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THERMOCHEMISTRY 2005 by 60Lr1YGG

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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
   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
             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
   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 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 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
  substance’s heat capacity
   Thermal energy can be transferred from
    object to object through direct contact
    –   Molecules collide, transferring energy from
        molecule to molecule
   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
              FROM SOMETHING WITH A HIGHER
DEFINITION   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
 Theamount of energy it takes to raise the
 temp of a standard amount of an object
 1°C is that object’s 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)
  SUBSTANCE        SPECIFIC HEAT CAPACITY, CP
    WATER              4.18J/g°C OR 1cal/g°C
     ICE             2.10 J/g°C OR .502cal/g°C
    STEAM            1.87J/g°C OR .447cal/g°C
 MERCURY, Hg         .139 J/g°C OR .033cal/g°C
ALCOHOL (Ethyl)      2.40 J/g°C OR .580cal/g°C
  CALCIUM, Ca        .647 J/g°C OR .155cal/g°C
 ALUMINUM, Al        .992J/g°C OR .237cal/g°C
TABLE SALT, NaCl     .865 J/g°C OR .207cal/g°C
 AMMONIA, NH3        2.09 J/g°C OR .500cal/g°C
   SILVER, Ag        .235 J/g°C OR .056cal/g°C
   LEAD, Pb          .129J/g°C OR .031cal/g°C
 Specificheats 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
 Specificheat capacity values are used to
 calculate changes in energy for chemical
 rxns
  –   It’s important for chemists to know how
      much energy is needed or produced in
      chemical rxns
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
 Enthalpyis a value that also contains a
 component of direction (energy in or
 energy out)
  –   Heat gained is the out-of
      direction; ie exo-
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
 Enthalpyis 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-
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
  – 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
Exothermic rxn
 C3H8 + 5O2  3CO2 + 4H2O + 2043kJ
  – To a cold camper, the important
    product here is the heat energy
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.
 Endothermic rxn
NH4NO3+H2O+ 752kJ NH4OH+HNO3
  – Similar system as what is found in
    cold packs
 ENDOTHERMIC RXN




                         Activation Energy




                                             Energy Lost
  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.
          CHANGE IN ENTHALPY
 Most common measurement of the energy
  or enthalpy in a reaction is actually a
  change in enthalpy (H)
    – Hrxn = ∑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
   –    H values can be provided with a rxn
       equn and have magnitude & direction of
       transfer (+ or -)
     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
      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

                Molar mass
       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                   = 2.8kJ
               2molH2O2

                  Rxn equation
                 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 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?
C2H5OH+3O22CO2+3H2O H = -1366.7kJ
               1mol C2H5OH    -1366.7kJ
4.77g C2H5OH
               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
   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 @ 755 mmHg and 25.0°C?
 1 C2H5OH+ 3 O2 2 CO2+ 3 H2O
                                H= -1366.8 kJ
 We can also track energy changes due to
  temp changes, using H=mCpT:
                 SPECIFIC       FINAL TEMP –
  H =      MASS  HEAT          INITIAL TEMP

 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 it’s endothermic
  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)
   H= 160,000J
     H=(1000g)(4.18J/g•°C)(37°C)
   H=mCH2OT
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.

              H = mCT
              C = H/mT
     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?
 Enthalpy   is dependent on the conditions of
 the rxn
  –   It’s important to have a standard set of
      conditions, which allows us to compare the
      affect of temps, pressures, etc. On
      different substances
 Chemist’s   have defined a standard set of
 conditions
  –   Stand. Temp = 298K or 25°C
  –   Stand. Press = 1atm or 760mmHg
 Enthalpyproduced in a rxn under standard
 conditions is the standard enthalpy (H°)
 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
   –   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.
 Allof 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    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
              Hsys is the system or what
              is taking place in the main
HSYS=-HSUR chamber (rxn etc.) And Hsur
               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
        (-Hsurr)
         – Endothermic (+Hsys)
    –   When the water gains heat from the
        system it obtains a positive change
        (+Hsurr)
         – Exothermic (-Hsys)
                               Hsys         =
                                                 -Hsurr
                              + SIGN MEANS   - SIGN MEANS
                                HEAT WAS       HEAT WAS
                              ABSORBED BY    RELEASED BY
                                 THE RXN         WATER
  HEAT              HEAT




-H sys        =   Hsurr
- SIGN MEANS   + SIGN MEANS    HEAT              HEAT
  HEAT WAS       HEAT WAS
RELEASED BY    ABSORBED BY
   THE RXN        WATER
                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)
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   HAl
   - Tfinal of Al = 27°C
   - CAl = .992J/g°C (from table)

 HAl = 72g .992J/g°C     27°C-100°C
           HAl = -5214J
 We    can do the same calc with the water
 info
   –   Mass of H2O= 120g
   –   Tinitial of H2O= 16.6°C
                               HH2O
   –   Tfinal of H2O = 27°C
   –   CH2O= 4.18J/g°C (from table)

HH2O= 120g 4.18J/g°C 27°C-16.6°C
          HH2O= 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 calori-
meter, the temperature drops from 21.0°C
to 16.9°C. Calculate the energy involved
     in the dissolving of the NH4NO3.
     Hwater = (mwater)(Cwater)(Twater)
Hwater = (60g)(4.18J/g°C)(16.9°C-21.0°C)
          Hwater = -1.03 x 103 J
           - Hwater = HNH4NO3
         HNH4NO3 = 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 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?
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)

								
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