Lecture outline: Chapter 5 Th h...

Lecture outline: Chapter 5Th h iThermochemistry

1. The nature of energy1. The nature of energy2. First law of thermodynamics3 E th l i f ti3. Enthalpies of reaction4. Hess’ law5. Enthalpies of formation

S. Ensign, thermochemistry1

Chemical Reactivityy(1) Does a chemical reaction occur?(2) H idl d th ti ?(2) How rapidly does the reaction occur?(3) How far does the reaction go towards ( ) g

product(s) when all change has apparently ceased?apparently ceased?


Chemical Thermodynamics• The study of energy and it’s transformations

Thermochemistry• The energy changes that take place duringThe energy changes that take place during

chemical reactions


A working definition for energy as it relates to chemical reactionsto chemical reactions

The capacity to do work or supply heat

Energy = work + heat


Matter- The physical material of the universe. Anything that occupies space and has mass.Anything that occupies space and has mass.

Energy- Much more complicated with multiple l l f d fi itilevels of definitions.

•Potential to perform work (Ch. 5)•Potential energy (Ch 5)•Potential energy (Ch. 5)•Kinetic energy (Ch. 5)•Heat (Ch. 5)Heat (Ch. 5)•Radiant energy (Ch. 6)•Chemical energy (Ch. 5)•Free energy (Ch. 19)•Electrical energy (Ch. 20)N l (Ch 21)


•Nuclear energy (Ch. 21)

Kinetic and potential energyK E : Energy of motion; P E : stored energyK. E.: Energy of motion; P. E.: stored energy

21E 2k mv2E =


Potential energy in water


Credit: http://commons.wikimedia.org/wiki/Image:Hoover_dam_from_air.jpg , author, snakefisch

Chemical energy

A form of potential energy where chemical bonds of a molecule act as the storage mediumg

Chemicals release potential energy as heat, li h k h h d ilight, or work when they undergo a reaction to form more stable products with more stable b dbonds.


Thermal energy (Heat)Energy transferred from one object to anotherEnergy transferred from one object to another as a result of a temperature difference

•Temperature is a measure of the kinetic energy of molecular motion

T t i d b d t i i th di ti•Temperature is measured by determining the directionand magnitude of heat flow from one object to another


Units of energy?How much kinetic energy does a bicyclist with aHow much kinetic energy does a bicyclist with a weight of 110 lbs (mass =50 kg) and travelling at a speed of 22 miles/hour (10 m/s) possess?

2mv1E =

a speed of 22 miles/hour (10 m/s) possess?

k mv2E =


The S.I. unit of energy is the Joule

1joule1kg ⋅m2

1joule =s2s

Other units of energy:calorie: one calorie is the amount of energy required tocalorie: one calorie is the amount of energy required to raise the temperature of one gram of water by one degree celsius 1 cal = 4.184 Jg

Calorie: 1000 calories (1 kcal). The Calorie is what you see for food contents

1 cal 4.184 J


see for food contents.

Potential energy in food…

250 Cal = 250 000 cal =250 Cal = 250,000 cal = 1,045000 J = 1045 kJ


70 CalPotential energy in food…

70 kcal290 kJ290 kJ


Law of conservation of mass: atoms are not created or destroyed during chemical reactions. They simply rearrange. y p y g

Mass before = mass afterLaw of conservation of energy: energy is notLaw of conservation of energy: energy is not created or destroyed during reactions. It is

d f f hconverted from one form to another. energy before = energy after

Yaaaa… zees ist true on das macroscopic level..but remember, mass und energy can be interconverted according to das theory of


interconverted according to das theory of relativity, und E = mc2. nichts vergessen kinder!! Gesundheit!

All energy is potentialLaw of conservation of energy

Mix of kinetic and Where does gy ppotential energy energy go?


The first law of thermodynamicsThe first law of thermodynamicsThe energy of the gy

UNIVERSE is constant

Yaaaa… but remember E = mc2 means energy d b i t t d b and mass can be interconverted..so maybe

better to say combined E + mc2 is constant in das universe. Ich bin very smart-very much smarter dan du!! Nichts vergessen das.g


The “system” : the components of a chemical reaction of interest chemical reaction of interest

• ReactantsReactants• Products

I t l• Internal energy– Kinetic (thermal) energy– Potential (chemical) energy

h l l ( Ek E ) f The total internal energy (Σ Ek + Ep) of an isolated system is constant


y

System and surroundingsy g

ΔE = Efinal - Einitial

Energy out of system to

final initial

surroundings: (-) sign“Exothermic”

Energy into system from surroundings: (+) sign“Endothermic”


Einitial (potential energy)

Energy out of system to surroundings: (-) sign“Exothermic”

Efinal

Exothermic



2 H2(g) + O2(g) 2 H2O(g) + 483.6 kJ

Einitial (chemical energy)2 H2(g) + O2(g)

Energy out of system to surroundings: (-) sign“Exothermic”Exothermic

2 H2O(g) Efinal



Consider combustion reactions, incorporating energy in the balanced chemical equation

2 H2(g) + O2(g) 2 H2O(g) + 483.6 kJ

CH4(g) + 2 O2(g) 2 H2O(l) + CO2(g) + 890 kJ


What determines how much internal energy t h ?a system has?

• Chemical identityChemical identity• Sample size

T t• Temperature• Pressure• Physical state (s, l, g)

Th t f i t l “ t ” h iThe amount of an internal energy a “system” has is dependent on it’s present condition (state). It is notdependent on the “system’s” past history


p y p y

State function• A function or property whose value depends• A function or property whose value depends

only on the present state of the system, not the path used to arrive at that conditionpath used to arrive at that condition– Age– WeightWeight– Location

• For any state function the overall change isFor any state function, the overall change is zero if the system returns to its original condition


State functions resulting from a “change” in a system…

• A function or property whose value depends only on the present state of the system, not the path used to arrive at that condition

Age


– Age– Weight– Location

• For any state function, the overall change is zero if the system returns to its original condition

Box Elder peak

Wellsville peak cone

Mendon peak

Rattlesnake

EinitialEfinal

canyon

Deepcanyon

S. Ensign, thermochemistry25ΔE = Efinal – Einitial ~ 0

canyon

State functions and energy changes• Internal energy of a nonreacting system is a state function• Internal energy of a nonreacting system is a state function• The energy change that occurs during a chemical reaction

is a state functionΔE = Efinal - Einitial

• The contributions of work and heat to ΔE will vary depending on the experimental conditionsp g p

ΔE = q + w

2 H2(g) + O2(g) 2 H2O(g) + 483.6 kJ2(g) 2(g) 2 (g)


Electron flow with no conservation of energy

e-

e-


http://commons.wikimedia.org/wiki/File:Rjukanfossen07.JPG,licensed under the Creative Commons Attribution-Share Alike 3.0 Unportedlicense. Attribution: MB. Gustavsen (user:Hau-maggus)

Electron flow with conservation of energy

ee-

e-

Heggmoen kraftverk in Bodø, Norway. licensed under the Creative Commons Attribution 2.5 Generic license., Author: Røed


Energy fundamentals: common forms of energy

• Energy associated with physical objects– Potential energy: E ∝ mass, gravity and heightote t a e e gy ass, g a ty a d e g t– Kinetic energy: E ∝ mass and velocity2

• Energy associated with atoms and molecules• Energy associated with atoms and molecules– Chemical potential energy: stored in bonds

Thermal energy: kinetic energy of molecules– Thermal energy: kinetic energy of molecules• Manifestations of energy changes in physical

d h i l tiprocesses and chemical reactions:– Heat: “radiant” energy


– Work: some form of movement

Energy fundamentals: energy changes• Energy changes are referencedEnergy changes are referenced

relative to the “system”• The internal energy of the system

before the reaction is Ebefore the reaction is Einitial• The internal energy of the system

after the reaction is Efinal ΔE = (-) “n” kJfinal• The change in energy (ΔE) from

the perspective of the system is: ΔE = Efi l – Ei iti lΔE Efinal Einitial

• If ΔE is negative, the system lost energy to the surroundings

• If ΔE is positive, the system gained energy from the surroundings ΔE = (+) “n” kJ


g• E and ΔE are extensive properties

Energy is a “state function”• A function or property whose value depends only

on the present state of the system, not the path used to arrive at that condition

Einitial is a defined value for a given systemE i d fi d l f i tEfinal is a defined value for a given systemΔE = Efinal – EinitialΔE is a state functionΔE is a state functionΔE = q + wq and w are not state functions, but their sum must q ,equal the state function ΔE

• For any state function, the overall change is zero


if the system returns to its original condition

Simplifying the components of energy• ΔE = Efinal – Einitial = q + w• A difference in pressure is necessary for work to p y

occur in a chemical reaction• Conduct the chemical reaction in a vessel open

to the atmosphere so no pressure change can occur

• ΔEp = Efinal – Einitial = qp + 0• Enthalpy = ΔH = qp• -ΔH: heat flows out of system (exothermic)• +ΔH: heat flows into system (endothermic)


Stoichiometric relationships between coefficients and ΔH in a balanced chemical equation

2 H2(g) + O2(g) 2 H2O(g) + 483.6 kJ

Hinitial2 H2(g) + O2(g)

initial

Enthalpy (heat) out of system t di ( ) ito surroundings: (-) sign“Exothermic”

2 H O


2 H2O(g) Hfinal

What is the enthalpy change associated with the combustion in air of a balloon containing 3 liters (0.13 mols) of hydrogen gas?g ( ) y g g

2 H2(g) + O2(g) 2 H2O(g) + 483.6 kJ

ΔHforward reaction = - ΔHreverse reaction



H out H in

2 H2O(g) Hfinal2 H2O(g) Hfinal


ΔH depends on the physical states of reactants and productsof reactants and products


Determination of ΔH for a reaction:Determination of ΔH for a reaction:• Determine experimentally: calorimetry• Calculate from enthalpy data for other

reactions


ExperimentalExperimental determination of ΔH f tiΔH for a reaction

Tqcapacity Heat Δ

=T

p yΔ


Specific heatqh tifiΔTmass

qheat specific⋅

=

Wh t i th ifi h t f t if it t k 2090 What is the specific heat of water, if it takes 2090 J of energy to raise the temperature of 100 ml of water from 25° C to 30° C? (d = 1 00 g/ml)water from 25 C to 30 C? (dH2O = 1.00 g/ml)


Experimental determination of ΔH for a reaction A + B C + D• The system: A, B, C, D

Th di th t i id th• The surroundings: the water inside the calorimeter.

Exothermic reaction: heat flows from– Exothermic reaction: heat flows from system to water (T of water goes up)

– endothermic reaction: heat flows from water to system (T of water goes down

• By knowing the mass of the water, the t t h d th ifitemperature change, and the specific heat of the surroundings (water), you can determine ΔH for the reaction


can determine ΔH for the reaction

Experimental determination ofdetermination of ΔH for a reaction

• Carry out reactionM t t• Measure temperature change of reaction

• Know specific heat of calorimeter

• Know mass of heat absorbing material in


gthe calorimeter

Determine ΔH for the neutralization of H2SO4 with NaOH, given the following data:• 50 ml 1.0 M NaOH• 25 ml 1.0 M H2SO4• Ti = 25° C• Tf = 33.9° C• SH = 4 18 J/g K• SHsolution = 4.18 J/g·K• Dsolution = 1.00 g/ml

qh tifiΔTmass

qheat specific⋅

=

• Carry out reaction• Measure temperature change of reaction


p g• Know specific heat of calorimeter• Know mass of heat absorbing material in the

calorimeter

Determine ΔH for the neutralization of H2SO4 with NaOH, given the following data: qheat specific =• 50 ml 1.0 M NaOH• 25 ml 1.0 M H2SO4

T = 25° C

ΔTmasseatspec c

⋅

• Ti = 25° C• Tf = 33.9° C• SHsolution = 4.18 J/g·K• Dsolution = 1.00 g/ml


ΔH values should be reported relative to the stoichiometry of the balanced chemical equation

2NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2H2O(l)


50.0 ml of 0.100 M AgNO3 are mixed with 50.0 ml of 0.100 M HCl in a calorimeter. The temperature increases from 22.30° C to 23 11° C23.11 C.• What reaction occurred?• What is ΔH for the reaction?What is ΔH for the reaction?


Energy fundamentals: energy changes• Energy changes are referencedEnergy changes are referenced

relative to the “system”• The internal energy of the system

before the reaction is Ebefore the reaction is Einitial• The internal energy of the system

after the reaction is Efinal ΔE = (-) “n” kJfinal• The change in energy (ΔE) from

the perspective of the system is: ΔE = Efi l – Ei iti lΔE Efinal Einitial

• If ΔE is negative, the system lost energy to the surroundings

• If ΔE is positive, the system gained energy from the surroundings ΔE = (+) “n” kJ


g• E and ΔE are extensive properties

50.0 ml of 0.100 M AgNO3 are mixed with 50.0 ml of 0.100 M HCl in a calorimeter. The temperature increases from 22.30° C to 23 11° C23.11 C.• What reaction occurred?• What is ΔH for the reaction?What is ΔH for the reaction?


Hess’ Law• ΔH for a given reaction is the sum of the ΔH valuesΔH for a given reaction is the sum of the ΔH values

for each individual step in the reaction, or the sum of ΔH values for a sequence of reactions that add together to give the reaction of interest


Compare the ∆H values for the combustion of hydrogen gas to produce either water vapor or liquid y g g p p qwater


Determine ∆H for the reaction: C(s) + 2H2(g) CH4(g)Given: C + O CO ΔH = 393 5 kJG ven C(s) + O2(g) CO2(g) ΔH = -393.5 kJ

H2(g) + ½ O2(g) H2O (l) ΔH = -285.8 kJCH4(g) + 2O2(g) CO2(g) + 2H2O (l) ΔH = -890.3 kJC 4(g) O2(g) CO2(g) 2O (l) 890 3 J


Enthalpy of formation (ΔHf°)The change in enthalpy (heat input or heat output) associated with the formation of one mol )of a compound from its constituent elements in their “standard state” forms

Standard state ≡ 25° C and one atmosphere of pressurepressure

The “standard state form” of an element is the f t t bl t 25° C d t hform most stable at 25° C and one atmosphere of pressure


Element Std state form?Standard state ≡ 25° C and one atmosphere of pressure

Element Std. state form?H H2(g) (diatomic molecule)O O2(g) (diatomic molecule)N N2(g) (diatomic molecule)C C(s)graphite (empirical)Cl Cl2( ) (diatomic molecule)Cl Cl2(g) (diatomic molecule)I I2(s) (diatomic molecule)H H ( t lli lid)Hg Hg(l) (metallic solid)S S8(s) (octatomic molecule)


Standard enthalpies of formation for selected compounds at 298.15 K (25°)

Tabulated enthalpies of formation (ΔHf°)p p ( )

Substance ∆H°f (kJ/mol) Substance ∆H°f (kJ/mol)

CO2(g) -393.5 NH3(g) -46.19

CH4(g) -74.8 NaCl(s) -410.9CH4(g) 74.8 NaCl(s) 410.9

C6H12O6(s) -1273 C(s) diamond 1.88

CH3OH(l) -238.6 H2O(g) -241.8

C2H2(g) 226 7 H2O(l) -285 85C2H2(g) 226.7 H2O(l) 285.85

ΔHf° for an element in it’s standard state = 0


Using a table of enthalpies of formation to calculate the enthalpy change (ΔH°) for acalculate the enthalpy change (ΔH ) for a

chemical reaction of interest

ΔH°rxn = Σ nΔHf°(products) - Σ mΔHf°(reactants)

Appropriate stoichiometric coefficients in the balanced chemical equation



CO2(g) -393.5 NH3(g) -46.19

CH4(g) -74.8 NaCl(s) -410.9

C6H12O6(s) -1273 C(s) diamond 1.88

CH3OH(l) -238.6 H2O(g) -241.8


C2H2(g) 226.7 H2O(l) -285.85

What enthalpy change is associated with the production of glucose and oxygen from carbon dioxide and water (the reaction of photosynthesis)?

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


What enthalpy change is associated with the production of glucose and oxygen from carbon dioxide and water (the reaction of photosynthesis)?6CO2(g) + 6H2O(l) C6H12O6(s) + 6O2(g)



CO2(g) -393.5 NH3(g) -46.19

CH4(g) -74.8 NaCl(s) -410.9

C6H12O6(s) -1273 C(s) diamond 1.886 12 6( ) ( )

CH3OH(l) -238.6 H2O(g) -241.8

C2H2(g) 226.7 H2O(l) -285.85

ΔH° = Σ nΔH °(products) Σ mΔH °(reactants)


ΔH rxn = Σ nΔHf (products) - Σ mΔHf (reactants)

What enthalpy change is associated with the production of glucose and oxygen from carbon dioxide and water (the reaction of photosynthesis)?6CO2(g) + 6H2O(l) C6H12O6(s) + 6O2(g) Standard enthalpies of formation for selected compounds at 298.15 K (25°)

ΔH°rxn = Σ nΔHf°(products) - Σ mΔHf°(reactants)


CO2(g) -393.5 NH3(g) -46.19

CH4(g) -74.8 NaCl(s) -410.9

C6H12O6(s) -1273 C(s) diamond 1.88

CH3OH(l) -238 6 H2O(g) -241 8rxn f (p ) f ( ) CH3OH(l) -238.6 H2O(g) -241.8C2H2(g) 226.7 H2O(l) -285.85


Lecture outline: Chapter 5 Th h...

Documents

Transcript of Lecture outline: Chapter 5 Th h...