Experiment #: Experiment Name by ze0Chm

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									                 C125 – Experimental Chemistry 1

                   Experiment #6:
                  Thermochemistry
                             C125 Prelab



Click Here for Lecture Outline
Click Here for Printable Version
       Experimental Purpose
• Investigate what is meant by:
  – Changes in energy which accompany a
    chemical reaction
• Determine :
  – Heat capacity of a bomb calorimeter
  – Molar heat of neutralization for a strong and a
    weak acid
         Important Terms for This
               Experiment
•   Endothermic
•   Exothermic
•   Mode final temperature
•   ∆T
•   Heat capacity
•   Specific heat
•   Enthalpy
•   Heat of reaction
•   Constant Pressure calorimetry
    Definition of Endothermic
• Process or reaction that absorbs energy
  from its surroundings
• Process or reaction that becomes colder,
  requiring a gain of energy
     Definition of Exothermic
• Process or reaction that releases energy
  to its surroundings
• Process or reaction that becomes warmer,
  requiring a loss of energy
       Definition of Mode Final
            Temperature
• Peak temperature reached by the system
  before energy is then lost by the system
• In this experiment, the energy gained by
  the system is absorbed from the chemical
  reaction
• The temperature change in the system =
  mode final temperature – initial
  temperature
 Definition of Heat Capacity (C)
• The amount of heat required to raise the
  temperature of a given quantity of the
  substance in a specific calorimeter by one
  degree Kelvin
• C [=] J/K
• Does not take into account the mass of the
  substance
• In this experiment, you will find the heat
  capacity of bomb calorimeter
      q
  C
     T
Definition of Specific Heat (CSH)
• The amount of heat required to raise the
  temperature of one gram of the substance
  by one degree Kelvin
• CSH [=] J/g.K
• Takes into account the mass of the
  substance
• Specific heat of water = 4.184 J/g-K
           q
 C SH 
          mT
    Definition of Enthalpy (Q)
• A thermodynamic quantity used to
  describe heat changes taking place at
  constant pressure
• Heat flows from warmer to cooler objects
• Q = mCSH∆T or CCal ∆T
   Definition of Heat of Reaction
               (∆Hrxn)
• The difference between the enthalpies of
  the products and the enthalpies of the
  reactants
• Also known as enthalpy of reaction
• ∆Hrxn = ∑Q
  – Negative signs indicate heat lost
  – Magnitude for heat lost = magnitude for heat
    gained
 Constant Pressure Calorimeter
• Used to determine the heat changes for
  non-combustion reactions
• The heat change for the process =
  enthalpy change
• A closed system so that heat transferred
  from hot to cold objects is not lost
  -qhot = qcold
• Heat gained by the calorimeter
  (qCal)= CCal ∆T
                  Experimental Purpose
    • Investigate what is meant by:
         – Changes in energy which accompany a chemical
           reaction
    • Determine :
         – Heat capacity of a bomb calorimeter
           Mass and            Graph Temp vs Time        Heat Capacity of
       temperature of hot        -qhot = qcold + qcal    Calorimeter ( 0)
         and cold water            Q = mCSHT
                                   Qcal = CcalT

         – Molar heat of neutralization for a strong and a
           weak acid
Mass and temperature        Hrxn = CcalT + msol       Heat of Neutralization of a
 of solution, moles of      CSHT                        strong and weak acid,
    limiting reagent        Hrxn = Hneutralization     Determine which is the
                            Hneu/n =                       stronger reaction
                            Hmolar,neutralization
            Things to Note
• Record all physical observations in lab
  notebook
• Record all concentrations of acids and
  bases
• Record masses exactly to 4 decimal
  places
• Safety precautions with acids and base
• Always read the thermometer the same
  way to decrease error in calculations
• Clean up after yourself
              Equipment
• Thermometer – to record the system
  temperature
• 2 nested Styrofoam cups + serrated
  cardboard – bomb calorimeter
• Beaker – equilibrating acid to room
  temperature

In your notebook you should have :
• Table 1
• 2 copies of Table 2
          Experiment – Part I
         Calorimeter Calibration
• Purpose:
  – Determine the heat capacity of the calorimeter
• Procedure:
  – Construct the calorimeter with thermometer low
    enough to be below the surface of the liquid, not
    touching the sides or bottom
  – Add 50.0 mL DI water and allow to reach thermal
    equilibrium
  – Heat 50.0 mL DI water to an apex 40 degrees above
    room temperature
  – Add hot water to cold water
  – Read temperature immediately
  – Record temperature at 5, 10, 15 seconds, then every
    15 seconds for 3 minutes
  – Repeat until 2 good trials where: 0 < Ccal < 100 J/K
            Experiment – Part I
               Calculations
• 2 graphs: time versus temperature readings
   – In Excel, use scatter plot without best-fit trend line
   – Mark the initial and mode temperature
   – Mark the change in temperature
• Qlost = heat lost by the hot water
   Qlost = mhCsh∆Th
• Qgained = heat gained by cold water + calorimeter
  Qgained = mcCsh∆Tc + Ccal ∆Tcal

- Qlost = Qgained
          Experiment – Part I
             Calculations
- Qlost = Qgained
  -mhCsh∆Th = mcCsh∆Tc + Ccal∆Tcal

• How do we find the heat capacity of the
  calorimeter?
  – Mass of water is known
  – ∆Tcal = ∆Tc is calculated
  – ∆Th is calculated
  – Csh is known
       Experiment – Part IIA
    Neutralization of Strong Acid
• Purpose:
  – Determine the heat of reaction for a strong acid
• Procedure:
  – Record mass of dry, empty nested cups
  – Add 55._ mL NaOH to calorimeter, record mass
  – Allow the system to reach thermal equilibrium,
    measure temperature to 0.1 0C
  – Measure 54._ mL HCl in graduated cylinder
  – Add HCl to tared beaker to record mass and allow
    temperature to equilibrate to within 0.5 0C of NaOH
  – Add HCl to calorimeter, taking care not to splash
  – Record temperature immediately, 5, 10, 15 seconds,
    then every 15 seconds for 3 minutes
  – Repeat for 2 trials
       Experiment – Part IIB
     Neutralization of Weak Acid
• Purpose:
  – Determine the heat of reaction for a weak acid
• Procedure:
  – Record mass of dry, empty nested cups
  – Add 55._ mL NaOH to calorimeter, record mass
  – Allow the system to reach thermal equilibrium, measure
    temperature to 0.1 0C
  – Measure 54._ mL acetic acid in graduated cylinder
  – Add acetic acid to tared beaker to record mass and allow
    temperature to equilibrate to within 0.5 0C of NaOH
  – Add acetic acid to calorimeter, taking care not to splash
  – Record temperature immediately, 5, 10, 15 seconds,
    then every 15 seconds for 3 minutes
  – Repeat for 2 trials
             Experiment – Part II
                Calculations
• 4 graphs: time versus temperature readings
   –   2 graphs strong acid, 2 graphs weak acid
   –   In Excel, use scatter plot without best-fit trend line
   –   Mark the initial and mode temperature
   –   Mark the change in temperature
• Qlost = heat lost by the neutralization reaction
   Qlost = ∆Hrxn
• Qgained = heat gained by water + calorimeter
  Qgained = msolutionCsh∆Tsolution + Ccal ∆Tcal
         Experiment – Part II
            Calculations
- Qlost = Qgained
  -∆Hrxn= msolutionCsh∆Tsolution + Ccal∆Tcal

• How do we find the heat of reaction?
  – Mass of solution = mass of acid + mass of
    base
  – ∆Tcal = ∆Tsolution
  – Write out the balanced chemical equation
  – Csh is that of water
  – Heat of reaction = heat of neutralization
                  Experiment – Part II
                     Calculations
   • How do we find the molar heat of
     neutralization?
       – Calculate the number of moles of acid and
         base to determine the limiting reagent?
                  
       vol concentration  mol
                             mol n
       concentration(C )[ ]    
                              L   V
       n  C *V
       ∆Hneut,mol = ∆Hneut / nlimiting
Click Here for Sample Calculations for this experiment
              Data & Results
• Don’t forget to write down all significant figures
  from data collected
• Don’t round intermediate answers
• Pay attention to the number of significant figures
• Compose balanced chemical equations for
  Part II
• Accuracy points are assigned to this experiment
  based on the ability to calculate the heat
  capacity and heats of reaction twice
   Things to Know Before Going to
                Lab
• Understand thermochemistry, enthalpy, heat and
  calorimetry
• Understand why ∆Tcal = ∆Tc
• Know the definition of heat of neutralization, specific heat,
  and heat of reaction
• Understand why specific heat of water is used in the
  neutralization reactions
• Understand why the heat of reaction = heat of neutralization
• Know the difference between heat of neutralization and
  molar heat of neutralization
• Understand that Celsius and Kelvin scales are equivalent
• Know the chemical formulae for hydrochloric and acetic acid
                  Remember
• This week
  – Complete a prelab for this experiment in your lab
    notebook
• Prior to your arrival next week
  – This experiment’s report sheet and notebook pages
    due at the beginning of class
  – Complete the prelab lecture for the next experiment
  – Complete a prelab for the next experiment in your lab
    notebook
• Check Oncourse for announcements
  – Prepare as announcements instruct
                  References
• Silberberg, M.S. Chemistry: The Molecular Nature of
  Matter and Change. 3rd Ed. Burr Ridge, IL: McGraw Hill.
  2003. p221-237.
• ChemTeam: Time-Temperature Graph
  http://dbhs.wvusd.k12.ca.us/Thermochem/Time-
  Temperature-Graph.html
• ChemTeam: Time-Temperature Calculation
  http://dbhs.wvusd.k12.ca.us/Thermochem/Time-Temp-
  Calc3.html
• ChemTeam: Specific Heat
  http://dbhs.wvusd.k12.ca.us/Thermochem/Specific-
  Heat.html
• ChemTeam: ThermoChem Example Problems
  http://dbhs.wvusd.k12.ca.us/Thermochem/Thermochem-
  Example-Probs.html

								
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