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CALORIMETRY – EXPERIMENT A
ENTHALPY OF FORMATION OF MAGNESIUM OXIDE
INTRODUCTION
This experiment has three primary objectives:
- Find the heat capacity (Cp) of a calorimeter and contents (calibration).
- Determine the Hrxn, the enthalpy of reaction, in kJ/mol for several different reactions, including the reaction of an unknown with a solution of HCl.
- Calculate the Hf, the enthalpy of formation, of MgO using Hess’ Law (in kJ/mol).
We will assume that the energy exchanged between the calorimeter and the surroundings during and following the reactions is small and at a slow, constant rate. You will become familiar with calorimetry concepts, computer data collection, and calculations.
BACKGROUND
Calorimetry measures the energy that a reaction produces or consumes. For example, the major difference between gasoline grades is the octane number. Unleaded gas has an octane of 86, while Super Unleaded gas has a higher octane. Calorimetry could be used to measure the heat or energy produced when gasoline is burned. More heat (energy) would be produced by the super unleaded gas so it would have a higher enthalpy compared to just unleaded gas. Calorimetry could be used to see if a gasoline station is selling the grades of gasoline it advertises.
The calories in food have also been measured by calorimetry (hence the term calories). Usually this is a measurement of calories (cal) per gram of food. Remember that calories are easily convertible to joules (J) and grams can be converted to moles if it is a pure chemical.
Enthalpy , represented by the symbol H , is a property chemists use to describe the heat flow into or out of a system in a constant-pressure process. This is often the case since most processes that are carried out are exposed to the atmosphere as are the reactions carried out in this course. The enthalpy of a reaction , Hrxn , is defined as the difference between the enthalpies of the products and the enthalpies of the reactants. In other words, it is the change in energy for a given amount of a given reaction. The enthalpy of formation , Hf is defined as the enthalpy or heat change that results when one mole of a compound is formed from its elements. The standard enthalpy of formation is defined as the enthalpy of formation measured at 1 atm such that the elements are in their standard state.
If a reaction is exothermic, heat will be released, and the temperature of the system or reaction mixture will rise. (In this experiment the heat and temperature rapidly increase and then slowly decrease as heat is lost to the surroundings.) For endothermic reactions heat will be absorbed or used and the temperature will decrease. In this experiment we will use the experimentally measured enthalpy of reaction for a series of exothermic
reactions and Hess' Law to determine the heat of formation for magnesium oxide (MgO). We will also determine the enthalpy of reaction for an unknown metal oxide with an acid. For this experiment pressure will be constant so Enthalpy of Reaction and Heat of Reaction ( Hrxn) are assumed to be the same. The enthalpy of reaction, Hrxn, can be calculated using the equation:
−(𝐶𝑝)(∆𝑇) 𝑛
−(𝑘𝐽℃ )(℃) 𝑚𝑜𝑙
−𝑘𝐽 𝑚𝑜𝑙
A-
Where n is the moles of limiting reagent, T (C) is the change in temperature in of the calorimeter’s contents, and Cp (kJ/C) is the heat capacity of the calorimeter. The value for n can be determined knowing the amounts of starting material. The T for a reaction can be calculated using the temperatures before and after the reaction or the initial and final temperatures. The heat capacity, Cp, of the calorimeter has to be experimentally determined by doing a reaction where the Hrxn is known. The heat capacity of the calorimeter is primarily due to the solution in the cup.
Heat capacity (Cp) has units of kJ/C. Physically, this means that it takes the value of the Cp in energy to raise the calorimeter by 1C. For example, if a calorimeter has a Cp of 0.200 kJ/C, the calorimeter, including its contents, must absorb 0.200 kJ of energy to increase 1C. A 20 kJ/C calorimeter increases 1C with one hundred times more energy, or 20 kJ. Cp varies depending on the substance or system and describes how much energy is needed to change the temperature of that substance or system. The Cp of an ocean is huge (compared to a drop of water) such that the oceans of the world maintain the earth at temperatures that support life.
In this experiment, the calorimeter is defined as two nested styrofoam cups, the lid, magnetic stir bar, and the temperature probe tip, plus the 60.0 mL of the reaction mixture (mainly water). In order for the heat capacity of the calorimeter to remain constant, all of these must be present.
Figure A-1 Calorimeter Apparatus (ignore B-1)
NOTE: If less than 60 mL of reaction mixture was added, it would take less energy to increase the calorimeter and contents by 1 C. In other words, the heat capacity would decrease. If more than 60 mL of the mixture was added, more energy would be needed to increase the calorimeter and contents by 1 C. The heat capacity is then increasing.
CHEMICALS:
- 3.0M Hydrochloric acid (3033) HCl - about 200mL needed
- ~5.0M Sodium Hydroxide (3034) NaOH - see carboy for exact concentration, about 30 mL needed
- Magnesium (0220) Mg turnings - about 0.20g needed
- Magnesium oxide (1011) MgO - about 1.0 g needed
- Unknown A-xxxx (1012) in your unknown packet - need at least 2-3 grams
SAFETY CONCERNS:
Risk Assessment-Moderate to High (due to corrosive liquids)
- The HCl and NaOH are corrosive. The unknown and MgO are mildly corrosive and some are powders so avoid contact with solids and dust. Avoid contact, wear eye protection at all times when working with these chemicals, and wash hands after handling them. Do not rub your eyes when using these chemicals. Any small spills should be cleaned up immediately with a damp sponge.
- Any contact with HCl or NaOH should be rinsed for 15 minutes with water.
- All solid and liquid chemical waste should be disposed of in the “Corrosive Liquids” container.
- Goggles required. Lab coat or apron and gloves are recommended but not required.
EXPERIMENTAL PROCEDURE:
FIRST LAB PERIOD
The Determination of Hrxn of Mg with HCl and MgO with HCl
NOTE: If these links are not present, click on the icon, "Vernier Programs" and then “Data Logger”. Change the time to 400 seconds.
NOTE: Use the same, calorimeter, computer- temperature probe, and thermometer for every lab period if possible.
Calibration of the Temperature Probe:
Startup procedure
Select “Start”, “Programs”, “Chemistry Applications” and then click on “CHM152L-A Calorimetry”.
Calibration Check Procedure
Find your thermometer and three beakers 100mL or bigger. The beakers do not need to be clean. Fill one beaker 2/3 full with ice and add cold water to make a slush. Fill another beaker 2/3 full with hot tap water. Put the temperature probe tip and thermometer bulb together so they touch and place them into the hot beaker and let sit for one minute. The temperature can be
A two point calibration will need to be done using hot and ice water if either the temperature of hot or ice water are not within 0.5C of the temperature measured with the thermometer. Your TA will provide with a procedure to calibrate the temperature probe.
NOTE: Only one trial is needed for this reaction.
Avoid adding extra heat from hands, hot plate- stirrer; make sure the hot plate heat is turned off.
Why should you measure the mass of Mg this way?
read below the graph. You do not need to click on the collect button. Record the temperatures. If the temperatures are not within 0.5C, see your TA. Put the temperature probe and thermometer into the cold beaker and let sit for one minute. You do not need to push the collect button. Record the temperatures. If the temperatures are not within 0.5C, see your TA. Be sure to check the calibration again at the beginning of the second lab.
Mg Reaction. Determining the Enthalpy for Reaction ( H rxn ) of Mg with HCl (H+):
Mg(s) + 2 H+(aq) Mg2+^ + H2(g)
Clean a teflon stir bar, 50 mL beaker, 25mL and 50mL graduated cylinder and wash bottle. Add 25.0 mL of 3.0 M HCl (use pump dispenser, ok to check volume with 25mL graduated cylinder) and 35.0 mL of pure water to the calorimeter. Put the temperature probe in through the lid and place the lid on the calorimeter. Secure the probe with a clamp so that its tip is in the water and off the calorimeter bottom. Don’t allow the stir bar to hit the probe tip (see fig.A-1). Stir the solution with a teflon stir bar ( Do not heat.) Set stirrer so the solution is mixed vigorously but slow enough so that it is not splashed. Put between 0.15 to 0.20 g (not 1.5) of Mg metal turnings into a clean, dry 50 mL beaker. Record both the beaker’s mass and the mass of the beaker and Mg. Click the "Collect" button at the top of your screen to start graphing the temperature. Do not add Mg turnings yet. After about one minute, add the metal Mg turnings without removing the temperature probe from the solution. (Crack the lid open, add Mg (^) (s), and then close the lid, it is ok if residual Mg sticks to the inside of the beaker since you will reweigh it later to see how much Mg was added to the calorimeter). If any Mg is stuck on the sides of the calorimeter above the liquid carefully swirl the solution (holding the cup in your hand) to dissolve it. Reweigh and record the mass of the beaker that contained the Mg turnings. Subtract this mass from the mass of the beaker and Mg to get the mass of Mg used.
because the Mg turnings are added to the solution.) Somewhere within the blocked out region the reaction stops.
Part 3. The reaction has already stopped. Since the calorimeter isn’t a perfect insulator, heat is lost to the environment and, as a result, the temperature decreases. The temperature should be constantly decreasing. (For example, for Mg, all of the Mg has been converted to Mg2+.)
How do you get Temperature Change ( T) in C? T is equal to the
extrapolated final temperature minus the solutions initial temperature so T = Tf – Ti. Below is the same general graph from figure 1, but it has been extrapolated to find Tf (Figure 2). Tf can also be determined by doing a linear regression on the linear, right hand side of the curve (part 3) to determine the slope and y-intercept of line 2 and then solving for the temperature using the time at the start of the reaction (line 1).
Figure A-3. Extrapolation of a Temperature vs. Time Graph to Find T.
Line 1****. This line represents the time when the reactants were mixed and so the start of the reaction.
Line 2. This line helps us model what the final temperature would be if the reaction and temperature measurement were instantaneous. It compensates for the heat lost from the calorimeter so that we can determine the final temperature if the reaction and temperature measurement were instantaneous. This line is important because it compensates for heat lost to the environment while temperature is measured during and after the reaction.
Line 3. This line is drawn at a right angle to line 1 to intersect the point where lines 1 and 2 meet. It is there to help read the final temperature, Tf, at the y-axis. The interpolate function can also be used to get Tf.
Calculation: Temperature Change ( T) in C and mol of Mg
IMPORTANT: For nearly all calculations in this manual the value you are calculating will be in bold print. In this reaction you were trying to find the Hrxn for reaction of Mg with HCl. Unfortunately all of the calculations cannot be done until the Cp in equation A-1 is found after doing the NaOH reaction. The limiting reagent is Mg, so find the moles of Mg.
𝑀𝑎𝑠𝑠 𝑀𝑔 (𝑔)
𝑚𝑜𝑙 𝑀𝑔 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑀𝑔 (𝑔)
𝑔 𝑀𝑔 𝑀𝑀 𝑜𝑓 𝑀𝑔
= 𝑚𝑜𝑙 𝑀𝑔 A-
where g Mg is grams of Mg and MM of Mg is the molar mass of Mg (g/mol). Now n is found. Find T by as noted earlier but you should be able to do the extrapolation using a printed graph as shown in figure A-3 draw all lines with a pen and a ruler. Label Ti (initial), Tf (final), and T on your graph. All temperatures are in C. Do not convert to Kelvin (K)!
NOTE: Only one trial is needed for the MgO reaction.
NOTE: Never click on “New Graph” when starting a new trial. Instead just close the program and then reopen it. If you do click on “New Graph” the time scale will decrease to 200 seconds. Do not move the calorimeter or delete or stop the program if it does stop early! Get help from your TA! .
MgO Reaction. Determination of the Enthalpy for Reaction ( Hrxn) of MgO with HCl:
MgO(s) + 2 H+^ (aq) Mg2+(aq) + H 2 O(l)
Add 25.0 mL of 3.0 M HCl and 35.0 mL of pure water to the calorimeter. Clean the temperature probe by thoroughly spraying with your wash bottle into a large waste beaker. Put the temperature probe in the calorimeter as was done before and vigorously stir the solution (but don't splash) with a Teflon stir bar. (Do not heat) Put between 1.0 to 1.2 g of MgO powder into a clean, dry 50 mL beaker. Record the mass of both the beaker and the beaker with MgO. If Data Logger is open close it and select “Start”, “Programs”, “Chemistry Applications” and then click on “CHM152L-A Calorimetry”. Click the “Collect” button at the top of the screen to start graphing the temperature. Do not add MgO powder yet. After about one minute, add the white MgO powder without removing the temperature probe from the solution. (Crack the lid open, add MgO(s), and then close the lid. If any MgO is stuck on the sides of the calorimeter above the liquid carefully swirl the solution (holding the cup in your hand) to dissolve it). It’s ok if a residual amount of powder remains in the beaker since it will be reweighed later to determine the amount transferred to the calorimeter.
(cont.) What kind of glassware should you use for this? _______
NOTE: Never click on “New Graph” to start a new trial. Instead just close the program and then reopen it. If you do click on “New Graph” the time scale will decrease to 200 seconds. Do not move the calorimeter or delete or stop the program if it does stop early! Get help from your TA!
NOTE: Be sure to record the exact NaOH molarity of the carboy.
NOTE: At least two Cp trials need to be done. Avoid adding extra heat from your hands or hot plate.
NOTE: At least 2 trials also need to be done for reaction of HCl with the unknown. Avoid adding extra heat from hands, hot plate, etc
NOTE: If your unknown is sticky see your TA.
NOTE: The unknown metal oxide is assumed to have a formula weight of 120.0 g/mol!
Put the temperature probe in the calorimeter and stir the solution with a teflon stir bar. (Do not heat) Click the Collect button at the top of the screen to start graphing the temperature probe. Do not add HCl solution yet. After about one minute, add the 3.0 M HCl without removing the temperature probe from the solution. (Crack the lid open, add HCl(aq), and close the lid.) After about 10 seconds, briefly swirl the solution. Continue graphing data until a linear line (Figure A-2: see part 3 of this figure) is made. (At about 350 to 400 seconds.) Adjust scale of graph, do a linear fit, and find the T as was done before. Label the graph by clicking on the graph title as before. Enter your last name, experiment title, date, section letter, and NaOH with HCl, Trial 1 or 2, and molarity of NaOH. Save the run on your “Z” drive, my documents, or a thumb drive and print it. Clean, rinse, and dry the calorimeter and temperature probe. Repeat this once.
Enthalpy of Reaction ( Hrxn) of HCl with an Unknown Metal Oxide:
Add 25.0 mL of 3.0 M HCl and 35.0 mL of pure water to the calorimeter. Clean the temperature probe by rinsing well with your wash bottle into a 600 mL beaker. Put the temperature probe in the calorimeter and vigorously stir the solution with a teflon stir bar but avoid splashing. (Make sure the heat is off) Before using your unknown, make sure it is a powder. If it is clumpy, grind it up in a clean, dry mortar and pestle. Put between 1.0 to 1.2 g of unknown powder into a clean, dry 50 mL beaker. Record mass of beaker & contents. Click the "Collect" button at the top of your screen. Your temperature probe will display the data it is collecting on the graph. Do not add the unknown yet.
NOTE: Never click on “New Graph” to start a new trial. Instead just close the program and then reopen it. If you do click on “New Graph” the time scale will decrease to 200 seconds. Do not move the calorimeter or delete or stop the program if it does stop early! Get help from your TA!
After about one minute, add the white unknown powder without removing the temperature probe from the solution (Crack the lid open, add the weighed unknown, and then close the lid. If any the unknown is stuck on the sides of the calorimeter above the liquid carefully swirl the solution, holding the cup in your hand, to dissolve it). It is ok if a residual amount of powder remains in the beaker since it will be reweighed later to determine the amount transferred to the calorimeter. Reweigh the beaker with unknown powder not transferred into the calorimeter. Continue graphing data until a linear line (part 3 in fig A-1) is made and then click on "Stop". (At about 350 to 400 seconds.) Adjust the scale of graph, do a linear fit, and find the T as was done before. Label the graph by clicking on the graph title. Enter in your last name, experiment title, date, section letter, Unknown with HCl, Trial 1 or 2, & mass unknown. Save the run on your “Z” drive, my documents, or a thumb drive and print it. Do a second trial after cleaning the cup and probe. Clean and rinse all glassware and the temperature probe. Before leaving, trim graphs to size and tape into the notebook, and have TA sign and date notebook.
Calculation: Cp
The calibration of the calorimeter is now complete! Cp can now be calculated and used for all other calculations. First solve for the moles of NaOH (limiting reagent).
(mol/L NaOH) * (L NaOH) = mol NaOH = n Where M is molarity (exact molarity on carboy) and L is liters of NaOH that was used. (You need to convert from mL). Now that we have n (mol of NaOH) plug it into equation A-3. Also called the enthalpy of neutralization (Hrxn) of a strong base by a strong acid is a constant – 55.90 kJ/mol at 25C. This is shown by reaction A-2.
H+^ (aq) + OH-^ (aq) H 2 O (^) (l); Hrxn = -55.90 (kJ/mol) at 25C A-
With this value, the moles of limiting reactant (n), and after determining T from your graphs by extrapolation, the equation A-3 becomes a simple plug and chug.
−(∆𝐻𝑟𝑥𝑛)(𝑛) ∆𝑇
(55.90 (^) 𝑚𝑜𝑙𝑘𝐽)(𝑚𝑜𝑙) ℃
𝑘𝐽 ℃
A-
Calculation: Heat of Reaction ( Hrxn) for Unknown
The unknown metal oxide is assumed to have a molar mass of 120.0 g/mol. These are the same calculations as described in the reaction of Mg. The calculations are now for the reaction of unknown with HCl(aq). The unknown is the limiting reagent.
Now use the value for n (mol of unk), the T determined from the graph, the Cp and the equation below to calculate the enthalpy of reaction for the unknown with HCl:
−(𝐶𝑝)(∆𝑇) 𝑛
−(𝑘𝐽℃ )(℃) 𝑚𝑜𝑙
𝑘𝐽 𝑚𝑜𝑙
A-
Calculate this value for each trial and report the median, range, and relative % range.
Fill out the unknown report sheet at the end of this experiment making sure to put the unknown number list on the unknown vial that you used (A-xxxx where xxxx is you unknown number). DO NOT use the hazard code at the bottom of the label (HC-1012). Make sure that all calculations are done in your laboratory notebook using dimensional analysis and that the report sheet is complete. Be sure to complete a calculation check! When doing the calculation check, be aware that the heat of reaction is the same as enthalpy of reaction and that these values are negative numbers for exothermic reactions. Get a printout of the calculation check and staple it to the unknown report sheet. Points will be taken off for incomplete report sheets or resubmission/repeats of the unknown. Have your regular TA review the report sheet, sign and date it and give it to them for grading. The report sheet for the unknown must be turned in by the deadline listed in the syllabus to avoid late points. This same process will be used for the unknowns done in experiments D, E and F.
POST LAB QUESTION (In your lab notebook):
Do you trust your results?
Use the following information to answer this question:
Your calculated range and relative percent range for the Cp and Heat of Reaction or Enthalpy ( Hrxn) for the unknown. Your calculated error and relative error for the Molar Enthalpy of Formation of Magnesium Oxide, Hf. Possible limitations or error in the measurement of temperature and mass , experimental procedure , and graph interpretation. Do not say ‘Human Error”, “Calculation Error”, or “Instrument/Machine Error” in your answer. These are too broad and nonspecific. Instead, be specific, for example: “It was difficult to choose which points to do the linear regression on since the graph for MgO was not very linear resulting is a possible error in the measurement of Tf”.
