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PHASE TRANSISTIONS PHASE TRANSISTIONS ALSO CALLED CHANGES OF STATE, HAPPENS BY CHANGING THE TEMPERATURE AND/OR PRESSURE OF A SUBSTANCE. MELTING  SOLID.

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Presentation on theme: "PHASE TRANSISTIONS PHASE TRANSISTIONS ALSO CALLED CHANGES OF STATE, HAPPENS BY CHANGING THE TEMPERATURE AND/OR PRESSURE OF A SUBSTANCE. MELTING  SOLID."— Presentation transcript:

1 PHASE TRANSISTIONS PHASE TRANSISTIONS ALSO CALLED CHANGES OF STATE, HAPPENS BY CHANGING THE TEMPERATURE AND/OR PRESSURE OF A SUBSTANCE. MELTING  SOLID TO LIQUID: MELTING FREEZING  LIQUID TO SOLID: FREEZING CONDENSATION  GAS TO LIQUID: CONDENSATION EVAPORATION  LIQUID TO GAS: EVAPORATION SUBLIMATION  SOLID TO GAS: SUBLIMATION DEPOSITION  GAS TO SOLID: DEPOSITION

2 Intermolecular Forces  London Dispersion Forces:  London Dispersion Forces: Also called Induced dipole forces. An instantaneous dipole is created within the atom or molecule via the instantaneous movement of the electrons around the nucleus. All molecules have LDF.  Dipole-Dipole Forces:  Dipole-Dipole Forces: The attractive force between molecules due to the existence of an overall dipole moment. Polar molecules have d-d forces.  Hydrogen Bonding:  Hydrogen Bonding: The attractive force between a highly electronegative atom of one molecule with the hydrogen on another molecule also containing a very electronegative atom. N, O, F are the electronegative atoms.

3 Interrogating interactions

4 Properties dependent on the Intermolecular Forces SURFACE TENSION: SURFACE TENSION: describes the resistance that a liquid has to an increase in its own surface area. Answers why bugs can walk on water and why green strawberry plastic baskets float on water dispite the 1” open square “holes”. Intermolecular forces create surface tension – an invisible “skin” holding the molecules together Force must be applied to break the “skin”

5 Properties dependent on the Intermolecular Forces EVAPORATION: Evaporation of a liquid occurs when the average kinetic energy present within the liquid is greater than the intermolecular forces responsible for holding the substance in its liquid state. When the particles have enough kinetic energy to overcome these attractive forces, the particles will escape from the surface to become a gas.

6 Energy must be added to a system to overcome the attractive forces that are exerted among liquid molecules. When an equal quantity of vapor condenses to a liquid, an equal amount of energy is released.

7 Explain the physical process of boiling. At room temperature the water molecules have enough energy to allow the particles to move past each other but not enough to escape the surface tension. As the temperature of water increases, the heat energy (from the burner) is transferred to kinetic energy (for the molecules) leading to an increase in the molecular motion of the molecules. This action results in an increase in the vapor pressure above the surface of the liquid. When the vapor pressure of the water equals the external pressure, boiling begins. Now a sufficient amount of the molecules have enough energy to resist the attractive forces. Bubbles of vapor are formed throughout the liquid and these bubbles rise to the surface to escape.

8 Properties dependent on the Intermolecular Forces VAPOR PRESSURE: When a liquid evaporates in a closed container, the gaseous vapor that forms at the surface of the liquid eventually establishes an equilibrium with the particles remaining in the liquid state. Equilibrium is established when the rate of evaporation is equal to the rate of condensation. A VOLATILE substance evaporates readily, has a low surface tension, and a high vapor pressure at ambient temperature. The Intermolecular forces are weak. A NONVOLATILE substance requires a large amount of energy to evaporate, has a high surface tension, and a low vapor pressure. The intermolecular forces are strong.

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10 THE DISSOLVING PROCESS Intermolecular Forces: “Like dissolves like”

11 Miscible / Immiscible: Two liquids are miscible in each other if they readily mix to form a uniform solution. Two immiscible liquids will always separate out into two distinct layers.

12 PROPERTIES ASSOCIATED WITH WATER HYDRATES: Solids that contain water molecules as part of their crystalline structure. The water in the hydrate is known as the water of hydration or the water of crystallization. HYGROSCOPIC: A substance is hygroscopic if it readily absorbs water from the atmosphere and forms a hydrate. DELIQUESCENT: A substance is deliquescent if it absorbs water from the air until it forms a solution. DESICCANTS: Compounds that absorb water and are used as drying agents. EFFLORESCENCE: The process by which crystalline materials spontaneously lose water when exposed to air. HYDROPHOBIC / HYDROPHILLIC: Hydrophobic refers to nonpolar substances interacting with water. Hydrophillic (water loving) is in reference to the interactions polar substances have with water.

13 Some Physical Properties of Water  Water is colorless, odorless, and tasteless.  The normal boiling point is 100 o C and the normal melting point is 0 o C.  The heat of vaporization (  H vap ) is 2259 J/g or 540 cal/g and the heat of fusion (  H fus ) is 335 J/g or 80 cal/g.  The vapor pressure of water at 20 o C is 17.5 torr; this is relatively low when compared to volatile ethyl alcohol (43.9 torr) and very volatile ethyl ether (442.2 torr)  The density of water at 4.0 o C is 1.0 g/mL; the density of ice at 0 o C is g/mL.  The specific heat of water is 1.0 cal/g o C or J/g o C.

14 The Unusual Properties of Water  Water co-exists in all three states of matter naturally on earth.  The only common substance is a liquid at STP.  As a solid, it is less dense than its liquid form, that is “Ice floats”. Most substances contract upon solidifying.  It has a very high Heat Capacity. It stores a large amount of energy with very little atomic or molecular motion.  It requires a lot of heat energy (enthalpy) to change states.  It has a high boiling point for such a low molecular weight compound.  It is a universal solvent, as a good dissolving medium a large number of substances are soluble in water.

15 Why is Water so unusual? The fundamental explanation for water’s unusual properties relates to the polarity of its bonds. Polarity describes the partial charge associated with a bond or molecule. A polar bond or molecule has a charge distribution present (one end positively charged and the other end negatively charged) while a nonpolar bond or molecule has no distinct charge distribution (neutral). Water is composed of two polar covalent O-H bonds (the difference in electronegativity is 1.4) arranged in a “bent” molecular geometry. Each bond has a dipole moment pointing in an overall similar direction leading to the existence of an overall dipole moment. The oxygen atom pulls the pair of electrons closer towards itself (making it partially negative) and further from the hydrogen atoms (making them partially positive). - + HYDROGEN BONDING This charge distribution allows the partially positive hydrogen atoms from one molecule to be attracted to the partially negative oxygen atom of another molecule. This strong interlocking network between neighboring molecules is called HYDROGEN BONDING. The ability to form strong hydrogen bonds is the main reason for water’s unusual properties.

16 Why does ice float in water? Ice floats in its own liquid due to the intermolecular force, hydrogen bonding. As water freezes, the molecular motion of the molecules slow down and the partial positive end (hydrogen) of one water molecule is attracted to the partial negative end (oxygen) of another water molecule. Combine this event with the bent shape of water and the molecules become arranged in a 3-D hexagonal array. This array creates pockets of vacuum (empty space) in the lattice structure as well as a decrease in the number of molecules per unit volume. The mass is directly related to the number of molecules therefore, in the solid state, since there are less particles then there must be less mass per unit volume therefore the solid is less dense than the liquid.

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18 Explain if ice will float in ethyl alcohol (d = g/L)? Ice would not float in pure ethyl alcohol because the density of water is g/mL which is greater than g/mL for ethyl alcohol. Yet since ethyl alcohol also undergoes a small degree of hydrogen bonding, the sinking effect is not as dramatic as it would be with a nonpolar substance.

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20 Why does water have a relatively high boiling point? Water has a relatively high boiling point because of the amount of intermolecular forces present. Water experiences LDF (London Dispersion Forces) and d-d (dipole-dipole) forces, along with the additional attractive force, Hydrogen bonding. A large amount of heat energy is required to break all of these forces in order for a phase transition to occur, thus the high boiling point.

21 Water as a universal solvent Water is called the universal solvent because of its ability to dissolve many substances. The general solubility rule is “like dissolves like”. Since water is a polar molecule it will dissolve other polar substances as well as ionic compounds. Water will not dissolve or mix with nonpolar substances therefore water is immiscible in nonpolar substances. Description of how water dissolves an ionic salt (like NaCl) on the molecular level? Although the attractive force from the partial charge of a single polar molecule is not as strong as the charge from an ion, it is plausible that a multitude of polar molecules could react on a single ion effectively. The positive end (H + ) of several water molecules are attracted to the negative end of the salt crystal (Cl - ) while the negative end of several water molecules (O 2- ) are attracted to the positive end of the crystal (Na + ). The ionic bonds of the crystal are weakened by the solvating effect of the water molecules and the ions break away from the bulk crystal. The large number of water molecules in the container prevent the salt ions from re- combining.

22 PRACTICE PROBLEMS 1. Which contains less heat, ice at 0 o C or water at 0 o C? Explain your answer. 2. On the basis of KMT, explain why vapor pressure increases with temperature. Ice at 0 o C contains less heat than liquid water at the same temperature. Heat must be added to convert ice to water, so the water will contain that much more additional heat energy. Also the liquid state is in motion much more than the solid state. An increase in motion can only be accomplished by an increase in energy. According to the kinetic molecular theory, the vapor pressure of a liquid should increase with temperature because of the increase in collisions and kinetic energy that always accompanies an increase in heat energy (temperature). KE m = 3 / 2 RT. The increase in energy thus motion allows the liquid molecules to escape (overcome the surface tension and other cohesive forces maintaining the liquid state) from the surface of the liquid into the gas phase.

23 Surfactants and the laundry Surfactants in lungs have two tails which makes for a rectangular shape Surfactants with one tail will arrange in a spherical shape – micelles Micelles are soluble Detergents clean by dissolving grease inside the micelle


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