[8.5] Melting Points and Boiling Points of Solutions

Transcription

1 [8.5] Melting Points and Boiling Points of Solutions

2 Melting and Boiling Points We have talked about melting and boiling points earlier in the semester Now we are going to understand how intermolecular forces affect melting and boiling temperatures 6/13/2018

3 As energy is being added to the substance, it reaches a critical value allowing a phase change The substance is absorbing heat (energy) to break the intermolecular forces (the attraction between molecules) Thus the distance between particles increases but the molecule doesn t move faster

4 Melting and Boiling Points MP and BP are a result of intermolecular forces Two main IMFs acting on covalent molecules: Polar molecules have dipole-dipole forces and possibly hydrogen bonding forces. E.g. Ammonia Nonpolar and Polar molecule both have London Forces E.g. Gasoline

5 There is 2 types by location: Intramolecular inside of the molecule Intermolecular between 2 or more molecules (outside) 5

6 Intermolecular Forces Bonds between molecules. Not as strong as bonds between atoms inside the molecule. δ- δ+ δ+ δ- δ+

7 Intermolecular Forces Many types, we will learn three 1.Dipole-dipole forces 2.Hydrogen bonds 3.London forces 7

8 Dipole-Dipole Results from uneven sharing of electrons. Some atoms don t share well Maybe one pulls on the electron stronger than the other.

9 Dipole-Dipole

10 Dipole-Dipole Uneven sharing makes one side a little positive and one side a little negative. δ+ δ-

11 Dipole-Dipole The negative dipole of one molecule is attracted to the positive dipole of the other molecule. Cl is slightly negative, H - positive δ+ δ- δ+ δ-

12 Hydrogen Bonds Special case of dipole forces. Attractive force between the H attached to an electronegative atom (O, N, F) of one molecule and an electronegative atom of a different molecule. 12

13 London Dispersion Force Temporary dipoles caused by the movement of electrons around the nucleus. Sometimes the electrons are more to one side, or are uneven. Makes a temporary dipole, and neighbours are attracted. 13

14 London Dispersion Force The weakest intermolecular force. Cause nonpolar substances to condense to liquids and to freeze into solids when the temperature is lowered sufficiently.. Bigger molecule = more e- = stronger London forces. 14

15 Melting point & bonding? How they relate? stronger chemical bond of one atom to another atom the more energy is necessary to break it. Thus, higher melting and boiling points. 15

16 What pattern do you see with the total number of electrons, polar verses non-polar, and intermolecular forces? 1. More electrons (atomic number) the higher the boiling point 2. Polar molecules have a higher boiling point that N.P. molecules 3. Stronger the IMF, the higher the boiling point 16

17 1. Order the three non-polar molecules in increasing order of BP: N 2, O 2, Br 2 < <<< 2. Explain why the elements are ordered the way that they are:

18 1. Order the three non-polar molecules in increasing order of BP: N 2, O 2, Br 2 N 2 < O 2 <<< Br 2 2. Explain why the elements are ordered the way that they are: The only IMF effecting nonpolar molecules is London dispersion forces (LDF). For LDF, the chemical species with the most electrons (size) and largest contact area (shape) has the strongest LD force.

19 Why does NO have a higher melting point than O 2 even though O 2 has more electrons

20 Why does NO have a higher melting point than O 2 even though O 2 has more electrons. Answer: NO is a polar molecule with Dipole-Dipole & London Forces O2 is a nonpolar molecule with only London force. Generally polar molecules with Dipole-dipole forces have stronger intermolecular forces and therefore result in higher MP/BPs

21 NO has more electrons than H 2 O, however it has a lower boiling point. Explain why H 2 O has a higher boiling point than NO.

22 NO has more electrons than H 2 O, however it has a lower boiling point. Explain why H 2 O has a higher boiling point than NO. Answer: Water has hydrogen bonds. H-Bonds are generally stronger than other Dipole-dipole forces and London Forces and therefore result in higher Melting Point

23 electrons 1) As the number of (size) increase, London forces also increase, thereby increasing the melting (Mp) and boiling the chemical. 2) Polar molecules with dipole-dipole attraction generally have stronger IMFs than non-polar molecules. Therefore, polar molecules have higher BPs compared to non-polar molecules. 3) Similarly, since hydrogen bonding is generally the strongest dipole-dipole force, molecules with H-bonds have higher boiling points than what would be predicted. 4) Relative strength of IMFs: ionic bonds >hydrogen bonds > dipole-dipole force > London forces.

24 Homework Textbook: Hebden Page Questions: 1-4 Page 203 Questions: 13, 14, 16 6/13/2018