chapter 6 thermochemistry l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Chapter 6: Thermochemistry PowerPoint Presentation
Download Presentation
Chapter 6: Thermochemistry

Loading in 2 Seconds...

play fullscreen
1 / 58

Chapter 6: Thermochemistry - PowerPoint PPT Presentation


  • 298 Views
  • Uploaded on

Chapter 6: Thermochemistry. AP Chemistry. Energy. work. The capacity to do WORK or produce HEAT. No work. Forms of Energy. Potential Energy. Stored energy due to position or composition. Gravitational Potential Energy ( E g ).

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Chapter 6: Thermochemistry' - jana


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
energy
Energy

work

The capacity to do WORK or produce HEAT

No work

forms of energy
Forms of Energy

Potential Energy

Stored energy due to

position or composition

gravitational potential energy e g
Gravitational Potential Energy (Eg)

Stored energy due to an object’s height above the surface of the earth

chemical potential energy e chem
Chemical Potential Energy (Echem)

Methane

CH4

Landfill Gas Flare

Energy Stored in the bonds of molecules

forms of energy6
Forms of Energy

Kinetic Energy

Energy due to motion

slide7

Mechanical Kinetic Energy (Ek)

Ek = ½ mv2

Energy due to an object’s motion

thermal kinetic energy
Thermal Kinetic Energy
  • Energy due to molecular motion
first law of thermodynamics
First Law of Thermodynamics

AKA The Law of Conservation of Energy

Energy cannot be created or destroyed;

it can just change forms

james prescott joule
James Prescott Joule

Wealthy English Beer Baron (1818 - 1889)

joule s chamonix falls experiment
Joule’s Chamonix Falls Experiment

Potential Energy

(mechanical)

P.E. = m g h

Kinetic Energy

(mechanical and thermal)

K.E. = ½ m v2

Q = m c DT

Law of Conservation of Energy

Energy cannot be created or destroyed in only changes forms

how we look at reactions
How we look at reactions
  • system: The Reactants and Products
  • surroundings: The room, container, etc.

http://www.weldonryan.com/new_page_1.htm

internal energy e

DE = Efinal- Einitial

DE

E

R P

R P

Internal Energy, E
  • The TOTAL energy of the system (potential + kinetic energy)

DE

E

Energy GAINED by system Energy LOST by system

temperature vs heat
Temperature vs. Heat

Temperature—

Measure of the average kinetic energy of molecules

Cl2, N2, He

100, 300, 700K

heat q
Heat (q)

Transfer of thermal energy between the system and surroundings from high to low temps

work w
Work (w)

A force exerted over a distance

w = F•Dd

Examples: syringe, hatchback

internal energy
Internal Energy

surroundings

  • The energy of a system can be changed by a flow of heat (q), work (w) or both

System

DE = q + w

Note: q and w must have

a magnitude (#) and a sign (+ or -)

energy transfer of heat only no work
Energy transfer of heat only (no work)

EXOTHERMIC

ENDOTHERMIC

heat flows INTO the system heat EXITS the system

the temperature _________ the temperature ________

surroundings

system

system

energy transfer of heat only

ENDOTHERMIC

+DE

E

R P

R P

Energy transfer of heat only

EXOTHERMIC

-DE

E

Energy GAINED by system (+q) Energy LOST by system (-q)

demos identify the following as endo or exothermic
demos: Identify the following as endo- or exothermic.

1) 2 H2 (g) + O2 (g) 

2) Ba(OH)2•8H2O + NH4Cl  BaCl2 •2H2O + 2NH3 + 8H2O

D

3) CuSO4 • 5 H2O (s) 

4) CuSO4 (s) + H2O 

5) NaC2H3O2 (aq) 

Exo

H2O(g)

Endo

Endo

Exo

NaC2H3O2 (s)

Exo

when work is done by a gas

P =1 atm cnst

Dd

When work is done by a gas

F

P =

w = FDd

DV =ADd

surroundings

A

F = PA

System

wout = PADd

wout= PDV

Sign of w? Why?

-w= PDV

w= -PDV

sample problem
Sample Problem:
  • Calculate DE for an exothermic reaction in which 15.6 kJ of heat flows and where 1.4 kJ of work is done on the system.

DE = q + w

DE = -15.6 kJ + 1.4 kJ = -14.2 kJ

sample problem23
Sample Problem

A balloon is being inflated to its full extent by heating the air inside it. In the final stages of this process, the volume of the balloon changes from 4.00 X 106 L to 4.50 X 106 L by the addition of 1.3 X 108 J of energy as heat. Assuming that the balloon expands against a constant pressure of 1.0 atm, calculate DE for the process.

DE = q + w

w = -PDV

q = +1.3 X 108 J

w = - 1.0 atm (4.5 X 106 – 4.0 X 106 L)

w = -5 X 107atm • L

5 X 107 atm • L = ? J

DE = 1.3 X 108 J + -5.1 X 10-7 J = 8.0 X 107 J

units of energy
Units of Energy
  • calorie
  • Joule
  • 1 cal = 4.18 J
state function upper case
State Function—UPPER CASE

depends on the initial and final positions; not the path

depends on the current state; not how it got there

6 pts either way

Panthers

Visitors

Explain why Energy is a state function

path function lower case

start

finish

Path Function—lower case

depends on the pathbecause they have to do with a transition.

Explain why work is a path function

Explain why heat is a path function

state path
STATE path
  • Temperature
  • Pressure
  • Volume
  • Heat Capacity
  • Density
  • Energy
  • Displacement
  • Altitude
  • Heat
  • Work
  • Distance
demo state or path function

H2O2

Ep

H2O + O2

Demo: State or Path Function?

control

add KI

add MnO4

sign conventions for q w
Sign Conventions for q & w—

Take the point of view of the system!

  • +q:
  • - q:
  • +w:
  • -w:

heat flows into the system

heat flows out of the system

work is done ON the system

work is done BY the system

types of reaction systems
Types of Reaction systems

OPENCLOSEDINSULATED

Enthalpy; Endo/Exo Problems

Calorimetry

HARD! Engineering/Thermo Classes

enthalpy

Pressure • volume = work

Energy = q + w

Enthalpy (H)

Enthalpy

a property defined as internal energy + product of pressure and volume

H = E + PDV

DH = DE + PDV

benjamin thomson
Benjamin Thomson

AKA – Count Rumford

American?/English Scientist (1753-1814)

Heat is produced by friction that is generated by the process of doing work

comparing d e and d h
Comparing DE and DH
  • Reactions that do not involve gases

2KOH (aq) + H2SO4(aq)  K2SO4 (aq) + 2H2O(l)

DH = DE + PDV

What is DV?

With no gases, DV = 0

sooooooo

DH = DE

comparing d e and d h35
Comparing DE and DH
  • Reactions in which the moles of gas does NOT change

N2 (g) + O2 (g)  2NO(g)

DH = DE + PDV

What is DV?

same moles of gas, DV = 0

sooooooo

DH = DE

comparing d e and d h redo this add derivation
Comparing DE and DH—redo this; add derivation
  • Reactions in which the moles of gas DOES change

2H2 (g) + O2(g)  2H2O (g)

DH = DE + PDV

What is DV?

waaaayyyy smaller than DH so that it’s insignificant and

DH ≈ DE

how can we calculate heat q

Heat absorbed

q

J

C =

=

DT

DT

oC

How can we calculate heat (q)?
  • Heat Capacity (C):

quantity of heat needed to change it’s temperature by 1 oC

an extensive property—

depends on amount of the substance

how can we calculate heat q38

Heat

q

J

c =

=

m • DT

g • oC

m • DT

How can we calculate heat (q)?

2) specific heat capacity (c)

intensive property

does not depend on amount

can be used to identify a substance

The amount of heat needed to raise the temperature of 1 g of substance 1oC

cH2O(l) = 4.18 J/g •oC

demos
Demos

1) Which metal will melt more ice? How do you know?

cCu = 0.385 J/g oC

cPb = 0.128 J/g oC

cAl = 0.900 J/g oC

cSn = 0.21 J/g oC

All metals have the same mass (70 g) and Tinitial (100oC)

2) How much energy is in a match?

Heat Lost = Heat Gained

3) Dollar Bill

calorimetry
Calorimetry:

A calorimeter is a device used to experimentally determine the amount of heat associated with a chemical reaction (a device that measures calories)

consider a system at a constant pressure an insulated system check redo this derivation
Consider a system at a constant pressure (an insulated system)—check/redo this derivation

Recall: E = q + w and E = H -PDV

E = q - PDV

q - PDV = H - PDV

q - PDV = H - PDV

H = q

Conclusion: Enthalpy = heat AT A CONSTANT PRESSURE

DH = qp

bomb calorimetry
Bomb Calorimetry:

used to determine heat content in food/fuels

volume

constant __________

E = q + w

E = q - PDV

E = qv

w = 0 since constant volume

hess s law
Hess’s Law:

In going from a set of reactants to a set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps

hess s law problems
Hess’s Law Problems

Use the thermochemical equations to determine the enthalpy for the reaction:

C2H4O(l) + 5/2O2(g) 2CO2(g) + 2H2O(g)

2C2H4O(l) + 2H2O(l)2C2H6O(l) + O2(g) DH=610.5KJ

2CO2(g) + 3H2O(l)C2H6O(l) + 3O2(g) DH= 2056.5KJ

slide47
NO2(g) + 2H2(g) 1/2N2(g) + 2H20(l)

2NH3(g)N2(g) + 3H2(g) DH= 23KJ

2H2O(l)+NH3(g)NO2(g) + 7/2H2(g) DH=54KJ

things to remember
Things to Remember

1) If you reverse a reaction, you must change the sign of DH

2) If you multiply the coefficients by an integer, do the same to DH

standard state
Standard State
  • Symbol = o
  • gases are at 1 atm ex: NO(g)
  • solutions are at 1.0 M ex: NaCl(aq), Ba2+
  • Pure liquids and solids are in their condensed states ex: H2O (l)
standard heat of reaction d h o rxn
Standard Heat of Reaction (DHorxn)

The enthalpy taken in / given off in a reaction

standard enthalpies of formation d h o f
Standard Enthalpies of Formation (DHof)
  • WhenDH values cannot be obtained from constant pressure calorimetry experiments

The change in enthalpy to form

one mole of a compound

from its elements

with all substances at their

standard states.

standard enthalpies of formation d h o f53
Standard Enthalpies of Formation (DHof)
  • DHof for all elements at standard state = 0

(1 atm, the state they are at 25oC)

  • DHof of most compounds is negative—find on Thermochemical Ref Chart = 0

O2 (g) , Hg (l) , Fe (s)

heat of reaction d h rxn
Heat of Reaction, DHrxn

DHorxn = S(DHof)products - S(DHof)reactants

sample problem 1
Sample Problem 1:

The thermite reaction occurs when a mixture of powdered aluminum and iron (III) oxide is ignited with a magnesium fuse. It is used to weld massive steel objects such as ships’ propellers underwater. Calculate the standard change in enthalpy for the thermite reaction: 2Al(s) + Fe2O3(s)  Al2O3(s) + 2Fe(s)

DHorxn =

Energy given off when 54 g of Al reacts with excess Fe2O3?

Tf if reaction takes place in 1 L of water at 20oC?

Graph?

DH of reverse reaction?

sample problem 2
Sample Problem 2:

Calculate DHrxno for the following reaction:

2NH3 (g) + 3O2(g) + 2CH4 (g)  2HCN(g) + 6H2O (g)

the first law of thermodynamics energy is neither created nor destroyed
The First Law of ThermodynamicsEnergy is neither created nor destroyed

surroundings

DE= q + w w = F•d = PDV E = energy

DEsystem = heat in – work out P = pressure

DEsystem = q - PDV V = volume

H = enthalpy

Rearranging: q = DE+ PDV w = work

q = heat

Enthalpy definition: H = E + PV

For a given reaction: DH = DE+D(PV)

At constant pressure: DH = DE+PDV

DH = q

System

PATM

PATM

DV