Pre Lab Reports Required for Each of the Attached Lab Reports
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CALORIMETRY PART 1: SPECIFI C HEAT CAPACITY
I. PURPOSE In this experiment you will use calorimetry to experi- mentally measure the specific heat capacity of aluminum and a n unknown metal alloy. You will be using a digital thermom eter to take the most precise measurement of the unknown alloy.
II . BACKGROUND The amount of heat energy required to raise the tempera- ture of an object by one degree Kelvin (or Celsius) is the heat capacity of an o bject, which is txpressed in units of JIK (or Jl°C). The amount of heat (q) absorbed (or released) by an object can be quantified and expressed as the product of th e object's heat capacity and its change in temperature (.17), as described in equation l .
q = (heat capacity) X ti T ( 1) It is important to recognize that the change in tempera- ture (.17) is always defined as the difference between the
final (T) and the initial (T) temperatures (equation 2) . Conseq uen tly, .1 T can be positive o r negative .
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The amount of heat req ui red to raise the temperature of
one gram of material by one degree Kelvin is its specific heat capacity (c) and has units ofJ/g-K. Because the h eat capacity is rh e product of rhe specific hear capacity and mass of an object, equation l can be rearranged to be ex- pressed in terms of the mass of the object and the specific heat capacity, sh own in eq uation 3.
q = c X m ass X ti T (3) A calorimeter is a device wh ich ca n be used to measure the heat released o r absorbed by a p hysical o r chem ical
process. Calorimeters are thermally insulated containers that prevent (or at least limit) the tran sfer of heat with the
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56 EXPERIMENT 6
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outside environment Effectt'vely aki th If · d · · m ng em se -coma.me env1ronmenrs that arc not influenced by their surroundings.
You will use a calorimeter like the one shown in Figure 6-1 , constructed from:
A. Thermometer/probe
B. Cover
C. Two nested Styrofoam cups
D. 250 mL glass beaker
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Figure 6-1 . Calorimeter
Water placed in the Styrofoam cups constitutes the "surroundings" and the sample placed into the water will constitute rhe "system." You will be studying rhe exchange of hear between the surroundings and rhe sys tem.
For processes of interest in chemist!")', energy changes will occur primarily as a transfer of hear; the process will either absorb heat from rhe surroundings or release it into the surroundings as the process occurs. A physical change or chemical reaction is endothermic if ir absorbs hear (positive q) and exothermic if it releases hear (nega- tive q) . Nore rhar any physical process or chemical reaction rhar is endothermic in one direction wi ll be exoth ermic in the opposite direction (e.g., breaking versus forming bonds).
The heat transfer associated with a physical or chemical change is carried out enti rely inside rhe calorimeter and that, assuming that the calorimeter is a perfectly isolated system; the hea t exchanged to rhe outside of the calorimeter is zero. The hear change for rhe calorimeter (q ,1 will be equal in magn itude, bur opposi te in sign, to the Jlt"Olltlfltn hear exchange fo r a chemic reaction (q ) occurring within (eq uation 4):
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Any heat released by the reaction will be absorbed by the calorimeter; hear absorbed by the reacti on will have been taken from the calorimeter.
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CA LOR IM ETRY PAR T 1 SPEC IFIC H EAT CAPACITY
Measuring rhc change in remperarure for rhe calorimeter (L1 T,11rroundin) caused by a chemical reaction allows for rhe determination of q , using equation 5. The Jll"'1Untllflf \'alue of q
I fo r the reaction (o r process) of inreresc can then be determined from
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q , urrounding = m ,urround,ng X c , urrounding X /J. T , urround,ng (S) Your calorimeter is nor a perfectly insulated system; a small bur significant amount of hear may be lost ro (or gained from) outside che calorimeter during the reaction or process being studied. This "extra" hear transfer either our of or into th e calorim- eter will result in a systematic error in L1 T , such chat the measured value of
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made graphically from a plot of rhe temperature of the calorimeter vs. time, as shown for an endothermic reaction plotted in Figure 6-2. The corrected L1 T is measured at system the start time of the reaction by extrapolating back in a straight line .from the temperature measurements taken after the reaction is complete. Nore rhar if an exothermic process were plotted as in Figure 6-2 rhe remperarure would rise over time.
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Figure 6-2. Determination of f1 T .from calorimeter data
In this exp eriment, yo u will use calorimetry ro measure rhe heat transfers associated with a physical process: rhe warming of a block of aluminum. The heats will be used ro exp~rim enrally ~erermine values for the specific heat capacity of aluminum (cA). According ro equarwn 4 , th e heat energy which, enters the aluminum (the "system") is exactly eq ual co the heat energy, which leaves the water and the calorimete ( h "
d · ") Wi h f; r t e sur- roun mgs . e can t ere o re write equation 6 co mathematically describe chis process:
CAI X mAI X l'.1TAI = -(cw X V w X dJ L1Tw Where:
cAJ = specific heat of aluminum, J /g-K cw = specific hear o f water, J /g- K mAI = mass of aluminum, g Vw = volume of water, mL dw = density of warer, g/mL /J. ~ , = lj-- T;.AI ( T;.AI = initial Al temperature, O °CJ LIT = T,- T (T - · · · l
II' { i,1u i,U' - lnJfta Water tempertlfttre, °CJ lj = fin a solution temperature, °C
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58 EXPERIMENT 6
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For this equation you will be able to measure the various temperatures, thus the only remaining unknown is the specific heat of aluminum, which you will be able to calcu- late. The same principle applies to any unknown placed in the calorimeter.
Il l. SAF ETY PRECA UTI ONS • Safety glasses are always required as long as anyone in the lab is still performing
laboratory work!
• An apron or lab coat and closed-toe/closed-heel shoes are required.
•
No food or drinks in the lab at any time.
Handle the digital thermometer probe very carefully. Do not stir solutions in the calorimeter with the thermometerlprohe.' Instead, gently move the entire calo- rimeter in a d rcular motion.
Don't handle the calorimeter cover and the thermometer probe as one unit.
• When you assemble the calorimeter, place the cover on the calorimeter first and then insert the thermometer probe.
• When opening the calorimeter, remove the probe before removing the cover.
• There is no chemical waste to collect in this experiment.
IV. MATER IALS AND REAGENTS
QT Equipment (communal equipment)
Styrofoam cups, as set of two and a cover
Thermometer
Aluminum bar
Unknown alloy (check it out from your TA)
250 ml Beaker
100 ml Graduated cylinder
Ruler
NO HAZARDOUS WASTE
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CALO RI METRY PART 1: SPECIFIC HEAT CAPACIT Y 59
V. EXPERIMENTAL PROCEDURE : You will work in pairs for chis experiment.
The Styrofoam cups and calorimeter covers will be available in plastic cubs ar the front of the room. The temperature will be read on a digital thermometer. You must clean and return all equipment to rhese cubs when you are finished.
A. THE SPECIFIC HEAT CAPACITY OF ALUMINUM (CA,) 1. If you followed the special note in the previous lab, cake out your 1 L plastic bottle
filled wirh rap water chat is now exactly at room temperature. Use chis water for Pare A and B of chis experiment.
2. If you did not follow the directions of the special note in the last lab, fill a 1 L plastic botde with room temperature cap water and set it aside for lacer use. Do not collect very hot or cold water, as you will want the water to be the same tem- perature for your replicate experiments. Use this water for Pares A and B.
3. Obtain an aluminum block from the front desk, dry it if necessary, weigh it on an analytical balance, and accurately record the mass.
4. To adjust the temperature of the al uminum block to 0.0°C you will need to place the block in an ice bath. Do this by placing the block in the bottom of a 250 mL beaker. Fill the beaker with ice, then add just enough cold tap water co cover the block. Adding coo much water will prevent the block from cooling to 0.0°C.
5. As the aluminum block is cooling, assemble the calorimeter as shown in Figure 6-1 .
6. Then use your graduated cylinder to rransfer precisely 100 mL of the room tem- perature tap water to the calorimeter.
7. Record the temperature of the calorimeter every 10 seconds for a couple of min- utes or until the temperature appears co be constant.
8. After rhe water in the calorimeter has reached a constant temperature, remove che aluminum block from rhe ice with your tongs. Wipe off any pieces of ice with a paper towel (but don't cry to dry it completely).
9. Carefully place it in the calorimeter. Do not splash any water out of the calorim- eter; it is part of your calculations.
10. Place the lid back on the calorimeter and record the temperature and then again every 10 seconds thereafter for approximately 5 minutes in your laboratory man- ual. The calorimeter should be swirled gently by moving the beaker in a circu- lar/stirring motion berween temperature readings. Don't use the thermometer probe as a stirrer; it will break!
11. Repeat steps 4- 10 wirh the sa me aluminum block rwo more rimes.
12. C~nstruct a graph for calorimeter temperature vs. time for each trial on the sa me gnd and label each of them (Trial 1, 2, and 3).
13. Use the experimental data from the three trials to calculat th •fi h _ _ _ , e e spec1 1c ear capamy of alummum (c"!) m J/g,K. The specific heat capacity of water (c u) is 4. 184 J/g-K and rhe denmy of water is 1.00 g/mL Calcul th al
d h · ace e average v ue
an r e standard deviation from the three results.
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B. THE SPECIFIC HEAT CAPACITY OF AN UNKNOWN METAL ALLOY 1. There will be a different unknown for each student in the class. Pick out one un-
known from che front desk, dry it if necessary, weigh it on an analytical balance,
and accurately record the mass.
2. Record the mass and unknown number in your labo ratory noteb ook.
3 . Place the block in an ice-water mixture as you did for Pare A of this experiment.
4. Follow steps 4-1 0 from Part A to experimentally determ ine the temperature change fo r your unknown. Record your data in your laboratory notebook.
5. Repeat chis process two more times in order to obtain three replicate secs of data.
6 . Const ruct a graph of unknown metal tem perature vs. time and determine the temperature change of the unknown metal (.6 T k ) • Put all data trials for yo ur
un nown
unknown on the same graph and label each of them (exa mple: Trial 1, 2 , and 3) .
7 . Calculate the specifk heat capacity of the unknown metal from d ata from the three trials. The specific heat capacity of water (c w) is 4.184 J /g-K and the density of water is 1.00 g/mL. Calculate the average value and the stand ard d eviation from the three results.
VI. DISCUSSION/QUESTIONS I. Discussion: The actual value for the specific heat of aluminum is 0 .900 J/g·K.
From your expe rimental data on al uminum, you sh ould be able to assess the accuracy and precision of your experi mental procedure. Comment o n both the accuracy and precision of your experimental value for c Al.
2. Discussion: Discuss the accuracy, precision, systematic errors, and random errors of yo ur unknown data.
3 . What _change in_ your experimental data would result from usin g 200 mL of water 111 the calorimeter for the alum inum experiment?
REF ERE NC ES
C hem istry and Bioc~emistry D epartment. "Manual for C HEM 200/202 ." Laboratory Manual. San Diego State U niversity. San Diego. 2013 . Print.
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