Liquids and Solids H2O (g) H2O (s) H2O ()

The Three States of Matter

Heat & Changes of State

Heat & Changes of State sublimation sublimation boiling melting vaporization condensation freezing deposition

Heat & Changes of State Molar Heat of: Latent Heat of: Fusion Heat involved per mole Latent Heat of: Heat involved per gram Fusion Vaporization 0 C 100 C 80 cal/g 540 cal/g

Energy Requirements for changing state: In ice the water molecules are held together by strong intermolecular forces. The energy required to melt 1 gram of a substance is called the latent heat of fusion For ice it is 80.0 cal/g The energy required to change 1 gram of a liquid to its vapor is called the latent heat of vaporization For water it is 540 cal/g

It takes more energy to vaporize water than to melt it. This is because in melting you weaken the intermolecular forces. Here about 1/6 of the hydrogen bonds are broken. In vaporization you totally break them. All the hydrogen bonds are broken Fusion is when a solid melts to form a liquid Vaporization is when a liquid evaporates to form a gas.

Heating and cooling curve for water heated at a constant rates. A-B = Solid ice, temperature is increasing. Particles gain kinetic energy, vibration of particles increases. Ice

B-C = Solid starts to change state from solid to liquid B-C = Solid starts to change state from solid to liquid. Temperature remains constant as energy is used to break inter-molecular bonds. H2O (s)  H2O () energy required  80 cal/g 0ºC

C-D = temperature starts to rise once all the solid has melted C-D = temperature starts to rise once all the solid has melted. Particles gain kinetic energy. Liquid water

D-E = Liquid starts to vaporize, turning from liquid to gas D-E = Liquid starts to vaporize, turning from liquid to gas. The temperature remains constant as energy is used to break inter-molecular forces. H2O ()  H2O (g) energy required  540 calg 100ºC

E-F = temperature starts to rise once all liquid is vaporized E-F = temperature starts to rise once all liquid is vaporized. Gas particles gain kinetic energy. steam

Heating Diagram Calculating Energy Changes 2. DHf = 80 cal/g Temperature 2. DHf = 80 cal/g 4. DHv= 540 cal/g 3. cl = 1.00 cal/g • oC 5. cv = 0.50 cal/g • oC 1. cs = 0.50 cal/g • oC (oC) Heat Added

Calculating Energy Changes Phase change: Q(gained or lost) = m x L. H.(fusion/vaporization) Temperature change: Q(gained or lost) = m • c • T heat = mass specific (Tf - Ti ) heat

Problem How much energy is required to heat 25 g of liquid water from 25C to 100C and change it to steam?

Step 1: Calculate the energy needed to heat 25.0 g of water from 25C to 100C. Q = m  c  T Q = 25g  1.0 cal g-1 C –1  75 C =

Step 2: Vaporization: Use the Latent Heat to calculate the energy required to vaporize 25g of water at 100C Q = m  LH Q = 25.0 g  540 cal/g = .25g  1mol H2O / 18g mol-1 H2O = 1.4 mol H2O vap H (H2O) = 1.4 mol H2O  40.6kJ/mol = 57 kJ

Total energy change is: 1875 cal Q = 25.0g  1.0 cal / g • C  75 C = 13500 cal Q = 25.0 g  540 cal/g = 15375 cal

Calculating Energy Changes Calculate the total amount of heat needed to change 5.5 g of ice at -7 oC to steam at 125 oC. 5.5 g x 0.5 cal/g•oC x 25oC = 68.8 cal T 5.5 g x 540.0 cal/g = 2970 cal phase T 5.5 g x 1.0 cal/g•oC x 100oC = 550.0 cal phase 5.5 g x 80.0 cal/g = 440.0 cal T 5.5 g x 0.5 cal/g•oC x 7oC = 19.3 cal = 4048 cal

Intermolecular Forces Intra-molecular forces are (within the molecule) while inter-molecular forces are (between molecules) Types of inter-molecular forces dipole-dipole (1% as strong as covalent bonds) POLAR MOLECULES A special type of dipole-dipole force is the hydrogen bond. These form between molecules that contain a hydrogen atom bonded to a very electronegative element like N, O or F. Hydrogen bonds are very strong compared to an ordinary dipole-dipole bond. E.g HF, NH3, H2O all form hydrogen bonds Hydrogen bonding 10% as strong as covalent bonds

London dispersion forces (instantaneous and induced dipoles) NON-POLAR MOLECULES

Non-polar molecule This instantaneous dipole will effect any nearby molecules Movement of electrons causes an instantaneous dipole This induces a dipole in a nearby molecule

Water molecules are polar molecules Water molecules are polar molecules. The  - oxygen forms intermolecular bonds with the  + hydrogen of another water molecules. Water has a special type of intermolecular bond called a hydrogen bond. Inter-molecular forces

Ice molecules are locked in fixed positions, held by intermolecular-bonds. Ice is less dense than liquid water because the molecules are further apart than in liquid water.

Other properties of Liquids: Many of there properties are due to the forces between the particles. Why when you pour a liquid onto a surface does it form droplets? Why do some liquids exhibit capillary action? Hg H2O Why are some liquids more viscous than others?

Viscosity is the resistance to motion of a liquid. Maple syrup is more viscous than water. But water is much more viscous than gasoline or alcohol. The stronger the attraction between molecules of a liquid, the greater its resistance to flow and so the more viscous it is.

Surface tension This allows insects to walk on water! The inward force or pull which tends to minimize the surface area of any liquid is surface tension.

The smallest surface area a liquid can form is a sphere. Surface tension is caused by hydrogen bonding between water molecules. The more polar a liquid the stronger its surface tension. Hg pure H2O H2O with detergent Surfactants are chemicals that decrease the surface tension of water, detergents and soaps are examples. The smallest surface area a liquid can form is a sphere.

Capillary action is the spontaneous rising of a liquid in a narrow tube. Two forces are responsible for this action: Cohesive forces,the intermolecular forces between molecules of the liquid Adhesive forces, between the liquid molecules and their container If the container is made of a substance that has polar bonds then a polar liquid will be attracted to the container. This is why water forms a concave meniscus while mercury forms convex meniscus Hg H2O

Vapor Pressure If you place a liquid in a container, then some of the particles will have enough kinetic energy to evaporate. You will notice the amount of liquid decreasing. At the same time some of these gaseous molecules condense to reform liquid. In an open container all the liquid will eventually evaporate out if they have enough kinetic energy. As the temperature of the liquid increases the rate at which the particles escape the surface of the liquid (evaporate) increases.

In a sealed container, molecules will start to evaporate and the liquids volume will decrease. But some of these molecules will then condense and after a short time the volume of the liquid will not change. Has evaporation and condensation stopped? No, both evaporation and condensation continue. But an equilibrium has been reached. The rate of evaporation = the rate of condensation The pressure exerted above the liquid at this point is called vapor pressure.

Boiling point and Vapor pressure When water is heated bubbles of vapor form within it. The vapor pressure in the bubble is the same as the vapor pressure of the water at that temperature. As long as this vapor pressure is less than atmospheric pressure the bubbles collapse. When the temperature of the water reaches a point that the vapor pressure of the bubble equals atmospheric pressure, the bubbles don’t collapse, they get larger and more form and escape as steam. The water begins to boil.

Boiling point and Vapor pressure REMEMBER: vapor pressure of the bubble equals atmospheric pressure The water begins to boil. 450 mm The kinetic energy does not have to be as high to make the vapor pressure in the bubble equal to atmospheric pressure. Therefore the water can boil at a lower temperature!! Normal boiling point is the temp. that water boils at 1atm pressure 450 mm 58o C By reducing the atmospheric pressure,. The water begins to boil at a lower temperature.

Unusual Properties of Water Result from very strong H-bonding High MP and BP Contains H but is not acidic Takes large amounts of heat to change temp. and phase Density as solid is lower then density of liquid. High surface tension causing capillary action in trees Great polar solvent

Solids Solids materials have particles that are locked in a highly organized crystal structure. Properties of materials in the solid phase are due largely to the arrangement of its crystal structure.

Solids The simplest repeating pattern in a crystal structure is called a unit cell. Graphite Diamond

Solids Some materials such as glass appear to be solid, however they lack a crystal structure. These materials are called amorphous solids or super cooled liquids since they act like solids without a crystal structure. Crystalline Solid Amorphous Solid