Energy and Energy and Chemical ReactionsChemical Reactions

Goals:

1. Assess heat transfer associated with changes in temperature and changes of state.

2. Apply the first law of thermodynamics.3. Define and understand the state functions

enthalpy and internal energy.4. Calculate the energy changes occurring in

chemical reactions and learn how these changes are measured.

What is the relation of What is the relation of ENERGY and CHEMISTRY?ENERGY and CHEMISTRY?

• Energy accompanies both chemical and Energy accompanies both chemical and physical changes.physical changes.* Evaporation of water: heat is absorbed by the water molecules and the system is cooled down.* Photosynthesis: sun’s energy is stored as chemical energy in carbohydrates and oxygen from carbon dioxide and water. This energy can then be released.

6 CO2 (g) + 6 H2O (g) + energy C6H12O6 (s) + 6 O2(g)

C6H12O6 (s) + 6 O2 (g) 6 CO2 (g) + 6 H2O (g) + energy

What is Energy?What is Energy?

• Energy is ______________________.Energy is ______________________.• Energy is classified as Energy is classified as kinetic kinetic or or

potentialpotential..

Kinetic EnergyKinetic Energy – is associated with – is associated with ____________

____________.____________.

Potential EnergyPotential Energy – associated with – associated with ____________

____________.____________.• Energy can be converted between Energy can be converted between

potential and kinetic.potential and kinetic.

Ex. A waterfall, a turbineEx. A waterfall, a turbine

List forms of Kinetic EnergyList forms of Kinetic Energy

• • • •

translate

rotate

vibratetranslate

rotate

vibrate

List forms of Potential Energy:List forms of Potential Energy:

• • • •

Potential EnergyPotential Energy

NaCl — composed of NaCl — composed of NaNa++ and Cl and Cl-- ions. ions.

•Positive and negative particles (ions) attract one another.

•Two ions can bond. •As the particles

attract they have a lower potential energy.

What is Internal Energy?What is Internal Energy?

• Internal energy (E or U) = ______Internal energy (E or U) = ______• Internal Energy of a chemical system Internal Energy of a chemical system

depends ondepends on– number of particlesnumber of particles– type of particlestype of particles– temperaturetemperature

• The higher the T the higher the The higher the T the higher the internal energyinternal energy

• So, use changes in T (∆T) to So, use changes in T (∆T) to monitor changes in E (∆E).monitor changes in E (∆E).

PracticePractice

• When light shines on the solar panel, a small electric motor propels the car. What types of energy are involved in these setup?

What is the Law of What is the Law of Conservation of Energy?Conservation of Energy?

• OrOr

The first Law of Thermodynamics The first Law of Thermodynamics states that _______________________states that __________________________________________________.___________________________.

Thus, The total energy of the universe Thus, The total energy of the universe is constant.is constant.

THERMODYNAMICS is_______________________________.

Heat and TemperatureHeat and Temperature

• Heat is not the same as temperature.• The thermal energy (heat) of a given

substance depends not only on temperature but also on the amount of substance.

• Heat transfer occurs when two objects _____ ___________________________________

________________.– Heat transfer always occurs from an object at

_____ temperature to an object at a ________temperature = directionality.

– Transfer of heat continues until both objects are at _____________________ (the system has reached ______________________).

Thermal EquilibriumThermal Equilibrium

• No further temperature changes occurs and the temperature throughout the entire systems is the same.

• The quantity of heat lost by a hotter object and the quantity of heat gained by the cooler object when they are in contact are numerically equal (Law of Conservation of Energy). T measures the energy transferred.

Directionality of Heat TransferDirectionality of Heat Transfer

• Heat always transfer from _________ object to __________ one.

Heat energy is associated with Heat energy is associated with ______________.______________.

Exothermic and Endothermic Exothermic and Endothermic ProcessesProcesses

• EXOTHERMIC is when _______________________________________________.

T(system) goes T(system) goes downdownT(surr) goes upT(surr) goes up

• ENDOTHERMIC is when ______________________________

_________________.

T(system) goes upT(system) goes upT (surr) goes downT (surr) goes down

What is the relation of Energy What is the relation of Energy and Chemical Reactions?and Chemical Reactions?

All of thermodynamics All of thermodynamics depends on the law depends on the law of CONSERVATION of CONSERVATION OF ENERGY: The OF ENERGY: The total energy is total energy is unchanged in a unchanged in a chemical reaction.chemical reaction.

• If PE of products is If PE of products is less than reactants, less than reactants, the difference must the difference must be released as KE.be released as KE.

Reactants

Products

Kinetic Energy

PE

Reactants

Products

Kinetic Energy

PE

PE of PE of system system dropped. dropped. KE KE increased. increased. Therefore, Therefore, you often you often feel a T feel a T increase.increase.

What are the Units of Energy?What are the Units of Energy?

• 1 calorie = heat required to 1 calorie = heat required to raise temp. of ___________________raise temp. of ___________________

____________________________________________________________________..• 1000 cal = 1 kilocalorie = 1 kcal1000 cal = 1 kilocalorie = 1 kcal• 1 kcal = 1 Calorie (a food 1 kcal = 1 Calorie (a food

“calorie”)“calorie”)

• But we use the unit (SI) calledBut we use the unit (SI) called

the the JOULE; 1 J = 1 kg mJOULE; 1 J = 1 kg m22/s/s22

• As a rough guide, 1 joule is the absolute As a rough guide, 1 joule is the absolute minimum amount of energy required (on the minimum amount of energy required (on the surface of Earth) to lift a one kilogram surface of Earth) to lift a one kilogram object up by a height of 10 centimeters.object up by a height of 10 centimeters.

• 1 cal = _______ joules1 cal = _______ joules

James JouleJames Joule1818-18891818-1889

What is Heat Capacity?What is Heat Capacity?

It is the heat required to ___ ______________________

____________________.

Which has the larger heat capacity?Which has the larger heat capacity?

What is Specific Heat What is Specific Heat Capacity?Capacity?

It is the heat required to ________ __________________________.How much energy is transferred due to How much energy is transferred due to T difference?T difference?

The heat The heat (q)(q) “lost” or “gained” is “lost” or “gained” is related to related to

a)a) sample mass sample mass

b) change in T andb) change in T and

c) c) specific heat capacityspecific heat capacity

Specific heat capacity =

heat lost or gained by substance (J)

(mass, g)(T change, K)

Specific Heat CapacitySpecific Heat CapacitySubstanceSubstance Spec. Heat (J/g•K)Spec. Heat (J/g•K)

HH22OO 4.1844.184

Ethylene glycolEthylene glycol 2.392.39

AlAl 0.8970.897

glassglass 0.840.84

AluminumAluminum

Practice:Change sp.heat of Al from J/gK to J/molK

PracticePractice

When a 4.5 g sample of a pure element absorbs 31.5 J of energy, its temperature increases from 20oC to 74.3oC. Identify the element.

Specific Heata. Silver 0.129 J/goCb. Mercury 0.139 J/goCc. Copper 0.385 J/goCd. Iron 0.444 J/goC

If 25.0 g of Al cool from 310 If 25.0 g of Al cool from 310 ooC to 37 C to 37 ooC, how C, how many joules of heat energy are lost by the many joules of heat energy are lost by the

Al?Al?

Notice that the negative sign on q signals Notice that the negative sign on q signals heat “lost by” or transferred OUT of Al.heat “lost by” or transferred OUT of Al.

heat gain/lose = q = (sp. ht.)(mass)(∆T)

Specific heat capacity = heat lost or gained by substance (J)

(mass, g) (T change, K)

Heat TransferHeat TransferNo Change of StateNo Change of State

q transferred = (sp. ht.)(mass)(∆T)q transferred = (sp. ht.)(mass)(∆T)

Heat TransferHeat Transfer• Use heat transfer as a way

to find specific heat capacity, Cp

• 55.0 g Fe at 99.8 ˚CInitial T = 372.8 K

• Drop into 225 g water at 21.0 ˚C

Initial T = 294 K• Water and metal come to 23.1

˚CFinal T = 296.1 K

• What is the specific heat capacity of the metal?

Heat TransferHeat Transfer• Because of Because of conservation of energyconservation of energy,,

q(Fe) = –q(Hq(Fe) = –q(H22O)O)

(heat out of Fe = heat into H(heat out of Fe = heat into H22O)O)

or or q(Fe) + q(Hq(Fe) + q(H22O) = 0O) = 0

Heat TransferHeat TransferChange of StateChange of State

Changes of state involve energy Changes of state involve energy (at constant T)(at constant T)• Ice + 333 J/g (heat of fusion) -----> Liquid waterIce + 333 J/g (heat of fusion) -----> Liquid water

q = mass * heat of fusionq = mass * heat of fusion

Heat Transfer and Heat Transfer and Changes of StateChanges of State

• Requires energy

q = mass * heat of vaporization + energy

Liquid ---> VaporLiquid ---> Vapor

PracticePractice

The heat of vaporization of benzene, C6H6, is 30.8 kJ/mol at its boiling point of 80.1 °C. How much heat is required to vaporize 128 g benzene at its boiling point?a.4.04 kJb.18.8 kJc.19.3 kJd.50.5 kJe.4.04 x 103 kJ

Heating/Cooling Curve of Heating/Cooling Curve of WaterWater

Note that T is constant Note that T is constant as ice meltsas ice melts

Note that T is constant Note that T is constant as ice meltsas ice melts

What quantity of heat is required to melt What quantity of heat is required to melt 500 g of ice and heat the water to steam at 500 g of ice and heat the water to steam at

100 100 ooC?C?

Heat of fusion of ice = 333 J/gHeat of fusion of ice = 333 J/gSpecific heat of water = 4.2 J/g•KSpecific heat of water = 4.2 J/g•KHeat of vaporization = 2260 J/gHeat of vaporization = 2260 J/g

Heat of fusion of ice = 333 J/gHeat of fusion of ice = 333 J/gSpecific heat of water = 4.2 J/g•KSpecific heat of water = 4.2 J/g•KHeat of vaporization = 2260 J/gHeat of vaporization = 2260 J/g

+333 J/g+333 J/g +2260 J/g+2260 J/g

What quantity of heat is required to melt What quantity of heat is required to melt 500 g of ice and heat the water to steam at 500 g of ice and heat the water to steam at

100 100 ooC?C?1. To melt ice1. To melt ice

2.2. To raise water from 0 To raise water from 0 ooC to 100 C to 100 ooCC

3.3. To evaporate water at 100 To evaporate water at 100 ooCC

4. 4. Total heat energyTotal heat energy

Heat Transfer in a Heat Transfer in a Physical Physical ProcessProcess

COCO2 2 (s, -78 (s, -78 ooC) ---> COC) ---> CO2 2 (g, -78 (g, -78 ooC)C)Heat transfers from surroundings to system in endothermic process.

Heat Transfer in a Physical Heat Transfer in a Physical ProcessProcess

• COCO2 2 (s, -78 (s, -78 ooC) ---> COC) ---> CO2 2 (g, -78 (g, -78 ooC)C)

• A regular array ofA regular array of

molecules in a solid molecules in a solid -----> gas phase molecules. -----> gas phase molecules.

• Gas molecules haveGas molecules have

_______ (higher/lower)_______ (higher/lower)

kinetic energy than in the solid kinetic energy than in the solid phasephase..

Energy Diagram: Heat TransferEnergy Diagram: Heat Transfer

∆E = E(final) - E(initial) = E(gas) - E(solid)

COCO22 solid solid

COCO22 gas gas

• Gas molecules have higher Gas molecules have higher kinetic energy.kinetic energy.

• Also, Also, WORKWORK is done by the is done by the system in pushing aside the system in pushing aside the atmosphere.atmosphere.

11stst Law of Thermodynamics: Law of Thermodynamics: Energy is ConservedEnergy is Conserved

heat transfer outheat transfer out(exothermic), -q(exothermic), -q

heat transfer inheat transfer in(endothermic), +q(endothermic), +q

SYSTEMSYSTEMSYSTEMSYSTEM

∆E = q + w∆E = q + w

w transfer inw transfer in(+w)(+w)

w transfer outw transfer out(-w)(-w)

What is Enthalpy?What is Enthalpy?Enthalpy is ________________________.Enthalpy is ________________________.Most chemical reactions occur at constant Most chemical reactions occur at constant

Pressure (P), soPressure (P), soHeat transferred at constant P = qHeat transferred at constant P = qpp

qqpp = = ∆H∆H where where H = enthalpyH = enthalpy

Heat transferred at constant P = qHeat transferred at constant P = qpp

qqpp = = ∆H∆H where where H = enthalpyH = enthalpy ∆E = q + w

and so ∆E = ∆H + w (and w is usually small)

∆H = heat transferred at constant P ≈ ∆E

∆H = change in heat content of the system

∆H = Hfinal - Hinitial

EnthalpyEnthalpy

If If HHfinalfinal < H < Hinitialinitial then ∆H is negative then ∆H is negative

Process is Process is ________________________________

If If HHfinalfinal < H < Hinitialinitial then ∆H is negative then ∆H is negative

Process is Process is ________________________________

If If HHfinalfinal > H > Hinitialinitial then ∆H is positive then ∆H is positive

Process is Process is __________________________________

If If HHfinalfinal > H > Hinitialinitial then ∆H is positive then ∆H is positive

Process is Process is __________________________________

∆∆H = HH = Hfinalfinal - H - Hinitialinitial

•Select between endothermic and exothermic.

Energy Transfer and Chemical Energy Transfer and Chemical ReactivityReactivity

What drives chemical reactions? How do What drives chemical reactions? How do they occur?they occur?

The first is answered by The first is answered by THERMODYNAMICS THERMODYNAMICS and the second by and the second by KINETICSKINETICS..

Have already seen a number of “driving Have already seen a number of “driving forces” for reactions that are forces” for reactions that are PRODUCT-PRODUCT-FAVOREDFAVORED..

•• formation of a precipitateformation of a precipitate•• gas formationgas formation

•• HH22O formation (acid-base reaction)O formation (acid-base reaction)•• electron transfer in a batteryelectron transfer in a battery

Energy Transfer and Chemical Energy Transfer and Chemical ReactivityReactivity

But energy transfer also allows us But energy transfer also allows us to predict reactivity.to predict reactivity.

In general, reactions that In general, reactions that transfer energy to their transfer energy to their surroundings are product-surroundings are product-favored.favored.

Using EnthalpyUsing Enthalpy

Consider the formation of waterConsider the formation of water

HH22(g) + 1/2 O(g) + 1/2 O22(g) --> H(g) --> H22O(g) + O(g) + 241.8 kJ241.8 kJ

Exothermic reaction — heat is a Exothermic reaction — heat is a “product” and “product” and ∆H = – 241.8 kJ ∆H = – 241.8 kJ –> –>

product favoredproduct favored

Using EnthalpyUsing Enthalpy

Making Making liquidliquid H H22O from HO from H22 + + OO22 involves involves twotwo exothermic exothermic steps. steps. H2 + O2 gas

Liquid H2OH2O vapor

Using EnthalpyUsing Enthalpy

Making HMaking H22O from HO from H22 involves two steps. involves two steps.

HH22(g) + 1/2 O(g) + 1/2 O22(g) ---> H(g) ---> H22O(g) + 242 kJO(g) + 242 kJ

HH22O(g) ---> HO(g) ---> H22O(liq) + 44 kJ O(liq) + 44 kJ ------------------------------------------------------------------

HH22(g) + 1/2 O(g) + 1/2 O22(g) --> H(g) --> H22O(liq) + O(liq) + 286 kJ286 kJ

Example of Example of HESS’S LAWHESS’S LAW——

If a rxn. is the sum of 2 or more others, If a rxn. is the sum of 2 or more others, the net ∆H is ___________________the net ∆H is ___________________

_______________________________._______________________________.

What is the Hess’s Law?What is the Hess’s Law?

Hess’s Law:Hess’s Law:Forming H2O can occur in a single step or in a two steps. ∆Htotal is the same no matter which path is followed.

∆H is a State FunctionH is a State Function

• This equation is valid because ∆H is a This equation is valid because ∆H is a STATE FUNCTIONSTATE FUNCTION

• STATE FUNCTIONsSTATE FUNCTIONs depend only on ______ depend only on ______

___________________________________.___________________________________.• V, T, P, energy —V, T, P, energy —• Unlike V, T, and P, one cannot measure Unlike V, T, and P, one cannot measure

absolute H. Can only measure ∆H.absolute H. Can only measure ∆H.

∆∆H along one path =H along one path =

∆∆H along another pathH along another path

∆∆H along one path =H along one path =

∆∆H along another pathH along another path

What are Standard Enthalpy What are Standard Enthalpy Values?Values?

Most ∆H values are labeled Most ∆H values are labeled ∆H∆Hoo

Measured under Measured under standard conditions:standard conditions:

Pressure = ____________Pressure = ____________

Concentration = ________Concentration = ________

Temperature = _________Temperature = _________

with all species in with all species in standard statesstandard states

e.g., C = graphite and Oe.g., C = graphite and O22 = gas = gas

Enthalpy ValuesEnthalpy Values

HH22(g) + 1/2 O(g) + 1/2 O22(g) --> H(g) --> H22O(g)O(g)

∆∆H˚ = -242 kJH˚ = -242 kJ

2 H2 H22(g) + O(g) + O22(g) --> 2 H(g) --> 2 H22O(g)O(g)

∆∆H˚ = -484 kJH˚ = -484 kJ

HH22O(g) ---> HO(g) ---> H22(g) + 1/2 O(g) + 1/2 O22(g) (g)

∆∆H˚ = ______H˚ = ______

HH22(g) + 1/2 O(g) + 1/2 O22(g) --> H(g) --> H22O(liquid)O(liquid)

∆∆H˚ = -286 kJH˚ = -286 kJ

Depend on how the reaction is written and Depend on how the reaction is written and on phases of reactants and products:on phases of reactants and products:Depend on how the reaction is written and Depend on how the reaction is written and on phases of reactants and products:on phases of reactants and products:

Standard Enthalpy ValuesStandard Enthalpy Values

NIST (Nat’l Institute for Standards and NIST (Nat’l Institute for Standards and Technology) gives values ofTechnology) gives values of

∆∆HHffoo = standard molar enthalpy of = standard molar enthalpy of

formationformation

It is the enthalpy change when It is the enthalpy change when ______________________________________is formed from is formed from ______ ______

under _________________under _________________..See Table 6.2 and Appendix LSee Table 6.2 and Appendix L

Standard Molar Enthalpy of Standard Molar Enthalpy of FormationFormation

HH2 2 (g) + 1/2 O(g) + 1/2 O2 2 (g) --> H(g) --> H22O (g)O (g)

∆∆HHffoo (H (H22O, g)= -241.8 kJ/molO, g)= -241.8 kJ/mol

By definition, By definition,

∆∆HHffoo = 0 for elements in their = 0 for elements in their

standard standard statesstates..

Which of the following chemical equations Which of the following chemical equations corresponds to the standard molar enthalpy corresponds to the standard molar enthalpy

of formation of Nof formation of N22O?O?

NO(g) + 1/2 N2(g) N2O(g)

2 N(g) + O(g) N2O(g)

N2(g) + 1/2 O2(g) N2O(g)

N2(g) + O(g) N2O(g)

2 N2(g) + O2(g) 2 N2O(g)

Use ∆H˚’s to calculate enthalpy change for Use ∆H˚’s to calculate enthalpy change for HH22O(g) + C(graphite) --> HO(g) + C(graphite) --> H22(g) + CO(g)(g) + CO(g)

From reference books we find:From reference books we find:

HH22(g) + 1/2 O(g) + 1/2 O22(g) --> H(g) --> H22O(g) O(g)

∆∆HHff˚ of H˚ of H22O vapor = - 242 kJ/molO vapor = - 242 kJ/mol

C(s) + 1/2 OC(s) + 1/2 O22(g) --> CO(g)(g) --> CO(g)

∆∆HHff˚ of CO = - 111 kJ/mol˚ of CO = - 111 kJ/mol

Use ∆H˚’s to calculate enthalpy change for Use ∆H˚’s to calculate enthalpy change for HH22O(g) + C(graphite) --> HO(g) + C(graphite) --> H22(g) + CO(g)(g) + CO(g)

Using Standard Enthalpy Using Standard Enthalpy ValuesValues

In general, when In general, when ALLALL enthalpies of enthalpies of formation are known, formation are known,

∆Horxn = ∆Hf

o (products) - ∆Hf

o (reactants)∆Ho

rxn = ∆Hfo (products) - ∆Hf

o (reactants)

Remember that ∆ always = final – initial

Calculate the heat of combustion Calculate the heat of combustion of methanolof methanol

What is Calorimetry?What is Calorimetry?

• Constant Volume “Bomb” Calorimeter

• Burn combustible sample.

• Measure heat evolved in a reaction.

• Derive ∆E for reaction.

• Measuring Heats of Reaction by Calorimetry

Total heat evolved = qtotal = qwater + qbomb

CalorimetryCalorimetry

qtotal = qwater + qbomb

Some heat from reaction warms waterqwater = (sp. ht.)(water mass)(∆T)

Some heat from reaction warms “bomb”qbomb = (heat capacity, J/K)(∆T)

Calculate heat of combustion of Calculate heat of combustion of

octane.octane. CC88HH1818 + 25/2 O + 25/2 O22 --> 8 CO --> 8 CO22 + 9 H + 9 H22OO

Burn 1.00 g of octane; temp rises from 25.00 to 33.20 Burn 1.00 g of octane; temp rises from 25.00 to 33.20 ooCC

Calorimeter contains 1200 g water; Heat capacity of Calorimeter contains 1200 g water; Heat capacity of bomb = 837 J/Kbomb = 837 J/K• Step 1Step 1 Calc. heat transferred from reaction to water.Calc. heat transferred from reaction to water.

• Step 2Step 2 Calc. heat transferred from reaction to bomb.Calc. heat transferred from reaction to bomb.

• Step 3Step 3 Total heat evolvedTotal heat evolved

PracticePracticeWhen 14.2 g KBr is dissolved in 100.0 g of water in a coffee-cup calorimeter, the temperature drops from 28.88 °C to 23.38 °C. What is the enthalpy change per gram of KBr dissolved in the water? Assume that the solution has a specific heat capacity of 4.18 J/g·K.

a.28.6 J/gb.185 J/gc.263 J/gd.326 J/ge.2.63 x 103 J/g

PracticePractice

• Calculate ∆Ho for a reactions given thermochemical equations of individual steps (Hess’s Law)

• Identify reactions for standard molar enthalpy of formation, ∆Ho

f

• Calculate ∆Ho for a reaction given enthalpy of formation of reactants/products.

RememberRemember• Go over all the contents of your

textbook.• Practice with examples and with

problems at the end of the chapter.• Practice with OWL tutor – Homework

Tutors (just before end-of-chapter assignment).

• Work on your OWL assignment Chapter 6.

• Practice with the quiz on CD of Chemistry Now.