Surroundings

1
Exothermic
Surroundings
System
Endothermic
2
Flasks, beakers, solvent
Reaction
3
CH4 O2 ---------gt CO2 H2O
The PE of the reactants is higher than the the PE
of the products. The DPE is given
to surroundings as heat.
CH4 O2
CO2 H2O
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The study of energy and its Interconversions is
called Thermodynamics.
First Law
The energy of the universe is constant.
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Lets revisit the concept of total Energy
DE q w
Each of these quantities will have a numerical
value and an associated sign
Signs The sign reflects the systems Point of
view
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Heat flows into system q is
Heat flows out of the system q is -
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E flows into system E is
E flows out of the system E is -
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Work done on the system by surroundings w is
Work done on the surroundings by system w is -
Like an endothermic process
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The expanding CO2 pushes up the book
Heat is absorbed from the surroundings And work
is done on the surroundings
DE q w
DE () (-)
Thus DE ltq
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Calculate DE for a system undergoing an
endothermic process in which 15.6kJ of heat
flows and where 1.4 kJ of work is done on the
system.
DE q w
w
q
DE 15.6kJ1.4kJ
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P F/A
w F Dh
w PA Dh
F P A
P
For a piston, the change In volume DVA Dh
w PDV
V
This gives us the magnitude Of work.
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What about the sign?
If the gas is expanding it (the system) moves the
piston, and does work on the surroundings
From the systems point of view, The sign of the
work is (-)
Thus, for an expanding gas, DV is () and DV
and w have opposite Signs w -PDV
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How much work is associated with The expansion of
a gas from 46L to 64L at a constant P 15 atm?
w -PDV what is sign and magnitude
DV 18L
Thus w -15 atm 18L -270L-atm
14
We need to fill a hot air balloon. The volume
changes from 4.00106L to 4.5106L. 1.3108 J
of energy needed to accomplish this. Assuming
constant P of 1 atm, what is the DE for
the process?
1L-atm 101.3J
DE q w
(sign of q?)
w -PDV w -1 atm (5105)L
E 1.3108J - 5.1107J
E 8107 J
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PV work of gas
In most laboratory work, in plants and animals,
reactions often take place at constant pressure.
Enthalpy
H
H E PV E internal energy of system P
pressure of system V volume of system
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Since E,P and V are state functions WHAT DOES
THIS MEAN?
H is also a state function.