Intermolecular Forces OR WHY IS WATER SPECIAL?

Transcription

1 Intermolecular Forces OR WHY IS WATER SPECIAL?

2 Define the prefixes Inter Between, as internet, interstate Intra Inside, as intramural

3 Intermolecular Forces (2) Forces between covalent molecules These are always electrostatic forces, based on the uneven location of electrons. Just like when you build up a static charge on a balloon and make it stick to a wall

4 Intermolecular Forces (3) Always weaker than Intramolecular forces

5 Intramolecular Forces (4 & 5) Forces that hold a molecule together. Ionic (Large difference in electronegativity) Polar covalent (Medium difference in electronegativity) Nonpolar covalent (Small difference in electronegativity)

6 Types of Intermolecular Forces Sometimes these are called bonds, although they are much weaker than the bonds between two atoms that hold a compound together

7 Dispersion forces (6) Because electrons are always moving, sometimes they are unevenly spread around an atom. This causes one side of the atom to be slightly positive, and the other side slightly negative. ALL ATOMS DO THIS

8 Dispersion forces (6) The slightly negative side will push other electrons away from it. So you ll have 2 atoms with slightly positive and negative sides. The positive and negative sides stick together, just a little.

9 Dispersion forces (6) Because these charges are weak and temporary, dispersion forces are very weak. Individual atoms (for example noble gases) and non-polar molecules experience dispersion forces.

10 Dispersion forces Because dispersion forces are very weak, they cannot hold atoms or molecules together very well. Thus, non-polar molecules do not stick to each other well. They have low melting and boiling points.

11 Dispersion forces Although dispersion forces cause very weak intermolecular attraction, the intramolecular forces of non-polar molecules are very strong

12 Dipole-Dipole interactions (7) ALL Polar covalent compounds can be thought of as tiny magnets. They have a permanent positive side, and a permanent negative side. If you have two atoms in a polar covalent bond, which one will be more negative? Which one will be more positive?

13 Dipole-Dipole interactions (7) When two atoms are in a polar covalent bond, which one will be more negative? The one with the higher electronegativity Which one will be more positive? The one with the lower electronegativity

14 Dipole-Dipole interactions (7) Dipole-dipole forces are stronger than dispersion forces. Hydrogen Chloride has dipole-dipole forces holding its molecules to each other. ALL polar covalent compounds have dipole-dipole interactions

15 Hydrogen bonding (8) Hydrogen bonding is a special type of dipole-dipole bonding. It occurs when hydrogen is bonded to fluorine, nitrogen or oxygen Not all polar covalent compounds do this

16 Hydrogen bonding (8) Fluorine, nitrogen and oxygen have very high electronegativities, and hydrogen easily shifts its one electron mostly over to them. This makes the hydrogen side of the molecule VERY positive, and the other side VERY negative

17 Hydrogen bonding (8) These molecules stick to each other really well. They have relatively high melting and boiling points. Water is the prime example of hydrogen bonding.

18 What kind of bond? (9 and 10) Choose the strongest bond that it makes All bonds have dispersion forces All polar covalent bonds have dipole dipole forces Only polar covalent compounds where hydrogen is attached to nitrogen, oxygen or fluorine can have hydrogen bonding

19 What kind of bond? (9 and 10) H 2 H 2 S HCl HF Only Dispersion Dipole-dipole Dipole-dipole Hydrogen Bonding

20 Rank the Bond types from weakest to strongest (9 ) Dispersion forces Dipole-dipole forces Hydrogen bonds Nonpolar covalent bonds Polar covalent bonds Ionic bonds

21 What is the only intermolecular force acting in a nonpolar covalent bond? (10 ) Dispersion

22 What kind of bond do you need for a permanent dipole?(11) Polar covalent

23 Which of the species below can form a hydrogen bond? (12) C 2 H 6 CH 3 NH 2 KCl CH 3 CH 2 CH 2 OH CH 3 OCH 3

24 Rank the Intermolecular bond types from weakest to strongest (13) I 2 =N 2 H 2 S H 2 O

25 List the dominant (Strongest) IMF for each and then rank bond strength Substance IMF Strength HBr Dipole-dipole Middle (2) O 2 Dispersion Weakest (3) CH 3 OH Hydrogen Bonding Strongest (1)

26 How do intermolecular forces impact the phase of a substance at room temperature? (15 and 16) The weaker the intermolecular forces, the more likely a substance is to be a gas at room temperature The stronger the intermolecular forces, the more likely it is to be a solid

27 Arrange the following in order of increasing boiling points. Give reasons (17A) These are all about the same mass CH 3 OCH 3 Bond types- nonpolar covalent and polar covalent Intermolecular forces dipole dipole CH 3 CH 2 OH CH 3 CH 2 CH 3

28 Arrange the following in order of increasing boiling points. Give reasons (17A) CH 3 OCH 3 Dipole-Dipole CH 3 CH 2 OH Bond types- nonpolar covalent and polar covalent, with O to H bonds Intermolecular forces hydrogen bonding CH 3 CH 2 CH 3

29 Arrange the following in order of increasing boiling points. Give reasons (17A) CH 3 OCH 3 Dipole-Dipole CH 3 CH 2 OH Hydrogen bonding CH 3 CH 2 CH 3 Bond types- nonpolar covalent Intermolecular forces dispersion

30 Arrange the following in order of increasing boiling points. Give reasons (17A) CH 3 OCH 3 Dipole-Dipole CH 3 CH 2 OH Hydrogen bonding CH 3 CH 2 CH 3 Dispersion

31 Arrange the following in order of increasing boiling points. Give reasons (17A) CH 3 CH 2 CH 3 Dispersion CH 3 OCH 3 Dipole-Dipole CH 3 CH 2 OH Hydrogen bonding

32 Arrange the following in order of increasing boiling points. Give reasons (17B) Br 2 Cl 2 I 2 These are all non-polar covalent, so rank them by mass, with the heaviest having the highest boiling point

33 Arrange the following in order of increasing boiling points. Give reasons (17B) Cl 2 Br 2 I 2

34 What is vapor pressure? (18) The pressure of vapor phase molecules of a substance above a liquid or solid phase of that same substance It is a measure of how eager a substance is to evaporate or sublime

35 How does a strong intermolecular force affect vapor pressure? (19) A strong intermolecular force will decrease vapor pressure. It will take more energy to overcome the intermolecular force and get the substance into the vapor phase.

36 Identify which in the pair of stubstance will have the higher vapor pressure CO 2 (nonpolar) or SO 2 (polar) SO 2 because it will have higher intermolecular forces CH 3 OH or CH 3 OCH 3 CH 3 OH because it will have hydrogen bonding

37 Review! wlp4t6d0