Water

1. Water Focus 1: Water is distributed on Earth as a solid, liquid and gas

2. Definitions • Solution: A homogeneous mixture of one substance dissolved in another. • Solute: The substance that is dissolved • Solvent: The substance that does the dissolving (usually a liquid) • Examples: salt in water, iodine in alcohol Outcome 1

3. Density • All these cubes have a volume of 1 cm3 lead iron Wood copper • Can you arrange them in order of mass? • The density of a substance is its mass per unit volume. 11.3g 7.9g 0.3g 8.9g Outcome 5

4. Calculating Density Density = Mass Volume Units: g/cm3 or g/mL Outcome 5

5. Calculating the density of water and ice • Discuss: How would you determine the density of: • Liquid water • Ice • Write down the steps you would take for each. • Carry out the investigation!! (Use worksheet) Outcome 5

6. Homework: • Use your textbook to answer outcomes 2-4 • Exercises 1 – 2 on p.185-186

7. Explaining the density of water vs ice – a model • Complete practical investigation “Using models to explain the densities of water and ice”

8. Investigating some properties of water • First we will compare these properties of water with other substances: • Melting and boiling points • Surface tension • Viscosity • Then we will study intermolecular forces in order to explain these unusual properties!

9. Comparing the melting and boiling points of the hydrides Use Excel and the worksheet “Boiling and Melting point data” to graph the melting and boiling points of different element hydrides. Outcome 15

10. Boiling points of hydrides Outcome 8-9

11. Boiling points of hydrides - conclusions • As you go down a group, the boiling points of the hydrides increase. • However, H2O, HF, and NH3 do not fit the pattern – we would expect them to have the lowest boiling points in their group, but instead they have anomalously high boiling points! Outcome 15 Tin Hydride – Melting point -146C - Boiling point -52C

12. Stations activity • Complete the investigations of two other important properties of water – surface tension and viscosity! Outcome 14

13. Surface Tension • the tendency of a liquid to resist increase in its surface area. • A high surface tension means the surface behaves like a taut skin and “holds in” the water inside. • The liquid forms spherical droplets (as this minimises the surface area) rather than spreading out as a thin film. • Measured in J/m2(it is the energy required to increase the surface area by 1m2) Outcome 14

14. Can you walk on water? Outcome 14

15. What shape has the lowest surface area for a given volume? • Answer: a sphere Outcome 14

16. Viscosity • The resistance of a liquid to flow • The higher the viscosity, the less easily the liquid flows. • Measured in N s /m2) (it is the force required in 1 second for the liquid to move 1m2) Outcome 14

17. Ballpoint pens Outcome 14

18. Motor oils contain “Polymeric viscosity index improvers” Outcome 14

19. Capillary action (non-syllabus) Outcome 14

20. Capillary action (non-syllabus) • The tendency of a liquid to rise up a tube against the pull of gravity. Outcome 14

21. Capillary action – why? • Cohesive forces = forces between the molecules of the liquid • Adhesive forces = forces between the liquid and the tube walls. • These two forces are enough to overcome gravity • In water, the adhesive forces between water and glass are stronger than the cohesive forces within the water, resulting in a convex meniscus. • In mercury, it is the opposite. Adhesion Cohesion Outcome 14

22. Capillary action in plants Outcome 14

23. Spillproof tablecloth? Outcome 14

24. Comparing the surface tension of water and other liquids

25. Comparing the viscosity of water and other liquids

26. Explaining these properties of water • Summary so far: Water has: • An unusually high melting and boiling point for its molecular weight • The highest surface tension of any molecular liquid • A high viscosity for its molecular weight • WHY???????????????????

27. Comparing water, ammonia and hydrogen sulfide 1. Construct Lewis Dot diagrams of: a. Methane b. Ammonia (NH3) c. Water d. Hydrogen sulfide (H2S) 2. Construct molecular models of compounds a - d The next section covers outcomes 9-14

28. Methane - tetrahedral 4 Bonding pairs H H C H H

29. Ammonia – Trigonal Pyramidal 1 non-bonding pair 3 Bonding pairs H N H H

30. Water - Bent 2 non-bonding pairs H O H 2 Bonding pairs

31. Hydrogen sulfide - Bent 2 non-bonding pairs H S H 2 Bonding pairs

32. H H Polar Covalent Bonds • When the two elements in a chemical bond have different electronegativities, the electrons will be shared unevenly between them; i.e the electrons will spend more time near one nucleus than the other. Unequal sharing Equal sharing O H δ+ δ- Electronegativity: 2.1 Electronegativity: 2.1 Electronegativity: 2.1 Electronegativity: 3.5 Dipole Outcome 8-9

33. Polar vsNonpolar Molecules • HF • CO2 δ+ δ- F H Shape = linear Net dipole present – molecule is polar δ- δ+ δ- Shape = Linear No net dipole – molecule is nonpolar O C O Outcome 8-9

34. Polar vsNonpolar Molecules δ- • H2O • NH3 O Shape – Bent Net dipole present – molecule is polar δ+ δ+ H H δ- N Shape: Pyramidal Net dipole present – molecule is polar H H δ+ δ+ H δ+ Outcome 8-9

35. Polar vsNonpolar Molecules δ+ H • CH4 • BF3 Shape – tetrahedral No Net dipole – molecule is nonpolar δ- C δ+ δ+ H H H δ+ δ- Shape – trigonal planar No Net dipole – molecule is nonpolar F δ+ F δ- B δ- F Outcome 8-9

36. Polarity - summary • A bond is polar if one end is slightly positive and one end is slightly negative, thanks to different electronegativities of the atoms. • A molecule is polar if one end of the molecule is slightly positive and the other is slightly negative due to the additive effect of the polar bonds. • If the molecule’s shape means that the dipoles of each bond cancel each other out, the molecule is nonpolar overall.

37. Polarity - summary • To decide whether a molecule is polar or not: • Use electronegativities to decide the polarity of the bonds • Use the shape of the molecule to decide whether the polar bonds cancel out or combine to produce a net dipole.

38. Intermolecular forces – take notes on w/s • Dispersion Forces – occur in all substances. • Dipole - Dipole Forces – occur in polar substances only • Hydrogen Bonding – occur in polar substances that have an H bonded to an F, O or N Strength Outcome 14

39. Dipole - Dipole Forces • Occurs between polar molecules only • Arise from the transient attraction of the positive pole of one molecule to the negative pole of the other. • Click here to view an animation of dipole-dipole forces Outcome 14

40. Hydrogen Bonding • A special type of dipole-dipole force • Occurs between molecules that have an H atom bonded to an N, O or F atom. • The H-N, H-O and H-F bonds are extremely polar, so the electron density is withdrawn strongly from H. • As a result, the partially positive H of one molecule is attracted strongly to the partially negative lone pair on the N, O or F of another molecule. • This attraction is called a hydrogen bond. • Click here to view an animation of H bonding • Another one! Outcome 14

41. Dispersion forces • Electrons are constantly moving, so at any instant they can be unevenly distributed across a molecule. • This can leave one end of a molecule slightly negative and the other end slightly positive – i.e there is a temporary (instantaneous) dipole. • This induces a dipole in a neighbour molecule, causing a short-lived attraction between them. F F Outcome 14

42. Comparing the four molecules Outcome 8-9

43. Comparing the four molecules- answers Outcome 8-9

44. Questions

45. What causes surface tension? • Molecules in the interior experience intermolecular attractions in all directions • Molecules on the surface experience intermolecular attractions only from below and the sides i.e a net attraction downwards and want to move into the interior. • The stronger the IMFs in a liquid, the greater its surface tension

46. What causes viscosity? • When a liquid flows, the molecules slide past one another. • Intermolecular attractions hinder this movement, resulting in viscosity (resistance to flow) • Large, long molecules have higher viscosity than small, spherical ones. • Due to strong H bonding, water has a much higher viscosity than its small molecular size might suggest.

47. Writing explanations • Explain what causes liquids to have surface tension. • Explain what causes liquids to be viscous • Melting and boiling point Game – matching properties to explanations

48. Polarity - Summary • Both ICl and Br2 have the same number of atoms and approximately the same molecular weight, but ICl is a solid whereas Br2 is a liquid at 0oC. Why? Polar compounds like H2S, NH3 and H2O have dipole-dipole forces present; nonpolar compounds like CH4 do not.

49. Homework: Outcomes 2-4

50. H H Polar Covalent Bonds • When the two elements in a chemical bond have different electronegativities, the electrons will be shared unevenly between them; i.e the electrons will spend more time near one nucleus than the other. Unequal sharing Equal sharing O H δ+ δ- Electronegativity: 2.1 Electronegativity: 2.1 Electronegativity: 2.1 Electronegativity: 3.5 Dipole