Transcript Thermodynamics PPT

INTRO TO THERMOCHEMISTRY
 Chemical rxns involve changes in energy
–
–
Breaking bonds requires energy
Forming bonds releases energy
The study of the changes in energy in
chem rxns is called thermochemistry.
 The energy involved in chemistry is real
and generally measurable, and can be
thought of as heat

–
–
Energy units are numerous, but our
focus will be Joule (SI base unit) and the
calorie
1 calorie = 4.184 Joules
WHAT IS HEAT?
Hot & cold, are automatically associated
with the words heat and temperature
– Heat & temperature are NOT synonyms
– The temperature of a substance is
directly related to the vibrational energy
of its particles, specifically its:
The Kinetic Energy defines the
temperature
– Particles vibrating fast = hot
– Particles vibrating slow = cold

Kinetic energy is transferred from one
particle to the next (a.k.a. conduction)
–
–
Sometimes this energy can be
transferred from one object to another
and influence physical properties
The more energy an object has
the more energy is transferred
An Ice Cold Spoon
A Hot Spoon

Thermal energy (q) is the total energy of
all the particles that make up a substance
–
–
Kinetic energy from vibration of particles
Potential energy from molecular attraction
(within or between the particles)
Thermal energy is dependent upon the
amount or mass of
2 Hot Spoons
material present
(KE =½mv2)
 Thermal energy is also
related to the type of
material


Different types of materials may have the
same temp, same mass, but different
connectivity
– Affected by the potential energy stored in
chemical bonds or the IMFs holding
molecules together
So it is possible to be at same temp
(same KE) but have very different thermal
energies
 The ability to hold onto or release thermal
(heat) energy is referred to as the
substance’s heat capacity


Thermal energy can be transferred from
object to object through direct contact
–
–
Molecules collide, transferring energy from
molecule to molecule
The flow of thermal energy is called heat
DEFINITION
THE FLOW OF THERMAL ENERGY
FROM SOMETHING WITH A HIGHER
TEMP TO SOMETHING WITH A LOWER
TEMP
UNITS
MEASURED IN JOULES OR CALORIES
THROUGH WATER OR AIR =
CONVECTION
TYPES
THROUGH SOLIDS = CONDUCTION
TRANSFERRED ENERGY BY
COLLISION WITH PHOTON = RADIANT
ENERGY
HEAT CAPACITY
 The measure of how well a material
absorbs or releases heat energy is its
heat capacity
–

It can be thought of as a reservoir to hold
heat, how much it holds before it overflows
is its capacity
Heat capacity is a physical property
unique to a particular material
–
–
Water takes 1 calorie of energy
to raise temp 1 °C
Steel takes only 0.1 calorie
of energy to raise temp 1 °C
SPECIFIC HEAT CAPACITY
 The amount of energy it takes to raise the
temp of 1 gram of an object 1°C is that
object’s specific heat capacity (C or s)
 Specific heats can be listed on data tables
–
Smaller the specific heat  the less
energy it takes the substance to feel hot
•
–
They heat up quickly and cool down
quickly
Larger the specific heat  the more
energy it takes to heat a substance up
(bigger the heat reservoir)
•
They heat up slowly and cool down slowly
SPECIFIC HEAT CAPACITY
Specific Heat Equation Q = MCDT
Q = Quantity of heat (joules)
M = Mass of substance (grams)
C = Specific heat capacity
DT = Change in temperature
Tfinal – Tinitial = Change in temp.
http://www.youtube.com/watch?v=XLWP03
pwTYY
SUBSTANCE
SPECIFIC HEAT CAPACITY, CP
WATER
ICE
STEAM
4.18J/g°C OR 1cal/g°C
2.10 J/g°C OR .502cal/g°C
1.87J/g°C OR .447cal/g°C
.139 J/g°C OR .033cal/g°C
2.40 J/g°C OR .580cal/g°C
.647 J/g°C OR .155cal/g°C
.992J/g°C OR .237cal/g°C
.865 J/g°C OR .207cal/g°C
2.09 J/g°C OR .500cal/g°C
.235 J/g°C OR .056cal/g°C
.129J/g°C OR .031cal/g°C
MERCURY, Hg
ALCOHOL (Ethyl)
CALCIUM, Ca
ALUMINUM, Al
TABLE SALT, NaCl
AMMONIA, NH3
SILVER, Ag
LEAD, Pb

There are three methods used to transfer
heat/thermal energy
– Conduction – transfer of heat through
direct contact
– Convection – transfer of heat through a
medium like air or water
– Radiant – transfer of heat by
electromagnetic radiation
CHANGE IN HEAT ENERGY (ENTHALPY)
 The energy used or produced in a chem
rxn is called the enthalpy of the rxn (DHrxn)
–
Burning a 15 gram piece of paper
produces a particular amount of thermal
energy or heat energy (enthalpy)
 Enthalpy
is a value that also contains a
component of direction (energy in or
energy out)
–
–
Heat gained by the surroundings
is the out-of/exo direction
Heat lost by the surroundings
is the in-to/endo direction
HEAT
HEAT
HEAT
HEAT
Chemical rxns can be classified as either:
– Exothermic  a reaction in which heat
energy is generated (a product)
– Endothermic  reaction in which heat
energy is absorbed (a reactant)
Exothermic rxns typically feel warm as the rxn
proceeds (from the perspective of the
surroundings)
– Give off heat energy (light, fire, heat)
Endothermic rxns typically feel cooler the
longer the rxn proceeds (from the perspective
of the surroundings)
– Absorb heat energy, sometimes enough to
get very cold
http://www.youtube.com/watch?v=XgiCn1Ipvz
M&list=PL65159266CFC74682
Exothermic rxn
C3H8 + 5O2  3CO2 + 4H2O + 2043kJ
– To a cold camper, the important
product here is the heat energy
C 3H 8 + O 2
In an exothermic process the amount of energy
given off is more than the initial energy invested.
So the products are always lower in energy
than the reactants.
 Endothermic rxn
NH4NO3+H2O+ 752kJ  NH4OH + HNO3
– Similar system as what is found in
cold packs
NH4OH + HNO3
NH4NO3 + H2O
In an endothermic process more energy is required
to cause the rxn to proceed than obtained in return.
So the products are always higher in energy
than the reactants.
CHANGE IN ENTHALPY
 Most common measurement of the energy
or enthalpy in a reaction is actually a
change in enthalpy (DH)
– DHrxn = ∑Hproducts - ∑Hreactants
 The enthalpy absorbed or gained
(changed) in a rxn is dependent on the
number of moles of material reacting
–
–
We can stoichiometrically calculate
how much energy a rxn uses or produces
DH values can be provided with a rxn
equn and have magnitude & direction
of transfer (+ or -)
USING DH IN CALCULATIONS
 Chemical reaction equations are very
powerful tools.
Given a rxn equation with an energy value,
We can calculate the amount of energy
produced or used for any given amount of
reactants.
(For Example)
How much heat will be absorbed
for 1.0g of H2O2 to decompose in
a bombardier beetle to produce
a defensive spray of steam
–
2H2O2 +190kJ 2H2O + O2
2H2O2 +190kJ  2H2O + O2
Analyze: we know that if we had 2 mols of
H2O2 decomposing we would use
190kJ of heat, but how much would it
be if only 1.0 g of H2O2
Therefore: we have to convert our given
1.0 g of H2O2 to moles of H2O2
1.0g H2O2
1mol H2O2
34g H2O2
Molar mass
= .02941 mol
2H2O2 +190kJ  2H2O + O2
Therefore: with 2 moles of H2O2 it requires the
use of 190 kJ of energy, but we
don’t have 2 moles we only have
.02941 mols of H2O2, so how much
energy would the bug require?
190kJ
.02941 mol
2molH2O2
Rxn equation
= 2.8kJ
Example #2
How much heat will be released when 4.77 g
of ethanol (C2H5OH) react with excess O2
according to the following equation:
C2H5OH+3O2 2CO2+3H2O DH = -1366.7kJ
4.77g C2H5OH
1mol C2H5OH
-1366.7kJ
46g C2H5OH
1mol C2H5OH
= -142 kJ
Classroom Practice 1
1. Ethanol, C2H5OH, is quite flammable and
when 1 mole of it burns it has a reported
DH of -1366.8 kJ. How much energy is
given off in the combustion of enough
ethanol to produce 12.0 L of Carbon
dioxide @ 755 mmHg and 25.0°C?
1 C2H5OH+ 3 O2 2 CO2+ 3 H2O
DH= -1366.8 kJ
 We
can also track energy changes due to
temp changes, using DH=mCDT:
DH =
 If
–
–
 if
–
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SPECIFIC
MASS
HEAT
FINAL TEMP –
INITIAL TEMP
the temp difference is positive
The rxn is exothermic because the final
temp is greater than the initial temp
So the enthalpy ends up positive
the temp change is negative
the enthalpy ends up negative
the rxn absorbed heat into the system,
so it’s endothermic
Example: If you drink 4 glasses of ice
water at 0°C, how much heat energy is
transferred as this water is brought to
body temp? Each glass contains 250 g
of water & body temp is 37°C.
 mass of 4 glasses of water:
– m = 4 x 250g = 1000g H2O
 change in water temp:
– Tf – Ti = 37°C - 0°C
 specific heat of water:
– CH2O = 4.18 J/g•C°(from previous slide)
DH=mC
DH=
DH=(1000g)(4.18J/g•°C)(37°C)
160,000J
H2ODT
Example 2:
500 g of a liquid is heated from 25°C to
100°C. The liquid absorbs 156,900 J of
energy. What is the specific heat of the
liquid and identify it.
DH = mCDT
C = DH/mDT
C = 156,900J/(500g)(75°C)
C = 4.184 J/g°C
H2O
Classroom Practice 2
2. An orange contains 445 kJ of energy.
What volume of water could this same
amount of energy raise from a temp of
25.0°C to the boiling point?
3. Water at 0.00°C was poured into 30.0g of
water in a cup at 45.0°C. The final temp of
the water mixture was 19.5°C. What was
the mass of the 0.00°C water?
2. An orange contains 445 kJ of energy. What volume of
water could this same amount of energy raise from a temp
of 25.0°C to the boiling point?
DH= 445 KJ x 1000 = 445000J
DT = 100.0 – 25.0 = 75.0
C = 4.18 J/g°C
M = DH / C DT
445000J / (4.18 x 75.0) = 1420 g H2O = 1420 ml H2O
 Enthalpy
the rxn
–
is dependent on the conditions of
It’s important to have a standard set of
conditions, which allows us to compare
Cthe affect of temps, pressures, etc. On
different substances
 Chemist’s
conditions
–
–
have defined a standard set of
Stand. Temp = 298K or 25°C
Stand. Press = 1atm or 760mmHg
 Enthalpy
produced in a rxn under standard
conditions is the standard enthalpy (DH°)
 Standard
enthalpies can be found on
tables measured as standard enthalpies
of formations, enthalpies of combustion,
enthalpies of solution, enthalpies of fusion,
and enthalpies of vaporization
–
–
–
Enthalpy of formation (DHf) is the amount
of energy involved in the formation of a
compound from its component elements.
Enthalpy of combustion (DHcomb) is the
amount of energy produced in a
combustion rxn.
Enthalpy of solution (DHdiss) is the amount
of energy involved in the dissolving of a
compound
–
–
Enthalpy of fusion (DHfus) is the amount
of energy necessary to melt a substance.
Enthalpy of vaporization (DHvap) is the
amount of energy necessary to convert
a substance from a liquid to a gas.
 All
of these energies are measured very
carefully in a laboratory setting under
specific conditions
–
At 25 °C and 1atm of pressure
 These
measured energies are reported in
tables to be used in calculations all over
the world.
 Calorimetry
heat energy
–
–
is the process of measuring
Measured using a device called a
calorimeter
Uses the heat absorbed by H2O to measure the heat given off by a rxn or an object
 The
amount of heat soaked up by the
water is equal to the amount of heat
released by the rxn
DHsys is the system or what
is taking place in the main
DHSYS=-DHSUR chamber (rxn etc.) And DHsur
is the surroundings which
is generally water.
A COFFEE CUP
CALORIMETER
Used for a reaction
In water, or just a
transfer of heat.
A BOMB
CALORIMETER
used when trying
to find the amount
of heat produced by
burning something.

CALORIMETRY
With calorimetry we use the sign of what
happens to the water
–
–
When the water loses heat into the
system it obtains a negative change
(-DHsurr)
– Endothermic (+DHsys)
When the water gains heat from the
system it obtains a positive change
(+DHsurr)
– Exothermic (-DHsys)
DHsys
HEAT
-DH sys
- SIGN MEANS
HEAT WAS
RELEASED BY
THE RXN
HEAT
=
+ SIGN MEANS
HEAT WAS
ABSORBED BY
THE RXN
=
-DHsurr
- SIGN MEANS
HEAT WAS
RELEASED BY
WATER
DHsurr
+ SIGN MEANS
HEAT WAS
ABSORBED BY
WATER
HEAT
HEAT
CALORIMETRY
 You
calculate the amount of heat absorbed by the water (using DH= mCDT)
 Which leads to the amount of heat given
off by the rxn
–
–
–
you know the mass of the water (by
weighing it)
you know the specific heat for water
(found on a table)
and you can measure the change in the
temp of water (using a thermometer)
A chunk of Al that weighs 72.0g is heated
to 100°C is dropped in a calorimeter
containing 120ml of water at 16.6°C.
the H2O’s temp rises to 27°C.
- mass of Al = 72g
- Tinitial of Al = 100°C
DHAl
- Tfinal of Al = 27°C
- CAl = .992J/g°C (from table)
DHAl = 72g .992J/g°C 27°C-100°C
DHAl = -5214J
 We
info
–
–
–
–
can do the same calc with the water
Mass of H2O= 120g
DH
H2O
Tinitial of H2O= 16.6°C
Tfinal of H2O = 27°C
CH2O= 4.18J/g°C (from table)
DHH2O= 120g 4.18J/g°C 27°C-16.6°C
DHH2O= 5216J
Equal but opposite, means that the Al decreased
in temp, it released its stored heat into the H2O,
causing the temp of the H2O to increase.
When a 4.25 g sample of solid NH4NO3
dissolves in 60.0 g of water in a calorimeter, the temperature drops from 21.0°C
to 16.9°C. Calculate the energy involved
in the dissolving of the NH4NO3.
DHwater = (mwater)(Cwater)(DTwater)
DHwater = (60g)(4.18J/g°C)(16.9°C-21.0°C)
DHwater = -1.03 x 103 J
- DHwater = DHNH4NO3
DHNH4NO3 = 1.03 x 103 J
Classroom Practice 3
4. A coffee-cup calorimeter with a mass of 4.8 g
is filled with water to mass of 250 g. The
water temperature was 24.2C before 3.2 g
of NaOH pellets was added to the water.
After the NaOH pellets had dissolved the temp
of the water registered 85.8C. How much
heat did the H2O absorb, and how much heat
did the NaOH produce?
5. 41.0g of glass at 95°C is placed in 175 g of
Water at 19.5°C in a calorimeter. The temps
are allowed to equalize. What is the final temp
of the glass/water mixture? (Water =
4.18J/g°C; Glass = 8.78J/g°C)