Section 10.2: Kinetic Energy and Changes of State

Main Idea: Matter changes states when energy is added or removedA) Interpret changes in temperature and changes of state of a substance in terms of the kinetic theory of matterB) Relate Kelvin and Celsius Temperature scalesC) Analyze the effects of temperature and pressure on changes of state

Temperature and Kinetic EnergyParticles move in random directions at different rates Temperature- measure of the average kinetic energy of the particles that make up the material As gas is heated, the average kinetic energy and speed of its particles increases → temperature increases As gas is cooled, the average kinetic energy and speed of its particles decreases → temperature decreases

Kelvin ScaleKelvins= SI unit of temperature (divisions on a Kelvin

scale) Water freezes at 273.15 K and boils at 373.15 K The temperature at which a substance would have zero

kinetic energy is called Absolute zero Absolute zero has never been reached because

submicroscopic particles are in constant motion It is defined so that the temperature of a substance is

directly proportional to the average kinetic energy of the particles and so the zero on the Kelvin scale corresponds to zero kinetic energy

Celsius Scaleused throughout the world

Water freezes at 0°C and boils at 100°C

Fahrenheit Scaleused by weather reporters, household ovens Water freezes at 32°F and boils at 212°F

Temperature ConversionsThe divisions of the Fahrenheit and Celsius are

called degrees, but the divisions of the Kelvin scale are called Kelvins

 Celsius to Kelvin:Tk = (Tc +273) K Kelvin to Celsius:Tc = (Tk - 273) °Chttp://education-portal.com/academy/lesson/

temperature-units-converting-between-kelvins-and-celsius.html#lesson

Examples:

25°C = ? K 298 K =?°CTk = (25 +273) K Tc = (298 - 273) °CTk = 298 K Tc = 25°C

Changes of State

Dependent on temperature Include: Evaporation, Sublimation,

Condensation, Melting, Freezing, Deposition

Evaporationthe process by which particles of a liquid

form a gas by escaping from the surface

The area of the surface, temperature, and humidity affect the rate of evaporation

 Liquids that evaporate quickly are volatile

liquidsExample: perfume

Sublimationprocess by which solid goes to a gas occurs when the solid to liquid state is

skipped Example of material that sublimes: dry ice

(solid CO2)

DepositionThe opposite of sublimation  Gas goes into a solid

Condensationthe reverse of evaporation (gas → liquid) the gas particles come closer together

(condense) and form a liquid

Melting Pointthe temperature of the solid when its crystal

lattice begins to disintegrate When more heat is applied after the melting

point, energy is used until the crystal lattice collapses and becomes a liquid

Freezing PointIf a liquid substance is cooled, the

temperature falls, and the liquid becomes a solid

 The temperature of a liquid when it begins to

form a crystal lattice and becomes a solid

During Phase Changes…Because energy is always conserved, energy

is released when vapor changes to a liquid As with boiling and condensing, the kinetic

energies of the particles of a substance do not change during melting or freezing

Mass and Speed of ParticlesParticles of greater mass have greater kinetic

energy

Particles with greater speed have greater kinetic energy

Motions of gas particles cause them to spread out to fill containers uniformly

Diffusionprocess by which particles of matter fill a

space because of random motion (ex: food coloring moving in water)

 The rate of diffusion of a gas depends upon

its kinetic energy- mass and speed of its molecules

http://education-portal.com/academy/lesson/diffusion-and-effusion-grahams-law.html#lesson

Vapor PressureThe liquid water that is left in a closed

container will not all evaporate. The liquid in a closed container comes to equilibrium with its vapor

When equilibrium is reached, the pressure exerted by vapors reaches its final, maximum value (volume of liquid will not change)

 

Vapor Pressure (cont)Vapor pressure - The pressure of a substance in

equilibrium with its liquid (rates of evaporation and condensation are equal)

 The value of vapor pressure of a substance

indicates how easily the substance evaporates:High vapor pressure = more volatileLow vapor pressure = less volatile

Higher temperatures = greater vapor pressureLower temperatures = less vapor pressure

Boiling pointTemperature of the substance when its vapor

pressure equals the pressure exerted in on the surface of the liquid

 Normal boiling point is the temperature at

which liquid boils in an open container at normal atmospheric pressure

http://www.youtube.com/watch?v=XoOQNwcrDWE

Boiling point (cont)Boiling point of a liquid increases when pressure

increases Boiling point of a liquid decreases when pressure

decreases

Example: Sea level: 100°C High altitude: 96°C Because the temp of the boiling water is lower at

high elevations, it takes longer to cook foods. ↑ altitude = ↓ pressure = ↓ boiling point

Heat of Vaporizationenergy absorbed when 1 kg of a liquid vaporizes at

its normal boiling point

Joule (J) -SI unit of energy required to lift a 1-kg mass 1 meter against the force of gravity

2.26 x 106 J is the energy needed to move molecules in 1 kg of water far enough apart that they form water vapor

Heat of vaporization of water = 2.26 x 106 J/kg

Heat of Vaporization (cont)Example: How much energy is absorbed if a 500g sample of water vaporizes? 

0.5 Kg x 2.26 x 106J = 1.13 x 106 J1 Kg

Heat of FusionThe energy released as 1 kg of a substance

solidifies at its freezing point

Heat of fusion of water = 3.34 x 105 J/kg

Heat of Fusion (cont)Example: How much energy is released if a 5000g sample of water solidifies?

5 Kg x 3.34 x 105J = 1.67 x 106 J 1 Kg

Heating and Cooling curves

http://education-portal.com/academy/lesson/phase-changes-and-heating-curves.html#lesson