Transcript Thermodynamics

Thermodynamics
Thermodynamics is the
study of systems involving
energy in the form of heat
and work.
First Law of Thermodynamics

The change in internal energy of a
system (DU) must be related to the
energy exchange of heat (q) and work
(w).
DU = q + w
DU, q and w are measured in joules (J) the
metric unit for energy
First Law of Thermodynamics

q is positive if heat is added to the
system, and negative if heat is
released.

w is positive if work is done on the
system, and negative if work is done
by the system.
What does it mean for the
system to do work?

Work is simply a force multiplied by the
distance moved in the direction of the
force.

A good example of a thermodynamic
system that can do work is the gas
confined by a piston in a cylinder.
An example of how work is done
by a thermodynamic system

If the gas is heated, it
will expand and push
the piston up, doing
negative work on the
piston.

If the piston is pushed
down, the piston does
positive work on the
gas.
Pressure-Volume Work
Work = Force x distance
For a chemical system the expansion or
contraction of a gas is one type of work.
 So , Work (w) = -PDV

*Vf -Vi will be negative when work is done on the
system, but work done on the system must be
positive thus the negative sign.
DU = q – DVP
Sign Conventions

Positive sign – energy is absorbed
from the surroundings, and work is
done on the system

Negative sign - energy is released
to the surroundings and work is
done by the system.
Calorimetry

Heat capacity- the quantity of heat needed to
change the temperature of the system 1K
•

Cp = q/ DT (units are J/k or J/oc)
Specific Heat capacity – the quantity of heat
need to raise the temperature of 1 gram of a
substance 1 oC
•
q = C m DT
Specific Heat


The quantity of heat required to raise 1
gram of water 1oC.
The higher the specific heat the harder it
is to change the temperature of the
substance. C (J/goC)
• Al
• Cu.
• Ethanol
• Water
.092
.385
2.46
4.182
Calorimetry

Measures the amount of heat generated
from a chemical reaction by letting the heat
generated flow into a mass of cooler water.
q= m DT C
q = heat
m = mass
DT = Tf – Ti
C = specific heat
(J/g oC)
Coffee Cup Calorimeter


When a chemical reaction
occurs in the coffee cup
calorimeter, the heat of
the reaction if absorbed
by the water.
The change in the water
temperature is used to
calculate the amount of
heat that has been
absorbed or evolved in
the reaction.
For example,
A chemical reaction occurs in 200 grams of water with
an initial temperature of 25.0°C. As a result of the
reaction, the temperature of the water changes to
31.0°C. The heat flow is calculated:
 qwater = 4.18 J/(g·°C) x 200 g x (31.0°C - 25.0°C)
 qwater = +5.0 x 103 J

ΔHreaction = -(qwater)
Bomb Calorimeter


A bomb calorimeter is
used to measure heat
flows for gases and
high temperature
reactions.
In a bomb calorimeter,
the reaction takes place
in a sealed metal
container, which is
placed in the water in
an insulated container.
Bomb Calorimeter




Analysis of the heat flow is more
complex than for the coffee cup
calorimeter because the heat flow into
the metal parts of the calorimeter must
be accounted for (heat capacity, Cp):
qreaction = - (qwater + qbomb)
where qwater = 4.18 J/(g·°C) x mwater x Δt
qbomb = Cp x Δt
Calorimetry

A 1.5886 g sample of glucose (C6H12O6)
was ignited in a bomb calorimeter. The
temperature increased by 3.682oc. The
heat capacity of the calorimeter was 3.56
kJ/oc, and the calorimeter contained 1.00
kg of water. Find the molar heat in
kJ/molrxn
C6H12O6 + 6 O2  6 CO2 + 6 H2O
Calorimetry
qbomb = 3.562 kJ/oc (3.682oc) = 13.12 kJ
qwater = 1000g (4.184 J/goc) (3.682oc) =
15400 J
Qtotal = qbomb + qwater
qtotal = - 28.52 kJ (exothermic)
per molrxn
1.5886 g / 180.16g/mol = .0088177 mol
-28.52 kJ/ .0088177 mol = -3234 kJ/molrxn
Enthalpy
Enthalpy is a measure of the
total emery of a thermodynamic system.
 Most chemistry reactions take place at
constant pressure, open to the
atmosphere.
So, Enthalpy (H) is used to describes
these types of reactions.
H = U + PV

Enthalpy (H) –
The Heat of the Reaction

The change in enthalpy (DH) is equal to the
difference in the heat of the reaction.
DH = DHfinal – DHinitial
= DHproducts – DHreactants
Heat of formation (DHof) - the change in
enthalpy when a compound forms from its
pure elements. (table in back of text book)
 DHof of a pure element = 0

Enthalpy and Internal Energy
DH = DU + DVP

DVP = DngasRT @ const. T and P
DH = DU + DngasRT
 DU and DH are very close to the same
value and are the same when no gas is
generated by the reaction.
Hess’s Law
The heat of a reaction is equal to the
sum of the individual DH values for each
step.
Example:
What is the DH for C + ½ O2  CO?
CO + ½ O2  CO2
DH = -283.0 kJ
C + O2  CO2
DH = -393.5 kJ

What is the DH for
C + ½ O2  CO?
CO2  CO + ½ O2
C + O2  CO2
DH = +283.0 kJ
DH = -393.5 kJ
C + ½ O2  CO
DH = -110.5 kJ