THERMOCHEMISTRY
OR
Thermodynamics
OR
The Bane Of Chem Majors Everywhere
ENERGY
The capacity to do
work OR to cause a
heat transfer
LAW OF CONSERVATION OF ENERGY
Energy
can be
transferred, but not
created or destroyed.
TWO FORMS OF ENERGY
GPE: gravitational potential
energy
Any energy due to POSITION – an
object above the earth’s
surface that has the potential
to fall due to gravity. (the earth
LOVES us due to our mass)
1.
2. EPE: ELASTIC POTENTIAL ENERGY
Elastic potential energy is energy
that is stored up in an object that
causes/enables it to work or move.
THINK: spring or rubber band;
bouncy ball, etc.
KINETIC ENERGY
Energy due to the motion of
an object; the kinetic energy
is dependent on the MASS of
an object
2
KE = ½ mv
WHICH BALL HAS MORE GPE?
a
b
WHEN BALL A ROLLS DOWN THE HILL, GPE IS
CONVERTED TO KE AND ENERGY IS
TRANSFERRED TO BALL “B” IN AN ELASTIC
COLLISION
a
b
HEAT VS. TEMPERATURE
These two concepts are VERY
different.
TEMPERATURE: reflects the
random motions of particles
in a particular substance.
Temperature is a measure of
the average Kinetic Energy
HEAT
Heat
involves the
TRANSFER of energy
between two objects due to
a temperature difference.
WORK
WORK is a force acting
over a distance
CHEMICAL ENERGY
1. EXOTHERMIC
 “Exo” means “out” and “thermic” means heat
 Energy flows OUT of the system
 Ie. Methane gas CH4 + O2 → CO2 + H2 O
 Here – the SYSTEM loses energy but the energy
is gained by the surroundings, so the
surroundings get warmer.

2. ENDOTHERMIC
“Endo” means “into” and “Thermic” means
“heat”
 Energy flows INTO the system
 N2 + O2 + energy → 2NO
 The System gains energy and the surroundings
LOSE energy, so the surroundings get

THERMODYNAMICS

The study of energy and its interconversions.
st
1
The
Law of Thermodynamics
energy of the universe is
constant (does this sound like
another familiar law in chemistry)
ΔE = Q + W
 ΔE
is the change/total energy
q = heat
 W = work
 Note that chemists call this the “Law
of Conservation of Energy” and
physicists call it the “First Law of
Thermodynamics”
(physicists couldn’t handle chemistry so they chose
physics!!)
MATTER AND ITS FORMS (S, L, AND
GASES) DEPENDS ON ENERGY
LIQUIDS – differ in the cohesive
forces of attraction (COHESIVE
forces are between the
molecules; ADHESIVE forces are
between the molecule and
something else)
DEMONSTRATION
 Compare
the cohesive forces of water,
alcohol, and acetone (finger nail polish
remover)
Which has the highest vapor pressure?
Which has the greatest evaporation rate?
Which has the greatest cohesive forces?
SOLIDS
Low
potential energy
Molecules vibrate about a fixed
position.
LIQUIDS
More
potential energy than solids
Molecules relatively free moving
GASES
Great
potential energy
Molecules are free moving
ENTHALPY (H)
 Measures
thermal energy (it is
another word for heat!!)
 Enthalpy is a measure of the internal
energy (both KE and PE) of
molecules.
 The properties of matter are
dependent on energy and forces.
VAPOR PRESSURE
 The
pressure a vapor exerts – an upward
force!!
 Vapor pressure is another word for
EVAPORATION.
 A High VP means the liquids evaporates
readily.
 A LOW VP means there is very slow/little
evaporation
PHASE CHANGES
 Solid
↔ Liquid ↔ Gas
 WHAT CAUSES PHASE CHANGES???????
 Changes in temperature and/or
pressure!!!
 Fill in in your notes: condense, melt,
vaporize, solidify, freezing, evaporation,
sublimation, deposition.
TWO PHASES MAY EXIST IN
THE SAME SYSTEM
TWO TYPES OF SYSTEMS:
1. CLOSED SYSTEM: not open to the
surroundings.
2. OPEN SYSTEM: the system is
infinite; it is open to the
surroundings.
Draw an example of a closed system
1.
2.
Will evaporation occur indefinitely in a
closed system?
Will evaporation occur indefinitely in an
open system?
In closed system, EQUILIBRIUM will
be established.
EQUILIBRIUM
A condition in which two opposing processes occur
at equal rates.
 Ie. Evaporation ↔ Condensation
 When a system is in equilibrium, the concentration
of the substances remains constant

A
+ X ↔ AX
 A + X are forming AX at the same time
that AX is decomposing to form A + X
FACTORS AFFECTING
EQUILIBRIUM
Closed or Open System
2. Changes in temperature
3. Changes in Pressure
4. Changes in Concentration
These are called “stresses” in the
reaction.
1.
THREE IMPORTANT LIQUID
GRAPHS/DIAGRAMS
(Often on the ACT)
1. Vapor Pressure curve
2. Phase diagram (solid, liquid,
gas)
3. Heating/Cooling Curve
1. VAPOR PRESSURE GRAPH
USE THE GRAPH TO ANSWER…
1.
2.
3.
4.
5.
The vapor pressure of acetic acid at 80°C
The vapor pressure of chloroform at 40°C
The boiling point of ethyl alcohol at an
atmospheric pressure of 500 mm Hg.
The boiling point of water at an atmospheric
pressure of 100 mm Hg.
The normal boiling point of chloroform.
REMEMBER…
Boiling
occurs when vapor
pressure equals outside
atmospheric pressure.
2. PHASE DIAGRAM OF WATER
PHASE DIAGRAM STUFF…
 Tc
= Critical Temperature – the temperature
above which the gas or vapor cannot be
condensed (or liquified) no matter what
pressure is applied.
 Pc = Critical Pressure – the pressure required
to produce liquification (condensation) at the
critical temperature.
 The critical point of water is 374°C and 218
atm.
3. HEATING CURVES
F
D
E
AB – solid phase; increased kinetic energy.
BC – solid ↔Liquid; no change in kinetic energy
but big change in potential energy.
HEAT OF FUSION: the enthalpy change that
occurs to melt a solid (80 Cal/g)
CD – large increase in Kinetic energy (warming
the liquid to boiling point)
DE – large increase in potential energy; liquid ↔
gas
HEAT OF VAPORIZATION: the energy required to
vaporize or evaporate one mole of a liquid (540
Cal/g)
EF - increase in kinetic energy; gas molecules
colliding more – high temperature = high
kinetic energy.
As you go from A to F on the heating/cooling
curve, the ENTROPY of the system increases
(or it becomes more chaotic!!)
ENTROPY: THE TOTAL DISORDER OR
CHAOS OF A SYSTEM (ΔS)
SOLIDS: GREAT ORDER, LOW ENTROPY
LIQUIDS: LESS ORDER, HIGHER ENTROPY
GASES: GREAT DISORDER, HIGH
ENTROPY
SECOND LAW OF THERMODYNAMICS
The
total entropy of the universe is
increasing (in other words, chaos
reigns!!)
So…you now have scientific data
to back up your reasoning NOT to
clean your bedroom when mom
asks!
SUMMARY OF PHASE CHANGES
Phase Change
Name
Energy Change
 SOLID TO LIQUID Melting
Endothermic
 LIQUID TO SOLID Freezing
Exothermic
 LIQUID TO GAS
Vaporization
Endothermic
 GAS TO LIQUID
Condensation Exothermic
 SOLID TO GAS
Sublimation
Endothermic
 GAS TO SOLID
Deposition
Exothermic

CALORIMETRY
DEFINED: the science of measuring heat flow.
 You must observe the temperature change that
occurs when a body absorbs or releases heat.


HEAT CAPACITY: The amount of energy
required to raise the temperature of a
substance 1°C
HEAT CAPACITY(C) = HEAT ABSORBED/Δ TEMPERATURE

CH2O = 4.18 Joules/g - °C = 1 Cal/g-°C

CHg

Energy released = (heat capacity)(mass)(ΔT)
=
0.14 Joules/g-°C
Energy
released = (C) (g) (ΔT)
50 MLS OF HCL AND 50 MLS OF NAOH AT 25°C IS MIXED IN A CALORIMETER.
THE TEMPERATURE AFTER MIXING RAISES TO 31.9°C. HOW MUCH ENERGY
WAS RELEASED?
INTERPRETING VALUES OF ΔH
-ΔH = exothermic reaction
+ΔH = endothermic reaction
ONE LITER OF 1-MOLAR BARIUM NITRATE SOLUTION AT 25°C IS ADDED TO 1 LITER OF
A 1-MOLAR SODIUM SULFATE SOLUTION AT 25°C IN A CALORIMETER. THE
TEMPERATURE INCREASES TO 28.1°C WHEN THE TWO ARE MIXED. ASSUMING THE
SOLUTIONS ARE AQUEOUS, CALCULATE THE ENTHALPY RELEASED.
DRIVING FORCES OF CHEMICAL REACTIONS
All reactions in nature are driven by two factors:
1. Tend towards lower energy (enthalpy)
water flows downhill, heat flows from hot to cold
objects; exothermic changes represent this
driving force.
2. Tendency towards higher entropy (chaos)
a reaction becomes more chaotic as time
progresses; it is difficult to keep your bedroom
clean; when you eat food, it becomes more
chaotic as it leaves your body.
A CHEMICAL REACTION WILL BE SPONTANEOUS
(OR REACT) IF…
The energy decreases
2. The entropy increases
At equilibrium: energy ↔ entropy
To determine if change is
spontaneous, we must determine
if there is FREE ENERGY (G)
1.
ΔG = ΔH - TΔS
After the calculation:
Positive (+) ΔG = Not spontaneous
(the reaction will not proceed)
Negative (-) ΔG = Is spontaneous
(the reaction will proceed or it will react)
HESS’S LAW OF HEAT SUMMATION
This
law states that the heat of
a reaction is the algebraic sum
of the component heats of the
reaction.
HESS’S LAW PROBLEMS – 5 STEPS
Balance the equation.
2. Determine the reactions to be
added.
3. Adjust the positions.
4. Adjust the amounts.
5. Add the reactions together.
1.
2SO2 + O2 → 2SO3
NH3 + O2 → NO2 + H2O (G)
GRAPH OF ENDOTHERMIC REACTION
GRAPH OF EXOTHERMIC REACTION
HESS’S LAW – SHORT METHOD

Formula:

Use table 14 to get values of Heat of Formation
2SO2 + O2 → 2SO3
USING HEATS OF FORMATION TO
PREDICT STABILITY
VERY STABLE: high negative
heats of formation(it took a lot
of heat to form it, so it takes a
lot of energy to break it apart)
Ie. MgSO4 = -307.1 Kcal
1.
2. SLIGHTLY UNSTABLE
 Low
positive or low negative values of
heat of formation.
 The range is -10 Kcal to +10 Kcal
 Ie. H2 S = -4.82 Kcal
3. EXTREMELY UNSTABLE
High + (positive) values of
Heat of Formation
The higher the positive
value, the more explosive.
Ie. CS2 = +27.98 Kcal