar: that is the question? • Polar molecules must contain polar bonds • Determine bond polarity from electronegativity • The polar bonds must not cancel out • Determine molecular shape • Examples: • O2 nonpolar (no polar bond) • HCl polar (one polar bond) • CHCl3 polar (three polar bonds) • CCl4 nonpolar (four polar bonds but they all cancel)

15. Hydrogen bonding • The ultimate expression of polarity • Small positive H atom exerts strong attraction on O atom • Other H-bonding molecules: HF, NH3 • H2O is the supreme example: two H atoms and two lone pairs per molecule

16. Terms in solution • Solute: the dissolved substance • Solvent:the dissolving substance

17. Molarity • Concentration is usually expressed in terms of molarity: • Moles of solute/liters of solution (M) • Moles of solute = molarity x volume of solution

18. Example • What is molarity of 50 ml solution containing 2.355 g H2SO4? • Molar mass H2SO4 = 98.1 g/mol • Moles H2SO4 = .0240 mol • Volume of solution = 50/1000 = .050 L • Concentration = moles/volume = .0240/.050 = 0.480 M

19. Dilution • More dilute solutions are prepared from concentrated ones by addition of solvent M1V1 = M2V2 Molarity of new solution M2 = M1V1/V2 To dilute by factor of ten, increase volume by factor of ten

20. Trace quantities • Percent means one in a hundred • PPM measures trace amounts – 1 in a million • Iodized salt contains tiny amounts of KI – 7.6 x 10-5 g in 1 g of salt • 7.6 x 10-5 g = 7.6 x 10-2 mg = 76 μg

21. Milligrams per liter • Units for impurities in drinking water • 0.38 mg lead in 250 mL water • Concentration in mg/L

22. Something about water • High boiling point compared with similar compounds • Liquid at earth temperature • Solid less dense than liquid • Essential for life on earth • High heat capacity • Modifying influence on climate • Universal solvent

23. H2O has optimum combination of lone pairs and H atoms

24. H bonding generates three-dimensional network

25. Ice floats! • Something so familiar we might believe all solids float on their liquids. Not so. Water is the exception.

26. Hydrogen bonding and life • hold the two strands of the DNA double helix together • hold polypeptides together in such secondary structures as the alpha helix and the beta conformation • help enzymes bind to their substrate • help antibodies bind to their antigen • help transcription factors bind to each other • help transcription factors bind to DNA

27. Implications for life on earth • Without H-bonds molecules like DNA would not exist • H-bonds hold the two strands together • Comparative weakness of bonding allows for DNA replication

28. Unusual variation of the density of water with T: maximum density at 4°C

29. Effects of density on lakesseasonal cycling of waterreplenishment of oxygen Warm water on top Cold water on top

30. Water contamination • Biological • Human and animal waste – bacteria leading to hepatitis, cholera, typhoid, dysentery • Chemical • Organic • Benzene • Chlorohydrocarbons • Inorganic • Asbestos • Nitrates • Lead • Mercury • Radioactivity • Uranium • Tritium spills

31. Legislating cleanliness: The Safe Drinking Water Act 1974 • Establish maximum contaminant levels (MCLs) for 84 contaminants • All water supplies must pass • Periodic sampling required • Too much or too little? Water treatment costs money

32. Don’t trust the EPA? Treat at home • Active carbon filters • Effective on organic contaminants • Need regular replacement • Water softeners • Specific to hard water • Ion exchange using zeolites • Reverse osmosis • Apply pressure to push water from saline to pure side of membrane