7 – Shapes of Molecules & Intermolecular Forces

1. 7 – Shapes of Molecules & Intermolecular Forces Leaving Certificate Chemistry

2. Shapes of covalent Molecules

3. Using the Valence Shell Electron Pair Repulsion Theory The shape of a molecule depends on the number of pairs of electrons in the outer shell of the central atom 2007 Q5 (b) (6) 2005 Q4 (b) (6) 2004 Q4 (b) (6) 2003 Q5 (b) (12)

4. Using the VSEPR Theory What is the shape of a BeCl2 (beryllium chloride) molecule? 2 bonding pairs No Lone Pairs Cl Cl Be Beryllium chloride (BeH2) is a linear molecule 2 Bonding pairs

5. Beryllium Chloride Bond Angle = 180o Negatively charged electrons repel each other and arrange themselves as far apart as possible

6. Using the VSEPR Theory What is the shape of a BCl3 (boron trichloride) molecule? 3 bonding pairs No Lone Pairs Cl Boron trichloride (BCl3) is a triangular planar molecule B Cl Cl

7. Boron trichloride Bond Angle = 120o Negatively charged electrons repel each other and arrange themselves as far apart as possible

8. Using the VSEPR Theory What is the shape of a CH4 (methane) molecule? H 4 bonding pairs No Lone Pairs H H C Methane (CH4) is a tetrahedral molecule H

9. Methane Bond Angle = 109.5o Negatively charged electrons repel each other and arrange themselves as far apart as possible

10. Using the VSEPR Theory What is the shape of a NH3 (ammonia) molecule? Lone Pair of electrons 3 bonding pairs 1 Lone Pair H N H Ammonia (NH3) is a pyramidal molecule H

11. Ammonia (NH3) Bond Angle = 107o The presence of the lone pair of electrons distorts the tetrahedral shape to become a pyramidal shape

12. Using the VSEPR Theory What is the shape of a water molecule? 2 bonding pairs 2 Lone Pairs O Water (H2O) is a V-Shaped Molecule H H

13. Water (H2O) Bond Angle = 104.5o The presence of the 2 lone pair of electrons distorts the tetrahedral shape to become a v-shaped molecule

14. Learn the following table off by heart

15. Today’s objectives • To learn about the difference between a polar bond and a polar molecule • To predict whether a molecule will be polar

16. Symmetry & Bond Polarity Some molecules may have polar bonds but they are not polar molecules . . . .This is due to symmetry

17. ∂ ― Cl ∂ ― ∂ 2+ ∂ ― O C O ∂ 3+ B ∂ ― Cl Cl Cl ∂ ― ∂ 4+ ∂ ― Cl C Cl ∂ ― ∂ ― ∂ ― Cl Since the centre of the partial negative charges coincides with that of the partial positive charges, none of these molecules are polar

18. Since the centre of the partial negative charges does not coincide with that of the partial positive charges, none of these molecules are polar despite the presence of polar bonds

19. Learning objectives • Difference between intermolecular and intramolecular forces • 3 types of intermolecular forces and relative strengths • Relationship between intermolecular forces and melting/ boiling points

20. Intramolecular Bonding Water O H H Intramolecular bonding is the term used to describe the bonding between atoms in a molecule

21. Intermolecular Forces Intermolecular forces are attractions between different molecules 2008 Q5 (a) (5)

22. Intermolecular Forces Hydrogen Bonding Van Der Waals Forces Dipole-Dipole Interaction

23. Van Der Waals Forces Let’s look at neon atoms. There are 10 electrons in every neon atom. Ne + + + Ne This temporary force is called a Van Der Waals Force

24. Van Der Waals Forces

25. Van Der Waals Forces Intermolecular Forces They result from the ebb and flow of electrons within these molecules, which causes temporary polarity in a molecule These weak forces are the only main forces between like non-polar molecules like N2, O2, H2, I2, CH4. The bigger the molecule the more electrons so the greater the strength of the VDW forces.

26. Van Der Waals Forces – used by Gecko’s? Intermolecular Forces

27. Dipole-Dipole Forces A dipole-dipole interaction exists between polar molecules. They are permanent and much stronger than Van Der Waals forces. δ- δ+ H Cl δ- Cl δ- Cl δ+ H H Dipole-Dipole Forces Created δ+

28. Dipole-Dipole Forces

29. Hydrogen Bonding When molecules that have a hydrogen atom bonded to nitrogen, oxygen or fluorine atom come together a very strong dipole-dipole interaction is setup due to the high electronegativity difference between the atoms. δ+ δ2- H δ+ H O δ2- O δ2- O H δ+ δ+ H H H Hydrogen Bonding Created δ+ δ+

30. Hydrogen Bonding Hydrogen bonding forces are permanent. They are and much stronger than normal Dipole-Dipole Forces δ+ δ2- H δ+ H O δ2- O δ2- O H δ+ δ+ H H H Hydrogen Bonding Created δ+ δ+

31. Learning check… • What is the difference between a intermolecular force and an intramolecular force? • Name three different types of intermolecular force • Which is the strongest? • Which is temporary?

32. Particle arrangements: solids This animation shows a 2-D view of the motion of the atoms in a 3-D solid.

33. Particle arrangements: liquids This animation shows a 2-D view of the motion of the atoms in a liquid. There is no order.

34. Particle arrangements: gases

35. Hydrogen Bonding Hydrogen bonding in water high boiling point Strong forces attract molecules together and a lot of energy is needed to separate the particles to turn it to a gas

36. Hydrogen Bonding Kevlar is a type of fibre that is extremely strong and flexible. It is strong because long chains of molecules line up parallel to each other and are held together strongly by hydrogen bonding

37. Changes of state

38. Check your learning.. • What is the relationship between boiling point and intermolecular bonding?

39. Boiling Points & Forces • If a substance has a very high boiling point then the forces between molecules of that substance are very strong – ionic, dipole-dipole forces or hydrogen bonding. • If a substance has a very low boiling point then the forces between molecules of that substance are very weak – usually Van Der Waals forces

40. Why does oxygen (O2) have a much higher boiling point than hydrogen (H2)? • Both are non-polar • Therefore both molecules only experience weak Van Der Waals forces. • But the Van Der Waals forces are stronger in oxygen because it is a molecule with more electrons (16 versus 2). • More Electrons  Greater VDW Forces  Higher Boiling Point for oxygen. 2004 Q4 (c) (6)

41. Why does methanal have a much higher boiling point than ethene? • Ethene is non-polar  Weak Van Der Waals forces present • Methanal is polar  Stronger dipole-dipole forces present • Therefore methanal has a much higher boiling point then ethene. δ+ δ-

42. Why does water have a much higher boiling point than hydrogen sulfide (H2S)? Contains very stronghydrogen bonding intermolecular forces Contains very weakDipole-Dipole intermolecular forces 2007 Q5 (c) (12)