Experiment 9: Calorimetry, Heats of Reaction, and Hess's Law PDF

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CHEM 1310 Freshman Chemistry Program Student Notes, Experiment 9 Fall 2016 Experiment 9: Calorimetry, Heats of Reaction, and Hess’s Law Post-lab Assignment or Report The lab report for this experiment is due at the beginning of the following lab period. Learning Objectives Students will be able to… ...

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CHEM 1310 Student Notes, Experiment 9

Freshman Chemistry Program Fall 2016

Experiment 9: Calorimetry, Heats of Reaction, and Hess’s Law Post-lab Assignment or Report The lab report for this experiment is due at the beginning of the following lab period. Learning Objectives Students will be able to…  Apply basic principles of thermodynamics to think about heat and energy transfer inside a calorimeter.  Calibrate a homemade calorimeter by determining heat losses to the walls of the calorimeter in a controlled manner.  Run a reaction in a calorimeter using precise amounts of reagents and good temperature control.  Use temperature changes due to a reaction in a calorimeter to calculate the standard enthalpy change associated with the reaction (ΔHºrxn).  Apply Hess’s law to determine the ΔHºrxn of a reaction that cannot be studied experimentally. Talking Points for Pre-lab Discussion  Discuss the fundamentals of chemical thermodynamics.  How do the terms system and surroundings apply to chemical reactions?  Why are most chemical reactions thought of as constant-pressure situations?  Discuss the first law of thermodynamics and its relation to chemical reactions at constant pressure. In a constant-pressure situation, a reaction may do pressure-volume work!

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Introduce the calorimeter and discuss how it is used to measure heat transfer during chemical reactions. Why is it necessary to calibrate our calorimeters? What procedure will we use to calibrate them? Diagram the calorimeter and ask students to think about places where heat transfer occurs during the calibration procedure. Briefly mention Hess’s law, which will be applied in post-lab data workup to calculate the ΔHºrxn of the reaction of magnesium metal with elemental oxygen to form MgO.

This equation refers to enthalpy changes of formation, but the general idea applies to any sum of reactions! Lab Logistics  Students should work in pairs.  Pause after completing part A and determine Ccal for your calorimeter. This will make later calculations easier.  Weigh down magnesium ribbon with a “basket” of copper wire to ensure that it reacts rapidly and completely.

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CHEM 1310 Student Notes, Experiment 9

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Freshman Chemistry Program Fall 2016

Ccal can vary considerably (between 10-200 J/K). Scratching, etc. can affect Ccal profoundly!

Lab Report Rubric I. Cover Sheet (5 pts) a. Title b. Name c. Lab partner’s name d. Date e. Honor pledge II.

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Data and Results(16 pts) a. Data tables and plots (see Data & Results on T-Square)(16 pts) Discussion (64 pts) a. Discussion of Part A i. State the technical objective(s) of part A of the experiment.(4 pts) ii. Identify your mean Ccal value and its standard deviation.(2 pts) iii. For one trial, show how Ccal was calculated from measured temperatures. Explainconceptually the setup of the equation used to calculate Ccal.(6 pts) b. Discussion of Part B i. State the technical objective(s) of part B of the experiment.(4 pts) ii. Identify the mean and standard deviation of Hºrxn for the reaction of Mg with aqueous HCl.(2 pts) iii. For one trial, show how Hºrxnfor the reaction of Mg with aqueous HCl was calculated.(4 pts) iv. Calculate a theoretical value for Hºrxn by applying Hess’s law to the reaction Mg(s) + 2 HCl(aq) → Mg2+(aq) + 2 Cl–(aq) + H2(g).(4 pts) v. Comment on the accuracy and precision of your H measurements for part B.1.(6 pts) vi. Identify the mean and standard deviation of Hºrxn for the reaction of MgO with aqueous HCl.(2 pts) vii. For one trial, show how Hºrxn for the reaction of MgO with aqueous HCl was calculated.(4 pts) viii. Calculate a theoretical value for Hºrxn by applying Hess’s law to the reaction MgO(s) + 2 HCl(aq) → Mg2+(aq) + 2 Cl–(aq) + H2O(l).(4 pts) ix. Comment on the accuracy and precision of your H measurements for part B.2.(6 pts) c. General Discussion i. Identify Hºf for solid magnesium oxide based on your data and Hºf for liquid water.(2 pts) ii. Show how the enthalpy of formation of solid magnesium oxide was calculated from your values in items b.ii and b.v along with Hºf for liquid water.(6 pts) iii. Identify a literature value for the enthalpy of formation of solid magnesium oxide.(4 pts)

CHEM 1310 Student Notes, Experiment 9

Freshman Chemistry Program Fall 2016

iv. Comment on the accuracy of the value in item i.(4 pts) IV.

References (5 pts) a. Reference to the lab manual in ACS format b. Any other references used, with in-text citations(bit.ly/1pHbUry) where they are relevant

Frequently Asked Questions (FAQs) I can’t find theoretical Hºrxn values for the reactions in parts B.1 and B.2. Can I calculate these? Yes! These enthalpy changes can be calculated from standard enthalpies of formation, which are tabulated in a number of places (including a Wikipedia data page and the NIST Chemistry WebBook). Keep in mind that Hºf = 0 for an element in its standard state, and Hºf for an ionic compound in aqueous form is equal to the sum of Hºf’s for all of its aqueous ions. In general, we can find Hºrxn by subtracting the enthalpies of formation of the reactants from those of the products. Mathematically,

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