Intermediate bonding and bond polarity

1. Date Intermediate bonding and bond polarity

2. BIG picture • What skills will you be developing this lesson? • ICT • Numeracy • Literacy • Team work • Self management • Creative thinking • Independent enquiry • Participation • Reflection • How is this lesson relevant to every day life? (WRL/CIT)

3. Is the pair of electron shared equally by the two H atoms? What is the type of bonding in hydrogen? H:H How will the electrons be shared in HCl? In a hydrogen molecule, one pair of electrons is shared between the two hydrogen atoms. The attraction between the nuclei and electrons is same from each atom as they both are same. The pair of electrons in the bond is equally shared. These type of molecules are called NON-POLAR.

4. HCl In a Hydrogen chloride molecule, one pair of electrons is shared between the hydrogen and chlorine atom. The attraction between the nuclei and electrons is different from each atom as they both are different. The pair of electrons in the bond is unequally shared. These type of molecules are called POLAR.

5. How do we know if the molecule is Polar or Non-polar? • We can predict the relative attracting powers of the nuclei of atoms of different elements by using the concept of electronegativity. • What is Electronegativity?

6. Electronegativity Electronegativity is a measure of how strongly an atom or atoms attracts the bonding electrons in a covalent bond. The higher the electronegativity, the stronger an atom's attraction for bonding electrons. We use the Pauling Scale of Electronegativities. It has no units and runs between 0 – 4.

7. Pauling Scale of Electronegativities Write any three trends that you see.

8. Electronegativity Trends What about the electronegativity of noble gases? They have zero electronegativity as they rarely form covalent bonds and they have no electron affinity.

9. 1) Of the commonly occurring elements, this makes _________ the most electronegative and ______________the least. 2) Atoms of the same electronegativity joined by a covalent bond give an electronegativity difference of zero - i.e. H2 2.1 - 2.1 = 0. This indicates a normal nonpolar covalent bond. Calculate the electronegativity difference between C and H. How can we use the electronegativity difference to calculate the percentage of ionic and covalent bonding (Bond character)?

10. How to calculate the Bond character?

11. 100% covalent character 59% ionic character and 41% covalent character 89% ionic character and 11% covalent character COVALENT POLAR IONIC What is the relation between electronegativity difference and percentage ionic character?

13. Questions?

14. Answers

15. Polar covalent bond Non-Polar covalent bond Polar covalent bond

16. Ionic bonds can be polarised by the cations strongly attracting the outer shell electrons of the anion. What does the polarising power of a cation depend on? Its charge density, which depends on the size of the ions and its charge. Smaller the ionic radius/greater the charge – the greater the charge density. For an anion, larger the radius, more easily polarised it is. Polarisation of Ionic bond

17. Questions CLUE: Ionic compounds tend to have high melting and boiling points and dissolve in polar solvents only.

18. Answers 1 HF as there biggest difference in electronegativity. 2 3 Difference in electronegativity between lithium and iodine is 1.5 (2.5 - 1.0). From Table 2.1.1, percentage ionic character = 43%. So possibly lower melting and boiling points, and soluble in an organic solvent.

19. Task 1: Review Go back to your lesson outcome grid and fill out the ‘How I did’ and the ‘Targets’ column.

20. What is a dipole? • Dipole = a molecule (or part of a molecule) with a positive end and a negative end. • E.g. HCl molecules have a dipole. • When a molecule has a dipole it is polarised.

21. + - H Cl Dipole Moment • Is it possible to have several polar bonds in a molecule? • Does this make the whole molecule polar? • Polarity of a molecule is measured as Dipole moment. • It is defined as the product magnitude of charge on the atoms and the distance between the two bonded atoms. Its common unit is debye D. • Direction of the polar bond in a molecule: • Arrow points toward the more electronegative atom.

22. F BF3 B F F Determining Molecular Polarity • Nonpolar Molecules • Dipole moments are symmetrical and cancel out.

23. O net dipole moment H2O H H Determining Molecular Polarity • Polar Molecules • Dipole moments are asymmetrical and don’t cancel . Resultant dipole Because water is an angular molecule the dipoles do not cancel and a resultant dipole moment is produced.

24. Molecules with 3 Atoms Even though the C-O bond is polar, the bonds cancel each other out because the molecule is linear the dipole moments are equal and in opposite directions.Therefore CO2 is non-polar. CO2 The dipole moment between H-C points in the direction of C. The dipole moment points between C-N points in the direction of the N. Therefore the dipole vectors are additive and HCN is polar HCN SO2 SO2 is a polar molecule because the S-O dipole Moments don’t cancel each other out due to the angle

25. H net dipole moment CHCl3 Cl Cl Cl Determining Molecular Polarity • Therefore, polar molecules have... • asymmetrical shape (lone pairs) or • asymmetrical atoms

26. Molecules with 4 Atoms Non-polar CCl4 is non-polar Polar CHCl3 is polar

27. Which of the following molecules are polar (have a dipole moment)?H2O, CO2, SO2, and CH4 O O S H H H O O O C H H C H dipole moment polar molecule dipole moment polar molecule no dipole moment nonpolar molecule no dipole moment nonpolar molecule 10.2

28. Polar or non-polar? ? ?

29. Task 2: Review Go back to your lesson outcome grid and fill out the ‘How I did’ and the ‘Targets’ column.

30. Experiment • To carry out experiments to determine the effect of an electrostatic force on jets of liquids and use the results to determine whether the molecules are polar or non-polar • Equipment & chemicals: • Burette with filling funnel • Glass rod and dry piece of cloth • Beaker, 600 cm3, or large diameter container • A range of organic hydrocarbons, alcohols and ketones for which hazard information is available, • 50 cm3 each

31. Procedure

32. New Information for Task 3

33. Observations and interpretations • What happens to the jet of your liquid? • What happens with other liquids? • Why does this happen?

34. Task 3: Review Go back to your lesson outcome grid and fill out the ‘How I did’ and the ‘Targets’ column.

35. Review

36. Homework • Homework task: • Due date: • Criteria for Grade C: • Criteria for Grade B: • Criteria for Grade A/A*:

37. Information Properties of Covalent compounds: http://misterguch.brinkster.net/covalentcompounds.html Properties of Ionic compounds: http://misterguch.brinkster.net/ionic.html

38. .. .. Polar Bonds .. F N O H Cl H H H B H H F F Polar Polar Nonpolar Polar Cl Cl F F F H Cl C C Xe Cl F F Cl H F F Cl H Polar Nonpolar Nonpolar Polar A molecule has a zero dipole moment because their dipoles cancel one another.