1. No category

Unit 13Thermochemistry
AHS Chemistry
2012-2013
WHAT IS ENERGY?
Energy is the ability of a physical system to
perform______.
Work
The First Law of Thermodynamics states that
Destroyed
energy cannot be Created
_______ or ________.
Transformed
However, it can be ________
___into
another type of energy! This is also referred
Law of Conservation of Energy
to as the ____________________________
Types of Energy
1. Kinetic Energy-Energy of Motion
*Remember, temperature is a measure of average internal kinetic energy.
So, the HIGHER the temperature, the MORE MOVEMENT there is in the
substance!
Heat (Q) also known as Thermal Energy is a form of kinetic energy
transfer of kinetic energy from a substance at a HIGHER temperature to a
system at a LOWER temperature.
2. Potential Energy-STORED energy.
*In chemistry, potential energy is usually due to one of two things – either
BONDING or INTERMOLECULAR FORCES.
Chemical Energy-potential energy that is made available during a chemical
reaction in which old bonds are broken and new bonds are formed.
 Energy is RELEASED when bonds form.
 Energy is REQUIRED to break bonds.
**Label as KE, PE, TE, CE
 Energy of motion.
 Energy stored in chemical bonds.
 Heat energy.
 Temperature is a measure of average _____ energy
 Stored energy, or energy of position.
 Changes when a substance is heated or cooled.
 Changes during a phase change.
Transfer of Heat
 CONDUCTION is the transfer of heat between objects by
direct contact with one another (meaning they have to
touch).
 CONVECTION is the transfer of heat through a fluid (liquid
or gas) caused by the circulation or movement of
molecules.
 RADIATION is energy that is transmitted in the form of
waves (electromagnetic radiation) and transformed into
kinetic energy. Also, it can travel through a vacuum –
like outer space!
**Label as Conduction, convection or
radiation
 Must have direct contact
 Travels as waves
 Travels only through fluids
 Matter must be present (2 answers)
 Can travel through a vacuum
 Heat from the sun.
 Heat from hot water to cold water.
 Heat from your hand to a piece of ice.
Units of Heat Energy (Q)
• Joule is the SI Unit of heat
• A calorie is defined as the amount of energy needed to raise the
temperature of 1 gram of water by 1 °C.
It takes 4.184 joules of energy to do the same thing.
1 calorie (cal) = 4.184 joules (J)
1000 calories (cal) = 1 Calorie (Cal) = 1 kilocalorie (kcal)
Types of Phase Changes
Physical changes!!!
Vaporization: liquid changing to vapor
(evaporation) ENDOTHERMIC
Condensation: vapor changing to liquid
(formation of dew) EXOTHERMIC
Melting: solid changing to liquid (ice to
water) ENDOTHERMIC
Freezing: liquid changing to solid (water
to ice) EXOTHERMIC
Sublimation: solid changing to vapor (dry
ice) ENDOTHERMIC
Deposition: vapor changing to solid
(formation of frost from water vapor in
the air) EXOTHERMIC
Phase Diagram – shows what state of matter is present at different
Temperatures and Pressures.
PHASE EQUILIBRIUM: where one
phase change is occurring at an
EQUAL rate to the opposite phase
change.
The NORMAL BOILING POINT is
the boiling point that occurs at a
pressure of 1 atm (100°C for
water).
The NORMAL MELTING POINT is
the melting point that occurs at a
pressure of 1 atm (0°C for water).
TRIPLE POINT: the point at
which all three states of matter
coexist together.
Video of triple point:
http://www.youtube.com/watch?v=BLRqpJN9zeA
** Phase diagram Answer the questions about the phase diagram for CO2
shown below.
If a sample of CO2 has a temperature of – 100°C and a pressure of 100 atm, what state
is that sample in?
If you take the sample from part a and decrease the pressure to 0.01 atm, what phase
change has been made?
If you take the sample from part a and increase the temperature to – 20°C, what phase
change has been made?
Heating/Cooling curves
DIAGONAL LINES = HEATING/COOLING OF ONE PHASE
*Kinetic energy (velocity of the molecules) is changing
HORIZONTAL LINES = PHASE CHANGES
*Potential energy is changing (intermolecular bonds are
strengthening or weakening)
*We can determine the boiling point and melting/freezing
point from the graph
Thermal (heat)
energy is a type of
kinetic energy
however, kinetic
energy does NOT
change during a
phase change!
**Heating/Cooling Curve
Label the diagram with the following
information: solid, liquid, gas, boiling,
melting, freezing, condensation,
temperature, time
Do phase changes occur on the diagonal
sections or the horizontal (flat) sections of the
curve?
What happens to the temperature of the
molecules during a phase change?
Describe how the kinetic energy of the
molecules changes during a phase change.
Describe how the potential energy of the
molecules changes during a phase change.
18
Calculating Specific Heat
The Greek letter Δ means “change in”
Specific Heat capacity-Cp
Specific heat capacity of water: is the amount of heat required to raise
ONE GRAM of a substance ONE DEGREE CELCIUS.
 intensive physical property
Low Cp: able to change temperature relatively rapidly because they
conduct heat very well.
EX: most metals (cp of gold = 0.129 J/g•°C
High Cp: take longer to heat up because they can absorb large quantities
of heat. Because they do not conduct heat well, these substances make
good insulators.
EX: water has a cp = 4.184 J/g•°C
CALCULATING ENTHALPY CHANGES OF
A PHASE CHANGE
MCAT
H (or Q) = mcp(T)
ENTHALPY OF FUSION
ENTHALPY OF VAPORIZATION
ΔH = (moles) x (Hfus)
ΔH = (moles) x (Hvap)
Ex 1: Calculate the Enthalpy change that occurs when 10.0 g of
steam at 150.oC cools to liquid water at 50.0 C
Enthalpy (∆H or q)
a certain amount of energy is
required to get the reaction started is
called the ACTIVATION ENERGY
(Ea),
The ACTIVATED COMPLEX is a
short-lived structure that exists while
old bonds are breaking and new
bonds are forming.
The ENTHALPY OF REACTION
(ΔH) is the energy lost or gained by
the system. It is calculated as the
difference between the energies of
the products and the reactants.
A CATALYST speeds up a reaction
by DECREASING the activation
energy
ENDOTHERMIC
EXOTHERMIC
Verbally
…
Energy is
Energy is
Observa
tionally
…
The temperature of
the surroundings
DECREASED
The temperature of
the surroundings
INCREASED
Symboli
cally…
∆H = +# KJ
∆H = ─# KJ
In a
Reactio
n…
Graphic
ally…
Heat + A + B  C
+D
A+B C+D+
Heat
**Enthalpy calculations:
1. Calculate the amount of energy needed to vaporize 19.2 mol of liquid water.
Hvap = 40.79 kJ/mo
2. Calculate the amount of energy RELEASED when 1044.2 g of water are cooled
from 98.6°C to 32.3°C. Cp(liq water) = 4.184 J/g•°C
3. The temperature of a 95.4-g piece of copper increases from 25.0oC to 48.0oC
when the copper absorbs 849 J of heat. What is the specific heat of copper
4. When 435 J of heat is added to 3.4 g of olive oil at 21oC, the temperature
increases to 85oC. What is the specific heat of the olive oil?
Reaction Energy Diagram
 1. Label the following on the
energy diagram:
* Activated Complex, Reactants (R),
Products (P), Activated Complex (*), Ea,
ΔH
 2. Calculate the activation
energy.
 3. Calculate the enthalpy of the
reaction.

4. Draw a dashed line to show
what would happen if a catalyst
reduced the activation energy by
200 kJ/mol.
ENTHALPY OF REACTION (ΔHrxn) :
The Basics
Is the reaction endothermic or exothermic?
For exothermic reactions, heat is listed as
a PRODUCT(ΔH value would be negative!)
2 H2 + O2  2 H2O + 483.6 kJ
ΔH = -483.6 kJ
For endothermic reactions, heat is listed as
a REACTANT(ΔH value would be positive!)
483.6 kJ + 2 H2O  2 H2 + O2
ΔH = +483.6 kJ
The Basics cont’d
When the enthalpy change for a reaction is given, it references the
heat change for those exact stoichiometric amounts (always assumed
to be the equation balanced with the lowest whole number ratio
unless specified otherwise):
89.3 KJ + C(s) + 2S(s)  CS2(l)
Ex 1: Calculate the amount of heat (in kJ) absorbed when 4.13
moles of carbon disulfide are formed.
Ex2: Calculate the amount of heat (in kJ) absorbed when 16.83 g of
sulfur is reacted with excess carbon to form carbon disulfide.
**Identify the following reactions as exothermic or endothermic and write
ΔH with the correct sign.
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) + 2219.2 kJ
180.7 kJ + N2 (g) + O2 (g)  2 NO (g)
**Calculate the amount of heat (in kJ) absorbed when 16.83 g of sulfur is
reacted with excess carbon to form carbon disulfide (WE DID THIS ON
THE LAST SLIDE  )
The production of iron and carbon dioxide from iron(III) oxide and carbon monoxide has a
enthalpy value of ΔH = –26.3 kJ. How many kilojoules of heat are produced when 3.40 g of CO
reacts with an excess of Fe2O3? (HINT: Place the energy as a reactant or product first!)
Fe2O3(s) + 3CO(g)  2Fe(s) + 3CO2(g)
Calorimetry
Calorimetry is a technique used to measure the HEAT TRANSFERRED in a
reaction or process. It is typically just an insulated container measured
at either constant pressure or constant volume.
Ex 1: A 0.303 gram sample of sucrose sugar (C12H22O11) was burned in
a calorimeter containing 240. grams of water. During the process, the
temperature of the water changed from 22.53°C to 27.55°C
** Calorimetry
A 1.000 g sample of octane (C8H18) is burned in a bomb calorimeter
containing 1200.0 grams of water at an initial temperature of 25.00°C.
After the reaction, the final temperature of the water is 33.20°C. The
specific heat of water is 4.184 J/g °C. Neglect the heat absorbed by the
calorimeter.
Calculate the heat released when the 1.000 g sample is combusted.
mCalculate the heat of combustion of octane in kJ/mol.
Calculating the Heat of Reaction
(ΔHrxn)
Steps to calculate ΔHrxn:
1. Multiply the ΔHf of each product and multiply it by its coefficient (coefficient
= # moles).
2. Add the numbers in step #1 together.
3. Multiply the ΔHf of each reactant and multiply it by its coefficient (coefficient
= # moles).
4. Add the numbers in step #3 together.
5. To find ΔHrxn, take Step 2 – Step 4. The unit is usually kJ.
Use standard heats of formation values given to calculation the enthalpy change
for the combustion of propane (C3H8). Is the reaction endothermic or exothermic?
C3H8 (g) + 5 O2 (g)  3 CO2(g) + 4 H2O(g)
substance
C3H8 (g)
CO2(g)
H2O(g)
∆Hfo (kJ/mol)
─104.70
─393.50
─241.82
**heat of reaction
Use standard heats of formation values given to calculation the enthalpy change
for the formation of sucrose (C12H22O11) and oxygen in photosynthesis.
Is the reaction endothermic or exothermic?
12 CO2(g) + 11 H2O(g)  C12H22O11 (s) + 12 O2 (g)
substance
C12H22O11 (s)
CO2(g)
H2O(g)
∆Hfo (kJ/mol)
─2226.00
─393.50
─241.82