Introduction to Introduction to thermochemistrythermochemistry

Heat, work, energy and the First Heat, work, energy and the First LawLaw

Learning objectivesLearning objectives

• Define energy and identify types of energyDefine energy and identify types of energy• Compare and contrast heat and workCompare and contrast heat and work• Describe internal energy and how it changes Describe internal energy and how it changes

during a processduring a process• Describe basic properties of state functionsDescribe basic properties of state functions• Apply first law of thermodynamics to Apply first law of thermodynamics to

determine heat flow and workdetermine heat flow and work• Define enthalpyDefine enthalpy

Behind it allBehind it all• Why do things happen in chemistry?Why do things happen in chemistry?• Substances spontaneously move towards a Substances spontaneously move towards a

position of greater stability – in energy termsposition of greater stability – in energy terms• A high energy state is unstable with respect A high energy state is unstable with respect

to a state of lower energyto a state of lower energy• A simple (but incomplete) analogy is a ball A simple (but incomplete) analogy is a ball

rolling downhillrolling downhill

EnergyEnergy

• Is capacity to perform workIs capacity to perform work• Mechanical work is application of force Mechanical work is application of force

over distanceover distance• Heat is energy transferred by virtue of Heat is energy transferred by virtue of

temperature gradient – associated with temperature gradient – associated with molecular motionmolecular motion

• Joule demonstrated experimentally that Joule demonstrated experimentally that heat and work are interchangeable heat and work are interchangeable forms of energyforms of energy

Energy: formsEnergy: forms

• KineticKinetic energy is the energy of motion energy is the energy of motion

• PotentialPotential energy is energy stored – by energy is energy stored – by position, within a spring, within a chemical position, within a spring, within a chemical bond, within the particles of a nucleusbond, within the particles of a nucleus

2

2

1mvEK

mghEP

Energy: unitsEnergy: units

• From the definition of kinetic energy (1/2mvFrom the definition of kinetic energy (1/2mv22), ), we get the units of energy:we get the units of energy:

kg mkg m22/s/s22

• S.I. unit for energy is the joule (J) = 1NmS.I. unit for energy is the joule (J) = 1Nm• Another common unit is the calorie (cal): the Another common unit is the calorie (cal): the

energy required to raise the temperature of 1 energy required to raise the temperature of 1 g of water by 1g of water by 1ºCºC

1 cal = 4.184 J1 cal = 4.184 J• Note the food calorie (Cal) = 1 000 calNote the food calorie (Cal) = 1 000 cal

Interchange and conservationInterchange and conservation

• Energy in its many forms can be Energy in its many forms can be changed from one to anotherchanged from one to another– A stationary ball on a hill has potential A stationary ball on a hill has potential

energy (P.E.) by virtue of position but no energy (P.E.) by virtue of position but no kinetic energy (K.E.). As it rolls down, it kinetic energy (K.E.). As it rolls down, it gains K.E. at the expense of P.E.gains K.E. at the expense of P.E.

Energy conservationEnergy conservation

• There is no gain or loss:There is no gain or loss:

Energy cannot be created or destroyed; it can Energy cannot be created or destroyed; it can only be changed from one form to anotheronly be changed from one form to another

– Chemical processes involve conversion of Chemical processes involve conversion of chemical potential energy into other forms and chemical potential energy into other forms and vice versavice versa

– Energy never goes away, but in some forms it is Energy never goes away, but in some forms it is more useful than othersmore useful than others

– Efficient energy use means maximizing the useful Efficient energy use means maximizing the useful part and minimizing the useless partpart and minimizing the useless part

Some like it hotSome like it hot

• Thermal energy is the kinetic energy of Thermal energy is the kinetic energy of molecular motionmolecular motion– Temperature measures the magnitude of the Temperature measures the magnitude of the

thermal energy thermal energy

• Heat is the transfer of thermal energy from a Heat is the transfer of thermal energy from a hotter to a cooler bodyhotter to a cooler body– Temperature gradient provides the “pressure” for Temperature gradient provides the “pressure” for

heat to flowheat to flow

• Chemical energy is the potential energy Chemical energy is the potential energy stored in chemical bondsstored in chemical bonds

System and surroundingsSystem and surroundings

• Any process can be divided into the SYSTEM Any process can be divided into the SYSTEM contained within the SURROUNDINGScontained within the SURROUNDINGS– When energy changes are measured in a chemical When energy changes are measured in a chemical

reaction, the system is the reaction mixture and reaction, the system is the reaction mixture and the surroundings are the flask, the room, and the the surroundings are the flask, the room, and the rest of the universe. rest of the universe.

Internal energyInternal energy

• Internal energyInternal energy is the sum of all of the types is the sum of all of the types of energy (kinetic and potential) of the of energy (kinetic and potential) of the system. It is the capacity of the system to do system. It is the capacity of the system to do workwork

• Typically we don’t know the absolute value of Typically we don’t know the absolute value of U for the systemU for the system– (Internal energy usually has symbol U. Other (Internal energy usually has symbol U. Other

sources use E)sources use E)• We can measure the We can measure the changechange to the internal to the internal

energyenergyΔΔU = UU = Ufinalfinal - U - Uinitialinitial

Work and internal energyWork and internal energy

• Work done Work done onon system system increasesincreases its its internal energyinternal energy

• Work done Work done byby system system decreasesdecreases its its internal energyinternal energy

ΔΔUU = = ww

Workin’ for a livin’Workin’ for a livin’

• Mechanical work is force applied over a Mechanical work is force applied over a distancedistance

W = F x dW = F x d

• In chemical process release of gas In chemical process release of gas allows work to be done by systemallows work to be done by system

Work done at constant pressureWork done at constant pressure

• Gas generated in reaction pushes against the Gas generated in reaction pushes against the piston with force: P x Apiston with force: P x A

• At constant P, volume increases by At constant P, volume increases by ΔΔV and V and work done by system is: work done by system is:

w = w = -P-PΔΔVV ( (ΔΔV = A x d)V = A x d)– Work done Work done byby system is system is –ve–ve in expansion ( in expansion (ΔΔV > 0)V > 0)

• ΔΔU U < 0 (< 0 (ΔΔV > 0, -PV > 0, -PΔΔV < 0)V < 0)

– Work done Work done byby system is +ve in contraction ( system is +ve in contraction (ΔΔV < V < 0)0)

• ΔΔUU > 0 ( > 0 (ΔΔV < 0, -PV < 0, -PΔΔV > 0)V > 0)

Expansion workExpansion work

• Work done by gas expanding:Work done by gas expanding:

ww = -P = -PexexΔΔVV• In expansion the In expansion the ΔΔV > 0; V > 0; ww < 0 < 0

ΔΔU < 0U < 0

• In contraction, In contraction, ΔΔV < 0; V < 0; ww > 0 > 0

ΔΔU > 0U > 0

Heat and internal energyHeat and internal energy

• Heat is transfer of energy by virtue of Heat is transfer of energy by virtue of temperature gradienttemperature gradient

ΔΔU = U = qq

• If system is cooler than surroundingsIf system is cooler than surroundings

qq > 0 > 0

• If system is hotter than surroundingsIf system is hotter than surroundings

qq < 0 < 0

Deposits and withdrawalsDeposits and withdrawals

• Process is always viewed from perspective of Process is always viewed from perspective of systemsystem

• Energy leaving system has negative sign Energy leaving system has negative sign – (decreases internal energy – lowers the chemical bank (decreases internal energy – lowers the chemical bank

balance)balance)

• Energy entering system has positive sign Energy entering system has positive sign – (increases internal energy – increases chemical bank (increases internal energy – increases chemical bank

balance)balance)

• Useful process is one where change is negativeUseful process is one where change is negative• Energy is in the form of heat or workEnergy is in the form of heat or work

– ΔΔU = q + wU = q + w

First Law of ThermodynamicsFirst Law of Thermodynamics

Total internal energy of isolated Total internal energy of isolated system is constantsystem is constant

– Energy change is difference between final Energy change is difference between final and initial states (and initial states (ΔΔU = UU = Ufinalfinal – U – Uinitialinitial))

– Energy that flows from system to Energy that flows from system to surroundings has surroundings has negativenegative sign (U sign (Ufinalfinal < < UUinitialinitial,),)

– Energy that flows into system from Energy that flows into system from surroundings has surroundings has positivepositive sign (U sign (Ufinalfinal > > UUinitialinitial.).)

Functions of stateFunctions of state

• State FunctionState Function

A property that depends only on present A property that depends only on present state of the system and is independent state of the system and is independent

of pathway to that stateof pathway to that state

• Internal energy is a state function, as Internal energy is a state function, as are pressure, volume and temperatureare pressure, volume and temperature

Significance of state functionsSignificance of state functions

• Change in state function between two Change in state function between two states is independent of pathwaystates is independent of pathway

• Given two states of a system:Given two states of a system:– ΔΔU is always the sameU is always the same– q and w depend on type of changeq and w depend on type of change

Heat and workHeat and work

• Any chemical process may have Any chemical process may have associated with it heat and work termsassociated with it heat and work terms

• The total internal energy change will be The total internal energy change will be the sum of the contributions from eachthe sum of the contributions from each

ΔΔU = q + w = q - P U = q + w = q - P ΔΔVV

q = q = ΔΔU + P U + P ΔΔVV

• In a sealed system In a sealed system ΔΔV = 0, so q = V = 0, so q = ΔΔUU

Cracked pots and enthalpyCracked pots and enthalpy

• Most reactions are conducted in open Most reactions are conducted in open vessels where P is constant and vessels where P is constant and ΔΔV ≠ 0V ≠ 0

• The heat change at constant pressure isThe heat change at constant pressure is

qqPP = = ΔΔU + P U + P ΔΔVV

• EnthalpyEnthalpy (H) is defined as: (H) is defined as:

H = U + PVH = U + PV

Heats of reaction and enthalpyHeats of reaction and enthalpy

• Absolute enthalpy of system is not knownAbsolute enthalpy of system is not known• Enthalpy Enthalpy changechange is measured is measured• Enthalpy change is known as heat of reactionEnthalpy change is known as heat of reaction

ΔΔH = H = qqPP = = ΔΔU + P U + P ΔΔVV– If reaction is exothermic and involves If reaction is exothermic and involves

expansion:expansion:• ΔΔU < 0, U < 0, ΔΔV > 0 V > 0 ΔΔH H less negativeless negative than than ΔΔUU

• Enthalpy change is portion of internal energy Enthalpy change is portion of internal energy available as heat after work is doneavailable as heat after work is done

• If no work done, all the internal energy change is If no work done, all the internal energy change is enthalpyenthalpy

Comparing Comparing ΔΔH and H and ΔΔUU

• In reactions involving volume change In reactions involving volume change at constant P, at constant P, ΔΔH and H and ΔΔU are different. U are different. How big is it?How big is it?

• Consider reaction:Consider reaction:

• 1 additional mole of gas is produced1 additional mole of gas is produced

ΔΔU = - 2045 kJ, U = - 2045 kJ, ΔΔH = - 2043 kJH = - 2043 kJ

PPΔΔV = + 2kJV = + 2kJ

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