Phase Change Diagram 1) Phase changes when matter changes from one phase to another- this is a PHYSICAL change. Label the phase changes: (color code) melting, vaporization, sublimation condensation, freezing, deposition
Add these to your graph! Fusion (melting)…S L…endothermic Crystallization (freezing)…L S…exothermic Vaporization (boiling)…L G…endothermic Condensation …G L…exothermic Sublimation…S G…endothermic Deposition…. G-S exothermic
2) Physical Equilibrium A dynamic condition in which 2 opposing changes occur at equal rates in a closed system.
3) Physical Equilibrium equations Vapor Liquid + Heat Energy Exothermic Or Liquid + Heat Energy Vapor Endothermic
4) What is equilibrium vapor pressure? Pressure exerted by a vapor above a liquid in equilibrium with that liquid at a given temp. At a constant temp., evaporation/condensation becomes constant.
5) Equilibrium vapor pressure continued By increasing the temp., the avg. amount of k.e. increases and the particles move faster. The equilibrium is disturbed and the vapor concentration increases, which in turn increases the condensation and a new equilibrium is established.
6) What is vapor pressure? The pressure of a vapor above the liquid Vapor pressure is directly related to temperature and also depends on the strength of the intermolecular forces.
Question 13 Heath bookS 7) Acetone has a greater vapor pressure than ethyl alcohol at 25.0° C. Predict which substance has stronger intermolecular forces? Why? Acetone C 3 H 6 O Ethanol C 2 H 5 OH B.pt 56.0°CB.pt 78.0°C The stronger the intermolecular forces, the lower the vapor pressure and the slower the particles will evaporate. The weaker the IMF, the faster the particles will evaporate, lower the boiling point. Ethanol exhibits hydrogen bonding. Summary: Polar materials have lower vapor pressures as compared to nonpolar materials.
8) What is the meaning of volatile in terms of liquids? Volatile liquids evaporate quickly. (acetone, alcohol) These liquids have very high vapor pressures and evaporation occurs at room temperature. WHY? Both ethanol and acetone are volatile…which one is more volatile? How do you know?
9) Boiling! Boiling – Vaporization throughout the entire liquid. The boiling point defined as -the temperature at which a L G. Normal boiling point = boiling point at 1 atm Boiling points are related to external or atmospheric pressure(s). If an increase in altitude occurs, air pressure decreases and the temperature at which something boils decreases. ( requires less k.e. to cause the boiling so it occurs at a lower T) And food would cook slower. (it is the heat that cooks food) At higher altitudes, intermolecular forces can break more easily, allowing the liquid to turn into vapor more easily.
10) Why does a pressure cooker cook food faster? The cooker exerts an increased pressure and the water boils at a higher temperature than normal (more heat energy) and the food cooks faster. stop
11) New definition of boiling Temperature at which the vapor pressure pushing up equals the external pressure pushing down on the liquid DEMO Boiling in Ice Water.
12) The boiling point of ethanol is 78.5*C. Will the boiling point of acetone be lower or higher and why? Refer back to your drawings. Lower Acetone is Non Polar and needs less kinetic energy to vaporize meaning to break the intermolecular forces between the molecules.
13) Melting S L The stronger the intermolecular forces, the higher or lower the melting point? Higher Why?
14) What type of substance with almost equal masses, polar or non-polar, have higher melting points? Polar Why? It takes more K.E. to overcome the stronger attractive forces.
15) Why do materials with ionic and metallic bonds (not covalent) have very high melting points? It takes much K.E. to overcome intra- molecular forces between ions as compared to overcoming the intermolecular forces that hold covalent molecules together.
Water is a different- The Freak of nature! Liquid water is in a Bent geometry, while ice is linear…liquid water takes up less volume …more dense …solid water floats on liquid water Most dense temp. is 4 °C Ice is linear & polar, and forms a crystal lattice Density: L>S>G Liquid H 2 O Solid H 2 O
Background Info Energy…measured in calories (cal) in metric and Joules (J) in SI * 1 cal = 4.18 J * 1 Cal = 1000 cal (food calories)
Heat of Phase Changes 1) Relationship between ________ & __________during the phase changes of water. The conditions for this diagram are for 1 mole of water (18.02 g) @ 1 atm. Heat is being added to the system at a rate of 100. J of energy/minute, which is a very small amount of heat energy. This would be like lightly blowing air on a small piece of ice. Heating Curve for Water Q= mH f Freezing Melting
2) Molar heat of fusion? The energy needed to change 1 mole (18.02g) of ice into 1 mole (18.02g) of liquid water at 1 atm and 0 o C and vice versa. Molar heat of fusion symbol? H f Values for the H f = 333.5 J/g – Convert 6.009 kJ/mol This applies to freezing and melting on the diagram.
3) According to this diagram, water has strong H – bonding and it takes 60 minutes for the ice to completely melt. At this point, only 15% of the H-bonds have been broken. Enough to melt.
4) Molar Heat of Vaporization The energy needed to change 1 mole (18.02 g) of liquid water into 1 mole of(18.02 g) vapor at 1 atm and 100. o C and vise versa. Molar heat of Vaporization symbol = H v Values = 2264 J/g or 40.79 kJ/mol This applies to vaporization and condensation Q= mH V Vaporization Condensation
5) Why does it take longer to vaporize one mole of water rather than melting 1 mole of water at the same conditions? The rest of the H-bonds (85%) must be broken in order for the liquid to vaporize.
Heat of Phase Changes 6) Equations and symbols Q = heat energy (J or kJ) H f = molar heat of fusion H v = molar heat of vaporization Q = mol x H f or H v or Q = amount (mol or mass) x H f or H v
Heat lost = Heat gained Heat lost = Heat gained mc t = mc t Q lost = Q gained Q lost = mc t Q gained = mc t m= mass t = temperature c = specific heat capacity Specific Heat – the amount of energy needed to change the temperature of 1 g of a substance by 1 °C for liquid water = 1 cal/g°C = 4.18 J/g°C
Heating Curve for Water mc t = mc t Q= mcΔT Q= mH f Q= mH v Temp C Heat added
Assignments # 5,6,7 Specific heat examples – see book