Energy, work and heatEnergy, work and heatAll processes in nature involve All processes in nature involve

energyenergy

Learning objectivesLearning objectives

• Define heat and energy and differentiate among Define heat and energy and differentiate among different types of energydifferent types of energy

• Describe main uses of energy nationally and globallyDescribe main uses of energy nationally and globally• Apply units of energy to simple calculations of Apply units of energy to simple calculations of

energy changes in chemical processesenergy changes in chemical processes• Define heat capacity and use in calculations of Define heat capacity and use in calculations of

energy consumptionenergy consumption• Describe exothermic and endothermic reactionsDescribe exothermic and endothermic reactions• Describe principle of entropy and the “heat tax” and Describe principle of entropy and the “heat tax” and

the limitations of heat enginesthe limitations of heat engines• Describe the main sources of energyDescribe the main sources of energy• Describe basic principles of the Greenhouse effect Describe basic principles of the Greenhouse effect

and global warmingand global warming

Heat and chemical reactionsHeat and chemical reactions

• The burning candle is a chemical The burning candle is a chemical reactionreaction

• It releases energy which heats up the It releases energy which heats up the air moleculesair molecules

• HeatHeat is the flow of energy due to a is the flow of energy due to a temperature differencetemperature difference

Energy from chemical reactions Energy from chemical reactions performs workperforms work

• In what way is a human like a car?In what way is a human like a car?

• It’s unreliableIt’s unreliable

• Both burn fuel to runBoth burn fuel to run– Humans burn glucoseHumans burn glucose

• CC66HH1212OO66 + O + O22 = CO = CO22 + H + H22O + energyO + energy

– Cars burn petrolCars burn petrol• CC88HH1818 + O + O22 = CO = CO22 + H + H22O + energyO + energy

Where would we be without it?Where would we be without it?

We like to use itWe like to use it

Energy is capacity to do workEnergy is capacity to do work

• Work is done in different waysWork is done in different ways

• KineticKinetic energy is energy due to motion energy is energy due to motion

• PotentialPotential energy is energy due to energy is energy due to position or stateposition or state– HeightHeight– ChemicalChemical– ElectricalElectrical

Energy is mercurialEnergy is mercurial

• Processes convert Processes convert energy from one form energy from one form to anotherto another– Falling down stairs Falling down stairs

• (potential (potential → kinetic → → kinetic → pain)pain)

– Chemical reaction Chemical reaction • (potential → heat/light)(potential → heat/light)

– Battery Battery • (potential → electrical)(potential → electrical)

– Engine Engine • (potential →heat→ (potential →heat→

kinetic)kinetic)

But it never goes awayBut it never goes away

• Energy is conserved in any processEnergy is conserved in any process– None is lostNone is lost– None is gainedNone is gained– But it goes from one place to anotherBut it goes from one place to another

• Law of Conservation of Energy:Law of Conservation of Energy:Energy is neither created nor destroyed in a chemical Energy is neither created nor destroyed in a chemical

reactionreaction• Also known as the First Law of ThermodynamicsAlso known as the First Law of Thermodynamics• System and SurroundingsSystem and Surroundings

– The process under study is the The process under study is the systemsystem– Everything else is the Everything else is the surroundingssurroundings

The First Law says that The First Law says that “perpetual motion” can’t be“perpetual motion” can’t be

• If a friend asks you If a friend asks you to invest in his new to invest in his new free energy machine free energy machine – don’t– don’t

• Visit the museum of Visit the museum of unworkable unworkable machines for a machines for a history of conmen history of conmen and futile energy and futile energy ideasideas

Heat and workHeat and work

• The industrial revolution was founded on The industrial revolution was founded on conversion of heat into mechanical motionconversion of heat into mechanical motion

• Joule (1843) was first to recognize Joule (1843) was first to recognize connectionconnection

• Heat: energy in transit – molecular motionHeat: energy in transit – molecular motion– Calorie is energy required to raise temperature of Calorie is energy required to raise temperature of

1 g water by 11 g water by 1ºCºC

• Work: force applied over a distanceWork: force applied over a distance– Joule is a force of 1 Newton applied for 1 meterJoule is a force of 1 Newton applied for 1 meter

1 cal = 4.18 J1 cal = 4.18 J

Heat enginesHeat engines

• Newcomen steam engine invented 1712Newcomen steam engine invented 1712

• Watt improves design 1760Watt improves design 1760

• Carnot (1820) described the operation Carnot (1820) described the operation of heat engines in abstract terms – the of heat engines in abstract terms – the Carnot cycle: the foundation of modern Carnot cycle: the foundation of modern thermodynamicsthermodynamics

• All engines based on burning fuels are All engines based on burning fuels are heat enginesheat engines

Rules of the roadRules of the road

• First law: energy is conservedFirst law: energy is conserved

• Is that all there is to it?Is that all there is to it?

• Heat engines convert heat into Heat engines convert heat into mechanical workmechanical work

• Question: Can all heat be converted Question: Can all heat be converted into work with 100 % efficiency?into work with 100 % efficiency?

• First Law says yesFirst Law says yes

• Second Law says No!Second Law says No!

Limitations on heat engines: the Limitations on heat engines: the Carnot cycle and entropyCarnot cycle and entropy

• Limit on efficiency of a heat engine is Limit on efficiency of a heat engine is ruled by temperature differenceruled by temperature difference

• Entropy and energy dispersalEntropy and energy dispersal• The second law of thermodynamicsThe second law of thermodynamics

Entropy of the universe is always Entropy of the universe is always increasingincreasing

• Processes only occur spontaneously Processes only occur spontaneously when energy becomes more dispersed when energy becomes more dispersed – spread out– spread out

Various spontaneities: Various spontaneities: dispersaldispersal

• Matter disperses – gas fills a container, Matter disperses – gas fills a container, two liquids mixtwo liquids mix

• Heat disperses – hot object cools on Heat disperses – hot object cools on cold surfacecold surface

• Motion disperses – a ball stops Motion disperses – a ball stops bouncingbouncing

• These processes These processes nevernever reverse reverse spontaneouslyspontaneously

Socks and spontaneitySocks and spontaneity

• Would you be stunned Would you be stunned if the tumble dryer if the tumble dryer matched the socks?matched the socks?

• Okay, you never match Okay, you never match the socks anywaythe socks anyway

• Chaos in the sock Chaos in the sock drawer is naturaldrawer is natural

• The same principles The same principles apply to chemical apply to chemical change (sort of)change (sort of)

Consequences for efficiencyConsequences for efficiency

• All processes use some of the energy All processes use some of the energy in dispersalin dispersal

• More energy is lost due to inefficiency More energy is lost due to inefficiency – friction, wind resistance etc.– friction, wind resistance etc.

Measuring energy: calories are Measuring energy: calories are case sensitivecase sensitive

• ccalorie is the energy required to raise temperature of 1 alorie is the energy required to raise temperature of 1 g of water 1 degree Cg of water 1 degree C

• CCalorie is the food version = 1,000 calalorie is the food version = 1,000 cal– Raises temperature of 1 pint of water 3.8Raises temperature of 1 pint of water 3.8ºFºF

• Joule is SI unit derived from mechanical work: the work Joule is SI unit derived from mechanical work: the work done when a force of 1 newton is applied for 1 meterdone when a force of 1 newton is applied for 1 meter

1 cal = 4.18 J1 cal = 4.18 J

Measurements of energy useMeasurements of energy use

What’s watt?What’s watt?

• Watts measure the rate of delivery of energy or Watts measure the rate of delivery of energy or powerpower

1 W = 1 J/s1 W = 1 J/s• How many Mars Bars to power a 100 W bulb for How many Mars Bars to power a 100 W bulb for

one minute?one minute?– 1 min = 60 s x 100 W = 6 kJ1 min = 60 s x 100 W = 6 kJ– 6 kJ = 1.4 kcal = 1.4 Cal (just a nibble)6 kJ = 1.4 kcal = 1.4 Cal (just a nibble)

Common energy conversionsCommon energy conversions

• 10 g of octane is burnt to produce 8500 J. 10 g of octane is burnt to produce 8500 J. How much is that in calories?How much is that in calories?

• 1 cal = 4.18 J1 cal = 4.18 J

Power consumptionPower consumption• An air conditioner is rated at 1,500 W. How An air conditioner is rated at 1,500 W. How

many kWh are used per month if it operates 6 many kWh are used per month if it operates 6 h per day?h per day?

• What is cost at $0.15 per kWh?What is cost at $0.15 per kWh?

Energy: in or out?Energy: in or out?

• Do chemical processes always create Do chemical processes always create heat?heat?

• If you think yes then how does a cold If you think yes then how does a cold pack work?pack work?

• Answer: some processes absorb heat Answer: some processes absorb heat from the surroundingsfrom the surroundings

ExoExo-thermic and -thermic and endoendo--thermicthermic

• HH22 + O + O22 gives out heat – gives out heat – exothermicexothermic

• NN22 + O + O22 absorbs heat - absorbs heat - endothermicendothermic

Enthalpy and chemical reactionsEnthalpy and chemical reactions

• Enthalpy of reactionEnthalpy of reaction ( (ΔΔHH) measures heat of ) measures heat of the reactionthe reaction

CHCH44 + 2O + 2O22 = CO = CO22 + 2H + 2H22O O ΔΔH = -11.8 kcal/gH = -11.8 kcal/g

The sun as our energy source: The sun as our energy source: directly and indirectlydirectly and indirectly

• Indirect:Indirect:– Solar radiation provided energy for fossil fuelsSolar radiation provided energy for fossil fuels– Heats the air (wind power)Heats the air (wind power)– Evaporation of water (hydro power)Evaporation of water (hydro power)

• Direct:Direct:– Solar panelsSolar panels– Photovoltaic cellsPhotovoltaic cells

• Nonsolar:Nonsolar:– NuclearNuclear– Geothermal Geothermal

Energy sourcesEnergy sources

• 85 % comes from fossil fuels85 % comes from fossil fuels• Fossil fuels are hydrocarbonsFossil fuels are hydrocarbons

– PetroleumPetroleum– CoalCoal– GasGas

• 15 % is everything else15 % is everything else– Nuclear (8)Nuclear (8)– Hydro (3)Hydro (3)– Renewables (3)Renewables (3)

Electrical generationElectrical generation

• More than 70 % comes from fossil fuelsMore than 70 % comes from fossil fuels

• Heat of combustion boils waterHeat of combustion boils water

• Steam turns a turbineSteam turns a turbine

• Turbine generates electricityTurbine generates electricity

FinitudeFinitude

• Gambling on the “Other”: how to turn 3 % Gambling on the “Other”: how to turn 3 % into 85 % without hurting anybodyinto 85 % without hurting anybody