Thermochemistry

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Thermochemistry
The heat of the matter
Energy
• The capacity to
do work or
produce heat
Law of Conservation of Energy
• Energy can be
converted from
one form to
another, but
cannot be
created nor
destroyed.
Potential Energy
• Potential energy is
due to position or
composition
Examples
• Water behind a dam
that may push a
turbine
• Gasoline
Kinetic Energy
• Energy due to the
mass and speed of an
object
• KE=1/2 mv2
Heat
• The transfer of energy
from kinetic to heat.
• Energy cannot be
created nor destroyed
so where does it go
when the ball hits the
ground?
HEAT
aka frictional heating
Heat
• Heat involves the transfer of energy
between two objects observed through
temperature changes
Work
• Work is the force
acting over a distance
• However, the way the
energy transfer is
divided between heat
and work depends on
certain conditions or
the PATHWAY.
• Regardless of the
pathway, the total
energy remains
constant. Why?
State Function
• A state function refers
to the property of the
system that depends
only on its present
state. It doesn’t
matter how you got
there, only that you
are there.
• Energy is a state
function.
Chemical Energy
• In discussing
reactions we need to
identify our present
state.
• System
• Surroundings
• Universe
Chemical Energy
• Exothermic: Energy
flows out of the
system into the
surroundings.
Expressed as (-)
• Endothermic: Energy
flows from the
surroundings into the
system. Expressed
as (+)
Exothermic and Endothermic
Thermodynamics
The First Law of
Thermodynamics:
• The energy of the
universe is constant
Internal Energy
• The internal energy of a system is the sum
of the kinetic and potential energies of all
the “particles” in the system. The internal
energy of a system can be changed by a
flow of heat or work or both.
ΔE=q+w
• ΔE is the change in energy
• q is the heat and w is the work
Example
• Calculate ΔE for a system undergoing an
endothermic process in which 15.6 kJ of
heat flows and where 1.4 kJ of work is
done on the system.
Result
ΔE=q+w
• q= +15.6kJ (endothermic) w=+1.4kJ
ΔE=15.6kJ + 1.4 kJ = 17.0 kJ
Work and Pressure
• Work may be done by a gas (inflation) and
work may be done on a gas (compression).
• Pressure is Force per unit area
P=F/A
• Work is force applied over a distance.
Work=force * distance=F * Δh (height)
Work and Pressure
• Since P=F/A or F=P*A then,
Work = F* Δh* = P*A* Δh
• This results in a change in volume,
ΔV= final volume – initial volume =A* Δh
• Substitute ΔV =A* Δh and
Work = P*A* Δh= PΔV
What about the sign of work? (+ or -)
Work and Pressure
• When the gas is expanding, work is done
on the surroundings by the system.
w= -PΔV
• When the gas is compressed, work is
done on the gas by the surrounding.
w= PΔV
Example
• Calculate the work associated with the
expansion of a gas from 46 L to 64 L at a
constant external pressure of 15 atm.
Result
• w= -PΔV as the gas is expanding.
• P=15 atm, ΔV = 64-46 = 18 L
• w= - 15atm *18L = -270 atm
To do tonight
Pressure from Work
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