Kinetic Molecular Theory and the the Nature of Fluids

Kinetic Molecular Kinetic Molecular Theory and the the Theory and the the

Nature of FluidsNature of Fluids

A model for liquids/EvaporationA model for liquids/EvaporationVapor PressureVapor PressureBoiling PointBoiling Point

Kinetic Molecular TheoryKinetic Molecular Theory

States that that all substances are made States that that all substances are made of tiny particles (atoms and molecules) of tiny particles (atoms and molecules) that are in constant motionthat are in constant motion

The kinetic energy and motion of these The kinetic energy and motion of these particles can help us to understand the particles can help us to understand the nature of fluids and phase changesnature of fluids and phase changes

A model for Liquids and A model for Liquids and gases- Kinetic Theorygases- Kinetic Theory

Liquids and Gases have kinetic energy---allows Liquids and Gases have kinetic energy---allows flowflow

No attraction between gas particlesNo attraction between gas particles Intermolecular attraction between liquid Intermolecular attraction between liquid

particles keeps them togetherparticles keeps them together Interplay between disruptive motions of Interplay between disruptive motions of

particles in a liquid and attractions among particles in a liquid and attractions among particles determines physical properties of particles determines physical properties of the liquid.the liquid.

Phase Changes using Kinetic Theory

Solid Liquid Gas

Low KE Medium KE Highest KE

IM Forces greater than KE so substance stays together

KE Forces overcome the IM forces to escape

Density and PressureDensity and Pressure

Liquids much more dense than gas due Liquids much more dense than gas due to intermolecular attractionto intermolecular attraction

Increased pressure has little effect on Increased pressure has little effect on liquids and solids volumeliquids and solids volume

Solids and Liquids are then called Solids and Liquids are then called condensed states of mattercondensed states of matter

EvaporationEvaporation

Conversion of liquid to gas = Conversion of liquid to gas = vaporizationvaporization Most molecules don’t have enough kinetic Most molecules don’t have enough kinetic

energy to break free energy to break free When vaporization is at the surface without When vaporization is at the surface without

boiling = boiling = evaporationevaporation During evaporation only those molecules During evaporation only those molecules

with a certain minimum kinetic energy can with a certain minimum kinetic energy can escape from the surface of the liquidescape from the surface of the liquid

Further EvaporationFurther Evaporation

Some escaping particle rebound back in Some escaping particle rebound back in off of air particlesoff of air particles

Heating increases kinetic energy which Heating increases kinetic energy which increases evaporationincreases evaporation

Removal of these higher energized Removal of these higher energized particles leaves a lower average energyparticles leaves a lower average energy

THEREFORE: evaporation is a cooling THEREFORE: evaporation is a cooling processprocess

Evaporation exampleEvaporation example

Sweating uses evaporation as a cooling Sweating uses evaporation as a cooling processprocess

Liquid on your skin takes heat energyLiquid on your skin takes heat energy Liquid evaporates taking that energy with Liquid evaporates taking that energy with

itit Leaves you with a lower temperatureLeaves you with a lower temperature

Vapor PressureVapor Pressure

Vapor Pressure – measure of the force Vapor Pressure – measure of the force exerted by the gas leaving the surface of exerted by the gas leaving the surface of a liquida liquid

Over time they increase and particles Over time they increase and particles condense ---eventually return to liquid condense ---eventually return to liquid statestate


In a system of constant vapor pressure, a In a system of constant vapor pressure, a dynamic equilibrium exists between dynamic equilibrium exists between vapor and liquidvapor and liquid

Equilibrium because Equilibrium because rate of evaporation = rate of condesation rate of evaporation = rate of condesation


As temperature increases, vapor pressure As temperature increases, vapor pressure increasesincreases

Particles in liquid have increased kinetic Particles in liquid have increased kinetic energyenergy

More KE, able to escape surfaceMore KE, able to escape surface Collide with “walls” Collide with “walls” Vapor pressure measured with a manometerVapor pressure measured with a manometer Vapor increases, difference between levels Vapor increases, difference between levels

increasesincreases

Boiling PointBoiling Point

Rate of evaporation increases as a liquid Rate of evaporation increases as a liquid is heatedis heated

KE increases as temperature increasesKE increases as temperature increases When a liquid is heated to a When a liquid is heated to a

temperature at which particles in the temperature at which particles in the liquid have enough KE to vaporize, the liquid have enough KE to vaporize, the liquid begins to boil.liquid begins to boil.


Boiling point- temperature at which the Boiling point- temperature at which the vapor pressure of the liquid is equal to vapor pressure of the liquid is equal to the external pressure on the liquidthe external pressure on the liquid

Not all liquids have same boiling pointNot all liquids have same boiling point Changes in altitude also affect boiling Changes in altitude also affect boiling

pointpoint


Boiling similar to evaporationBoiling similar to evaporation Particles with most KE rise to surface and Particles with most KE rise to surface and

break freebreak free

Temperature of boiling liquid never rises Temperature of boiling liquid never rises above boiling pointabove boiling point Escaping particles take growing energy with Escaping particles take growing energy with

themthem


Interesting fact:Interesting fact: Burn from steam more severe than a burn Burn from steam more severe than a burn

from waterfrom water Steam particles can carry more energy than Steam particles can carry more energy than

water particleswater particles Collides with skin harderCollides with skin harder

Boiling PointBoiling Point NORMAL BOILING POINT tableNORMAL BOILING POINT table

ASSESSMENTASSESSMENT

What factors determine the physical What factors determine the physical properties of liquids?properties of liquids?


Explain how evaporation lowers the Explain how evaporation lowers the temperature of a liquid.temperature of a liquid.


What is vaporization?What is vaporization?


Define vapor pressure.Define vapor pressure.


What is a normal boiling point?What is a normal boiling point?

Kinetic Molecular Theory and the the Nature of Fluids

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