Liquids & Solids. For the condensed states the ave KE is less than the attraction between molecules so they are held together.

Transcription

1 Liquids & Solids Intermolecular Forces Matter exists in 3 states. The state of matter is influenced by the physical properties of a substance. For liquids & solids, the condensed states, many of the physical properties are the result of the attraction between the molecules in the substance. Most solids are ionic substances what holds them together? Most liquids are covalent molecules. What is the primary reason for the difference btw gases & the condensed states? The state of matter of a substance depends on the relationship between 1) the KE ( keeps the molecules apart) & 2) attraction between the particles (draws molecules together) The stronger of the 2 wins. In gases the molecules are moving fast enough ( high KE) that the attraction between molecules can t hold the molecules together. COMPARE TO MAGNETS. For the condensed states the ave KE is less than the attraction between molecules so they are held together. A change in state of matter can be done in two ways 1) increase KE (T) 2) increase P brings molecules closer together & attractive forces are greater when closer Intermolecular forces (IMF) or van der Waals forces IMF - Attractive forces between molecules that causes the molecules to come together. These only work when molecules are in close proximity. As the distance increases, the force between molecules is weaker. AGAIN LIKE MAGNETS IMF are much weaker than bond energies (force of the bond between atoms) Eg. HCl H H-Cl = 431 kj H vap = 75 kj IMF s ARE LISTED IN ORDER OF DECREASING STRENGTH Ion dipole btw Ionic & polar substance requires a salt & covalent liquid Attractive force between an ion & the polar end (dipole) of a polar molecule Eg. NaCl in H 2 O

2 Dipole-dipole- between polar molecules Attractive force between opposite dipoles in a polar substance. In 2 molecules of similar size & mass, the greater the polarity of the molecule the stronger the dipole-dipole force. Helps explain BP & MP of substances. If a substance is more polar then the attraction between molecules is greater due to dipole-dipole attractions. Thus more E is required to break the attraction. MAGNET stronger magnetic attraction needs faster motion to separate (higher T = Higher KE) What are in the bubbles of a liquid that is boiling? Hydrogen bonding - a type of dipole-dipole attraction H bonded w/ atom w/ high electronegativity Explains the properties of water. In hydrogen bonding the shared e- basically belongs to the more electronegative atom. So H is just a little ol proton hanging out there. Because there are no electrons around the H interfering with the dipole it is attracted to, the H can get VERY close to the dipole & since they are closer the attractive force is stronger. There is a very strong dipole compared to the size of the molecule. London dispersion forces among nonpolar molecules & have to be very close together. Explains how nonpolar substances can form liquids & solids. Also influences polar substances. The movement of electrons around the atoms causes an instantaneous dipole moment. That is e- are concentrated on one side of the atom & because they are close to another atom it forces that atom into a dipole because e- repel or move to the opposite side of the neighboring molecule. Analogy toddlers running around Polarizability how easily an atom can be forced into a dipole moment Larger molecules have higher polarizability because of all their electrons. Thus they have larger LDF. If molecules have the same degree of polarity then the larger molecule has larger IMF due to the larger LDF Eg. Place the following in order according to increasing boiling point Ne, H 2, Co, BaCl 2 Where does HF fit in the above?

3 Viscosity & surface tension Viscosity resistance to flow Molasses very viscous, large IMF Motor oil 40 more viscous than 30 Cohesive force keeps the molecules together Adhesive forces adhering to other surfaces Surface tension E required to increase the surface area of a liquid (ie. To overcome the inward forces & spread out.) Surface molecules experience a net inward force, so they bead up or pack close together. Vapor Pressure Vapor pressure pressure a vapour exerts on it s liquid at the equilibrium point between evaporation & condensation in a closed container OVHD SIMULATION COMPUTER SIMULATION As liquid molecules escape (evaporate) the # of molecules that condense starts to increase because since more are evaporating going into the gas state then there is a greater likelihood that a gas will hit the surface of the liquid & reenter the liquid state. When the rate of condensation = evaporation (dynamic equilibrium) the pressure the gas exerts on the liquid is the vapour pressure. Relationship between vapour pressure & temperature OVHD Volatile a substance that evaporates quickly, low IMF, so high VP because lots of the gas molecules were able to escape & there are lots pushing down on the liquid. Nonvolatile does not evaporate quickly due to high IMF, so low VP because few molecules escaped to the gas state, few had enough energy to overcome the IMF & escape into gas state Larger molecules tend to be less volatile due to larger LDF Boiling point When the vapour pressure = external pressure then the liquid will boil. Why? DEMO- balloon. Why stay this shape? How do we get it to increase in size? Why didn t it blow up? COLLAPSE balloon This is like a liquid. The bubbles that form in a liquid are of the vapour of that liquid. The bubbles can t form liquid can t boil until it overcomes the external pressure exerted on the liquid/vapour in liquid. So boils when the vapour pressure (counts for inside liquid too) is = or > external pressure. Normal boiling point the temp at which a liquid boils when the external pressure is 1 atm DEMO Boiling water w/ ice. WHY?

4 COMPUTER SIMULATION OVHD ad wes 24 Two things going on the # of gas molecules able to escape to surface depend on 1) temp higher T more escape 2) external pressure on the liquid - lower external P more escape SO if the external pressure is low, it is easier for the molecules to escape & the T does not have to be as high to get them to have enough energy to boil. The reverse is also true. EXAMPLES use graph to read T boil at given P, & reverse APPLICATIONS High altitude cooking at lower atmospheric pressure, can bake at lower temp, but needs to bake longer Pressure cooker time required to cook something usually depends on the BP of water. As long as there is water present the T will not go any higher. So the food is cooking at the T = BP of H 2 O. Pressure cooker allows a larger pressure to be exerted on the water in the food. If the P is higher the BP will be higher. Food is cooking at a higher T so does not need to cook at long. DO NOT WORRY ABOUT ANY OF THE CALCULATIONS IN THIS SECTION Heating Curves Shows the change in temp of a substance as heat is applied As you go from solid to gas is the process endothermic or exothermic? Why does sweating cool the body? Gas to liquid? Why does steam burn? OVHD PH 28 why is there a straight line during change of state? Which requires more energy to change it s temp, solid or liquid water? How do we determine the amount of energy it takes to change 25 g water from Let s say 25 o C to 50 o C? How do we determine the enthalpy change of melting 25 g of water? H fus problem ( H fus 6.01 kj/mol) Let s put it together. Calculate the H when 32 g of ice is changed to steam. Use values in Brown LeMay Appendix B Individual Problem solving Brown & LeMay #25,26

5 Melting point Normal melting point - temperature, when the external pressure is 1 atm, when the solid & liquid states have the same vapour pressure. Phase diagrams Defines the state of matter as relationship between T & P for a closed system OVHD Ph 29 At low temp state is a solid, as pressure increase more likely to be in solid state At higher temp state depends on pressure, lower pressures gas, higher pressures liquid, related to vapour pressure Triple point T &P combination where all 3 states exist. Critical point where the liquid/vapour line ends. The temperature & pressure combination above which no amount of pressure can force the gas into the liquid state. OVHD Zumdahl 133 As you increase P, what happens to the space between the molecules? Denstity? So is it more likely to be a solid or liquid? Notice the slope of the line that demonstrates this. EXCEPTION OVHD Zumdahl 131/ ad wes 27 Notice the slope of the solid/liquid phase line. Which is more dense liquid water or ice? So as pressure increases will it shift toward water or ice? CD-Rom Eg. Describe any changes in phases present when water is kept at 0 o C while the pressure is increased from 0.61 to 105 kpa. P = 101 kpa & temp changes from-5 to 50 o C. Eg. OVHD Zum 132, problem 104 You will be expected to be able to draw a phase diagram based on information given (except for line curvature). Some of your homework problems provide you w/ this practice. USE graph paper. Solid structure Amorhous solid molecular arrangement lacks a regular pattern See fig Crystalline solid the arrangement of atoms, molecules or ions shows a regular repetition of a pattern Lattice 3-D representation of the position of atoms, molecules or ions Unit cell smallest repeating unit of a lattice Cubic unit cell types - Ionic solids - points on the lattice consist of ions; held in place by ionic bonds. Eg. NaCl, CaBr 2

6 Molecular solids points on the lattice consist of molecules; held together by IMFs Eg. Ice, sucrose Atomic solids points on the lattice consist of atoms Network solids nonmetal elements held together by covalent BONDs Eg. Diamond, silica Can have different properties due to the position of the covalent bonds. Metallic solids metal atoms are the points on the lattice; held together by attraction to delocalized e- That is, the shared electrons are not owned by any one atom but all atoms Eg. Copper, gold Explains the ease with which they can be shaped, the atoms can easily glide past one another, but not broken away from each other due to the sharing of a sea of e- (so high melting points) Conductivity is better understood as well all kinds of e- to move around strength of metal increases as it shares more e-, eg. Na vs Cu Closest packing atoms take on an arrangement so that they take up the least amount of space, easier to Share e- Alloys substitutional interstitial Group 8A solids solids of the noble gases; held together by LD