Lab 11

Thermochemistry: Enthalpy of Formation of Magnesium Hydroxide

Unit 2⏱ 90 minGuided Inquiry

Overview

How can the heat of a reaction be determined from Hess' Law and calorimetry?

Objectives

How can the heat of a reaction be determined from Hess' Law and calorimetry?
Connect observations to AP Chemistry models and calculations.

⚠️ Wear protective goggles and an apron. Follow your teacher’s safety directions for all chemicals and equipment.

Materials Needed

Balance a 10.0 cm strip of Mg ribbon Weigh boats 50 grams of Mg(OH)2 Graduated cylinder mL Digital Thermometer Styrofoam cups 2 samples of 50 mL of 1M HCl Observations - Write down all relevant observations Wear protective glasses at all times. WARNING: HCl(aq) can cause severe burns

Procedure

Prepare a data table like the one provided by your teacher.

Record all masses to the nearest 0.01 g and all temperatures to the nearest 0.1oC unless otherwise noted.

Design TWO experimental procedures that will determine the heat released or absorbed for Equation #1 and Equation #2 in the introduction.

You may use all the materials listed in the Materials section of his lab.

Using calorimetry, the facts listed in the introduction and the data you collect, determine the heat evolved from each reaction.

Then, use Hess' Law in coordination with and Equation #3 to determine the heat of formation of magnesium hydroxide.

Show your proposed procedure to your teacher before attempting the experiment yourself.

Pre-Lab Questions

1. Answer the objective clearly and completely. What was the ΔHf of Mg(OH)2 and the final experimental error for the lab? How do the 1st Law of Thermodynamics apply to this lab? How was Hess' Law used in this lab? Were the reactions that you did endothermic or exothermic, and how can you tell? Identify at least three sources of error in this lab that would affect your results. How could these errors be reduced? Would the errors you mentioned cause the heat of reaction to be higher or lower than the accepted value? Explain.

Post-Lab Questions

1. How many grams of magnesium hydroxide would be produced if the heat evolved from the combustion of magnesium was -108 kJ? How many grams of magnesium was burned?