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

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$MATH 35 FINAL EXAM REVIEW 1. Simplify. 1(3 7)2 + 8Ã· 4 2. Solve ...$

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

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