C105 Enthalpy of Formation of Ammonium Chloride Post-Lab
Write your answers to these in a new Word document.
Include a heading with your name, date, and section number at the top of the page.
Include section headings for each section of your post-lab report. (Data, Calculations, etc.).
Number your answers with the corresponding question number.
Save your document as a PDF and upload to Blackboard in the Pre-lab assignment area for this
Don’t re-write the questions when you go to answer these. Just make sure your answers have numbers that match up to the original questions’ numbers.
For your data, you will follow the procedure given in the experiment’s background and procedure document. Record—either by typing or writing—any parts of your data and/or procedure you are required to. Your sheet will specify some things you must include as well as those things you do not have to include. Paste those things you recorded into the post-lab you submit. If you elected to hand- write your data, attach clear (key word clear) images of them to the first pages of your post-lab assignment. Keep in mind that if your TA cannot read your data, they can’t award you any credit for them.
You will have received a separate document containing some of the data you need. This document contained a number in its file name that indicated which data set it was, e.g., “Lab Measurements Data Set 1.” Indicate on your data which set you received.
Plot Creation & Calculations
Follow the guidelines in the background to create the data plots you’ll need to determine the temperature changes your reactions’ surroundings underwent. Naturally you’ll need to construct two plots for this: one for the NH3 + HCl reaction and one for the dissolving of ammonium chloride into water. Paste the plots you make into the document you submit.
Use your data and the plots you made to calculate ΔH for the two reactions you studied in this experiment.
Use the two ΔHs you just calculated and the information in the background to calculate the enthalpy of formation of solid ammonium chloride.
1. Write a brief summary of your experiment. As part of this, you have to describe:
What you were trying to accomplish
What your experimental approach was (or how you did it).
c. What your results were (include the enthalpy of formation you calculated)
Look up the enthalpy for the formation of solid NH4Cl and cite your source.
How close is your calculated enthalpy to the value you looked up? What might have happened to cause them to be different? Think about the different parts of your procedure and describe two specific things
The procedure you used in this experiment was not ideal because you did not do anything to check its accuracy or precision. Explain what you would have need to do to check your technique’s accuracy.
What would you have needed to do to check your technique’s precision?Enthalpy of Formation of Ammonium Chloride
Student Learning Objectives (SLOs)
Your lab report score will be a reflection of how well you meet the following SLOs:
Make, use, and interpret graphs.
Make observations, and use laboratory instrumentation to make measurements and obtain information about chemical systems.
Apply Hess’s Law to determine unknown ΔH values for a chemical reaction.
In this experiment, you will measure the temperature change that results from mixing aqueous ammonium and aqueous hydrochloric acid solutions, and from dissolving solid ammonium chloride in water. This information will ultimately enable you to calculate the enthalpy of formation of ammonium chloride from the elements nitrogen, hydrogen, and chlorine.
What are “energy”, “heat”, and “enthalpy”?
Since this lab is all about energy, heat, and something about the enthalpy involved when ammonium chloride forms, it might be a good idea to explain what these things are. Energy is commonly defined as the capacity to do work. However, if you’re not satisfied with this definition or find it ambiguous, you can think of it this way instead: energy is what is needed to make things move. A ball moving across a table, for instance, has a certain amount of energy associated with it, and it can transfer that energy to another ball by striking it. As soon as it does, the second ball will start moving.
There are a couple of different things that makeup the total “energy content” of a substance. One of these is related to that substance’s temperature. If the molecules that make up an object are at a higher temperature than those of an otherwise identical object, the higher-temperature one will have more energy than the lower. And just as a moving ball can transfer some of its energy to another ball by striking it, so can these different- temperature objects transfer energy to each other. This isn’t done by colliding, though, but by the next concept in our list.
Energy transfers between objects at two different temperatures are done by heat. This transfer will cause the temperatures of those things to change. That of the higher-temperature thing will become lower, and that of the lower-temperature one will become higher. Or if you’d rather think of it this way, the warmer object will get colder and the colder one will get warmer.
Whenever we measure heat transfers in lab, we usually identify a specific thing that will either gain or lose heat and call that thing the system. Everything around the system that it exchanges heat with is its surroundings. If the system gains energy as heat from its surroundings, it is said to have undergone an endothermic process. If it loses energy as heat to the surroundings, it has undergone an exothermic process.
Chemical reactions can also act as systems that exchange energy as heat with their surroundings. Reactions that “absorb” energy as heat are endothermic reactions, and those that release energy are exothermic reactions.
The quantity of energy the reactions absorb or evolve, symbolized q, is a positive number for endothermic reactions and a negative number for exothermic ones.
Whenever reactions absorb energy from their surroundings, the temperature of those surroundings fall. When they release energy to the surroundings, the surrounding’s temperature rises. Your hand gets cold when you hold an ice cube because the ice (the system) “takes” the energy as heat from your hand (surroundings) as it melts. This also explains why the temperature changes we measure/perceive are what they are. Since we can only measure the temperature of the surroundings (the thermometer or probe we measure with is part of those surroundings, after all), when an exothermic reaction occurs, we observe an increase in temperature (of the surroundings); when an endothermic reaction occurs, we observe a decrease in temperature.
Heat and Enthalpy
Putting together the concept of heat with that of energy, we can define a new term you can roughly think of as the “heat content” or “thermal energy” of a system: the system’s enthalpy. The enthalpy of a system can fall and rise as that system undergoes exothermic and endothermic processes respectively:
Although enthalpy and heat are related, though they are not synonymous. Heat is a transfer of thermal energy from one system or object to another. Enthalpy, a state function, is one component of a system’s total internal energy. The change in a system’s enthalpy will be equal to its heat flux, but only under conditions of constant pressure.
ΔH = qrxn (at constant pressure)
“Constant pressure” basically means the reaction is not constricted by a rigid, closed container; in other words, it is somewhere open and can freely expand.
In this experiment, you’re going to determine the amount of heat released or absorbed by a couple of chemical reactions. Both of these reactions will take place in water, which act as the reaction’s surroundings. Since we can measure the temperature change this water undergoes, we can calculate how much energy as heat it gains
or loses, and therefore how much energy as heat the reaction loses or gains. These reactions and their surroundings will take place in an insulated vessel, a calorimeter, to isolate them from the air, benchtop, etc. of
the lab room.
Your Procedure for Collecting Temperature Data
Getting the before-reaction and after-reaction temperatures will be little more involved than just measuring the water’s temperature before and after the reaction takes place. This is because your reaction will take a few moments to finish, and during this time the solution will already have started cooling off or warming back up to room temperature again. So, the final temperature you measure will either be a little colder or warmer than it would have been if the reaction happened instantly. To overcome this problem, you will take temperature and time data, plot these data, and use best-fit lines to determine the temperature change at the exact time the reagents are mixed. You will use one line to determine the initial temperature, and another to determine the final temperature. The difference between these will be your reaction’s real temperature change.
The two reactions you’re going to get ΔH values for are the neutralization of HCl with NH3, and the dissolving of ammonium chloride salt into water:
NH3 (aq) + HCl (aq) → NH4Cl (aq) and
NH4Cl (s) → NH4Cl (aq)
Getting Temperature Data for the NH3 + HCl Reaction
Note: be sure to make a record of what you did (or would have done) in your lab notebook. This includes things like the solutions you used (and their concentrations), how you measured them out, descriptions of equipment used, and how you took your data measurements. It also includes descriptions of the reagents, and any numerical data you would have recorded.
Here’s how you will get the data you will use to calculate ΔH for the first reaction. First, you’ll set up your calorimeter—two coffee cups nestled together—like the one in this picture and put 45-50 mL of ammonia solution into it. Besides the ammonia in the calorimeter, you’ll also need 45-50 mL of HCl solution hanging out somewhere close to hand.
The HCl and NH3 solutions have concentrations, which you’ll need when you do your calculations.
You will use a temperature probe to collect your
temperature and time data. Your TA (actually your provided data sheet) will tell you for how long to collect your data and how many samples/minute to collect.
Once your ammonia and HCl solutions are ready, make sure the end of the probe is immersed in the ammonia in the calorimeter and start collecting data. Your data will all have to be recorded in your notebook, preferably in a table with time and temperature readings in separate columns. If you record these data as the computer gives them to you, you won’t have to spend your time copying them when the program’s done.
You’ll want a table of temp and time in your notebook:
After 3 minutes, pour your HCl solution into the calorimeter and stir for a minute or so. You can use the temperature probe to stir provided you don’t drag it on the bottom of the cup.
Getting Temperature Data for the NH4Cl(s) to NH4Cl(aq) Reaction
Getting the data you need to determine ΔH for the NH4Cl (s) → NH4Cl (aq) thing is very similar to what you already did, with a few modifications.
First, put 100 mL of DI water into your calorimeter. Later you’re going to dissolve solid NH4Cl into this water.
Next, measure out some solid NH4Cl. A diagram will be posted somewhere in your lab room to help you figure out how much you’re going to need. At some point you’re going to need to know the exact mass of the NH4Cl you used, so weigh the bottle now, and again later after you’ve dispensed the NH4Cl from it.
Start Logger Pro collecting data just like you did before. At 3 minutes, dump the NH4Cl into the DI water and stir until it dissolves (it might take a good solid minute or more to dissolve).
When you’re done collecting data, rinse your cups out with DI water and leave them at your station (don’t throw them away).
A Few More Things
Before you leave, every piece of equipment in your drawer must be clean and organized like the picture. Especially check your weighing bottle for salt residue. Your TA may tell you to leave some things out to dry, but everything else has to be lined up where it belongs. If you’re missing something, get a replacement piece from the stockroom. Part of your grade will depend on how well you do this
Your Data Pages
Make sure you turn in your signed data page to your TA before you leave the lab. No credit will be given for data pages that have been either unsigned, or have been signed and left the lab before being turned in to your TAChemistry 105 Enthalpy of Formation of Ammonium Chloride
NOTE: As part of the data you record, provide basic, brief descriptions of how you performed your procedure. For this experiment, include what reagents you used, how you measured them out, how you collected your temperature data, and provide a description of your calorimeter. There is no need to record any of the numbers you are given.
The following video will show you what you would have seen if you had performed this procedure in lab: https://youtu.be/w43_I8q2ESU
Getting Temperature Data for the NH3 + HCl Reaction
NOTE: As part of the data you record, describe what you do in this part. Include such things as how you measure your solutions out, what exactly you do with these solutions, and how you take your temperature measurements. Also describe your calorimeter. There is no need to record any of the numbers you are given.
HCl solution concentration: 1.0 M NH3 solution concentration: 1.0 M Volume HCl solution used: 48.8 mL Volume NH3 solution used: 47.9 mL Temperature Data:
3 Solutions Mixed
Getting Temperature Data for the NH4Cl(s) to NH4Cl(aq) Reaction
As for the previous part, describe what you would have done in this part to collect your data. Volume water added to colorimeter: 98.4 mL
Mass weigh bottle + NH4Cl: 17.972 g
Mass weigh bottle after transfer: 15.275 g
3 Solid Added
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