Homework Private study work (bring notes to show me next lesson) ;

1. Homework • Private study work (bring notes to show me next lesson); • Read pages 62 – 65 in your text book • Complete the summary questions on London (page 63) • Look at the following websites • http://www.chemguide.co.uk/atoms/bonding/electroneg.html#top • http://www.chemnotes.org.uk/f321.html • Topic 5, concentrate on hydrogen bonding and London.

2. London forces and hydrogen bonding Wednesday, 01 January 2020

3. Thinking skills – Try to use key scientific words where possible. Stage 1: This picture definitely shows me… Stage 2: I think this picture shows me… Stage 4: The questions I need to ask about this picture are… Stage 3: This picture does not show me…

4. London forces and hydrogen bonding Learning Objective: • Describe the intermolecular interactions forces: permanent dipole – dipole forces, London forces and hydrogen bonding Learning Outcomes: • understand the nature of intermolecular forces resulting from the following interactions: i London forces (instantaneous dipole – induced dipole) ii permanent dipoles iii hydrogen bonds • understand the interactions in molecules, such as H2O, liquid NH3 and liquid HF, which give rise to hydrogen bonding • understand the following anomalous properties of water resulting from hydrogen bonding: i its relatively high melting temperature and boiling temperature ii the density of ice compared to that of water • be able to predict the presence of hydrogen bonding in molecules analogous to those mentioned above

5. Intermolecular Forces Strength of Bonds and Forces: • Ionic and covalent bonds are strong. • Ionic bonds hold ions together in a lattice so that at room temperature all ionic compounds are solid. • Covalent bonds hold atoms together by sharing electrons. Many covalent compounds are small molecules with strong covalent bonds within them. These are intra-molecular forces.

6. Intermolecular Forces Intermolecular Forces: is an attractive force between neighbouring molecules. • Intermolecular forces are weak compared to covalent bonds. • Intermolecular forces act betweendifferent molecules. They are caused by weak attractive forces between very small dipoles in different molecules. • Intra-molecular bonds act withinone molecule.

7. Intermolecular Forces Intermolecular Forces: There are three types of intermolecular forces; • Permanent dipole-dipole interactions • London forces (induced dipole forces) • Hydrogen bonding.

8. Permanent dipole-dipole interactions Apermanent dipole-dipole force: a weak attractive force between permanent dipoles in neighbouring polar molecules. Polar molecules have a permanent dipole. The permanent dipole of one molecule attracts the permanent dipole of another.

9. Permanent dipole–dipole forces If molecules contain bonds with a permanent dipole, the molecules may align so there is electrostatic attraction between the opposite charges on neighbouring molecules. Permanent dipole–dipole forces (dotted lines) occur in hydrogen chloride (HCl) gas. The permanent dipole–dipole forces are approximately one hundredth the strength of a covalent bond.

10. London forces London forces (or induced dipole-dipole interactions) act between all molecules, whether they are polar or non-polar. • They are the weakest intermolecular force. • They act between very small, temporary dipoles in neighbouring molecules.

11. London forces • Electrons are always moving in an atom. • Would it be possible for a non-polar molecule or atom to produce a dipole? • Why or why not?

12. London forces Temporary dipoles

13. What will happen if two molecules or atoms are near each other and one has a temporary dipole?

15. London forces are attractions between temporary dipoles.

16. What factors might affect the strength of the London forces? • The greater the number of electrons the larger the induced dipole  the greater the London forces.

17. London forces – Boiling Points • London forces are the only attractions between non-polar molecules. • No. of e- increases • London forces increase • Boiling point increases If there were no London forces it would be impossible to liquefy the noble gasses or non polar molecules.

18. Strength of London forces 200 The strength of London forces increases as molecular size increases. 150 100 50 0 boiling point (°C) -50 -100 This is illustrated by the boiling points of group 7 elements. -150 -200 F2 Cl2 Br2 I2 element Atomic radius increases down the group, so the outer electrons become further from the nucleus. They are attracted less strongly by the nucleus and so temporary dipoles are easier to induce.

19. Strength of London forces The points of contact between molecules also affects the strength of London forces. butane (C4H10) boiling point = 272K 2-methylpropane (C4H10) boiling point = 261K Straight chain alkanes can pack closer together than branched alkanes, creating more points of contact between molecules. This results in stronger London forces.

20. How can a gecko’s feet stick to almost any surface? Write down your ideas.

22. HYDROGEN GRAPH • In groups, try to come up with an explanation for the pattern of each graph.

23. The boiling point of compounds of hydrogen and group 4 elements. Why does it increase?

24. How hydrogen bonding affects boiling points Look at the the boiling points of compounds of hydrogen and group 5,6 and 7 elements. What is unusual?

25. DIFFERENCE IN ELECTRONEGATIVITY H 2.1 Li Be B C N O F 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Na Mg Al Si P S Cl 0.9 1.2 1.5 1.8 2.1 2.5 2.9 K Br 0.8 2.2

26. what is hydrogen bonding? • They occur when hydrogen is bonded to either oxygen, nitrogen or fluorine. • There is a large difference in electronegativity between H and O,N,F which results in a strong permanent dipole. • Hydrogen bonding is the intermolecular force that occurs between molecules containing these permanent dipoles. • Hydrogen bonding is the strongest type of intermolecular force.

27. Hydrogen Bonding Hydrogen bonding: A strong intermolecular force between a δ+ hydrogen atom covalently bonded to F, O or N and a lone pair of electrons on the δ- F, O or N atom of a nearby molecule.

28. Occurs in O-H, N-H bonds and H-F

29. Hydrogen bonding In molecules with OH or NH groups, a lone pair of electrons on nitrogen or oxygen is attracted to the slight positive charge on the hydrogen on a neighbouring molecule. hydrogen bond lone pair Hydrogen bonding makes the melting and boiling points of water higher than might be expected. It also means that alcohols have much higher boiling points than alkanes of a similar size.

30. Why do hydrogen bonds only form between O-H, N-H and F-H? It might help to sketch the shape and dipoles of the molecules containing these atoms (such as H2O)

31. What is hydrogen bonding? When hydrogen bonds to nitrogen, oxygen or fluorine, a larger dipole occurs than in other polar bonds. This is because these atoms are highly electronegative due to their high nuclear charge and small size. When these atoms bond to hydrogen, electrons are withdrawn from the H atom, making it slightly positive. The H atom is very small so the positive charge is more concentrated, making it easier to link with other molecules. Hydrogen bonds are therefore particularly strong examples of permanent dipole–dipole forces.

33. DEMONSTRATE hydrogen bonding between HF molecules • Following a similar procedure, show by way of diagram, the hydrogen bonding between: a) two water molecules b) two ammonia molecules • Extension: Why do you think water is considered to be a ‘perfect example’ of hydrogen bonding? HELP SHEET AVAILABLE TO HELP YOU STRUCTURE YOUR ANSWER

34. Why do icebergs float?

35. Hydrogen Bonds • What happens to the volume of water when it freezes? • How does this differ from other liquids? • What causes this?

36. Ice has open lattice, H-bonds hold water molecules apart. • When ice melts, H2O molecules move closer together.

38. How much does volume increase?

39. Many simple molecular structures are gases at room temperature but H2O is a liquid – why?

40. Other molecules are held together by London forces. • However, water molecules are held together by hydrogen bonds which are stronger and harder to overcome.

42. Other • H-bonds also give water relatively high surface tension and viscosity. • H-bonds are important in organic compounds containing O-H and N-H bonds (alcohols, carboxylic acids etc) • They are responsible for shape of proteins and even DNA.

43. DNA

44. DNA

45. DNA Double Helix

46. Hydrogen bonding explains the higher boiling point of hydrogen fluoride compared to the other hydrogen halide compounds.

47. Exam questions (new sample material)

48. Exam questions (new sample material)

49. Exam questions 1 mark each (total two marks)

50. Exam questions