Thermochemistry

Objective To explore heat transfer through calorimetry. To use calorimetry to determine the enthalpy of reaction of a strong acid and a strong base. To use Hess's Law of Heat Summation to determine the heat of hydration in calcium chloride. To explore a common use of heat of reaction in real life.

Grading Pre-Lab (10%) Lab Report Form (80%) TA Points (10%)

Background Information When two objects at different temperatures are brought into physical contact, thermal energy will spontaneously transfer from the warmer object to the colder object until both objects have achieved the same temperature. Assuming the two objects are thermally insulated from their surroundings, the heat lost by the warm object is identical to the heat gained by the cold object. This is a manifestation of the Law of Conservation of Energy. The heat transfer, q, is a function of the mass of the object (m), the change in temperature undergone by the object ( ΔT size 12{ΔT} {}) and the object's specific heat ( Cs size 12{C rSub { size 8{s} } } {}). This statement can be expressed mathematically as q = mCsΔT size 12{ ital "mC" rSub { size 8{s} } ΔT} {} Temperature change is always defined as Tfinal−Tinitial size 12{T rSub { size 8{ ital "final"} } - T rSub { size 8{ ital "initial"} } } {}, which means that q for the hotter object ( qhot size 12{q rSub { size 8{ ital "hot"} } } {}) is negative and q for the colder object ( qcold size 12{q rSub { size 8{ ital "cold"} } } {}) is positive. If energy is conserved, then q hot + q cold = 0 size 12{q rSub { size 8{ ital "hot"} } +q rSub { size 8{ ital "cold"} } =0} {} and ( mC s ΔT ) hot + ( mC s ΔT ) cold = 0 size 12{ \( ital "mC" rSub { size 8{s} } ΔT \) rSub { size 8{ ital "hot"} } + \( ital "mC" rSub { size 8{s} } ΔT \) rSub { size 8{ ital "cold"} } =0} {} Now consider dropping an ice cube into water just warm enough to melt the ice cube but not warm enough to further heat the water from the cube. The observation is that the ice cube melts and the warm water cools to 0 C. It is important to recognize that during the phase change, the temperature of the ice cube does not change. Therefore, it is not possible to use the preceding equation to determine the heat transferred. Rather, the energy transferred to the ice cube from the warm water affects the phase change. The energy equation is now adjusted to incorporate the enthalpy required to melt the ice cube, ΔHf size 12{ΔH rSub { size 8{f} } } {} (where f stands for fusion): ( mC s ΔT ) warm + ΔH f = 0 size 12{ \( ital "mC" rSub { size 8{s} } ΔT \) rSub { size 8{ ital "warm"} } +ΔH rSub { size 8{f} } =0} {} It is also possible to have thermal energy when chemical reactions occur. The amount and direction of heat flow is dependant on the chemicals reacting. Using a calorimeter, it is possible to experimentally determine the heat of reaction.

Calorimetry In the technique known as constant-pressure calorimetry, enthalpies of phase changes or chemical reactions are determined indirectly by measuring temperature (at constant pressure) changes in a medium, most often water, surrounding the materials undergoing the change. That is, by measuring ΔT size 12{ΔT} {} of the water one can use the preceding equation to calculate ΔH size 12{ΔH} {} for the process of interest. Of course, this means one must know the mass of the water used and water's specific heat: Cwater=4.18J/(gK) size 12{C rSub { size 8{ ital "water"} } =4 "." "18"J/ \( ital "gK" \) } {}. Today in Part I, you will add a strong base to a strong acid, measure the temperature change in the water as the two react, and use that information to calculate the heat of reaction per gram of NaOH. Then convert your experimental value into an enthalpy in kJ/mol (of NaOH). The enthalpy of a reaction might be difficult to obtain directly by experiment but can be determined by measuring (or looking up in tables) the enthalpy changes of reactions which contain the reactants and products in a process governed by Hess's Law of Heat Summation. In Part II of this experiment you are asked to find the enthalpy of hydration of CaCl2(s) size 12{ ital "CaCl" rSub { size 8{2} } \( s \) } {}  the "target reaction" in this manner. Target reaction: CaCl2+6H2O=CaCl2⋅6H2O size 12{ ital "CaCl" rSub { size 8{2} } +6H rSub { size 8{2} } O= ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O} {} It cannot be measured directly because of the slow kinetics of the reaction in the solid state. However, the heats of dissolution of CaCl2 size 12{ ital "CaCl" rSub { size 8{2} } } {} and CaCl2⋅6H2O size 12{ ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O} {} can be determined and, using Hess's Law, the enthalpy of hydration can be calculated. Theoretical value of the enthalpy of hydration of CaCl2 size 12{ ital "CaCl" rSub { size 8{2} } } {} anhydrous is -81.33kJ/mol. Theoretical value of the enthalpy of hydration of CaCl2⋅6H2O size 12{ ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O} {} is 15.79 kJ/mol

Experimental Procedure

Materials Required two Styrofoam cups cardboard cover conventional or digital thermometer stirring motor and stir bar two large Ziploc bag two plastic spoons

Setup of the MicroLab Thermistor Open the MicroLab Program by clicking on the Shortcut to MicroLab.exe tab on the desktop. On the “Choose an Experiment Type” Tab, enter a name for the experiment, and then double click on the MicroLab Experiment icon Click “Add Sensor”, Choose sensor = Temperature (thermistor) Choose an input, click on the red box that corresponds to the port that your thermistor is connected to. Label = Thermistor, sensor units = °C, click next Click “Perform New Calibration” Click “Add Calibration Point” place the thermistor and thermometer in an ice water bath. Wait until the temperature is constant and then read the temperature on the thermometer (to the nearest 0.1 °C) and enter it into the “Actual Value” box in MicroLab and hit “ok”. Again, Click “Add Calibration Point” place the thermistor and thermometer in warm water bath. Wait until the temperature is constant and then read the temperature on the thermometer (to the nearest 0.1 °C) and enter it into the “Actual Value” box in MicroLab and hit “ok”. Under Curve Fit Choices , click on “First order (linear)” and then “Accept and Save this Calibration”, when prompted to enter units, set as deg C. Save as your name-experiment-date. Click “Add Sensor”, Choose sensor = Time Choose an input, click on the red box that corresponds any of the Timers. Label = Time 1, click next, click Finish. Left click on thermistor and drag to the Y-axis over “data source two”, also click and drag to column B on the spreadsheet and also click and drag to the digital display window. Left click on Time and drag to the X-axis over “data source one”, also click and drag to column A on the spreadsheet and also click and drag to the digital display window. When you are ready to obtain data, click start. When you are finished collecting data, click stop. To run another trial, click repeat experiment.

Part I. Reaction of Strong Acid with Strong Base Weigh the calorimeter WITH a stir bar. Accurately measure 50 mL of 1 M HCl into your calorimeter. Turn on the stirring motor to the medium speed. Fit the thermistor though the cardboard lid to a length such that its tip goes deep into the solution but misses the stir bar. Cover the calorimeter quickly. Start the Acquisition program. You should see successive, constant temperature readings of near room temperature. Accurately measure 50 mL of 1 M NaOH and quickly add it to the acid solution in the Styrofoam cups. Cover the calorimeter. Continue monitoring the temperature change until thermal equilibrium is reached (the temperature stops changing or starts decreasing). Stop the Acquisition program. SAVE YOUR FILE. Remove the cardboard lid, reweigh the calorimeter and record the information on your lab report. Repeat the same procedure (Steps 1-4) two more times. Save all your files. Give them different, distinguishable names. Print off your graphs and use the data to determine the initial and final temperatures. Record all the temperatures to the nearest 0.1oC. If a slight downward trend appears on the final temperature plateau, use the maximum value achieved. Calculate the average enthalpy of the reaction and standard deviation for the three trials. Then combine your data with the data obtained from the rest of the group and calculate average enthalpy of the reaction and standard deviation for all trials together. Compare your average and standard deviation with that of the larger set and comment on the results obtained using a larger data set. The theoretical value of the enthalpy of reaction is -55.1 kJ/mol

Part II. Enthalpy of Reaction Tare your calorimeter and pour approximately 75 mL of cold water in it, add your stir bar. Record the mass to 3 decimal places. Place the cups on the magnetic stirrer and turn on the stirring motor to a medium rate. Fit the thermistor though the cardboard cover to a length such that its tip goes deep into the water but misses the stir bar. Cover the calorimeter. Start recording data on the MicroLab interface. You should see successive, constant temperature readings of near room temperature. Run 10 seconds before opening the lid. Approximately 5 grams of anhydrous CaCl2 size 12{ ital "CaCl" rSub { size 8{2} } } {} has been weighed for you, record the weight of this plus the weight of the weighing bottle. Then weigh the dry empty weighing bottle. Quickly add CaCl2 size 12{ ital "CaCl" rSub { size 8{2} } } {} to the water and reposition the thermistor and cover assembly. Be sure to shake well to loosen any material that has adhered to the sides of your calorimeter Note: Since anhydrous CaCl2 size 12{ ital "CaCl" rSub { size 8{2} } } {} absorbs moisture rapidly from the air, close the lids of the bottles securely immediately after using. Dry the spatula each time before weighing the powder and clean the balance of any solid. Continue monitoring the temperature change until thermal equilibrium is reached (the temperature stops changing or starts decreasing). Stop collecting and SAVE YOUR DATA and/or print it. Repeat steps 1-4 two more times. Don’t forget to save your data each time you do a run because it is lost as soon as the next run begins. Repeat the process but replace CaCl2 size 12{ ital "CaCl" rSub { size 8{2} } } {} anhydrous with CaCl2⋅6H2O size 12{ ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O} {}. Use the data to determine the initial and final temperatures. Record all temperatures to the nearest 0.1o C. If a slight downward trend appears on the final temperature plateau, use the maximum value achieved. Calculate the average enthalpy of dissolution and standard deviation. Use Hess’s Law to calculate the heat of hydration of CaCl2 size 12{ ital "CaCl" rSub { size 8{2} } } {} and % error. Then combine your data with the data obtained from the rest of the class and calculate the average enthalpy of dissolution and hydration.

Calculations Calculations are similar to those done for the acid-base neutralization reaction. The calculation of ΔHdissolution size 12{ΔH rSub { size 8{ ital "dissolution"} } } {} is the same as the calculation of ΔHrxn size 12{ΔH rSub { size 8{ ital "rxn"} } } {}, that is: ΔH dissolution = − q water size 12{ΔH rSub { size 8{ ital "dissolution"} } = - q rSub { size 8{ ital "water"} } } {} qwater size 12{q rSub { size 8{ ital "water"} } } {} can be calculated using water’s specific heat, mass of water and temperature change of water solution: q water = m water ⋅ C water ⋅ ΔT size 12{q rSub { size 8{ ital "water"} } =m rSub { size 8{ ital "water"} } cdot C rSub { size 8{ ital "water"} } cdot ΔT} {} In order to account for the mass of CaCl2 size 12{ ital "CaCl" rSub { size 8{2} } } {} anhydrous or hexahydrate, divide – qwater size 12{q rSub { size 8{ ital "water"} } } {} by the mass of CaCl2 size 12{ ital "CaCl" rSub { size 8{2} } } {} anhydrous or CaCl2⋅6H2O size 12{ ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O} {} to get ΔHdissolution size 12{ΔH rSub { size 8{ ital "dissolution"} } } {} in J/g. Then convert to J/mol.

Part III. Chemistry of Life Hot packs and cold packs are a real life example of thermochemistry. Anhydrous magnesium sulfate and ammonium chloride can be used to make “hot/cold” packs similar to those used for sports injuries and in hospitals. Your TA will make a pack from each of the two compounds and pass them around and answer some fundamental thermochemical questions about the reactions involved.

TA Procedure Fill two Ziploc bags half way full with water. Put two spoons full of anhydrous magnesium sulfate into one bag and two spoons full of ammonium chloride into the other bag, zip them closed, and shake. Pass the packs around to students and observe the temperature change.

PreLab: Thermochemistry (Total 10 Points) Hopefully here for the Pre-Lab Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students’ work (including your laboratory partner) or misrepresent your own data (see honor code). Name(Print then sign): ___________________________________________________ Lab Day: ___________________Section: ________TA__________________________ For calculations on heat transfer, use the equations: q H 2 O = mC w ΔT size 12{q rSub { size 8{H rSub { size 6{2} } O} } = ital "mC" rSub {w} size 12{ΔT}} {} where q = heat in J, m = mass of water in grams, Cw size 12{C rSub { size 8{w} } } {} = specific heat of water in, 4.18 J/(g°C), ΔT size 12{ΔT} {} = change in temperature in °C and ΔHsolution size 12{ΔH rSub { size 8{ ital "solution"} } } {} = [ qsalt size 12{q rSub { size 8{ ital "salt"} } } {} / g of salt] [formula weight in grams / 1 mole] where qsalt=−qH2O size 12{q rSub { size 8{ ital "salt"} } = - q rSub { size 8{H rSub { size 6{2} } O} } } {} to find the following heats of solution and, in the last question, heat of fusion. Heat of Fusion - Melting of Ice. When 6.4 g of ice was added to 100.0 g of water, there was a temperature drop from 24.31 C to 18.86 C. Find the values of q for ice and water and then ΔHfusion size 12{ΔH rSub { size 8{ ital "fusion"} } } {}. Remember, during the phase change the temperature of the ice does not change. Heat of Solution of Magnesium Sulfate ( MgSO4 size 12{ ital "MgSO" rSub { size 8{4} } } {}). 5.096 g of magnesium sulfate was added to 97.3 g of water and a temperature rise was observed from 24.67 °C to 31.86 °C. Hint: find q then ΔH size 12{ΔH} {}, for MgSO4 size 12{ ital "MgSO" rSub { size 8{4} } } {}. Original (unbalanced) Reaction Rewritten (1) CaCl2(s)→Ca2++2Cl− size 12{ ital "CaCl" rSub { size 8{2} } \( s \) rightarrow ital "Ca" rSup { size 8{2+{}} } +2 ital "Cl" rSup { size 8{ - {}} } } {} (2) CaCl2⋅6H2O(s)→Ca2++2Cl− size 12{ ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O \( s \) rightarrow ital "Ca" rSup { size 8{2+{}} } +2 ital "Cl" rSup { size 8{ - {}} } } {} target reaction: CaCl2(s)+6H2O(g)>CaCl2⋅6H2O(s) size 12{ ital "CaCl" rSub { size 8{2} } \( s \) +6H rSub { size 8{2} } O \( g \) > ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O \( s \) } {}

You will perform reactions (1) and (2) and use your experimentally determined enthalpy values to calculate ΔH size 12{ΔH} {} for the target reaction. Find the value for the heat of reaction for the rewritten reactions 1 and 2 and compute the heat of reaction for the target reaction.

Report: Thermochemistry (Total 80 Points) On my honor, in preparing this report, I know that I am free to use references and consult with others. Hopefully here for the Report Form Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students’ work (including your laboratory partner) or misrepresent your own data (see honor code). Name(Print then sign): ___________________________________________________ Lab Day: ___________________Section: ________TA__________________________ Part I. Reaction of a Strong Acid with a Strong Base 1. Write a balanced chemical equation for this reaction. 2. Calculate both the initial and final number of moles of HCl, NaOH, H2O size 12{H rSub { size 8{2} } O} {} and NaCl involved in the reaction. Hint: Reaction between a strong acid and a strong base goes to completion. 3. Is the reaction exothermic or endothermic? Explain. Individual Data Set (you and your partner) DATA CALCULATIONS m (solution), g T(initial), C T(final), C q ( water), J ΔH̲ size 12{ {underline {ΔH}} } {} (rxn), J/g (NaOH) ΔH̲ size 12{ {underline {ΔH}} } {} (rxn), kJ/mol (NaOH) Trial 1 Trial 2 Trial 3 AVERAGE % ERROR (use average) STD. DEV.

Group Data Set DATA CALCULATIONS m(solution), g T (initial), C T (final), C q ( water), J ΔH̲ size 12{ {underline {ΔH}} } {} (rxn), J/g (NaOH) ΔH̲ size 12{ {underline {ΔH}} } {} (rxn), kJ/mol (NaOH) Group 1Trial 1 Group 1Trial 2 Group 1Trial 3 Group 2Trial 1 Group 2Trial 2 Group 2Trial 3 Group 3Trial 1 Group 3Trial 2 Group 3Trial 3 Group 4Trial 1 Group 4Trial 2 Group 4Trial 3 Group 5Trial 1 Group 5Trial 2 Group 5Trial 3 Group 6Trial 1 Group 6Trial 2 Group 6Trial 3 Group 7Trial 1 Group 7Trial 2 Group 7Trial 3 AVERAGE % ERROR (use average) STD. DEV.

4. Don't forget to include the printouts of your temperature curves! 5. Compare the average and standard deviation of your data with that of the larger set. Which data is more accurate? Which is more precise? Comment. If your result is not what you would expect then explain. Part II. Enthalpy of Reaction Note: In preparing this report you are free to use references and consult with others. However, you may not copy other students’ work (except to compile the group data set) or misrepresent your own data. Individual Data Set (you and your partner) Dissolution of CaCl2 size 12{ ital "CaCl" rSub { size 8{2} } } {} Anhydrous mH2O size 12{m rSub { size 8{H rSub { size 6{2} } O} } } {}, g mCaCl2 size 12{m rSub { size 8{ ital "CaCl" rSub { size 6{2} } } } } {}, g Tinitial size 12{T rSub { size 8{ ital "initial"} } } {}, oC Tfinal size 12{T rSub { size 8{ ital "final"} } } {}, oC qH2O size 12{q rSub { size 8{H rSub { size 6{2} } O} } } {}, J ΔH1 size 12{ΔH rSub { size 8{1} } } {}, kJ/mol Trial 1 Trial 2 Trial 3 AVERAGE % ERROR STANDARD DEVIATION

Dissolution of CaCl2⋅6H2O size 12{ ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O} {} mH2O size 12{m rSub { size 8{H rSub { size 6{2} } O} } } {}, g mCaCl2⋅6H2O size 12{m rSub { size 8{ ital "CaCl" rSub { size 6{2} } cdot 6H rSub { size 6{2} } O} } } {}, g Tinitial size 12{T rSub { size 8{ ital "initial"} } } {}, oC Tfinal size 12{T rSub { size 8{ ital "final"} } } {}, oC qH2O size 12{q rSub { size 8{H rSub { size 6{2} } O} } } {}, J ΔH2 size 12{ΔH rSub { size 8{2} } } {}, kJ/mol Trial 1 Trial 2 Trial 3 AVERAGE % ERROR STANDARD DEVIATION

Hess’s Law ΔH1 size 12{ΔH rSub { size 8{1} } } {}, kJ/mol CaCl2→Ca2++2Cl− size 12{ ital "CaCl" rSub { size 8{2} } rightarrow ital "Ca" rSup { size 8{2+{}} } +2 ital "Cl" rSup { size 8{ - {}} } } {} ΔH2 size 12{ΔH rSub { size 8{2} } } {}, kJ/mol CaCl2⋅6H2O→Ca2++2Cl− size 12{ ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O rightarrow ital "Ca" rSup { size 8{2+{}} } +2 ital "Cl" rSup { size 8{ - {}} } } {} ΔHhydration size 12{ΔH rSub { size 8{ ital "hydration"} } } {}, kJ/mol CaCl2+6H2O→CaCl2⋅6H2O size 12{ ital "CaCl" rSub { size 8{2} } +6H rSub { size 8{2} } O rightarrow ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O} {} THEORETICAL VALUE % ERROR

Group Data Set Hess’s Law Average ΔH1 size 12{ΔH rSub { size 8{1} } } {}, kJ/mol CaCl2→Ca2++2Cl− size 12{ ital "CaCl" rSub { size 8{2} } rightarrow ital "Ca" rSup { size 8{2+{}} } +2 ital "Cl" rSup { size 8{ - {}} } } {} Average ΔH2 size 12{ΔH rSub { size 8{2} } } {}, kJ/mol CaCl2⋅6H2O→Ca2++2Cl− size 12{ ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O rightarrow ital "Ca" rSup { size 8{2+{}} } +2 ital "Cl" rSup { size 8{ - {}} } } {} ΔHhydration size 12{ΔH rSub { size 8{ ital "hydration"} } } {}, kJ/mol CaCl2+6H2O→CaCl2⋅6H2O size 12{ ital "CaCl" rSub { size 8{2} } +6H rSub { size 8{2} } O rightarrow ital "CaCl" rSub { size 8{2} } cdot 6H rSub { size 8{2} } O} {} THEORETICAL VALUE % ERROR

Data Analysis 6. Evaluate the possible sources of error in this experiment and specifically how those errors would have affected your ΔH size 12{ΔH} {}values. Source of Error Increase or Decrease ΔH size 12{ΔH} {}? Explain

Part III. Chemistry of Life Based on your observations, which compound, magnesium sulfate or ammonium chloride, is used to make cold packs (hot packs)? Explain. Explain whether the two reactions are endothermic or exothermic: (1) ammonium chloride and water (2) magnesium sulfate and water. What sign will ΔH size 12{ΔH} {} have for the two reactions: (1) ammonium chloride and water (2) magnesium sulfate and water? New from the File menu, and then double-click your template.