Thermochemistry
Chapter 6
The Nature of Energy
Work = force used to move
an object a distance.

Heat= transfer of energy due
to temperature differences
Energy is the capacity to do work or
produce heat.
Total energy of the universe is constant.
Energy lost = Energy gained by something else.
Potential Energy = energy due to position= mgh
Kinetic Energy = energy of motion = ½ mv2
Heat is a transfer of energy

System vs Surroundings
System
Surroundings

State Function – a property that depends only on
the present state of the system…not on the
changes it has or will experience.




Internal energy
Pressure
Volume
Energy
Heat and work
are NOT state
functions!
Thermodynamic Quantities
Consist of two parts
1) the number – indicates how much
2) the sign- indicates direction of flow
HEAT
q
WORK
w
ENTHALPY
H
Internal
Energy
E
Negative Values = flow out of system
Positive Values = Flow into system
Heat Lost & Heat Gained
Draw…
…a graph of energy vs. reaction time
for a reaction that gives energy to
the surroundings
AND
a second one for a reaction that
absorbs energy from the surroundings
Exothermic and Endothermic
Exothermic Reaction
 Feels hot
 Heat transferred to
surroundings (lost
by system)
 Negative enthalpy
and heat values
Endothermic Reaction
 Feels cold
 Heat transferred to
system (gained by
system)
 Positive enthalpy
and heat values
PV Work
Work (w) = Force*displacement
W = F * d = F * Dh
W = P * A * Dh
P = Force/Area
When pressure of system
does not change
VOLUME!
W = -PDV
Try Me Problem


A balloon is inflated to its maximum capacity by heating. If
the volume changes from 4.0 x 106L to 4.5 x 106L by addition
of 1.3 x 108J energy as heat. Assuming that the balloon
expands against constant 1.0 atm pressure. Calculate
Internal Energy.
(1 L*atm = 101.3 J)
How is enthalpy different?

Enthalpy, H, is the amount of energy
capable of doing work in a system.


The amount of energy contained within the
bonds of chemicals involved in the system.
H = E + PV
Answer
Now
Compare the equation for total
internal energy with the equation
for enthalpy listed above.
How can you alternately define
Enthalpy?
Enthalpies of Formation
The enthalpy of formation (Hfo) for an
element in its standard state is ZERO.
The more negative the value of Hfo,
the more stable the compound.
DHrxn = S[np(DHfoprod)]-S[nr(DHforct)]
Sample Problems

Try Me 1

Try Me 2
Find the enthalpy for the
reaction:
Find the enthalpy for the
reaction:
4NH3(g) + HCl(l)  4NH4Cl(s)
2Al(s) + Fe2O3(s)  Al2O3(s) + 2Fe(s)
Experimental Determination of
Heat & Enthalpy

Specific/Molar Heat of Combustion
q = mcDT
q = ncDT
-The heat needed to raise the temperature
of 1 g (or 1 mol) of substance 1 degree K.
Another way to find DHrxn

Experimentally!
 DHrxn = DHproducts – DHreactants

How do you measure this stuff?

CALORIMETRY!
Hess’s Law

Enthalpy is a state function

The value will be the same regardless of how many
steps are needed to complete the reaction.
Hess’s Law States:
The enthalpies of individual steps
in a reaction mechanism can be
added together to calculate the
enthalpy of the overall reaction.
Fundamentals for Applying Hess’s Law



Reverse the reaction, reverse the sign
on enthalpy.
Multiply the reaction by a coefficient,
multiply the enthalpy by the same
coefficient.
Add the reactions together, add the
enthalpies together.
Try Me!
Overall:
N2O4(g)=>N2(g) + 2O2(g)
Reaction Mechanism:
NO2(g)  ½ N2(g) + O2(g)
2NO2(g)  N2O4(g)
This
is fun! Let’s do
some more!
H=-84.75 kJ
H=-145.5 kJ