2- Heats of Reaction

Background:

This week, you will be tasked with investigating the solubility of several chemical compounds

Chemists routinely measure the heat involved in a chemical change either experimentally or from enthalpy changes observed in other related reactions. In order to do the latter, the application of Hess’s law is required:

If a process can be written as the sum of several stepwise processes, the enthalpy change of the total process equals the sum of the enthalpy changes of the various steps.

In this experiment, you will determine and compare the quantity of heat energy released in three exothermic chemical reactions through application of Hess’s law.

Reaction 1: NaOH(s) → Na+(aq) + OH(aq) + x1 kJ
Reaction 2: NaOH(s) + HCl(aq) → H2O(l) + Na+(aq) + CL(aq) + x2 kJ
Reaction 3: NaOH(aq) + HCl(aq) → H2O(l) + Na+(aq) + Cl(aq) + x3 kJ

In order to accurately measure the heat released in each reaction, we will be using a calorimeter. As discussed in the textbook, a calorimeter is a device used to measure the amount of heat involved in a chemical or physical process; the calorimeter used in this experiment will be a Styrofoam cup.

In the reactions above, the variables x1, x2, and x3 are the heats that are evolved during the reactions. The change in temperature that occurs for each reaction will be used to calculate the energy released in kJ/mol of NaOH used. We will assume for our calculations that any heat transferred to the Styrofoam cup and surrounding air will be negligible; we will also assume that the specific heat of water is 4.184 J/g∙°C. Thus, the applicable equation is simply

q_reaction=-m∙C_water∙∆T

There are several objectives of this lab:

Explore the technique of calorimetry, and
Calculate and interpret heat and related properties using typical calorimetry data.

Before you start this laboratory assignment, you are encouraged to review Section 5.2 (p. 238) and Section 5.3 (p. 251) in the textbook. Throughout this laboratory assignment, you will be required to perform and thoroughly document your data and calculations. Be sure to record all observations and any relevant notes that you think you will need to include in your laboratory report; refer to the end of this document for information that must be included in your final report.

 
Procedure:

Preparing the Lab I

From the course home page, access the lab environment by clicking on the Virtual Lab link.
After the lab environment loads, click ‘File’ then ‘Load an Assignment.’
Select the ‘Thermochemistry’ category.
Select the ‘Heats of Reaction’ assignment.
Review the data table for this lab (see the end of this document).
Evaluate the reactions shown in the ‘Background’ section above and draft a hypothesis based on this information; write your hypothesis in your notes for reference later.
At this point, you have prepared the laboratory for the experiment with the required supplies to complete your experiments.

Performing the Reactions I

In the ‘Glassware’ menu, place a 50-mL graduated cylinder and a foam cup onto the workbench. From the ‘Tools’ menu, add a balance (scale) to the workbench. Then, move the distilled water and solid NaOH to the workbench as well.

Transfer 50.0 mL of distilled water to the foam cup. To do this, you must first transfer the water to the graduated cylinder then to the foam cup. After dragging the water to the graduated cylinder, enter ‘50.0’ and click ‘Pour.’ Record the initial temperature of the water in your data table.

Tare the balance after adding the foam cup, then weigh 1.00 g of solid NaOH pellets directly into the foam cup. Remove the scale from the workbench at this point.

Pour the entire contents of the graduated cylinder into the foam cup and immediately record the highest temperature observed and the mass of the contents of the cup. In a real laboratory, you have a temperature probe (like a thermometer) inserted into the foam cup to record the maximum temperature.

Performing the Reactions II

Take the 0.50 M HCl solution, a foam cup, the solid NaOH, a 50-mL graduated cylinder, and a balance from the from the Stockroom and place it on the workbench.

Repeat Steps 7-10 but replace the 50 mL of water with 50 mL of the 0.50 M HCl solution. After mixing the HCl solution with the solid NaOH, immediately record the highest observed temperature.

Clear the workbench.

Performing the Reactions III

Take out another graduated cylinder, a fresh foam cup, the 1.0 M HCl and 1.0 M NaOH solutions and add them to the workbench.

Measure 25.0 mL of the 1.0 M HCl solution into the foam using the graduated cylinder. Using a clean graduated cylinder (to avoid cross-contamination), measure out 25.0 mL of the 1.0 M NaOH solution.

Pour the NaOH solution into the foam cup and immediately record the highest observed temperature.

Clear the workbench.

Data Analysis

Complete the table shown below in the ‘Data’ section by filling in the values recorded during the experiments or those produced from the required calculations.

Write out the net ionic equations for all three reactions referring to the initial reactions shown in the ‘Background’ section of this template. Be sure to use proper notation when writing your equations as you will include these in the final draft of your report.

The energy, x1, in Reaction 1 represents the energy of solution for one mole of solid NaOH. Look at the net ionic equations for Reactions 2 and 3, and make a similar statement concerning the significance of x2 and x3 and their connection to Hess’s law. Record your statements in the ‘Notes’ section below for review later.

Find the difference between the value of x2 and the sum of x1 plus x3.; let x4 be equivalent to the sum of x1 and x3 and let x5 be the difference between x2 and x5. This calculation scheme is shown below. Make notes about any similarities or differences between the values in your notes.

x4 = x1 + x3
x5 = x2 – x4

Calculate the percent difference between x2 and x4 according to the equation below. (Assume x2 to be the accepted value.) Record the calculation in your notes.

% difference=|(experimental value-accepted value)/(accepted value)|×100%

% difference=|(x_4-x_2)/x_2 |×100%
 

Notes
This section should include notes about any observations or data collected during the lab.
Begin typing here.
 
Data
This section will include all data collected during the lab.

Thermochemical Data
Tinitial (°C) Tfinal (°C) ΔT (°C) moles NaOH qreaction (kJ) ΔHrxn
Reaction 1
Reaction 2
Reaction 3

*Note: You can copy and paste your data table into your report later.

Insert the net ionic equations here:

Reaction 1:

Reaction 2:

Reaction 3: 

Report Requirements
This section contains key information that must be included in your typed report.

Define the problem/goals in a manner that is clear and insightful.
Identification of the strategies/procedures used during the lab.
Clear hypothesis statement and other potential solutions that identify any relevant contextual factors (i.e. real-world costs). Consider alternative ways to confirm Hess’s law experimentally.
Clear presentation of data including any tables or other figures that are relevant to understanding your stated conclusions at the end of the report. The following items are required in your report:
Data table
Net ionic equations
Percent difference calculation.
Clearly stated results (evidence confirming Hess’s law) and discussion possible improvements to the procedure.
Conclusive statements arguing in favor of your findings.

Note: All reports will be graded using the rubric embedded within the course.

Here are some questions to consider as you write your report:

Does my problem statement make sense?
Have I summarized my strategies/procedures well enough to be replicated by an outsider?
Did I have a valid hypothesis at the start of the lab? Have I expressed this in my report?
Do my tables and/or graphs make sense?
Are my conclusions valid based on my supplied data?
Did I thoroughly summarize my laboratory experience in a concise, factual way such that the reader can understand my processes and findings in the conclusion section alone?