Purpose: The purpose of this lab is to calculate the ∆H of the reaction of

ammonia (NH3) and hydrochloric acid (HCl) to form ammonium chloride (NH4CL) using the principals of Hess’s law applied to calorimetry.

Background: Thermochemistry is the study of the energy change associated with

chemical reactions. Every substance contains stored energy, or enthalpy. As a chemical reaction occurs, there is a quantitative relationship between the reaction and the heat that is either released or absorbed during the reaction. In an exothermic reaction where heat is released, the resulting products contain less heat than the starting reactants. Conversely, in an endothermic reaction where heat is absorbed, the change in enthalpy will be positive.

While there is no way to directly measure the gain or loss of heat during a reaction, the change in heat can be calculated based on mass, temperature change, and the specific heat of the substance that is either gaining or losing heat. The equation used to calculate the heat lost or gained is:

Q = mC∆T

Materials: 2 Molar HCL 2 Molar NaOH 2 Molar NH3

2 Molar NH4CL 3 Styrofoam cups CBL 2

Temperature probe (2) 50mL graduated

cylinders Distilled water bottle Magnetic stirrers Hot plate

Procedure:1) Measure 25mL of HCl and 25ml of NaOH, and record initial

temperatures of the solutions. 2) Set up Datamate application on graphing calculator so that a data

sample is taken every 10 seconds 18 times for a total time of 180 seconds or 3 min.

3) Make calorimeter by cutting off a little more than half of a Styrofoam cup and inserting it in another so that it fits snugly.

4) Calculate specific heat of calorimeter. 5) Mix HCL and NaOH in calorimeter and turn the stirrer on.6) Immediately press the “START” button on the calculator so that data

samples of the change in temperature of the solution are taken and graphed.

7) Sync CBL 2 to a computer, title the graph, and print the graph.8) Pour the solution down the sink and calculate ∆T of the reaction.9) Repeat Steps 1-7, excluding Step 3, using HCL and NH3.10) Repeat Steps 1-7, excluding Steps 3 and 4, using HCL and NH4CL.

Post Lab Questions:1. What is meant by calorimetry?

Calorimetry is the science of measuring the heat of chemical reactions or physical changes.

2. How does the graphical temperature analysis improve the accuracy of your data?

Graphical temperature analysis’ linear regression capabilities means that the data will be more accurate as well as precise because it makes collecting the numerous data samples needed for a good graph feasible and quick at the same time.

3. How could you modify the method to achieve greater accuracy?Greater accuracy in lab can be achieved by using a better

calorimeter rather than one constructed from styrofoam, as the homemade calorimeter was not a perfect closed system by any means.

4. The equation used to find the heat evolved in each reaction is Q = mC∆T. What does the negative sign mean?

The negative sign means that the reaction is exothermic, so water is the surrounding for the reaction and gains heat because heat lost by the system = heat gain by the surroundings.

Conclusion:The ∆Hrxn of ammonia (NH3) and hydrochloric acid (HCl) to form

ammonium chloride (NH4CL) is kJ/mol was -15.6 kJ/mol. The ∆H of the first and second reactions were -23.3kJ/mol and -7.7kJ/mol, respectively. Since the reaction of NH3 and HCl to form NH4Cl was carried out in a series of steps and no external work was done, the principals of Hess’s Law, which states that ∆H or the heat energy involved in a chemical reaction will be equal to the sum of the enthalpy changes for the individual steps and is the same whether the reaction occurs in one or several steps, can be employed to find the enthalpy change. In other words, regardless of how the reaction occurs, enthalpy change is only dependent on the start and end states of the reaction.