copyright©2000 by houghton mifflin company. all rights reserved. 1 energy the capacity to do work...

Copyright©2000 by Houghton Mifflin Company. All rights reserved.

1

EnergyEnergy

The capacity to do work or to produce heat.


2

Law of Conservation of Energy

Law of Conservation of Energy

Energy can be converted from one form to another but can neither be created nor destroyed.

(Euniverse is constant)


3

The Two Types of EnergyThe Two Types of Energy

Potential: due to position or composition - can be converted to work

Kinetic: due to motion of the object

KE = 1/2 mv2

(m = mass, v = velocity)


4

Temperature v. HeatTemperature v. Heat

See fig 6.1, p. 242. Note changes in KE/PE Why does B not get as high as A?• Frictional heating: hill gets hotter

Temperature reflects random motions of particles, therefore related to kinetic energy of the system.

Heat involves a transfer of energy between 2 objects due to a temperature difference


5

Work

• In fig 6.1, B gains PE because work was done on B by A.

• Work: f x d.• So, energy can be transferred two ways:

through work and through heat.• For ball A, PE lost as work or heat depends

on pathway.


6

State FunctionState Function

Depends only on the present state of the system - not how it arrived there.

It is independent of pathway.


7

A note about state functions

• Would you agree that the distance between Chicago and L.A. is fixed?

• This “fixed distance” is analogous to a State Function.

• But the pathway, how we get to L.A. from Chicago, is not fixed.

• Energy is a state function

• Work and Heat are not state functions


8

System and SurroundingsSystem and Surroundings

System: That on which we focus attention

Surroundings: Everything else in the universe

Universe = System + Surroundings


9

System/Surroundings

• Example:

• CH4 + O2 --> CO2 + 2H2O + heat

• System: the reaction inside your furnace

• Surroundings: everything else (furnace,

house, everything else


10

Exo and EndothermicExo and Endothermic

Heat exchange accompanies chemical reactions.

Exothermic: Heat flows out of the system (to the surroundings).

Endothermic: Heat flows into the system (from the surroundings).

Where does the the heat come from?

Fig 6.2/6.3 pg 244


11

Figure 6.2The Combustion of Methane


12

Figure 6.3: The Energy Diagram for the Reaction of Nitrogen and Oxygen to Form Nitric Oxide


13

First LawFirst Law

First Law of Thermodynamics:

The energy of the universe is constant.


14

First LawFirst Law

E = q + w

E = change in system’s internal energy

q = heat

w = work


15

Signs

• We always take they systems p.o.v.

• q= + for endothermic ( energy flows

into the system

q = - for exo, (energy flows out system)

w = - (system does work on surr)

w = + (surr do work on the system)


16

Figure 6.4The Volume of a Cylinder


17

PV WorkPV Work

(look at fig 6.4 page 246)

work = force distance, force x h

since P = force / area, (force = P x area)

work = (pressure x area) x h

w = PV expanding gas: work is neg, work done on surr compressed gas: work is pos (V is neg), work done on sys


18

EnthalpyEnthalpyEnthalpy = H = E + PV

E = H PVH = E + PV

At constant pressure,

E = qP PV or

qP = E + PV,

where qP = H at constant pressure

H = energy flow as heat (at constant pressure)


19

Change in Enthalpy

• Flow of heat is change in ethalpyH = Hproducts Hreactants

H = neg exothermicH = pos endothermic


20

Calorimetry

• Science of measuring heat.

• Calorimeter -->• Heat capacity: energy

required to change the temperature of a substance.

• C = heat absorbed (Joules) Increase in temp


21

Figure 6.6A Bomb Calorimeter


22

Heat CapacityHeat Capacity

C = heat absorbed

increase in temperature =

JC

or JK°


23

Heat Exchange Terms Heat Exchange Terms

specific heat capacityheat capacity per gram = J/°C g or J/K g

ex. Water: 4.18 J/ °C g

molar heat capacityheat capacity per mole = J/°C mol or J/K mol


24

Hess’s LawHess’s Law

Reactants Products

The change in enthalpy is the same whether the reaction takes place in one step or a series of steps.


25

Figure 6.7The Principle of Hess’s Law


26

Calculations via Hess’s LawCalculations via Hess’s Law

1. If a reaction is reversed, H is also reversed.

N2(g) + O2(g) 2NO(g) H = 180 kJ

2NO(g) N2(g) + O2(g) H = 180 kJ

2. If the coefficients of a reaction are multiplied by an integer, H is multiplied by that same integer.

6NO(g) 3N2(g) + 3O2(g) H = 540 kJ


27

Standard StatesStandard States

Compound For a gas, pressure is exactly 1 atmosphere. For a solution, concentration is exactly 1 molar. Pure substance (liquid or solid), it is the pure liquid or

solid.

Element The form [N2(g), K(s)] in which it exists at 1 atm and

25°C.


28

Change in EnthalpyChange in Enthalpy

Can be calculated from enthalpies of formation of reactants and products.

Hrxn° = npHf(products) nrHf(reactants)


29

Figure 6.10A Pathway for the Combustion of

Ammonia


30

Figure 6.11Energy Sources Used in the

United States


31

Petroleum/nat. gas

• From marine organisms 500 my ago• Petroleum: liquid mix of hydrocarbons• Natural Gas: methane, ethane, butane, propane.• Petroleum separated by separated by boiling point.

Table 6.3/6.4• Kerosene the original: replacewhale oil/animal

fats in oil lamps


32

Fractional Distillation

QuickTime™ and a decompressor

are needed to see this picture.


33

Coal

• From plants, dead, buried with heat/pressure. ( 300 my ago)

• Cellulose (CH2O empirical) from plants

slowly becomes more pure in C.

• 300 my ago nothing evolved yet to eat

cellulose.

• Use expected to increase - problems


34

Figure 6.12The Earth’s Atmosphere


35

Figure 6.13Atmospheric CO2 Concentration


36

What other energy sources do we have?

• Coal gasification:syngas

AK could benefit

• Hydrogen.

• Oil shale

• Nuclear


37

Figure 6.14Coal Gasification


38

Hydrogen

• H2 + .5O2 --> H2O ∆H˚= -286 kJ

• Much greater than methane

• Problems:

production, storage, transport

Production: a. CH4 + H2O --> 3 H2 + CO

∆H˚ = 206 kJ

( b. Decomp water ∆H˚= 286 kJ

copyright©2000 by houghton mifflin company. all rights reserved. 1 energy the capacity to do work...

Documents

houghton mifflin company

system q

heat flows

system w

endothermic energy flows

energy diagram

transfer of energy

kinetic energy