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Hydrogen Bonding Learning intention Learn about this strong type of intermolecular forces which exists between molecules containing N- H, O-H or F-H bonds.

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Presentation on theme: "Hydrogen Bonding Learning intention Learn about this strong type of intermolecular forces which exists between molecules containing N- H, O-H or F-H bonds."— Presentation transcript:

1 Hydrogen Bonding Learning intention Learn about this strong type of intermolecular forces which exists between molecules containing N- H, O-H or F-H bonds.

2 Relating physical properties to intermolecular forces Learning intention Learn how to explain differences in physical properties such as viscosity, melting point and boiling point in terms of differences in strength of intermolecular forces.

3 Intermolecular - Hydrogen Bonding Consider the compounds formed between elements in group 4 of the Periodic table and hydrogen The group 4 hydrides are CH 4, SiH 4, GeH 4, SnH 4 They are all covalent molecular so have low melting points and boiling points.

4 The boiling point increases as you go down the group.

5 As you go down the group the central atom gets bigger. There are more electrons so a greater chance of an uneven distribution of electrons within the atom. The London’s forces between the molecules gets stronger as you go down the group. More energy is needed to separate the molecules from each other.

6 Intermolecular – Hydrogen Bonding A similar pattern would be expected in the other covalent molecular hydrides The group 5 hydrides NH 3, PH 3, AsH 3 and SbH 3 The group 6 hydrides H 2 O, H 2 S, H 2 Se and H 2 Te The group 7 hydrides HF, HCl, HBr and HI

7 NH 3, has a higher boiling point than expected.

8 H 2 O has a higher boiling point than expected.

9 HF has a higher boiling point than expected.

10 Intermolecular - Hydrogen Bonding NH 3 H2OH2O HF It is more difficult to separate NH 3, H 2 O and HF molecules from each other than expected.

11 Intermolecular - Hydrogen Bonding These compounds all have H atoms directly bonded to very electronegative atoms. In HF the H-F bond is polar covalent. The F has a much higher electronegativity than H. The pair of shared electrons in the covalent bond spend more time closer to the fluorine than the hydrogen. The H-F bond is polar.H δ+ - F δ-

12 Intermolecular - Hydrogen Bonding The HF molecules can attract each other H δ+ - F δ- This is called hydrogen bonding. Hydrogen bonding is weak but is stronger than very weak London’s forces.

13 Intermolecular - Hydrogen Bonding NH 3 has H atoms directly bonded to very electronegative N atoms. N  - H  + H  + H  + H+H+ N  - H  + H  + N  - H  + H  + There are Hydrogen bonds as well as London’s forces between the ammonia molecules.

14 Intermolecular - Hydrogen Bonding H 2 O has H atoms directly bonded to very electronegative O atoms. O  - H+ H+H+ H+ H+O-H+O- H  + O  - H  + H  + There are Hydrogen bonds as well as London’s forces between the water molecules.

15 Proteins consist of long chain atoms containing polar C=O and H-N bonds. Hydrogen bonds help give enzymes their shape.

16 Water Oxygen has 2 lone pairs of electrons which can form a hydrogen bonds with two hydrogen atoms. Each water molecule, in theory, could be surrounded by 4 hydrogen bonds O HH -- ++ ++

17 Hydrogen bonding in water

18 Water Water has its greatest density at a temperature of 4 o C. When, as water cools further, the molecules start to move further apart, due to the hydrogen bonding, until a more open structure is formed at its freezing point. So ice floats!! Density of water New Higher Chemistry E Allan J Harris

19 Video trends\3dwater.wmv Water 3D model

20 Hydrogen bonding in ice Hydrogen bonding in solid water gives rise to an open structure. This is why ice is less dense than liquid water.

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22 Hydrogen bonding is also responsible for holding the two strands of nucleic acids together in DNA

23 Viscosity Density of water Viscosity is related to the molecular mass and the number of –OH present. Hydrogen bonding between the molecules will increase its viscosity. New Higher Chemistry E Allan J Harris

24 Water Water has a high surface tension. The molecules on the surface have in effect, hydrogen bonds. This has the effect of pulling the surface molecules closer together. Surface tension

25 Bond Strengths Bond TypeStrength (kJ mol –1 ) Metallic80 to 600 Ionic100 to 500 Covalent100 to 500 Hydrogen40 Dipole-Dipole30 London’s forces1 to 20

26 Behaviour in electrical fields New Higher Chemistry E Allan J Harris Video clip

27 Cloth nappies cost between £100-£400 as opposed to disposable at £800-£1,200 for the 2.5 years of normal nappy use. 3 billion nappies are thrown away in the UK each year with 90% going to landfill. They can take up to 500 years to decompose. Disposables make up 4% of total household waste and up to 50% of that of families with one baby Disposable nappies use up to 5 times more energy to produce than cotton ones – that's including the washing process. Seven million trees are felled every year in Canada and Scandinavia to supply the pulp for disposables sold in the UK. Nappies

28 Disposable nappy

29 Chemical Background Groups called sodium carboxylate are attached along the backbone. sodium carboxylate Sodium polyacrylate is a polymer with a molecular weight of over one million!

30 Sodium poly(acrylate) absorbs 500 times its own mass of water. - water + + Na+

31 Sodium poly(acrylate) absorbs 500 times its own mass of water

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33 Predicting solubility from solute and solvent polarities Learning intention Learn how the polarity of both the solute and solvent molecules influences solubility.

34 Solvent Action

35 Water is a polar molecule so it is a polar solvent. A liquid that a substance dissolves in is called a SOLVENT. Solvents can be either polar or non-polar molecules. Immiscible liquids do not mix, e.g. oil and water, however, non-polar liquids are miscible with each other. Polar solvents will usually dissolve polar molecules. Non-polar solvents will usually dissolve non-polar molecules. Water has a polar covalent bonding between O and H. O H H -- ++ ++ ++ ++ --

36 Dissolving in Water Hydrated ions Ionic Compound dissolving in water -- ++ ++ -- ++ + -- ++ ++ -- ++ ++ -- ++ ++ ++ -- ++ ++ + -- ++ -- ++ ++

37 Dissolving in Water

38 Hydrated ions Pure Hydrogen chloride is polar covalent. When water is added it breaks to produce ions Cl - -- ++ ++ -- ++ ++ -- ++ ++ ++ -- ++ ++ H+H+ -- ++ -- ++ ++ H Cl ++ -- -- ++ ++ -- ++ ++

39 Dissolving in Water Generally, covalent molecules are insoluble in water. However, small molecules like ethanol (C 2 H 5 OH), with a polar O-H functional group, will dissolve, -- ++ ++ H2OH2O Ethanol ++ O H HH H H CC H --

40 Dissolving salt solubility KMnO4 and I2(upper layer is water solvent lower layer is hexane)solubility KMnO4 and I2


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