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
• These energy changes can be in the form of heat
• Heat is the flow of chemical 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

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WHAT IS HEAT?

• Hot cold, are automatically associated with the
  words heat and temperature
• Heat temp 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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• Vibrational energy is transferred from one
  particle to the next
• One particle collides with the next particle and
  so on and so on down the line

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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 or the
  intermolecular forces
• 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

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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

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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

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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

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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

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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 (Cp)
• 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, H2O 4.18J/gC OR 1cal/gC
ALUMINUM, Al .992J/gC OR .237cal/gC
TABLE SALT, NaCl .865 J/gC OR .207cal/gC
SILVER, Ag .235 J/gC OR .056cal/gC
MERCURY, Hg .139 J/gC OR .033cal/gC
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SPECIFIC HEAT CAPACITY

• 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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CHEMICAL RXNS

• There are 2 types of chemical rxns
• Exothermic rxns ? rxns in which heat energy is a
  product
• Endothermic rxns ? rxns in which heat energy is a
  reactant
• Exothermic rxns typically feel warm as the rxn
  proceeds
• Gives off heat energy, sometimes quite alot
• Endothermic rxns typically feel cooler the longer
  the rxn proceeds
• Absorbs heat energy, sometimes enough to get very
  cold

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EXOTHERMIC
ENDOTHERMIC
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• Exothermic rxn

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

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• Endothermic rxn

NH4NO3H2O 752kJ ?NH4OHHNO3

• Similar system as what is found in cold packs

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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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CHANGE IN ENTHALPY

• Most common version of enthalpy is when we have a
  change in enthalpy (?H)
• The enthalpy absorbed or gained (changed) in a
  rxn is dependent on the amount of material
  reacting
• Amount is usually in the form of moles
• We can use the coefficient ratios to energy
  ratios to calculate how much energy a reaction
  used or produced

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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 released if
1.0g of (H2O2) decomposes in a bombardier beetle
to produce a defensive spray of steam
2H2O2? 2H2O O2 ?Hº 190kJ
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2H2O2? 2H2O O2 ?Hº 190kJ
Analyze we know that if we had 2 mols of H2O2
decomposing we would produce 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
1molH2O2
1.0g H2O2
.02941 mol
34gH2O2
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2H2O2? 2H2O O2 ?Hº 190kJ
Therefore with 2 moles of H2O2 we would produce
190 kJ of energy, but we dont have 2 moles we
only have .02941 mols of H2O2, so how much energy
would the bug produce?
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 C2H5OH 3O2? 2CO2
3H2O ?Hº-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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C2H5OH 3O2? 2CO2 3H2O ?Hº-1366.7kJ
1mol C2H5OH
4.77g C2H5OH
.1037 mol
46g C2H5OH
Therefore with 1 mole of C2H5OH we would
produce 1366.7 kJ of energy, but we dont have 1
mole we only have .1037 mols of C2H5OH, so how
much energy would the reaction produce?
-1366.7kJ
-142 kJ
.1037 mol
1mol C2H5OH
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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 is positive

• if the temp change is negative
• makes the enthalpy 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
• Cp 4.18 J/gC (from previous slide)

?HmC?T
?H(1000g)(4.18J/gC)(37C)
?H 160,000J
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• Enthalpy is dependent on the conditions of the
  rxn
• Its important to have a standard set of
  conditions
• This allow 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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• Stand enthalpies can be found on tables of data
  measured as standard enthalpies of formations
• Standard enthalpies of form-ations are measured
  values for the energy to form chemical compounds
  (?Hf)
• H2 gas O2 gas can be ignited to produce H2O and
  a bunch of energy
• The amount of energy produced by the rxn is 285kJ
  for every mol of water produced

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STANDARD ENTHALPIES OF FORMATION STANDARD ENTHALPIES OF FORMATION STANDARD ENTHALPIES OF FORMATION
SYMBOL FORMULAS ?HfkJ/mol
AlCl3(s) Al 3/2Cl2 ? AlCl3 -705.6
Al2O3(s) 2Al 3/2O2 ? Al2O3 -1676.0
CO2(g) C O2 ? CO2 -393.5
H2O(g) H2 1/2O2 ? H2O -241.8
C3H8(g) 3C 4H2 ? C3H8 -104.7
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CALORIMETRY

• Calorimetry is the process of measuring heat
  energy
• 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

?Hsys is the system or what is taking place
in the main chamber (rxn etc.) And ?Hsur is
the surroundings which is generally
water.
?HSYS-?HSUR
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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 (-) sign

SIGN MEANS HEAT WAS ABSORBED BY THE RXN
- SIGN MEANS HEAT WAS RELEASED BY WATER
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WATER SURROUNDINGS
SYSTEM
ENDOTHERMIC
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CALORIMETRY

• With calorimetry we use the sign of what happens
  to the water
• When the water gains heat from the system it
  obtains a () sign

- SIGN MEANS HEAT WAS RELEASED BY THE RXN
SIGN MEANS HEAT WAS ABSORBED BY WATER
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WATER SURROUNDINGS
SYSTEM
EXOTHERMIC
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CALORIMETRY

• You calculate the amount of heat absorbed 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)

??HSYS
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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)

?HSUR
Equal but opposite, means that the al since it
decreased in temp, it released heat causing the
H2O to increase in temp.