thermodynamics and phase changes

Thermodynamics and Thermodynamics and Phase ChangesPhase Changes

EnergyEnergy EnergyEnergy – E – the combination of the amount of – E – the combination of the amount of

work and heat a sample is able to transferwork and heat a sample is able to transfer The study of energy changes is thermodynamicsThe study of energy changes is thermodynamics Energy is measured in JoulesEnergy is measured in Joules

WorkWork – w – the energy required to move an – w – the energy required to move an object by applying a force over a distanceobject by applying a force over a distance Chemists generally don’t do work.Chemists generally don’t do work.

HeatHeat – q – the transfer of energy due to a – q – the transfer of energy due to a difference in temperaturedifference in temperature Heat is a verb and not a noun in scienceHeat is a verb and not a noun in science Heat and Temperature are not the same thing.Heat and Temperature are not the same thing.

Heat vs. TemperatureHeat vs. Temperature

Heat and Temperature are not the same Heat and Temperature are not the same thing.thing.Heat is a measurement of energy flow from Heat is a measurement of energy flow from

one object to another in Joules.one object to another in Joules.Temperature is a measurement that allows us Temperature is a measurement that allows us

to determine if heat CAN flow from one object to determine if heat CAN flow from one object to another. (Different temperatures or the to another. (Different temperatures or the same temperature) same temperature)

Heat is energy flow.Heat is energy flow.Temperature is an average energy amount of Temperature is an average energy amount of

an object.an object.

First Law of ThermodynamicsFirst Law of Thermodynamics

ΔΔEEuniverseuniverse = 0 = 0

The amount of energy in the universe is The amount of energy in the universe is constant. Energy is not created or constant. Energy is not created or destroyed.destroyed.

Thermodynamic ConventionsThermodynamic Conventions

A few standard definitions and conceptsA few standard definitions and conceptsUniverseUniverse – everything in existence – everything in existenceSystemSystem – the part of the universe that is being – the part of the universe that is being

studiedstudiedSurroundingsSurroundings – everything in the universe – everything in the universe

except the systemexcept the system

Thermodynamic ConventionsThermodynamic Conventions All thermodynamic numerical values have a number and All thermodynamic numerical values have a number and

a sign.a sign. The sign shows the direction of energy flowThe sign shows the direction of energy flow A positive sign means energy is added to the system.A positive sign means energy is added to the system. A negative sign means energy is released from the A negative sign means energy is released from the

system.system.

ReactionsReactions

Consider the combustion of methaneConsider the combustion of methaneFeels hotFeels hotTransfer of heat from reaction to surroundingsTransfer of heat from reaction to surroundingsq = negativeq = negativeExothermicExothermic – heat “leaves” the system and – heat “leaves” the system and

goes into the surroundingsgoes into the surroundings

ReactionsReactions

Consider an instant ice packConsider an instant ice packFeels coldFeels coldHeat is transferred from surroundings into the Heat is transferred from surroundings into the

system.system.q = positiveq = positiveEndothermicEndothermic – heat “goes into” the system – heat “goes into” the system

from surroundingsfrom surroundings

EnthalpyEnthalpy

Enthalpy = HEnthalpy = H

If we consider a system at constant If we consider a system at constant pressure:pressure:Everyday existenceEveryday existence

ΔΔH = HH = Hfinalfinal – H – Hinitialinitial = q = heat = q = heat

HeatHeat When no phase changes or reactions are When no phase changes or reactions are

involvedinvolved

Where:Where: q = heat (in Joules)q = heat (in Joules) m= massm= mass CCpp = = specific heatspecific heat – energy required to change – energy required to change

the temperature of one gram of substance by the temperature of one gram of substance by 11°C or 1K (units = J/g °C or J/g K)°C or 1K (units = J/g °C or J/g K)

ΔΔT = change in temperature = TT = change in temperature = Tfinalfinal - T - Tinitialinitial

TmCq p

Practice ProblemsPractice Problems

How much heat in kJ is required to heat a How much heat in kJ is required to heat a 100.0g sample of water from 20.0100.0g sample of water from 20.0°C to °C to 80.0°C?80.0°C?


If 330J of heat is removed from a 10.0g If 330J of heat is removed from a 10.0g block of zinc at block of zinc at 20.020.0°C, what will be the °C, what will be the final temperature?final temperature?

Determining Heat TransferDetermining Heat Transfer CalorimetryCalorimetry – process for – process for

measuring heat transfersmeasuring heat transfers Uses the temperature change Uses the temperature change

of an object with a known of an object with a known mass, and specific heat to mass, and specific heat to calculate the heat absorbed calculate the heat absorbed or released in a process.or released in a process.

Usually water is used as the Usually water is used as the known objectknown object

Sometimes referred to as Sometimes referred to as coffee cup calorimetrycoffee cup calorimetry

Under constant pressure q = Under constant pressure q = ΔΔHH

PracticePracticeA 4.57g sample of an unknown metal is heated in boiling A 4.57g sample of an unknown metal is heated in boiling

water bath at 98.1°C. The metal is then placed in a water bath at 98.1°C. The metal is then placed in a coffee cup calorimeter with 15.20g of water which is coffee cup calorimeter with 15.20g of water which is initially at 22.3°C. The mixture’s temperature peaks at initially at 22.3°C. The mixture’s temperature peaks at 27.5°C. What is the specific heat of the metal? 27.5°C. What is the specific heat of the metal?

PracticePracticeA 9.31g piece of an unknown metal is placed in a boiling A 9.31g piece of an unknown metal is placed in a boiling

water bath at 99.3°C. It is then placed in a coffee cup water bath at 99.3°C. It is then placed in a coffee cup calorimeter with 25.31g of water. The initial temperature calorimeter with 25.31g of water. The initial temperature of the water in the calorimeter is 24.1°C and it rises to of the water in the calorimeter is 24.1°C and it rises to 27.4°C once the metal is placed in. What is the identity 27.4°C once the metal is placed in. What is the identity of the unknown metal?of the unknown metal?

Phase ChangesPhase Changes


Relate heat to temperature changeRelate heat to temperature change q = mCq = mCppΔΔTT Applies where there is a change in temperature.Applies where there is a change in temperature. AB, CD, EFAB, CD, EF


To change solid to a liquidTo change solid to a liquid Weaken some of the intermolecular forcesWeaken some of the intermolecular forces Input of energyInput of energy Enthalpy of fusion, Enthalpy of fusion, HHff

q = mHq = mHff


To change liquid to a gasTo change liquid to a gas Totally break the intermolecular forcesTotally break the intermolecular forces Input of energyInput of energy Enthalpy of vaporization, Enthalpy of vaporization, HHvv

q = mHq = mHvv


How much heat is required to melt 50.0g How much heat is required to melt 50.0g of ice at 0.0of ice at 0.0°C?°C?


What heat flow is produced by condensing What heat flow is produced by condensing 25.0g of steam at 100.0°C to water?25.0g of steam at 100.0°C to water?


What is the heat flow for converting 105g What is the heat flow for converting 105g of steam at 120°C to ice at -15°C?of steam at 120°C to ice at -15°C?

Water in A Vacuum PumpWater in A Vacuum Pump

Phase DiagramPhase Diagram

Triple PointTriple Point – where all three states exist together. – where all three states exist together. Critical PointCritical Point – point where a gas can no longer be liquefied – point where a gas can no longer be liquefied

= supercritical fluid= supercritical fluid

Water Phase DiagramWater Phase Diagram

Carbon Dioxide Phase DiagramCarbon Dioxide Phase Diagram

Dry ice sublimes Dry ice sublimes normally because at 1 atm normally because at 1 atm you find the transition you find the transition between solid and gas.between solid and gas.

Phase DiagramsPhase Diagrams

Phase DiagramsPhase Diagrams

The slope of the liquid-solid boundary is The slope of the liquid-solid boundary is negative in waternegative in water Liquid water is more dense than ice.Liquid water is more dense than ice. This is not normal.This is not normal.

Reaction Pathway Reaction Pathway DiagramsDiagrams

Reaction Pathway DiagramsReaction Pathway Diagrams

Plot energy of Plot energy of substance versus its substance versus its place in the progress place in the progress of the reactionof the reaction

Reaction coordinateReaction coordinate – – reaction progressreaction progress


Consider Bunsen Consider Bunsen Burner flameBurner flame

What term do we use What term do we use to describe reactions to describe reactions that “feel” hot?that “feel” hot?

ExothermicExothermic – – releases heat to releases heat to surroundingssurroundings


EndothermicEndothermic – – absorbs heat from absorbs heat from surroundingssurroundings

Feels coldFeels cold

Ice packs = Ice packs = endothermic reactionendothermic reaction

Activation EnergyActivation Energy

Reactions need a Reactions need a certain amount of certain amount of energy to start them.energy to start them.

Sometimes it is very Sometimes it is very small, sometimes it is small, sometimes it is very largevery large

Activation energy (EActivation energy (Eaa))

– energy needed to – energy needed to start a reactionstart a reaction

Transition StateTransition State

Transition stateTransition state – the high energy state between the – the high energy state between the products and reactantsproducts and reactants

Transition states are unstable, they always go on to form Transition states are unstable, they always go on to form products or decay back into starting materialsproducts or decay back into starting materials

Labeling Reaction DiagramsLabeling Reaction Diagrams

CatalysisCatalysis

CatalystCatalyst – compound that speeds up a – compound that speeds up a reaction but is not consumed by the reactionreaction but is not consumed by the reaction

CatalysisCatalysis Catalysts must work Catalysts must work

by changing the path by changing the path a reaction goes a reaction goes through.through.

Black is the normal Black is the normal reaction profilereaction profile

Orange is the Orange is the catalyzed profilecatalyzed profile

Catalysis changes the Catalysis changes the transition states and transition states and lowers activation lowers activation energyenergy

thermodynamics and phase changes

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