The Application Of Hess's Law In Coffee-Cup Calorimetry

1. Introduction

In the study of Thermochemistry, reactions are quantitatively analyzed to determine the amount of heat that has been transferred, whether released or absorbed, between the system and its surroundings. Such data is important in realizing the properties of different types of reactions along with the elements and compounds of which they are comprised. However, it can be difficult to derive the exact enthalpy in a reaction when multiple processes occur simultaneously. A method to circumvent this problem is outlined in Hess’s Law which was established in 1840. Hess’s Law states that the steps taken to determine the enthalpy of a reaction do not matter because the end results will be the same. This is the principle used for both parts of this experiment.

In Part I of the experiment, two different reactions are performed to determine the enthalpy of formation for magnesium oxide. One reaction will involve only Mg, and the other will involve MgO вЂ" both will react with 1M HCl. Using the change in heat for both of these reactions, the enthalpies can be determined. With the use of Hess’s Law, the enthalpies of both reactions along with the enthalpy of water can be used to determine the enthalpy of formation for magnesium oxide. Such reactions will take place in a coffee-cup calorimeter which is made up of two Styrofoam cups and a top. It is assumed that the insulation provided by the cups will suffice in retaining the heat within the reaction so that an accurate reading can be derived. This assumption is essential to the accuracy of the results in both Part I of the experiment as well as Part II.

Part II of the experiment will be used to test the accuracy of this experiment against Hess’s Law. Two separate reactions will be induced: the first will be the reaction between 2M NaOH and 2M HCl; the second will be between 2M NaOH and 2M NH4Cl. Once the data is collected from these reactions, the enthalpies can be calculated and Hess’s Law can be used to determine the enthalpy of a third reaction: the reaction between 2M HCl and 2M NH3. In order to test the accuracy of Hess’s Law вЂ" or the inaccuracy of the experiment вЂ" the third reaction will be performed, the data will be collected, and the enthalpy will be determined. This experiment will yield a greater understanding of the laws of thermodynamics and the challenge of testing them.

2. Experimental

In starting Part I of the experiment all materials were obtained, which included a 250mL and 600mL beaker, a 50mL graduated cylinder, two Styrofoam cups, a plastic cover for the cups, a sample of Mg and MgO, and 1M HCl. Other required instrumentation necessary in performing this experiment was a computer and a Vernier LabPro interface along with a temperature probe which measured the temperature changes in the solution over the course of the reactions. Before beginning the experiment, batteries were put into the LabPro interface and the LabPro was set-up on the computer using the Logger Pro Program. The interface system was set-up to collect one sample of data every second for 570 seconds. Once the LabPro was on, which is confirmed by three blinking LED lights, and ready, the procedure could then proceed.

The procedure began with the taring of two weighing dishes followed by the weighing of both the Mg and MgO samples on the weighing dishes in order to calculate the weight of the samples alone. Once the weights of the samples were know, the coffee cup calorimeters were prepared. In order to reduce contamination and error, the Styrofoam cups were washed, weighed, and placed into the 250mL beaker. Being that this experiment requires a closed system, a top for the Styrofoam cups was needed with a hole in the center to allow the temperature probe to pass through. Using the 50mL graduated cylinder, exactly 25.00mL of the 1M HCl was measured and poured into the calorimeter (HCl must be handled carefully because it is corrosive and toxic if inhaled).

Once the HCl is poured, the temperature probe was placed into the acid through the hole allowing the LabPro to measure the temperature for 4.5 minutes. At 5.0 minutes the magnesium metal was placed into the calorimeter with the HCl aqueous solution where the LabPro continued recording the temperature until the yellow LED light ceased blinking, which was roughly 4 minutes (the reaction between HCl and Mg metal produces H2 gas which is flammable, it is best to combine the compounds under a hood and to avoid sparks). The LabPro was then taken to the computer to present the data in a graph in the form of Temperature (ЛљC) versus Time (s). In the meantime, the temperature probe was placed in the 600mL beaker that was filled with 400mL of deionized water for 2 minutes for cleansing purposes. It is important to watch the time carefully during the experiment to preserve the condition of the probe; if the temperature probe is submerged in the HCl for more than 10.0 minutes, it can be damaged.

Once the data was collected, the final solution along with the calorimeter was weighed. The solution was then discarded into the appropriate labeled receptacle and preparations began for the second task of Part I. Such preparations included washing the Styrofoam cups for the calorimeter, making sure that the probe was clean, and restarting the LabPro interface. Once completed, the preceding procedures were followed for the reaction involving the magnesium oxide sample and the HCl aqueous solution. The procedure for only Part I will be revealed in this section, however, the results of Part II will be analyzed in the Results and Discussion portion of this report.

3. Results and Discussion

In order to calculate the enthalpy of formation for magnesium oxide вЂ" which is the purpose of Part I вЂ" a few other calculations must precede. First off, the mass of the samples must be measured in order to calculate the number of moles. The method we used to determine the masses of the Mg and MgO samples is shown in Table 1:

Table 1: Masses of Mg and MgO samples

mass of mass of sample + (mass sample + dish) -

Sample weighing dish weighing dish (mass weighing dish) Mass of Sample

Mg 5.122g 5.371g 5.371g вЂ" 5.122g