INTRODUCTION TO
THERMOCHEMISTRY
TYPES OF ENERGY
Energy is an intrinsic (internal) property of
matter
 There are two classifications of energy:



Kinetic energy is energy of motion of objects
Potential energy is energy that is stored due to space
between objects
Energy Type
Definition
Example
Chemical (PE)
Energy Stored in
Chemical Bonds
Battery
Electrical (KE)
Energy of moving charges
”Plugging into
an outlet”
Electromagnetic
(KE and PE)
Radiant Energy
Light from the
Sun
Thermal (KE)
Energy due to random
motion of atoms and
molecules
Internal Body
Temperature
Mechanical (PE +
KE)
Sum of Potential and
Kinetic Energy
A wind up toy
moves
Nuclear (PE)
Energy stored in nucleus
Nuclear
Explosion
Visual
KINETIC MOLECULAR THEORY
Matter is made of tiny particles (atoms, ions,
and/or molecules)
 These particles are in constant motion
 Temperature measures the average kinetic
energy.


This means that the hotter a substance is, the more
kinetic energy it has.
APPLYING THE IDEAS

What are 2 differences between the molecules of
solid water and of liquid water?
APPLYING THE IDEAS

What are 2 differences between the molecules of
liquid water and of vapor water?
HEAT

Heat
 Heat is a transfer of energy
 Heat flows from “warm” to “cold”
 Typically thermal energy is being transferred.
 Measured in Joules or Calories.
 Represented by the symbol: q
WHICH WAY DOES THE HEAT FLOW?
WHICH DIRECTION IS THE HEAT
TRANSFER?
WHICH DIRECTION IS THE HEAT
TRANSFER?
WHICH DIRECTION IS THE HEAT
TRANSFER?
QUESTIONS TO PONDER…


In order to cool things in a refrigerator, heat
must be transferred away from the food. Where
do you think this heat ends up going?
Is ice releasing heat or taking heat from the
environment when it melts?
TEMPERATURE

Temperature
 Temperature is a measure of the kinetic
energy of the particles in a sample.
 Temperature is NOT a form of ENERGY.
THERMOMETERS
Thermometers are filled with dyed
alcohol
 As the energy is transferred from the
sample to the thermometer, the
alcohol begins to gain energy.
 The higher energy matter takes up
more space (because it is moving
faster)
 The temperature will rise…

TEMPERATURE SCALES
 Temperature



Conversion Equation
What is the equation of °C in terms of Kelvin?
What is the value of Absolute Zero on the Celsius
Temperature Scale?
What are the two Fixed Reference Points on all
three temperature scales?
ENERGY
 In
order to enact changes on a sample,
energy needs to be applied or withdrawn.
 Remember, kinetic energy is the energy of
molecular motion and is a measured by
temperature.
 Potential energy is a measure of the space
between molecules.
ADDING ENERGY
If we wanted to add energy to a cup of
water to change its temperature from 50oC
to 75oC, what kind of energy is involved?
Kinetic or Potential?
ADDING ENERGY
If we wanted to add energy to turn a solid
piece of ice into a liquid sample of water AT
THE SAME TEMPERATURE, what kind of
energy is involved?
Kinetic or Potential?
ADDING ENERGY
What if we wanted to change a solid piece of
ice to liquid water that is 17oC warmer?
What kind of energy is involved here?
Kinetic or Potential?
ADDING OR REMOVING KINETIC ENERGY
Energy added to a sample that increases the
kinetic energy increases the temperature
 q= mC T





q= heat gained/lost
m= mass
T = change in temperature (Final – Initial)
C = specific heat capacity
The energy required to raise the temperature of 1 gram of
substance 1 degree Celsius
 For liquid water: 4.18 J/g°C

How many joules of energy are needed
to change 10 grams of water
by 15 degrees Celsius?
Given
Find
Formula
Solve
HOW MANY JOULES OF ENERGY ARE NEEDED TO
CHANGE 12 GRAMS OF WATER BY 34 DEGREES
CELSIUS?
Given
Find
Formula
Solve
HOW MANY JOULES OF ENERGY ARE NEEDED TO
CHANGE 50 GRAMS OF WATER BY 4 DEGREES
CELSIUS?
Given
Find
Formula
Solve
Notes
ENERGY AND CHANGES IN MATTER
When a phase change (state) occurs due to a gain
or loss of heat, the temperature of the sample
stays constant and the potential energy changes
 The amount of heat gained by 1 mole of a solid to
change into a liquid is the heat of fusion.

The amount of heat lost by 1 mole of a liquid to
change into a solid is the same heat of fusion
 q=mHf


The amount of heat gained by 1 mole of liquid to
change to a gas is the heat of vaporization
The amount of heat lost by 1 mole of gas to change to
a liquid is the same as the heat of vaporization
 q=mHv

HOW MANY JOULES OF ENERGY ARE NEEDED TO
CHANGE 10 GRAMS OF LIQUID WATER TO
VAPOR?
Given
Find
Formula
Solve
HOW MANY JOULES OF ENERGY ARE NEEDED TO
CHANGE 5 GRAMS OF ICE TO LIQUID?
Given
Find
Formula
Solve
HOW MANY JOULES OF ENERGY ARE NEEDED TO
CHANGE 74 GRAMS OF WATER FROM 8OC TO
64OC?
Given
Find
Formula
Solve
TERMS OF THERMOCHEMISTRY



Enthalpy: heat transferred during a chemical change under a
constant pressure
The heat of a reaction is equal to the change in enthalpy
 Measured by subtracting the energy of the products from the
energy of the reactants
 H = Hproducts - Hreactants
ΔH values are given on Table I of the Reference Tables
DESCRIBING REACTIONS

Enthalpy can determined by the nature of a
reaction

Endothermic- When ΔH is positive (+ ΔH)
Heat is absorbed
 Reaction feels cool to the touch


Exothermic- When ΔH is negative (- ΔH)
Heat is released
 Reaction feels warm or hot to the touch

ENERGY AS PART OF THE EQUATION

Law of conservation of energy:

However much energy and mass we start with, we must
end with
CONSIDER THE FOLLOWING EXAMPLE

Example: Consider the combination reaction
between two moles of hydrogen and one mole of
oxygen to create two moles of water:
2 H2 + O2  2H2O
ΔH = -483.6 kJ
We can view this reaction on a molecular level to
understand why this reaction releases so much heat:
 Step 1: 2 H-H bonds and 1 O-O bond are broken
(energy absorbed)
 Step 2: 2 O-H bonds formed (energy released)
Recall: Energy needs to be supplied to break bonds and
is released when bonds are formed. In this example,
the energy released by the bond formation is greater
than the energy required to break the bonds, so overall
energy is released.
USING TABLE I


When we change the amount of reactants or
products we also change the amount of energy
produced or consumed
Example:
2N2 + 6 H2  4 NH3
H=?
ENTROPY OF REACTION
Which of the following has higher entropy?
Circle the higher entropy part…
2H2O(l)  2H2(g) + O2(g)
N2(g) + 2O2(g)  2NO(g)
INDEX CARDS “CHEAT SHEETS”
Enthalpy
H
Heat of Reaction
(-) heat lost
(+) heat gained
Entropy
S
Randomness
solid – liquid – gas
Endothermic
H is positive
Energy on left
(reactants)
Exothermic
H is negative
Energy on right
(products)
Now take two colored pencils from the bin up front and
color in chart I on your reference table. Make a key
where one color is exothermic and the other is
endothermic. Also color in the corresponding boxes on
your card.
DEFINE THE TERMS ON 8-10 AND
8-11
Quiz on these phase changes tomorrow!
Heating Curves
Gas
Liquid/Gas
BP
Vaporization
T
(oC)
MP
Melting
Liquid
Solid/Liquid
Solid
Heat Added
Cooling Curves
Gas
BP
Condensation
T
(oC)
MP
Liquid
Freezing
Heat Removed
Solid


What is the boiling point of the sample?
In the space below, draw the correct particle
arrangement of this sample during the first minute of
heating.


What is the total time the sample is in the liquid
phase, only?
How long does it take for the sample to fully convert
from a liquid to a gas?


What is the temperature range of the sample
when it is in the liquid phase?
Describe the phase that is present at 11:30 min.


Describe the amount of potential energy found at 8
minutes compared to that found at 1 minute.
Describe the changes of potential and kinetic energy
between the times 2 minutes and 8 minutes.
DO NOW:
WHAT WE (SHOULD) KNOW…
 When
energy is given off (ΔH = _______), the
reaction is classified as an _______________
reaction, and the reaction feels
______________ to the touch
 When energy is absorbed (ΔH = _______), the
reaction is classified as an _______________
reaction, and the reaction feels
______________ to the touch
 Enthalpy is the _________________________
 The equation for the heat of a reaction is
__________________________
TERMS OF THERMOCHEMISTRY

Entropy: ΔS
 Describes the randomness and disorder in a reaction
 Nature wants to go to high disorder,
(large, positive ΔS values)
•
vs
Nature will go towards
low energy and high entropy.
POTENTIAL ENERGY DIAGRAMS
Energy of
Reactants
A+BC+D
Energy of
Products
Energy of Activated Complex
POTENTIAL ENERGY DIAGRAM
Activation Energy (Ea)
Ea = Eactivated complex - Ereactants
A+BC+D
POTENTIAL ENERGY DIAGRAM
Heat of Reaction (H)
H = Hproducts - Hreactants
A+BC+D
POTENTIAL ENERGY DIAGRAM
The Heat of Reaction = Energy of the Products
minus the heat of the Reactants
ΔHrxn = Hproducts – Hreactants
ACTIVATED COMPLEX
Activated complex: Where everything is ready to
react, the highest point of the P.E. diagram
At the activated complex the reactants can
either react to form the products or… Fail to
react and stay as the reactants
FAILED REACTIONS
 There
are a few reasons why the
reaction may fail when the reactants
collide together…
Atoms or molecules are not oriented the
right way (like trying to put a key into a
lock backwards)
 The particles have insufficient energy

ENDO- VS. EXOTHERMIC
Absorbs Energy
Endothermic
Releases Energy
Exothermic
FORWARD VS. REVERSE REACTIONS
Forward Reaction Reverse Reaction
Ea
A
B
+ΔHrxn
B
-ΔHrxn
Ea
A
The heat of reaction for the reverse reaction
is the inverse value of the forward reaction.
EFFECTS OF A CATALYST
A catalyst helps
things react
 Adding a catalyst
will decrease the
activation energy
only

EFFECTS OF A CATALYST
EFFECTS OF A CATALYST
A catalyst guides
the reaction to
happen more
efficiently.
WHICH SEGMENT REPRESENTS THE ENERGY OF
THE PRODUCTS?
WHICH SEGMENT REPRESENTS THE ENERGY OF
THE REACTANTS?
WHICH SEGMENT REPRESENTS THE HEAT OF
THE REACTION?
WHICH TWO SEGMENTS, WHEN ADDED TOGETHER,
EQUALS THE ENERGY OF THE ACTIVATED
COMPLEX?
WHICH SEGMENT REPRESENTS THE
ACTIVATION ENERGY?
WHICH SEGMENT REPRESENTS THE ENERGY OF
THE PRODUCTS IN THE REVERSE REACTION?
WHICH SEGMENT REPRESENTS THE ACTIVATION
ENERGY OF THE FORWARD REACTION?
INDEX CARDS “CHEAT SHEETS” AGAIN
Potential Energy Diagram
.
BACK OF CARD
Phase Change Diagram
NAME: ____________________
DO NOW
3/6/13
KMT
The kinetic molecular theory explains how the
pressure, volume, and temperature change when
one property is changed for an ideal gas.
 Relationship between Pressure, Volume, and
Temperature
 Combined gas law states that as the pressure
changes, the volume inversely changes and
temperature directly changes.



Mathematically:
Solving combined gas laws problems: use the
five step method we practiced with our heat
equation. **
RELATIONSHIP TO VELOCITY AND
COLLISIONS
Remember increase the temperature really is
an increase in the kinetic energy of molecules
 Kinetic energy is energy of motion (velocity)
 When you increase the temperature (directly
or by increasing pressure or decreasing
volume) the gas particles are moving faster.
 When the particles are moving faster they
collide more frequently.
