1 Hess's Law Lab: Determine the Heat of Formation of Magnesium ...

Hess’s Law Lab: Determine the Heat of Formation of Magnesium OxideLab ReportObjective: In this calorimetry experiment we are going to do two separate reactions,determine the H rxn experimentally for each reaction and use Hess’s Law to add thesereactions together to find a target reaction, which corresponds to the H f°of MgO. Theexperiment will take two lab periods to complete.Procedure: Part 1The first reaction you will perform is: Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g)Accurately weigh 0.10-0.12 g of magnesium metal. Weigh your calorimeter andthen add 40-50 mL of 1 M HCl and reweigh to determine the mass of the acid. Note: wewill be using 1 M HCl in place of the distilled water in the calorimeter for thisexperiment. Let the hydrochloric acid equilibrate to room temperature in the calorimeter.Just before you add the metal, record the temperature of the HCl solution.This is a chemical reaction, in addition to a heat transfer process. Look at the reactionand the states of the products: what will happen to the metal in this experiment? Whatthis means experimentally, is that you can consider magnesium as the limiting reactant,and that it will have to completely dissolve before equilibration will occur, so thisexperiment will take a little more time than the last experiments, which was simply a heattransfer between the metal and the water.When you add the metal to the acid, continuously stir the heterogeneous solution toincrease the rate (speed) of the reaction. Record the temperature at 30 second intervalsfor five minutes, and, if the temperature has not yet stabilized, continue recording thetemperature for another 5 minutes at 1 minute intervals.Note: The directions state that you should record your temperature readings for 5minutes. Do so. Do not stop when the temperature seems to stabilize...record thetemperature for 5 minutes for each trial. If the temperature has not yet stabilized, thenrecord the temperature for additional, 1 minute intervals until the temperature doesstabilize. We will consider “stable” when the temperature reading is the same (to thetenths place) for 3 or more consecutive readings for 30 second interval, or 2 or moreconsecutive readings at the 1 minute interval.Calculations:The specific heat of 1M aqueous HCl is 4.19 Joules / g °C. The heat that theexcess acidic solution absorbed (this is the surroundings) is equal in magnitude andopposite in sign to the heat that the products lost while being formed.1

Q excess HCl aqueous soln (surroundings) = -Q Mg+2HCl MgCl2 + H2 (system)Since this is true, the next equation is also true:Q soln = m soln c soln T soln = -Q rxn = - (m rxnc rxnT rxn)and som soln c soln T soln = - (m rxnc rxnT rxn)• Calculate the heat absorbed by the surrounding solution in each trial of yourexperiments, and this will also determine the change in enthalpy of the reaction that youjust completed (see the equations above).• Then determine the molar heat of reaction: how much heat would be lost if your startinglimiting reactant were one mole?• Check to see if your two answers are within five percent of each other. Ask theinstructor if you do not know how to do this.• Check your answers with the instructor before doing a third trial to see if you are on theright track.Procedure: Part 2The reaction for this part isMgO(s) + 2HCl(aq) MgCl 2 (aq) + H 2 O(l)The procedure is the same, but with different quantities of materials.Accurately weigh 0.3-0.4 g of magnesium oxide metal. Weigh your calorimeter and thenadd 30 mL of 1 M HCl and reweigh to determine the mass of the acid. While allowingThought Question: The experiment is the same, but the quantities of materials are different.What does this tell you about the magnitude (how large the number is) of the heat lost inthe first reaction compared to the heat lost in the second reaction?the solution to equilibrate, use your spatula to pulverize any lumps in the magnesiumoxide powder. Lumps are tightly compacted powder, and this decreases the surface areaof the substance available for reaction.Aside: [This is also true in baking. Sometimes corn starch or baking powder is socompacted that most of the substance doesn’t even react, and so you get tiny white(uncooked) lumps in your cake or pancake or muffins. This is why chefs sift the dryingredients, to make sure that all of the ingredients are fine powders, ensuring that theywill react completely.]2

When more surface area is available, the reaction will occur faster. The faster thereaction occurs, then there is less time for heat to be lost to the environment; thus, yourexperiment will be more accurate.When you add the metal oxide to the acid, continuously stir the heterogeneous solution toincrease the rate (speed) of the reaction. Record the temperature at 30 second intervalsfor 10 minutes. Check to see if all of the powder has reacted. If the temperature has notyet stabilized, or if the powder has not completely reacted, continue recording thetemperature for another 5 minutes at 1 minute intervals until the powder has completelyreacted (limiting reactant, again) and until the temperature has stabilized. Again, all ofthis data should be recorded! This will be part of your observations grade.Repeat this reaction so that you have 3 good trials, or 3 trials that yield a H rxn that arewithin 5% of each other.CalculationsQ excess HCl aqueous soln (surroundings) = -Q MgO+2HCl MgCl2 + H2O (system)• Calculate the heat absorbed by the surrounding solution in each trial of yourexperiments, and this will also determine the change in enthalpy of the reaction that youjust completed (see the equations above).• Then determine the molar heat of reaction: how much heat would be lost if your startinglimiting reactant were one mole?Lab ReportObjective: Write an appropriate objective that mirrors the main conclusion.Reactions (3)Data Table: - all data taken, but you do not have to include all of the times-temperaturevalues. You DO need to show the initial and equilibrium temperatures. You also need toinclude the reference “correct” values given in class, as part of your data. Both sets ofdata (data from both reactions) should be in this section; DO NOT set up the data as ifthis were two separate lab reports. If you include results in your data table, points will bededucted.Calculations: One sample for each type of calculation for EACH reaction.There should be a header for each type of calculation, and then a verbal for each type ofcalculation. IF the verbal is the same for BOTH reactions, then you only need to write itonce. We are half way through the semester. You need to follow these instructionsexplicitly. If you want to discuss the format with me BEFORE the due date of the report,you can do that through email or office hours.DO NOT include more than 2 trials per calculation or points will be deducted. Show theanswers to all 6 trials in the results table which is separate from the data table, and comes3

after the calculations section. Points will be deducted if you format your reportdifferently.Calculations include – again one calculation using Trial 1 data in part 1 and one usingTrial 1 data from part 2:• change in temperatures (T)• heat of reaction• molar heat of reaction• average molar heat of reaction• average deviation• H f°of MgCl 2 , kJ/mole (experimental)• % error of H f°of MgCl 2 , kJ/mole from literature value given in class.• To find the H f°of MgO, kJ/mole (experimental), use Hess’s Law. You willhave gotten your target reaction (H f ° reaction of MgO) by the end of the lab.You will need ONE more reaction, which you will be able to figure out, giventhe other two reactions plus the target reaction. You will use the text book value(from the appendix in the text) for this reaction. Using Hess’s Law and yourAVERAGE numbers from the experimental data for reactions 1 and 2,determine the heat of reaction of the target reaction. No credit will be given for“products – reactants” method.• Show the complete solution for the Hess’s Law calculation.• % error of H f°of MgO, kJ/mole from literature value given in class.Results Table: Put all of the results (answers to all calculations above) for all 6 trials inthis table. For the Hess’s Law calculation, just enter the one, final H rxn for the resultstable, and then the % error from the literature value.No Graphs.Discussion: Discuss any errors or difficulties from this experiment.Conclusion: Re-state in sentence form, the most important results listed in the resultstable. The numbers should be stated as “17 “ not “seventeen”. Also, percent errorsassociated with the main results should be incorporated in context. (Do not list % errorsseparately, but next to the result that each describes).4