Kinetic Energy & Changes of State

Kinetic Energy & Changes of State Chapter 10 Section 2 Page 348

Temperature & Kinetic Energy • The energy of moving objects is Kinetic Energy • The temperature of a material is a measure of the average kinetic energy of the particles that make up the material Kelvin Temperature Average Kinetic Energy

The Kelvin Scale • Could Kinetic Energy be as low as zero no particle motion at all ?!? • As a substance is cooled, it loses more of its kinetic energy. The temperature at which a substance would have ZERO Kinetic Energy is called ABSOLUTE ZERO. • No particles would be moving at all. • The speed & kinetic energy = zero

The Kelvin Scale • The Kelvin Scale is defined so that the temperature of a substance is directly proportional to the average kinetic energy of the particles • The zero on the Kelvin scale corresponds to zero kinetic energy • Therefore, absolute zero corresponds to zero on the Kelvin Scale

The Kelvin Scale • A zero reading on the Celsius or Fahrenheit scales does NOT correspond to zero kinetic energy - atoms are still moving at Oo C (freezing) and Oo F (it’s below freezing, but atoms are still moving!) K oCoF _____373_______100_______ 212__________________boiling pt. (water) _____273_______0__________32__________________freezing pt. (water) _____0_________-273_______-459_________________Absolute zero

Answer This! What is the advantage of using the Kelvin scale? No negative numbers makes calculations simple Has Absolute Zero ever been reached? No, but using a laser-cooling technique that could one day allow scientists to observe quantum behavior in large objects, MIT researchers have cooled a coin-sized object to within one degree of absolute zero. {ScienceDaily(Apr. 8, 2007) }

Temperature Conversions • Since the Celsius degree & the kelvin are the same size, and Kelvin readings are 273 degrees higher than Celsius readings, simply add 273 to the Celsius reading to convert to Kelvin: Tk= (Tc + 273) K

Temperature Conversions • Similarly, a Kelvin reading can be expressed as a celsius reading by subtracting 273: Tc = (Tk – 273) oC

Temperature Conversions Tk = (Tc + 273) K Tc = (Tk – 273) oC Complete the following conversions using the equations above:

Temperature Conversions • Because Fahrenheit degrees are smaller than Celsius degrees the conversion equation is a bit more complex: oC=5/9(oF-32) oF=9/5oC + 32

Temperature Conversions oC=5/9(oF-32) To convert from Fahrenheit TO Celsius 1) subtract 32 from the Fahrenheit temp. 2) multiply by 5 3) divide by 9 (Do what is in the parenthesis FIRST) Convert 120 oF into celsius:

Temperature Conversions oF=9/5oC + 32 To convert from Celsius TO Fahrenheit 1) multiply by 9 2) divide by 5 3) add 32 (NO parenthesis, so multiply & divide FIRST) Convert 22oC into Fahrenheit:

Temperature Conversions oC=5/9(oF-32) oF=9/5oC + 32 Using the above equations, convert the following:

HOMEWORK Complete the “Temperature Conversions” worksheet for TOMORROW

Mass & Speed of Particles • The Kinetic Energy of a moving object depends on both its massAND its speed • A bowling ball will have more kinetic energy & will knock over more pins the faster it is moving

Mass & Speed of Particles • A loaded shopping cart will have more kinetic energy than an empty cart being pushed at the same speed • It takes more work and more kinetic energy to move heavier objects.

Diffusion • The process by which particles of matter fill a space because of random motion. • The rate of diffusion of a gas depends upon its kinetic energy, that is, on the mass and speed of its molecules • The faster the speed, the faster the diffusion • The lighter the molecule, the faster its diffusion

Changing State • Physical change of matter from solid to liquid to gas. • Temperature plays an important role in changing states. • There are five processes involved in changing states: evaporation, sublimation, condensation, boiling, and freezing

Evaporation • The process by which particles of a liquid form a gas byescaping from the surface. • The area of the surface, as well as the temperature and humidity, affects the rate of evaporation • Which will evaporate faster?

Evaporation • Liquids that evaporate quickly, like perfume and paint, are called volatile liquids. • We spray perfume on our skin to increase the volatility and scent by increasing the size of the surface area where evaporation takes place.

Evaporation • As liquids evaporate, they cool • This is the reason we sweat on a hot day. As the liquid evaporates from our skin, it cools us down.

Sublimation • Some solids can change into a gas directly without melting first. • This process by which particles of a solid escape from its surface and form a gas is called sublimation

Sublimation • Dry ice (solid CO2) sublimes. • Ice can also sublime. Molecules of ice can leave the surface and become water vapor, which then freezes. This is the cause of freezer burn. • Potpourri molecules sublime into the air as fragrance.

Condensation • Condensation is the reverse of evaporation. • Gas particles come closer together (they condense) and form a liquid or a solid. • The term “vapor” is used to describe the gaseous state of a substance that is liquid at room temperature

Vapor Pressure and Boiling • In a closed container, when the particles of a substance are evaporating and condensing at equal rates, equilibrium has been reached. The pressure exerted by the vapor in equilibrium with its liquid is its vapor pressure.

Vapor Pressure and Boiling • The value of the vapor pressure of a substance indicates how easily the substance evaporates. • Volatile substances, like alcohols, have a high vapor pressure, which means they evaporate easily.

Vapor Pressure and Boiling • In the same way that kinetic energy and temperature are related, temperature and vapor pressure are also related. • The evaporation rate is higher at higher temperatures

Vapor Pressure and Boiling • The boiling pointof a liquid is the temperature when its vapor pressure equals the pressure exerted on the surface of the liquid. • In an open container, the temperature rises until the vapor pressure equals atmospheric pressure. Small bubbles of vapor form below the surface and rise. The liquid boils.

Vapor Pressure and Boiling • When the pressure being exerted on a liquid increases, it takes more energy to raise the vapor pressure of that liquid to get it to boil. The boiling point temperature also increases. • If there is lesspressure being exerted on a liquid, the boiling point temperature decreases.

Vapor Pressure and BoilingTHINK ABOUT IT! • Why do foods cooked in boiling water at higher altitudes need to be cooked LONGER?

Vapor Pressure and BoilingTHINK ABOUT IT! • Why do foods cooked in boiling water at higher altitudes need to be cooked LONGER? Because the LOWER atmospheric pressure at high altitudes allows the water to boil at a lower temperature requiring a longer time to cook.

Heat of Vaporization • As you heat a substance, the temperature increases. Once the substance begins to change state from a liquid to a gas, the temperature stops rising. The energy that is being applied is being used for the state change. Once the state change is complete, the temperature will once again rise. • Liquid water and water vapor have the same Kelvin temperature during boiling.

Heat of Vaporization Changes in Temperature as a solid is heated gas temp liquid solid time Heat of Vaporization is the amount of energy absorbed when 1kg of liquid vaporizes at its normal boiling point

Heat of Vaporization Changes in Temperature as a solid is heated gas temp liquid solid time What do the plateaus on the graph represent?

Heat of Fusion • As you cool a substance, the temperature decreases. Once the substance begins to change state from a liquid to a solid, the temperature stops decreasing. Once the state change is complete, the temperature will once again decrease.

Heat of Fusion Changes in Temperature as a gas is cooled temp time Heat of Fusion is the energy released as 1 kg of a substance solidifies at its freezing point

Melting point - The temperature of a solid when its crystal lattice begins to disintegrate and becomes a liquid • Freezing point – The temperature of a liquid when it begins to form a crystal lattice and becomes a solid

Kinetic Energy & Changes of State