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

Energy

The capacity to do work or to produce heat.


2

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 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. Heat

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

State Function

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

It is independent of pathway.


6

System and Surroundings

System: That on which we focus attention

Surroundings: Everything else in the universe

Universe = System + Surroundings


7

Exo 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).


8

Figure 6.2The Combustion of Methane


9

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


10

First Law

First Law of Thermodynamics:

The energy of the universe is constant.


11

First Law

E = q + w

E = change in system’s internal energyq = heatw = work


12

Work

work = force distance

since pressure = force / area,

work = pressure volume

wsystem = PV


13

Figure 6.4The Volume of a Cylinder


14

EnthalpyEnthalpy = H = E + PV

E = H PVH = E + PV

At constant pressure,qP = E + PV,

where qP = H at constant pressure

H = energy flow as heat (at constant pressure)


15

Heat Capacity

C = heat absorbedincrease in temperature

= JC

or JK


16

Some Heat Exchange Terms

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

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


17

Hess’s Law

Reactants Products

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


18

Calculations 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


19

Standard Enthalpies of Formation• See the C(gr) → C(d) example.

• The standard enthalpy of formation (H˚f)

of a compound is the change in enthalpy that accompanies the formation of one mole of the compound from its elements with all substances in their standard states.


20

Standard 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.


21

Change in Enthalpy

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

Hrxn° = npHf(products) nrHf(reactants)


22

Equation (6.1)H˚

rxn = ΣnpH˚f (products) - ΣnrH˚

f (reactants)

Elements are not included in this calculation because elements require no change in form.

• You can find H˚f values on p. A-21,

appendix.• See the methods box on p. 263.See S/E 6.9, p 264: let’s do this using 6.1


23

Figure 6.11Energy Sources Used in the

United States


24

An Opinion

• It has been proposed that the vast coal deposits formed from plants once living in the carboniferous period have resulted from the absence of any life on earth, especially the fungi or certain saprophytic bacteria, that possessed the ability to digest cellulose.

• So, they just piled up and were eventually buried… today’s coal.


25

Figure 6.12The Earth’s Atmosphere


26

Figure 6.13Atmospheric CO2 Concentration


27

Figure 6.14 Coal Gasification

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

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