Welcome to Chem 2010 General chemistry (I) For third Level

1. Welcome to Chem 2010General chemistry (I) For third Level Chemistry, Biology and Physics Departments Chapter 8

2. Chapter 8: STATES OF MATTER AND ENTERMOLECULARE FORCES

3. Matter Matter is anything that has mass and occupies space. Mass is a measure of the quantity of matter in a sample of any material. STATES OF MATTER gas solid liquid

4. In the solid state, substances are 1- rigid and have definite shapes. 2- Volumes of solids do not vary much with changes in temperature (Expansion on Heating slight) 3- Volumes of solids do not vary much with changes in pressure (slight compressability). 4- Intermolecular spaces are smoll 5- Attractive force between atoms is very big.

5. In the liquid state, 1- the individual particles are confined to a given volume. 2- A liquid flows and assumes the shape of its container up to the volume of the liquid. 3- Expansion on Heating slight 4- Liquids are very hard to compress. (slight compressability) 5- Intermolecular spaces are middle. 6- Attractive force between atoms is middle.

6. In the gas state, 1- Gasesare much less dense than liquids and solids. 2- They occupy all parts of any vessel in which they are confined. 3- Gases are capable of infinite expansion 4- Gases are compressed easily. 5-Intermolecular spaces are big 6- Attractive force between atoms is very small

7. States of Matter LIQUID GAS SOLID Close together with no regular arrangement. Vibrate, move about, and slide past each other Well separated with no regular arrangement. Vibrate and move freely at high speeds Tightly packed, in a regular pattern Vibrate, but do not move from place to place 7 Lecture7: state of matter & intermolecular forces

8. PHASE CHANGES Physical changes that occur among the three states of matter. Sublimationis the conversion of a solid directly to a gas without passing through the liquid state; the reverse of that process is called deposition. The changes shown in blue are endothermic (absorb heat); those shown in red are exothermic (release heat). relatively fixed in position, but those in the liquid and gas can flow around each other.

9. INTERMOLECULAR FORCES

10. WHY? • Why is water usually aliquid and not a gas? • Why does liquid water boilat such a high temperature for such a small molecule? • Why does ice float on water? • Why do snowflakes have 6sides? • Why is I2 a solid whereasCl2 is a gas? • Why are NaCl crystals little?

11. The answers have to do with … INTERMOLECULAR FORCES

12. Intermolecular Forces

13. INTRAmolecularforces • —the forces holding atoms together to form molecules. INTERmolecular forces • Forces between molecules, between ions, or between molecules and ions.

14. Intermolecular Forces Intermolecular Forces Intramolecularforces determine such molecular properties as molecular geometries and dipole moments Intermolecular forces determine the macroscopic physical properties of liquids and solids

15. Intermolecular Forces INTERMOLECULAR ATTRACTIONS AND PHASE CHANGES Types of intermolecular forces( physical bond) 1- Ion–Ion Interactions (between ions in an ionic compound) 2- Dipole– Dipole Interactions (between 2 polar molecules) 3- Hydrogen Bonding (between 2 “special” polar molecules) 4- Dispersion Forces (between 2 non-polar molecules)

16. Intermolecular Forces 1- Ion-Ion interaction The attractive force between ions in an ionic compound. Ion – Ion interaction exist in ionic compounds such as NaCl, CaBr2, and K2SO4

17. Ion-Ion Forces for comparison of magnitude • Na+—Cl- in salt • These are the strongest forces. • Lead to solids with high melting temperatures. • NaCl, mp = 800 C0 • MgO, mp = 2800 Co

18. 2-Dipole-Dipole Forces Such forces bind molecules having permanent dipoles to one another.

19. Intermolecular Forces 2- Dipole-Dipole Forces Attractive forces between polar covalent molecules The positive end of the polar compound is attracted to the negative end of a nearby molecule. dipole–dipole interactions become less important as temperature increases.

20. Intermolecular Forces 3- Hydrogen bonding The hydrogen bond is a special case of dipole-dipole interaction. occurs between hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom.

21. Intermolecular Forces 4- Dispersion (London) Forces Non-polar molecules ( as hydrocarbons) do not have dipoles like polar molecules. How, then, can non-polar compounds form solids or liquids? 1- Because electrons are moving around the nucleus , sometimes, the charge around the nucleus is not symmetrical and a temporary dipole occur. 2- The temporary dipole forms in one atom or molecule, induce a dipole in the neighboring atoms.

22. Intermolecular Forces

23. Intermolecular Forces Dispersion (London) forces are present between all types of molecules in condensed phases The larger the molecule, the greater it’s Dispersion Forces are.

24. Intermolecular Forces Decane C10H22 has greater boiling point than C5H12 Why? Because the boiling point of alkanes increase with the length of the carbon chain. i.e. Long-chain alkanes have larger dispersion forces than short-chains.

25. Dispersion forces usually increase with molar mass. Intermolecular Forces

26. To detect the type of intermolecular forces in a compound , It is important to be able to tell whether a substance is ionic, nonpolar covalent, or polar covalent. Compound in Question Ionic or Molecular? ionic Molecular • has Ion –ion interactions Polar or NonPolar? Polar Non-Polar • Has dispersion forces • Has Dipole-dipole force Can it hydrogen bond ? hydrogen bonding will be formed if compound has H and O, F, or N

27. Intermolecular Forces Intermolecular forces ( physical bonds) are weaker than intramolecular forces( chemical bonds), Example 920 kJ is required to break the covalent bonds between H and O in one mole of water. Whiles only 40.7 kJ is required to convert one mole of liquid water into steam at 100°C.

28. What type(s) of intermolecular forces exist between each of the following molecules? O S O HBr 1- HBr is a polar molecule: dipole-dipole forces. 2- There are also dispersion forces between HBr molecules. CH4 CH4 is nonpolar: only dispersion forces. SO2 1- SO2 is a polar molecule: dipole-dipole forces. 2- There are also dispersion forces between SO2 molecules.

29. Properties of the liquid state viscosity Surface tension Capillary action Evaporation Vapor pressure We will briefly discuss the evaporation process and vapor pressure

30. Vaporization Vaporization ( evaporation), is the process by which molecules on the surface of a liquid break away and go into the gas phase. To break away, the molecules must have enough kinetic energy to overcome the intermolecular forces. The rate of vaporization increases as temperature increases.

31. Heat of vaporization “Amount of heat needed to change 1 gram of liquid to gas at its boiling point.” Boiling (Condensing) Point of Water = 100°C Heat of Vaporization (water) = 2260 J/g

32. The relation between the intermolecular forces and vapor pressure of a liquid • The very strong hydrogen bonding in water is the reason for its unusually low vapor pressure . • Dispersion forces generally increase with increasing molecular size, so substances composed of larger molecules have lower vapor pressures.

33. Exercise 1- Detect the intermolecular forces that are present in each of the following compounds. A, b, e Hydrogen bonding + dispersion force (a) CH3OH (methyl alcohol) (b) NH3 ( c ) HCl d Dispersion force (d) C6H14 (hexane) (e) H2O g Ion –ion interaction (f ) SO2 c, f, h Dipole- dipole interaction + dispersion force (g ) NaCl (h) BrF

34. Exercise Explain why? (a) CH3OH boils at 65°C while CH4 boils at -161°C . (b) Boiling point of C5H12 is greater than that of C2H6 ( c ) at definite temperature, vapor pressure of C5H12 is lower that vapor pressure C2H6