Transcript Thermodynamics

Thermodynamics
They study of energy and
its transformations
Energy
Defined as the ability to do work or
produce heat
 2 types of energy

– Kinetic Energy – energy of motion (thermal
energy or heat)
– Potential Energy – energy of position
(example – chemical bond energies)
– Units – Joules = kg m2/sec2
First Law of Thermodynamics
– Energy is always conserved. Energy
cannot be created or destroyed, it can
only be converted into matter or another
form of energy. E = mc2
 We must keep track of the energy
exchanged in a process. So we divide the
universe into two parts.

2 Parts of the Universe Are . . .
The SYSTEM - part of the universe under study
(generally a chemical process)
 The SURROUNDINGS – the rest of the
universe (like the container where a chemical
reaction is taking place.

In chemistry we always focus on the SYSTEM!!
In a closed system - energy can be exchanged
with the surroundings but not matter.
 In an open system – energy and matter can be
exchanged with the surroundings.


Internal Energy

Internal energy is the sum of the kinetic
and potential energy of a system.

INTERNAL ENERGY = KE + PE
Internal Energy
We can only measure a change in energy
(E)
 E = Efinal – Einitial

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It is a STATE FUNCTION – it only
depends on current conditions. It does
not matter how it got there.
Heat and Work

There are two ways for a system to
exchange energy with its surroundings
Heat (q) = the amount of energy
transferred between two objects
 Work (w) = Energy that is a force
acting over a distance w = F x d

Heat and Work
Heat and work are not state functions and
are therefore dependent on the pathway.
 The sum of heat and work = the change
in Internal energy of a system

E = q + w
Sign Conventions for work and heat

all thermodynamic variables include a number
and a sign.
Heat (q) = (+) then heat is absorbed by
the system (surroundings cool down
 Heat (q ) = (-)
then heat is released from
the system (and heats up the surroundings)
 Work (w) = (+) then work is done on the
system by the surroundings
 Work (w) = (-) then work is done by the
system on the surroundings

Change in Energy
E = (+) then the system gains energy
 E = (-) then the system loses energy
 Endothermic = energy is absorbed by
the system by the surroundings as heat
 Exothermic = energy is released by the
system to the surroundings as heat.

•Potential energy of
product bonds are less
than Reactants.
•Products have stronger
or more stable bonds
•Potential Energy of the
Products are higher than
reactants.
•Reactants have stronger
more stable bonds.
Pressure Volume Work
For chemical processes – work is done by a
gas (through expansion) or work is done
on a gas (by compression.)
 Using P = force/area and w= force x d
 you can derive
work = PV

(P = external pressure)
 For an expanding gas w = - PV
 (work is being done on the
surroundings)
Enthalpy
Enthalpy (H) is the amount of heat flow
under conditions when only Pressure
volume work is done.
 PV work is only done under conditions of
constant pressure with a volume
expansion (+V) or volume compression
(-V)

H = E + PV
H = E + PV
 (we also know E = qp + w)
 Since E = qp - PV
 (qp = heat at constant pressure)
 H = qp + PV - PV
so . . . . .


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H = qp
Which means we can measure enthalpy by
measuring the heat flow (or change in
temperature) at constant pressure!
Heat at Constant Volume
vs. Heat at Constant Pressure

Constant Volume
– Bomb Calorimeter
qv = ∆E
Volume must change for work to be done, so no
work term!

Constant Pressure
– Coffee Cup Calorimeter – volume of a gas can
change.
qp = ∆H
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Example -Hydrogen and oxygen gas are placed
together in a container with a moveable piston, and the
gases are ignited. As water is being produced, 1350
Joules of heat are lost to the surroundings. The gases
expand, and the piston is moved, producing 350 Joules
of work. What is the change in internal energy in this
system?
•Heat = q = -1350 J
•Negative because the heat is lost to the surroundings –
•Exothermic Reaction!
•Work = w = -350 J
•Negative because the work is done by the system
•The piston, part of the system, is expanding and working
against the surrounding pressure!
∆E = q + w

= (-1350J) + (-350J)

= -1700 J of energy transferred to
the surroundings.
 Exothermic Process! Gas expands to
keep constant pressure!
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H2
O2