THERMOCHEMISTRY

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THERMOCHEMISTRY
HEAT CAPACITY, SPECIFIC HEAT, ENDOTHERMIC/EXOTHERM
IC, ENTHALPY, STANDARD ENTHALPIES, CALORIMETERY
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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 a measurable value
• Energy units are numerous, but we will
  concentrate on the Joule (SI base unit) and the
  calorie (little c, big C is the food Calorie or a
  kilocalorie)
• 1 calorie 4.184 Joules

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• There are three methods used to transfer heat
  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

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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 energy of its particles,
  specifically its

• The Kinetic Energy defines the temperature
• Particles vibrating fast hot
• Particles vibrating slow cold

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• 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

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• Thermal energy 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
  material present
  (KE ½mv2)

• Thermal energy is also related to the type of
  material

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• Different type 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 different abilities to hold
  onto or release energy is
  referred to as the
  substances heat capacity

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• Thermal energy can be transferred from object to
  object through direct contact
• Molecules collide, transferring energy from
  molecule to molecule

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• Thermal energy can be transferred from object to
  object through direct contact
• Molecules collide, transferring energy from
  molecule to molecule

AKA HEAT
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DEFINITION THE FLOW OF THERMAL ENERGY FROM SOMETHING WITH A HIGHER TEMP TO SOMETHING WITH A LOWER TEMP
UNITS MEASURED IN JOULES OR CALORIES
TYPES THROUGH WATER OR AIR CONVECTION
TYPES THROUGH SOLIDS CONDUCTION
TYPES TRANSFERRED ENERGY BY COLLISION WITH PHOTON RADIANT ENERGY
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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

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SPECIFIC HEAT CAPACITY

• The amount of energy it takes to raise the temp
  of a standard amount of an object 1C is that
  objects specific heat capacity (C)
• The standard amount 1 gram
• Specific heats can be listed on data tables
• Smaller the specific heat ? the less energy it
  takes the substance to feel hot
• Larger the specific heat ? the more energy it
  takes to heat a substance up (bigger the heat
  reservoir)

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SUBSTANCE SPECIFIC HEAT CAPACITY, CP
WATER 4.18J/gC OR 1cal/gC
ICE 2.10 J/gC OR .502cal/gC
STEAM 1.87J/gC OR .447cal/gC
MERCURY, Hg .139 J/gC OR .033cal/gC
ALCOHOL (Ethyl) 2.40 J/gC OR .580cal/gC
CALCIUM, Ca .647 J/gC OR .155cal/gC
ALUMINUM, Al .992J/gC OR .237cal/gC
TABLE SALT, NaCl .865 J/gC OR .207cal/gC
AMMONIA, NH3 2.09 J/gC OR .500cal/gC
SILVER, Ag .235 J/gC OR .056cal/gC
LEAD, Pb .129J/gC OR .031cal/gC
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• Specific heats and heat capacities work for gains
  in heat and in losses in heat
• Smaller the specific heat ? the less time it
  takes the substance to cool off
• Larger the specific heat ? the longer time it
  takes the substance to cool off
• Specific heat capacity values are used to
  calculate changes in energy for chemical rxns
• Its important for chemists to know how much
  energy is needed or produced in chemical rxns

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CHANGE IN HEAT ENERGY (ENTHALPY)

• The energy used or produced in a chem rxn is
  called the enthalpy of the rxn
• Burning a 15 gram piece of paper produces a
  particular amount of heat energy or a particular
  amount of enthalpy
• Enthalpy is a value that also contains a
  component of direction (energy in or energy out)
• Heat gained is the out-of
  direction ie exo-

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CHANGE IN HEAT ENERGY (ENTHALPY)

• The energy used or produced in a chem rxn is
  called the enthalpy of the rxn
• Burning a 15 gram piece of paper produces a
  particular amount of heat energy or a particular
  amount of enthalpy
• Enthalpy is a value that also contains a
  component of direction (energy in or energy out)
• Heat gained is the out-of
  direction ie exo-

• Heat lost is the into
  direction ie endo-

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SURROUNDINGS
HEAT
HEAT
HEAT
HEAT
SYSTEM
SYSTEM
EXOTHERMIC
ENDOTHERMIC
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• 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
• Give off heat energy, sometimes quite alot
• Endothermic rxns typically feel cooler the longer
  the rxn proceeds
• Absorb heat energy, sometimes enough to get very
  cold

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• Exothermic rxn

• To a cold camper, the important product here is
  the heat energy

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In an exothermic process the amount of energy
given off is more than the initial energy
invested. So the products are less in energy
than the reactants.
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• Endothermic rxn

NH4NO3H2O 752kJ ?NH4OHHNO3

• Similar system as what is found in cold packs

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ENDOTHERMIC RXN
In an endothermic process more energy is required
to cause the rxn to proceed than obtained in
return. So the products are less in energy than
the reactants.
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CHANGE IN ENTHALPY

• Most common measurement of the energy or enthalpy
  in a reaction is actually a change in enthalpy
  (?H)
• 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 -)

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USING ?H 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
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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
1mol H2O2
1.0g H2O2
.02941 mol
34g H2O2
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2H2O2 190kJ ? 2H2O O2
Therefore with 2 moles of H2O2 it requires the
use of 190 kJ of energy, but we dont have 2
moles we only have .02941 mols of H2O2, so how
much energy would the bug require?
190kJ
2.8kJ
.02941 mol
2molH2O2
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Example 2
How much heat will be released when 4.77 g of
ethanol (C2H5OH) react with excess O2 according
to the following equation C2H5OH3O2?
2CO23H2O ?H? -1366.7kJ
analyze we know that if we had 1 mol of ethanol
(assuming coefficient of 1 in rxn equation)
burning we would produce 1366.7kJ of heat, but
how much would it be if only we only had 4.77 g
of ethanol?
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C2H5OH3O2?2CO23H2O ?H? -1366.7kJ
1mol C2H5OH
-1366.7kJ
4.77g C2H5OH
1mol C2H5OH
46g C2H5OH
-142 kJ
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Classroom Practice 1

1. Ethanol, C2H5OH, is quite flammable and when 1
   mole of it burns it has a reported ?H of -1366.8
   kJ. How much energy is given off in the
   combustion of enough ethanol to produce 12.0 L of
   Carbon dioxide _at_ 755 mmHg and 25.0C?

1 C2H5OH 3 O2? 2 CO2 3 H2O ?H -1366.8 kJ
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• We can also track energy changes due to temp
  changes, using ?HmCp?T

?H

• If 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

• if the temp change is negative
• the enthalpy ends up negative
• the rxn absorbed heat into the system, so its
  endothermic

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if you drink 4 glasses of ice water at 0C, how
much heat energy is transferred as this water is
brought to body temp? each glass contains 250 g
of water body temp is 37C.

• mass of 4 glasses of water
• m 4 x 250g 1000g H2O
• change in water temp
• Tf Ti 37C - 0C
• specific heat of water
• CH2O 4.18 J/gC(from previous slide)

?HmCH2O?T
?H(1000g)(4.18J/gC)(37C)
?H 160,000J
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Example 2 500 g of a liquid is heated from
25C to 100C. 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)(75C)
C 4.184 J/gC
H2O
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Classroom Practice 2

• An orange contains 445 kJ of energy. What
  volume of water could this same amount of energy
  raise from a temp of 25.0C to the boiling point?
• Water at 0.00C was poured into 30.0g of water in
  a cup at 45.0C. The final temp of the water
  mixture was 19.5C. What was the mass of the
  0.00C water?

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• Enthalpy is dependent on the conditions of the
  rxn
• Its important to have a standard set of
  conditions, which allows us to compare the affect
  of temps, pressures, etc. On different substances
• Chemists have defined a standard set of
  conditions
• Stand. Temp 298K or 25C
• Stand. Press 1atm or 760mmHg
• Enthalpy produced in a rxn under standard
  conditions is the standard enthalpy (?H)

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• 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 (?H?f) is the amount of
  energy involved in the formation of a compound
  from its component elements.
• Enthalpy of combustion (?H?comb) is the amount of
  energy produced in a combustion rxn.
• Enthalpy of solution (?H?diss) is the amount of
  energy involved in the dissolving of a compound

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• Enthalpy of fusion (?H?fus) is the amount of
  energy necessary to melt a substance.
• Enthalpy of vaporization (?H?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.

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• Calorimetry is the process of measuring heat
  energy
• Measured using a device called a calorimeter
• Uses the heat absorbed by H2O to meas-ure 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

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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.
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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
  (-?Hsurr)
• Endothermic (?Hsys)
• When the water gains heat from the system it
  obtains a positive change (?Hsurr)
• Exothermic (-?Hsys)

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CALORIMETRY

• You calculate the amount of heat absor-bed by the
  water (using ?H mC?T)
• 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)

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A chunk of Al that weighs 72.0g is heated to
100C is dropped in a calorimeter containing
120ml of water at 16.6C. the H2Os temp rises
to 27C.

• mass of Al 72g
• Tinitial of Al 100C
• Tfinal of Al 27C
• CAl .992J/gC (from table)

??HAl
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• We can do the same calc with the water info

• Mass of H2O 120g
• Tinitial of H2O 16.6C
• Tfinal of H2O 27C
• CH2O 4.18J/gC (from table)

?HH2O
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.
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When a 4.25 g sample of solid NH4NO3 dissolves in
60.0 g of water in a calori-meter, the
temperature drops from 21.0C to 16.9C.
Calculate the energy involved in the dissolving
of the NH4NO3.
DHwater (mwater)(Cwater)(DTwater)
DHwater (60g)(4.18J/gC)(16.9C-21.0C)
DHwater -1.03 x 103 J
- DHwater DHNH4NO3
DHNH4NO3 1.03 x 103 J
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Classroom Practice 3

• 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 dissolv-ed the temp of the water
  registered 85.8?C. How much heat did the H2O
  absorb, and how much heat did the NaOH produce?
• 41.0g of glass at 95C is placed in 175 g of
  Water at 19.5C in a calorimeter. The temps are
  allowed to equalize. What is the final temp of
  the glass/water mixture? (Water 4.18J/gC
  Glass 8.78J/gC)