Transcript Energy - Winona State University

Energy
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 neither be created nor
destroyed.
(Euniverse is constant)
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)
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
State Function
Depends only on the present state of the
system - not how it arrived there.
It is independent of pathway.
System and Surroundings
System: That on which we focus attention
Surroundings: Everything else in the universe
Universe = System + Surroundings
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).
First Law
First Law of Thermodynamics:
The energy of the universe is
constant.
First Law
E = q + w
E = change in system’s internal energy
q = heat
w = work
Work
work = force  distance
since pressure = force / area,
work = pressure  volume
wsystem = PV
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P=
P =
F
A
F
A
Area = A
A
h
(a) Initial
state
h
(b) Final
state
V
Enthalpy
Enthalpy = H = E + PV
E = H  PV
H = E + PV
At constant pressure,
qP = E + PV,
where qP = H at constant pressure
H = energy flow as heat (at constant pressure)
Heat Capacity
heat absorbed
J
J
C =
=
or
increase in temperature
C
K
Some Heat Exchange Terms
specific heat capacity
heat capacity per gram = J/°C g or J/K g
molar heat capacity
heat capacity per mole = J/°C mol or J/K mol
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Thermometer
Styrofoam
cover
Styrofoam
cups
Stirrer
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Stirrer
Ignition
wires
Thermometer
Insulating
container
Steel
bomb
Water
Reactants
in sample
cup
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.
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
Standard Enthalpies of
Formation
Change in enthalpy that
accompanies the formation of
one mole of substance from its
elements, with all substances in
their standard states.
Hfo
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.
Change in Enthalpy
Can be calculated from enthalpies of
formation of reactants and products.
Hrxn° = npHf(products)  nrHf(reactants)
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C(s)
CH4(g)
(a)
(c)
CO2(g)
2H2(g)
(d)
2O2(g)
(b)
Reactants
2O2(g)
Elements
Products
2H2O(l)
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Reactants
Elements
0 kJ
C(s)
2O2(g)
2O2(g)
75 kJ
2H2(g)
- 394 kJ
- 572 kJ
Energy
CH4(g)
= Reactants
= Elements
= Products
CO2(g)
2H2O(l)
Products
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2N2(g)
(a)
(c)
4NO2(g)
4NH3(g)
6H2(g)
(d)
(b)
7O2(g)
Reactants
7O2(g)
Elements
Products
6H2O(l)
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Reactants
Elements
0kJ
2N2(g)
7O2(g)
6H2(g)
7O2(g)
4 x 34 kJ
6 x (-286 kJ)
4 x (-64 kJ)
Energy
4NH3(g)
= Reactants
= Elements
= Products
4NO2(g)
6H2O(l)
Products
Infrared
radiated by
the earth
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Visible light
from the sun
CO2
and H2O
molecules
Earth
Earth’s
atmosphere