Hi, here's my (almost) 2-paged lab report introduction (hopefully you remember it from yesterday).
I went to class today, and actually a lot of my other classmates' introductions were over 2 pages! Now I'm feeling kind of insecure about mine, and I'm not even sure if 2 pages is sufficient!
Anyway, since it was our first introduction, our teacher helped us out by giving us the key terms we had to make sure to include (but not necessarily in that order - she said we need to put it in a correct order so that there's proper flow and the order is logical):
-enthalpy change
-standard molar of enthalpy formation
-formation reaction
-endothermic vs exothermic
-calorimetry
-Hess' Law
I put **s around the parts I was unsure of. (From your answer to my last question, I think maybe I should just omit the part about bonds? I guess that's not a really necessary part of the explanation.)
Also, my teacher didn't include it in the key terms list, but if I'm using terms like "system" and "surroundings", do you think I need to define that better and talk about how the total energy in the universe is constant, so energy can't be destroyed/created, and deltaEuniverse = 0, but energy can be transferred from one substance to another and converted into different forms?
Okay, the actual introduction now!:
The Enthalpy of Formation of NH4Cl(s)
Introduction
“Enthalpy”, H, is the total internal energy, or kinetic and potential energy, of a substance at a constant pressure. More significant, however, is the difference between the enthalpy of the reactants and the enthalpy of the products in a reaction, which is called “enthalpy change”, ΔH. In chemical reactions, enthalpy change is caused by the formation or breaking of chemical bonds as energy is stored within chemical bonds. Energy is required to break bonds, and when the bonds break, energy is absorbed. In contrast, when bonds form, energy is released. Whichever is greater in a reaction—the amount of energy absorbed, or the amount released—determines whether the reaction is “endothermic” or “exothermic”.
An “endothermic” reaction occurs when more energy is absorbed from the reactants’ bonds breaking than is released from the products’ bonds forming, resulting in a net absorption of energy. Because the enthalpy of the products are greater than the enthalpy of the reactants, as **the products have weaker bonds**, endothermic reactions have a positive enthalpy change value. They also cause a decrease in temperature as it absorbs heat from its surroundings. In contrast, if a reaction’s reactants have weaker bonds than the products, the reactants’ bonds will break more easily, and less energy will be required to break the bonds. As a result, less energy will be absorbed when the bonds break. Thus, the amount of energy released during the formation of the products’ bonds will be greater. When such a net release of energy occurs in a reaction, it is an “exothermic” reaction. Exothermic reactions have a negative enthalpy change value because the enthalpy of the reactants are greater than the enthalpy of the products since **the reactants have weaker bonds**. Exothermic reactions also cause an increase in temperature as it releases heat to its surroundings. Since the enthalpy change of a reaction is equal to its heat change at constant pressure, the enthalpy change can be determined experimentally through “calorimetry”. **(Why do weaker bonds mean more enthalpy/energy?)**-might just omit
“Calorimetry” is a method that allows one to measure the heat transfer in an isolated system, the reactants and products in a reaction; a calorimeter, a device with well-insulated walls that prevent heat exchange with its surroundings, is used in calorimetry. The initial temperature of the reactants and the final temperature, following the reaction, can be measured to find the temperature change of the system. The temperature change can then be used to calculate the amount of heat that is released or absorbed by the reaction, which is the opposite of any heat absorbed or lost by the surroundings:
Qsystem = -Qsurroundings
Q, the heat transfer in the isolated system, is measured in joules (J), and is calculated using the mass (m) of the substance, measured in grams (g); the specific heat capacity of the substance (m), measured in (J/g°C); and the temperature change (ΔT), measured in (°C):
Q = mc ΔT
Thus, the enthalpy change can be calculated using the number of moles and the negative value of Q, **as the heat lost by the reaction, or the system, is equal to the heat gained by the surroundings, or, for example, the solution, and vice versa.**
ΔH = -Q/n
However, the enthalpy change of a reaction can be determined using another method, or law, called “Hess’ Law of Heat Summation.”
According to Hess’ Law, the beginning and end conditions of the reactants and products are important for enthalpy change, which is unaffected by the pathway taken by the reaction. Therefore, the enthalpy change can be determined by calculating the sum of the enthalpy changes of all the steps, or intermediate reactions, that are taken to reach the final target equation. Thus, one can calculate the enthalpy change of a reaction without taking the direct measurements of the specific reaction using Hess’ Law by algebraically combining known chemical reactions and their enthalpy changes. However, “formation reactions”, where one mole of a compound is formed from its elements in their standard states at room temperature, may need to be determined if no known chemical reactions and enthalpy changes are readily available. The enthalpy change of a formation reaction is called the “standard molar enthalpy of formation”, and is the amount of energy absorbed or released when one mole of a compound is formed directly from its elements in their standard states. In many cases, Hess’ Law must be used to find the quantity of energy absorbed or released in a reaction, or the enthalpy change, rather than finding it experimentally through calorimetry as calorimetry is often impractical for many reactions, and is usually only used for dilute aqueous solutions. For example, in this lab, the enthalpy of formation (ΔH°f) of NH4Cl(s) is not determined directly as the elements that form NH4Cl(s) exist as a gas in their standard states:
½N2(g) + 2H2(g) + ½Cl2(g) → NH4Cl(s)
Moreover, the reaction would take too long to occur, and it would be impractical to use calorimetry. Thus, the purpose of this lab investigation is to find the enthalpy of formation of NH4Cl(s) using known chemical equations and their corresponding enthalpy change values as well as experimentally determined enthalpy change values of chemical equations required to calculate the target equation, the formation reaction of NH4Cl(s). If the reaction is exothermic, then energy will be released by the reaction to the surroundings and temperature will increase, because the reactants are gases, which have high kinetic energy, and they yield a stable solid, which would have low kinetic energy, resulting in energy being released as the reactants have greater enthalpy.
I would really appreciate any constructive criticism (feel free to be ruthless)!
OK. I am stunned. After reading your intro to your intro (the comments, instructions of your teacher and comrades in arms) I am inclined to think two pages is too little Your teacher doesn't want an introduction; she wants a book. It would take me two pages or more to describe calorimetry, let alone those other key points. I would take more than that to explain Hess' Law. I guess my conclusion is that I'm not the one to critique your introduction. What you wrote sounds ok to me. I don't see anything that is "wrong". Obviously you have a better feel for what is needed in that class than I.
Oh no! :( I tried to sum up the stuff as best as I could and be brief since I felt that two pages was way too long, but now I feel like I shouldn't have done that... I'll ask her tomorrow how much detail she expects, etc. because I don't really think mine is okay... The intro is worth the most out of the whole lab and I'm hoping to do well in this course. :(
Anyway, thank you so much for reading all of that and for looking over the whole thing! (even if it may not be enough, it is still a lot to read!) I'm beyond grateful.
damn dr bob kinda ruthless lmao
First of all, your introduction is well-written and provides a clear explanation of the key concepts related to enthalpy and calorimetry. However, there are a few areas where you could improve the flow and clarity of your explanation.
1. In the first paragraph, you mentioned that enthalpy is the total internal energy of a substance at a constant pressure. This is correct, but it might be helpful to briefly explain why constant pressure is important in measuring enthalpy changes. You can mention that most reactions occur at constant atmospheric pressure, and the pressure term cancels out in the calculation of enthalpy change.
2. In the paragraph where you explain endothermic and exothermic reactions, you could clarify that the amount of energy absorbed or released during the formation or breaking of bonds depends on the strength of those bonds. Stronger bonds require more energy to break and release more energy when formed. Weaker bonds require less energy to break and release less energy when formed.
3. In the paragraph discussing calorimetry, you could further explain the concept of an isolated system and how a calorimeter helps to measure heat transfer without exchange with the surroundings. You can mention that an isolated system allows us to determine the heat transfer solely between the reactants and the surroundings without any loss or gain from external factors.
4. In the paragraph where you introduce Hess' Law, you could explain the significance of this law in determining enthalpy changes. Note that Hess' Law allows us to use known enthalpy changes of other reactions to calculate the enthalpy change of a target reaction. This is particularly helpful when direct measurements are not feasible or practical.
5. Finally, in the last paragraph, you can briefly mention that the enthalpy of formation of a compound is determined by the enthalpy changes of the formation reactions of its constituent elements. This explanation will provide a better understanding of why the enthalpy of formation needs to be determined in this lab investigation.
Overall, your introduction provides a comprehensive explanation of the key concepts related to enthalpy change and calorimetry. By addressing the suggested improvements, you will enhance the clarity and understanding for your reader. Good luck with your lab report!