# The Application of Hess's Law in Coffee-Cup Calorimetry

Topics: Enthalpy, Thermodynamics, Heat Pages: 9 (3140 words) Published: April 27, 2008
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 – 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 – or the inaccuracy of the experiment – 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)....