Presentation on theme: "Hydrogen Bonding Learning intention"— 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 intentionLearn 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 hydrogenThe group 4 hydrides are CH4, SiH4, GeH4, SnH4They 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 hydridesThe group 5 hydrides NH3, PH3, AsH3 and SbH3The group 6 hydrides H2O, H2S, H2Se and H2TeThe group 7 hydrides HF, HCl, HBr and HI
10 Intermolecular - Hydrogen Bonding H2OHFNH3It is more difficult to separate NH3, H2O 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. Hδ+ - Fδ-
12 Intermolecular - Hydrogen Bonding The HF molecules can attract each otherHδ+ - Fδ-Hδ+ - Fδ-Hδ+ - Fδ-This is called hydrogen bonding.Hydrogen bonding is weak but is stronger than very weak London’s forces.
13 Intermolecular - Hydrogen Bonding NH3 has H atoms directly bonded to very electronegative N atoms.There are Hydrogen bonds as well as London’s forces between the ammonia molecules.N-H+ H+ H+H+N- H+N H+H+ H+
14 Intermolecular - Hydrogen Bonding H2O has H atoms directly bonded to very electronegative O atoms.There are Hydrogen bonds as well as London’s forces between the water molecules.O-H+ H+H+O-H+O H+H+
15 Proteins consist of long chain atoms containing polar C=O and H-N bonds. Hydrogen bonds help give enzymes their shape.
16 Water O H Oxygen has 2 lone pairs of electrons which can form -OH++Oxygen has 2 lone pairs of electrons which can forma hydrogen bonds with two hydrogen atoms.Each water molecule, in theory, could be surroundedby 4 hydrogen bonds
18 WaterDensity of waterWater has its greatest density at a temperature of 4oC. 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!!New Higher Chemistry E Allan J Harris
22 Hydrogen bonding is also responsible for holding the two strands of nucleic acids together in DNA
23 ViscosityViscosity is related to the molecular mass and the number of –OH present.Hydrogen bonding between the molecules will increase its viscosity.Density of waterNew Higher Chemistry E Allan J Harris
24 WaterSurface tensionWater has a high surface tension. The molecules on the surfacehave in effect, hydrogen bonds. This has the effect of pullingthe surface molecules closer together.
25 Bond Strengths Bond Type Strength (kJ mol –1) Metallic 80 to 600 Ionic CovalentHydrogen40Dipole-Dipole30London’s forces1 to 20
26 Behaviour in electrical fields Video clipNew Higher Chemistry E Allan J Harris
27 NappiesCloth 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 babyDisposable 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.27
35 Solvent ActionA 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 is a polar molecule so it is a polar solvent.Water has a polar covalent bondingbetween O and H.++-HO-H+
36 Dissolving in Water Ionic Compound dissolving in water - + - + -+-+--++-+Hydratedions