CfE Higher Chemistry. Unit 1: Chemical Changes and Structure. Intermolecular forces

Transcription

1 CfE Higher Chemistry Unit 1: Chemical Changes and Structure Intermolecular forces 05/09/2017

2 Van der Waal s Forces and London Dispersion Forces 05/09/2017 Learning Outcomes : I can explain the difference between intermolecular and intramolecular forces. I can explain how London dispersion forces arise. I can state where London dispersion forces arise. I can relate the strength of London dispersion forces to the number of electrons within the atom or molecule. I can relate London dispersion forces to other types of bonds in terms of strength.

3 Lesson Starter: Bonding What type of bonding is present in the following: a) Magnesium b) Calcium chloride c) Iron oxide d) Propane e) Copper f) Butene g) Iodine h) Gold i) Neon j) Lithium bromide

4 Intermolecular Bonding Intermolecular bonding is bonding between molecules Intermolecular bonding is weaker than intramolecular bonding. Melting point and boiling points depend on the strength of the interaction between molecules. Intermolecular bond Molecules Glow Science: Intermolecular Forces

5 1. London Dispersion Forces Very weak forces of attraction exist between all atoms and molecules e.g. when a noble gas is cooled sufficiently the atoms move together and form a solid. i.e. there must be forces holding the atoms together. He He He He Solid helium melts at -273 o C (0K) so the forces of attraction must be very weak. Pure covalent molecules have Van der Waals forces between them- m.pts and b.pts are low. London Dispersion forces increase with increasing atomic mass.

6 How do Van der Waals Forces Arise? The hydrogen atom At any instant one area of the atom is positive and one is negative i.e. an electric dipole exists. The electric dipole is temporary.

7 These dipoles in the atoms lead to instantaneous forces of attraction between atoms and molecules caused by the movement of electrons These are weak Van der Waals forces. i.e. the melting and boiling points of substances held together by Van der Waals forces are low because little energy is required to overcome the interaction.

8 Van der Waals Properties Consider the halogens F 2 Gas Cl 2 Gas Br 2 Liquid I 2 Solid Down a group size of molecules increase (there are more electrons / energy levels) so larger Van der Waals Melting and boiling points increase. The attraction increases as the molecules become closer.

9 Success Criteria: VdW and LDF Forces 05/09/2017 I can explain the difference between intermolecular and intramolecular forces. I can explain how London dispersion forces arise. I can state where London dispersion forces arise. I can relate the strength of London dispersion forces to the number of electrons within the atom or molecule. I can relate London dispersion forces to other types of bonds in terms of strength. Next Lesson: Permanent Dipole-Permanent Dipole Interactions

10 Permanent Dipole-Permanent Dipole Interactions 05/09/2017 Learning Outcomes : I can explain how permanent dipole-permanent dipole interactions arise. I can use spatial arrangement of polar covalent bonds to predict whether a molecule is polar or non-polar. I can relate permanent dipole-permanent dipole interactions to other types of bonds in terms of strength.

11 Lesson Starter: Polarity 2. Atoms of nitrogen and element X form a bond in which the electrons are shared equally. Element X could be: A) Carbon B) Oxygen C) Chlorine D) Phosphorus. 3. Some covalent compounds are made up of molecules that contain polar bonds but the molecules are overall non-polar. Which of the following covalent compounds is made up of non-polar molecules? A) Ammonia B) Water C) Carbon tetrachloride D) Hydrogen fluoride 05/09/2017

12 Polar molecules: Non-polar molecule Symmetrical molecule Polar molecule Non-symmetrical molecule A molecule is described as polar if it has a permanent dipole. In CCl 4 although there is a difference in electronegativity in the polar bonds, the charge is distributed around the central carbon atom with the positive and negative charges balancing out. The molecule has no overall dipole.

13 Propanone formula mass 58 boiling point 56 C POLAR Butane formula mass 58 boiling point 0 C NON-POLAR The stronger intermolecular forces are responsible for the relatively high boiling point of propanone compared with butane. 05/09/2017

14 Propanone formula mass 58 boiling point 56 C POLAR Butane formula mass 58 boiling point 0 C NON-POLAR The intermolecular forces between polar molecules containing polar bonds are stronger than LDF 05/09/2017

15 Propanone formula mass 58 boiling point 56 C POLAR Butane formula mass 58 boiling point 0 C NON-POLAR NOTE: We can use the boiling points of propanone and butane to compare the strengths of the intermolecular forces because these substances have the same relative molecular mass 05/09/2017

16 2. Permanent Dipole-Permanent Dipole Interactions Occurs between polar molecules. Polarity increases the strength of intermolecular bonding. Polar molecules have higher melting and boiling points. Stronger than Van der Waals. Attraction between a permanent dipole in one molecule and a permanent dipole in another molecule.

17 δ- δ+ δ- δ+ Permanent Dipole- Permanent Dipole intermolecular bondholds molecules together (weak) Intramolecular covalent bonds-holds atoms together (strong) e.g. Diatomic polar molecule

18 Notes: Permanent dipole-permanent dipole interactions act in addition to London Dispersion Force electrostatic attractions between polar molecules. PDPD forces are stronger than LDF attractions for molecules of equivalent size.

19 Success Criteria: PDPD Interactions I can explain how permanent dipole-permanent dipole interactions arise. I can use spatial arrangement of polar covalent bonds to predict whether a molecule is polar or nonpolar. I can relate permanent dipole-permanent dipole interactions to other types of bonds in terms of strength. 05/09/2017 Next Lesson: Hydrogen Bonding

20 Hydrogen Bonding 05/09/2017 Learning Outcomes : I can correctly describe hydrogen bonding as an example of intermolecular or intramolecular bonding. I can identify from the structural formula or the molecular formula compounds which have hydrogen bonds. I can relate hydrogen bonding to other types of bonds in terms of strength.

21 Lesson Starter:.Rank the following by from lowest to highest anticipated boiling point: C 2 H 4, CH 4, Ne, H 3 COCH 3..Explain why ethyl alcohol (C 2 H 5 OH) has a higher boiling point ( C) than methyl alcohol (CH 3 OH; C).

22 . Hydrogen Lesson Starter: cyanide, HCN, is highly toxic. Information about hydrogen cyanide is given in the table. Although hydrogen cyanide has a similar molecular mass to nitrogen, it has a much higher boiling point. This is due to the permanent dipole permanent dipole attractions in liquid hydrogen cyanide..what is meant by permanent dipole permanent dipole attractions?.explain how they arise in liquid hydrogen cyanide. 05/09/2017

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24 3. Hydrogen Bonding Strongest intermolecular bond. Occur due to high difference in electronegativity values of N-H, O-H andf-h bonds. δ- N-H δ+ δ- O-H δ+ δ+ H-F δ- Hydrogen bonds between water molecules

25 Eg. alcohols, water Eg. ammonia, amines

26 Substances with hydrogen bonds between the molecules have higher than expected boiling points.

27 e.g. Hydrogen bonds between a hydrogen atom and a strongly electronegative atom. e.g. methanol Covalent bond Hydrogen bond

28 Dimers and Trimers in Hydrogen Fluoride In hydrogen fluoride the bond is so polar that molecules of H-F move about in groups of two or three molecules bonded together. Dimer: two molecules linked together Hydrogen bonding

29 Trimer: three molecules linked together Hydrogen bonding Dimers can also occur in alkanoic acids.

30 Comparing the Strength of Intermolecular Forces Molecule Butane Formula Mass and Boiling Point 58 amu -1 o C Structure Polarity Intermolecular Forces Present Non-polar Only LDF Propanone 58 amu 56 o C Polar LDF and PDPD Propan-1-ol 60 amu 97 o C Polar LDF and Hydrogen bonding

31 INCREASING STRENGTH 1. Van der Waals (weakest) 2. Permanent Dipole- Permanent Dipole 3. Hydrogen (strongest)

32 Behaviour in electrical fields Expt 1.11 Video clip New Higher Chemistry E Allan J Harris

33 Bonding Summary Intramolcular (Within molecules) Pure Covalent (e.g. H 2, Cl) Same Electronegativity. Covalent Molecules. Polar Covalent (e.g. H Cl) Different electronegativities. Covalent Molecules. Ionic (between atoms of metals and non-metals not between atoms of covalent molecules) Intermolecular (Between molecules) Van der Waals (very weak) Permanent dipole- permanent dipole Hydrogen bonding (weak) e.g O H Water N H Ammonia F H Hydrogen fluoride 4 Metallic 5 Covalent Networks

34 Lesson Starter: Past Paper 2010

35 Relating Properties to Intermolecular Forces 05/09/2017 Learning Outcomes : I can explain the relationship between the type and strength of intermolecular bonds and melting point, boiling point and viscosity. I can predict the strength of intermolecular forces by considering the polarity of molecules and the number of electrons. I can explain patterns in melting and boiling points in terms of the strength of intermolecular forces. I can explain why the boiling points of ammonia, water and hydrogen fluoride are higher than expected given the number of electrons present in the molecules. I can explain how hydrogen bonding affects boiling points, melting points, viscosity and solubility (miscibility). I can explain why ice is less dense than water.

36 Lesson Starter: Past Paper Questions Which of the following elements exist as discrete molecules? A Boron B Carbon (diamond) C Silicon D Sulphur 2. In which of the following liquids does hydrogen bonding occur? A Ethanoic acid B Ethyl ethanoate C Hexane D Hex-1-ene

37 Properties of Polar Molecules 1. Melting and Boiling pts: Higher melting and boiling pts than non-polar molecules. Due to the strong intermolecular forces (permanent dipolepermanent dipole and hydrogen bonds). 2. Attraction to a charged rod Polar molecules are attracted to a charged rod whilst non-polar liquids are not.

38 3. Viscosity Viscosity is the thickness of a liquid. The more hydrogen bonds in a molecule the more the molecules will hold together and the more viscous the liquid will be. Consider the following molecules: So more O H bonds per molecule, the more hydrogen bonds the molecule can make and, therefore, so the more viscous the molecule is.

39 4. Solubility Polar solvents dissolve other polar substances (miscible) e.g. ethanol and water. The polar functional groups are able to hydrogen bond with water molecules Non-polar solvents dissolve other non-polar substances as there are only Van der Waals forces between the molecules e.g. hexane and iodine. Polar solvents will not dissolve nonpolar substances (immiscible) e.g. water (polar) and hexane (non-polar). Ionic compounds will dissolve in polar solvents but will not dissolve in nonpolar solvents. Like dissolves like!

40 Properties of Water The unusual properties of water are caused by hydrogen bonding. 1. Anomalous high boiling pt for such a small molecule. Boiling pt. of Group 6 Hydrides Boiling point ( o C) H 2 O H 2 S H 2 Se H 2 Te

41 2. Water is a solvent for ionic lattices The slightly negative ends of the polar water molecules are attracted to the positive ions in the crystal lattice while the slightly positive ends of the water molecule are attracted to the negative ions. The formation of electrostatic attractions between the ions and polar water molecules results in a release in energy. This energy overcomes the electrostatic attractions between oppositely charged ions in the lattice.

42 3. Lower density of solid ice. Water expands when it freezes. The Hydrogen bonds cause the molecules to adopt an open hexagonal shape. This means the ice is less dense than water and floats.

43 Predicting solubility from solute and solvent polarities 05/09/2017 Learning Outcomes : I can state the type of solvent which tends to dissolve ionic compounds and polar molecular compounds I can state the type of solvent which tends to dissolve nonpolar molecular compounds. I can select groups within molecules which imply hydrogen bonding. I can identify molecules with a permanent dipole by considering the spatial arrangement of polar covalent bonds.

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45 Practice Question Identify the main type of intermolecular force in each compound: 1) carbon disulphide 2) ammonia 3) oxygen 4) CH 2 F 2 5) C 2 H 6

46 Practice Question What is the strongest intermolecular force present for each of the following compounds? 1) water 2) carbon tetrachloride 3) ammonia 4) carbon dioxide 5) Phosphorus trichloride 6) nitrogen 7) ethane (C 2 H 6 ) 8) acetone (CH 2 O) 9) methanol (CH 3 OH) 10) borane (BH 3 )

47 Practice Question 1) nitrogen 2) carbon tetrachloride 3) H 2 S 4) sulfur monoxide 5) N 2 H 2 6) boron trihydride 7) CH 4 O 8) SiH 2 O

48 Practice Question a) FCN b) HCN c) C 2 H 6 d) CF 2 H 2

49 Practice Question Past paper 2006 (2)

50 Practice Question Past paper 2006 (2)

51 Practice Question Past Paper 2012

52 Practice Question Past Paper 2012

53 Practice Question Past Paper 2012

54 Practice Question Past Paper 2012

55 Starter 1. Write ionic formula for the following compounds: a) Sodium chloride b) Magnesium carbonate c) Aluminium phosphate d) Calcium oxide e) Aluminium carbonate f) Iron (II) nitrate