Tuesday
September 30, 2014

Homework Help: English

Posted by a Canadian on Thursday, February 21, 2013 at 6:16pm.

This is a chemistry lab report, but I was wondering if someone could edit the grammar and look at the sentence structure, etc.

I know it's really long and boring, but even if you just look over a bit of it, I would really appreciate it.

And I'm assuming you don't really know anything about the actual contents of this report, which is great. So if you could just include in your feedback if you think I explained well and you kind of have a general idea of what the terms mean, etc. after reading my explanations, that would great.

“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 and 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 reactants have stronger bonds and more energy is needed to break them, endothermic reactions have a positive enthalpy change value. They also result in 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 reactants less energy is needed to break the bonds of the reactants.. 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”.

“Calorimetry” is a method that allows one to measure the heat transfer in an isolated system, the system being the reactants and products in a chemical reaction; a calorimeter, a device with well-insulated walls that prevent heat exchange with its surroundings, is used in calorimetry. In this lab investigation, calorimetry was used to find the enthalpy change of the following reactions:

NH3(aq) + HCl(aq) → NH4Cl(aq)

NH4Cl(s) → NH4Cl(aq)

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:

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.

Thanks so much in advance!

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