CHE122 W5 Lab
CHE-122: Principles of Chemistry II
Laboratory 5 Procedure: Thermochemistry: Enthalpy and Entropy of
a Spontaneous Process
Background:
A spontaneous process can be defined as the time-evolution of a system in which it releases free energy, and it moves to a lower, more thermodynamically stable energy state
Chemistry
Share With
CHE122 W5 Lab
CHE-122: Principles of Chemistry II
Laboratory 5 Procedure: Thermochemistry: Enthalpy and Entropy of
a Spontaneous Process
Background:
A spontaneous process can be defined as the time-evolution of a system in which it releases freeenergy, and it moves to a lower, more thermodynamically stable energy state.
The sign convention for free energy is as follows: a release of free energy from the system corresponds to a negative change in the free energy of the system, and a positive change in the free energy of the surroundings.
Enthalpy is a total heat content in a thermodynamic system.
ΔH = positive value implies that the system is gaining heat. Endothermic process.
ΔH = negative value implies that the system is losing heat. Exothermic process.
Second Law of Thermodynamics states that “in an isolated system, natural processes are spontaneous when they lead to an increase in disorder, or entropy.”
We can apply the second law of thermodynamics to chemical reactions by noting that the entropy of a system is a state function that is directly proportional to the disorder of the system.
Ssys > 0 implies that the system becomes more disordered during the reaction.
Ssys < 0 implies that the system becomes less disordered during the reaction.
For an isolated system, any process that leads to an increase in the disorder of the system will be spontaneous. The following generalizations can help us decide when a chemical reaction leads to an increase in the disorder of the system.
Solids have a much more regular structure than liquids. Liquids are therefore more disordered than solids.
The particles in a gas are in a state of constant, random motion. Gases are therefore more disordered than the corresponding liquids.
Any process that increases the number of particles in the system increases the amount of disorder.
Figure 1: Enthalpy and Entropy vs Spontaneous Process
There are couple of objectives of this lab:
To enhance your understanding of spontaneity, entropy and enthalpy.
To gain deeper understanding of the second law of thermodynamics by exploring energy transfer between system and surroundings.
Before you start this laboratory assignment, you are encouraged to review Section 18.2 on page 609 in the textbook. Throughout this laboratory assignment, you will be required to analyze a chemical reaction in terms of energy changes. Be sure to record all observations and any relevant notes that you think you will need to include in your laboratory report.
Take a moment to formulate and write down a hypothesis describing the criteria of spontaneous reaction.
Pre-Lab questions:
In 3-5 sentences, using your own words, explain what does spontaneous process mean?
In 5-10 sentences explain why ice melts when dropped in a cup of warm water. Can you specify if it is a spontaneous process?
In 5-10 sentences explain if water in a glass container on your kitchen counter can decompose into hydrogen and oxygen? And if the process occurs is it a spontaneous process?
Procedure:
Preparing the Lab 5
From the course home page, click on the Virtual Lab Tutorial link to watch the overview of using the virtual lab.
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 ‘Camping Part 1’ assignment.
At this point, you have prepared the laboratory for the first experiment with the require supplies to complete your experiments.
If you haven’t already done so, formulate a hypothesis describing the criteria of spontaneous reaction as you will need to include this in your final report.
Performing the Experiment
Select the ‘Solutions’ tab in the stockroom if it is not already selected. Then, select the Erlenmeyer flask containing the ‘0.1 Reagent X’ to move it to the workbench.
Select the ‘Solutions’ tab in the stockroom. Then, select the Erlenmeyer flask containing the ‘0.1 Reagent Y’ to move it to the workbench.
From ‘Glassware’ select foam cup 0.2 L, and 10 mL pipette.
Transfer 10 mL of ‘Reagent X’ from the Erlenmeyer’s flask to the foam cup. Record its volume and the temperature.
Transfer 10 mL of ‘Reagent Y’ from the Erlenmeyer's flask to the foam cup.
Record the volume and the temperature of the reaction mixture.
After you record your data, clear the workbench but do not close the virtual lab!
Data Analysis
Defining Entropy
Based on our reaction where:
Reagent X + Reagent Y = ProductXY
does the entropy of the system increased or decreased as a result of synthesis of Reagent X with the Reagent Y?
Would ΔS for this change be positive or negative?
Did the synthesis reaction between Reagent X and Reagent Y happen spontaneously?
Defining Enthalpy
What was the final temperature of the reaction mixture?
Was the system losing or gaining thermal energy?
Is the ΔH of the system positive or negative?
Is the process endothermic or exothermic?
What happens in the surroundings during the process of thermal energy exchange?
Use 5-10 sentences to describe what happens to the molecules in the reaction mixture when thermal energy is transferred in exothermic process.
Explain using 5-10 sentences explain how exothermic process affects the entropy of the system?
Based on your findings is ΔS of the system positive or negative?
Complete the following:
In an exothermic reaction ΔS is.
In an endothermic reaction ΔS is.
Based on the data collected during the experiment calculate ΔH(q-heat) of this reaction. Assumethatspecificheatcapacityofthereactionmixtureequals4.18J/g°C,anddensity of the solution is 1.00 g/mL. (q=mCsΔT)
Notes
This section should include notes about any observations or data collected during the lab.
Report Requirements
This section contains key information that must be included in your typed report.
Define the problem in a manner that is clear and insightful.
Identify the strategies and procedures used during the lab.
Clear hypothesis statement and other potential solutions that identify any relevant contextual factors (i.e. real-world costs).
Clear presentation of data including any tables or other figures that are relevant to understanding your stated conclusions at the end of the report. Include any relevant calculations performed during the lab.
Clearly stated results and discussion of 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?
