# Chapter 6- Thermochemistry

Mechanics

Oct 27, 2013 (5 years and 1 month ago)

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Chapter 6
-

Thermochemistry

Energy

Energy

is the capacity to do work or to
produce heat

Law of conservation of energy
-

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

The energy of the universe is
constant

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2

Types of Energy

Potential energy
-

the energy due to position
or composition

Potential energy in chemicals is present in the
bonds between atoms

Kinetic energy
-

energy due to the motion of
the object
-

depends on the mass of the object
and its velocity

Equation: KE = ½ mv
2

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3

Converting Energy

Energy can be converted from one form to
another

Two ways to transfer energy: Through

heat

and

work

Heat
-

involves the transfer of energy between two
objects due to a temperature difference.

Work
-

a force acting over a distance.

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4

Energy Transfer and Pathway

How an energy transfer is divided between
heat and work depends on the
pathway

Pathway
-

the specific conditions involved

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5

State Function

State function or State Property
-

A state
function refers to a property of the system
that depends on only its present state.

It does not depend on the system’s past or
future

In other words, it does not depend on

how
the system arrived at its present state

It depends only on the characteristics of
the present state

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6

Chemical Energy

There are 2 parts to anything we study: the

system

and the

surroundings

System
-

the part of the universe on which we
wish to focus attention

Surroundings
-

everything else in the universe

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7

Exothermic

Exothermic
-

energy flows

out of

the
system and into the surroundings

The potential energy stored in the chemical
bonds is being converted to thermal energy

more

energy stored in their
bonds than the products.

This means the bonds in the products are
stronger than those of the reactants

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8

Exothermic

Bonds with more potential energy are LESS
STABLE than bonds with less energy, so the
more reactive something is, the more energy
it can release.

More stable bonds have

less

potential
energy than less stable bonds.

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9

Endothermic

Endothermic
-

reaction absorbs energy from
the surroundings. Heat flows

into

the system.

The reactants have

less

potential
energy than the products

The reactants are

more stable

than the products

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10

Thermodynamics

Thermodynamics
-

the study of energy and its
interconversions

The first law of thermodynamics
-

the energy of the
universe is constant

Internal energy

the sum of the kinetic and potential
energies of all the particles in the system.

It can be changed by a flow of work, heat, or both

Represented by E

∆E = q + w

q is heat

w is work

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11

Thermodynamic Quantities

Thermodynamic quantities always consist of 2
parts: a number and a sign

Number
-

gives the magnitude of the change

Sign
-

indicates direction of the flow from the

system’s

point of view

For heat:

Exothermic reactions have a

negative

sign

Endothermic reactions have a

positive

sign

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12

Thermodynamic Quantities

For work:

if the system does work on the surroundings
then w is

negative

If the surrounding do work on the system, then w
is

positive

If work is done by a gas, this is

expansion

If work is done to a gas, this is

compression

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13

Work

Equation: w =
-
P∆V

remember that change in volume is final volume

initial volume

If gas expands, change in volume is positive, so
work is negative.

If gas contracts, change in volume is negative, so
work is positive

Note that the pressure is the external pressure.

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14

Practice Problems

1
-
4, 8, 20, 22; 26, 28

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15

Enthalpy

Enthalpy: H
H

= E + PV

E is the internal energy of the system

P is the pressure of the system

V is the volume of the system

Since internal energy, pressure, and volume
are state functions, the enthalpy is also a

state function

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16

Enthalpy

For a process carried out at constant pressure
and where the only work allowed is from a
volume change: ∆H =
q
p

This allows for heat of reaction and change in
enthalpy to be used interchangeably for systems
at constant pressure

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Enthalpy

For any chemical reaction, change is enthalpy can be
determined as follows:

∆H =
H
products

H
reactants

If products have a greater enthalpy than the reactants
than change in enthalpy will be

positive

and the reaction will be

endothermic

If the enthalpy of the products is less than that of the
reactants, change in enthalpy will be

negative

and the reaction will be

exothermic

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18

Calorimetry

Calorimeter

device used experimentally to
determine the heat associated with a chemical
reaction

Calorimetry
-

the science of measuring heat
-

based on observing the temperature change
when a body absorbs or discharges energy as
heat

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19

Heat Capacity

Heat capacity
-

measure of the amount of heat
required to raise the temperature of a
substance by 1 degree Celsius

C = heat absorbed/ increase in temperature

C represents heat capacity

This value varies depending on

amount of
the substance

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20

Heat Capacity

Specific Heat Capacity
-

the heat capacity per
gram of a substance.

Units are:

J/
o
C

*g or J/K* g

Molar Heat Capacity
-

the heat capacity per
mole of a substance

Units are:

J/
o
C

mol or J/K mol

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21

Constant Pressure
Calorimetry

Constant Pressure
Calorimetry
-

measurement
of heat using a simple
calorimetry
-

when
atmospheric pressure remains constant during
the process

Used for determining changes in enthalpy for reactions
that occur in

solution

Memorizing the specific heat capacity of water may
prove beneficial: 4.18 J/
o
C

g

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Constant P
Calorimetry

Energy released by the reaction = energy
absorbed by the solution

Equation: Energy released = s x m x ∆T

s = specific heat capacity of the substance

m = mass of that substance

∆T = Change in temperature

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Heat of Reaction

Heat of reaction is an

extensive

property because it depends on the amount of
substances

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24

Constant Volume
Calorimetry

∆E =
q
v

Energy released by reaction = ∆T x
C
calorimeter

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25

Hess’s Law

Hess’s Law
-

enthalpy is a state function
-

when
going from a particular set of reactants to a
particular set of products, the change in
enthalpy is

the same

whether
the reaction takes place in one step or in a series
of steps.

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26

Using Hess’s Law

Add together the reactions and the enthalpies of
those reactions to get the reaction you want and
the enthalpy of that overall reaction.

If a reaction is reversed, the sign of ∆H is

reversed

The magnitude of ∆H is directly proportional to
the quantities of reactants and products in a
reaction. If the coefficients in a balanced reaction
are multiplied by an integer the value of ∆H is

multiplied

by the same integer.

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27

Hints for using Hess’s Law

Hints for using Hess’s Law

Work

backward

from the required reaction,
use the reactants and products to decide how
to manipulate the other reactions

Reverse any reactions as needed to give
required reactants and products

Multiply reactions to give the correct numbers
of reactants and products.

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28

Practice Problems

Questions 58, 60, 64

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29

Standard Enthalpy of Formation

Standard enthalpy of formation
-

H
f
o
-

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

standard states

The degree symbol means that the process
occurred under standard conditions

Standard state
-

reference state

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Standard States

Compound

For a gas, pressure is exactly 1 atmosphere.

For a solution, concentration is exactly 1 molar.

Pure substance (liquid or solid)

Element

The form [N
2
(
g
), K(
s
)] in which it exists under
conditions of
1
atm

and 25
°
C
.

Enthalpy

Enthalpies of formation are always given

per
mole

of the product with the product in its standard
state

The enthalpy change for a given reaction can be
calculated by subtracting the enthalpies of formation
of the reactants from the enthalpies of formation of
the products

Equation: ∆
H
f
o
reaction

= ∑
n
p
∆H
f
o
products

-

n
r
∆H
f
o
reactants

Elements are not included because elements require
no change in form

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Practice Problems

66, 70, 72

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33

Energy from the Sun

The energy in woody plants, coal, petroleum
and natural gas originally came from

the
sun

We obtain the energy by

burning

the
plants or the decay products of those plants

-

fossil fuels

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34

Petroleum

Petroleum
-

thick dark liquid composed mostly
of compounds called hydrocarbons, which
contain
H and C

Separation of petroleum occurs as different
substances are boiled off in a process called:

distillation

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35

Natural Gas and Coal

Natural gas
-

usually near petroleum deposits
-

consists of mostly methane (CH
4
) and contains
ethane, propane, and butane

Coal
-

will become more important as oil is
used up

Expensive and dangerous to mine

Burning causes air pollution that leads to acid rain

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36

Carbon Dioxide

Molecules in the atmosphere

H
2

O, CO
2

,
CH
4
back toward earth, so the earth is much
warmer than it would be without the
atmosphere

http://www.teachersdomain.org/resource/ph
y03.sci.phys.matter.co2/

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37

Alternative Energy Sources

3 things to consider when finding new energy
sources

Economic, climatic, and supply factors

Potential Sources
-

sun, nuclear processes,
biomass, and synthetic fuels

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38

Alternative Energy

Syngas
-

use coal gasification
-

reduce the size
of molecules by treating coal with oxygen and
steam at high temperatures to break many C
-
C
bonds. The bonds are replaced by C
-
H and C
-
O bonds as coal reacts with water and oxygen

The product is a mixture of CO and H
2

CO and H
2

can also be converted to methanol,
which is used to produce synthetic fibers and
plastics and is used as a fuel

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39

Hydrogen

H
2

as fuel

The heat of combustion for hydrogen is 2.5 times
that of natural gas

The only product of burning hydrogen is water

3 problems: cost of production, storage, transport

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40

Hydrogen as Fuel

Hydrogen is abundant but doesn’t exist as the
free gas, so treating natural gas with steam
produced hydrogen

This reaction is highly endothermic, so it is

not an efficient way

to obtain hydrogen for
fuel

Storage
-

hydrogen decomposes into atoms
when in contact with metal, enter the metal,
and make the metal brittle

http://www.teachersdomain.org/resource/eng06.sci.
engin.systems.fuelcells/