The Chemistry of Air Conditioning in Everyday Life
- Page ID
- 418923
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)This Exemplar will teach the following concepts from the ACS Examinations Institute General Chemistry ACCM:
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VI. A. 2. b. In thermodynamic treatments of chemical systems, the definition of the system of interest versus the surroundings is important.
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VI. C. 1. Heat exchange is measured via temperature change.
Introduction:
If you are not outside while reading this, it is likely that you are experiencing the wonders of air conditioning at this very moment. Although it is seldom necessary to pay any mind to the otherwise nebulous mechanisms that keep our indoor spaces comfortable, upon taking a step back, one realizes just how interesting the science that underlies AC truly is. Despite the common perception of air conditioners as just boxes of complicated electronics, there is a fair bit of chemistry that these boxes use as their basis. A large part of air conditioning chemistry applies the thermodynamics of gasses and liquids, in conjunction with chemical systems as a whole. Upon reading this article, it will become clear how this is the case.
Air Conditioning: Practice and Principles:
One of the more common methods of refrigeration is Vapor-Compression Refrigeration or VCR. This method revolves around a certain liquid refrigerant and its cycle through a series of compression and expansion, which in turn removes heat from a given area and redistributes it elsewhere [1]. You’ll find this cooling method employed in a large portion of everyday buildings and cars, as well as refrigerators [2]. In practice, this implementation is a multi-step complicated process through which the refrigerant evaporates and absorbs heat, then condenses and releases heat to the surroundings [3]. Even though the implementation may be complex, the underlying system is a quite elegant example of heat exchange and thermodynamics.
If you view the interior of a room (which you desire to be cooled) as a system, and the corresponding exterior as its surroundings, you can perform standard heat transfer calculations (molar heat problems, specific heat problems) to help visualize a wide array of changes between the systems and the substances, the most salient of which comes in the form of temperature. In order to simplify the thermodynamic processes that underlie AC, some assumptions must be made, the most important of which relates to work. The refrigerant must undergo frequent phase changes from liquid to gas and vice versa, and this is able to happen due to the work being done by the compressor. The pressurization of the gas does indeed require a sizable amount of energy to proceed, meaning that all in all, the \(\ ΔE\) of the process is not simply equal to q, but instead is equal to \(\ q+w\) , or \(\ q+pV\) . As a result, some forms of energy flow do indeed occur in reality, but can be ignored for the purposes of observing thermodynamics. Unsurprisingly, the most significant of these forms is electricity, which allows for the compressor to perform the necessary work [4]. Again, for the purposes of observing thermodynamics, the influence of work and other energy flow (like electricity) will not be discussed, and it will be assumed that this is being ignored. When looking at a system of interest and making these assumptions, based on the principles of the VCR model explained above and the tendency of the refrigerant to absorb/release heat, formulas such as \(\ q = mCΔT\) can be applied for the liquid refrigerant, while formulas such as \(\ q = nC_vΔT\) can be applicable for the gasses in the surroundings.
Assuming that no heat is lost during the transfers of this energy, and the knowledge that \(\ -q_{system} = q_{surroundings}\) , you can calculate the transfer of heat/energy and identify differences in temperature in an ideal system using these formulas. These underlying concepts help to illustrate the heating/cooling of gasses and liquids, which is what is implemented by most modern ACs in the un-ideal system that is real life. To help illustrate the relationships that are outlined above, some worked-out practice problems are provided.
Problems:
a) 1,1,1,2 Tetrafluorethane (also called r134a) is a refrigerant liquid used in air conditioning. its chemical formula is CH2FCF3. Given 102.03 grams of freon, its Cv of 24.386 \(\frac{J}{mol*K}\) [5] and the temp change from -100ºC to -80ºC. Calculate q for the change.
\[\ q = mC_vΔT\]
\[\ q = 1mol * 24.386 \ (\frac{J}{mol*K}\ ) * 20K \]
\[\ q = 487.72 J \]
b) Calculate the temperature change of 4.875 mols of air in a room of constant volume using the q calculated in part a. Assume Cv is trivial.
\[\ q = -q_{surroundings}\]
\[\ q = nC_vΔT \]
\[\ -487.72 J = 4.785 mols * 1\ (\frac{J}{mol*K}\ ) *ΔT \]
\[\ ΔT= \frac{-487.72 J}{4.785 mols * 1\ (\frac{J}{mol*K}\ )} \]
\[\ ΔT = -101.926 K\]
Explanation: This problem helps to illustrate a heat exchange for an ideal system in which no heat is lost during the transfer. The first part involves calculating the heat released by the liquid (\(\ q = mC_vΔT\) for the freon), and the second part involves setting the negative of that value of change equal to the heat absorbed by the corresponding gas (\(\ q = nC_vΔT\) for the air) in order to help visualize the heat exchange.
a) What conditions must be present in order for a phase change in the freon to occur?
In order to observe a phase change, enough heat must be applied to the substance in order to overcome the heat of vaporization for the given mass of that substance
b) The heat of vaporization of a given coolant is 2002 \(\frac{J}{gram}\). If a phase change is necessary for an air conditioner to work properly, and 11645.40 J of heat is added to 200.5 grams of coolant, will a phase change occur?
\[\ q_{phasechange} = m * ΔH_v \]
\[\ q_{phasechange} = 200.5 g * 2002 (\frac{J}{gram}\ )\]
\[\ q_{phasechange} = 401401 J \]
Since the amount of heat required to change phases is significantly higher than the amount of heat given to the system, a phase change will not occur.
Explanation: This problem focuses on one of the aspects that are necessary for an air conditioner to function which is the phase change of the refrigerant. In the case of most modern refrigerants, the phase change is from a liquid to a gas and vice versa. This means that the necessary thermodynamic calculations should be focused on the heat of vaporization of the given refrigerants which will in turn aid in the overall understanding of the thermodynamic properties of Air Conditioners.
References:
1. Heating and air conditioning: Characteristics.
https://www.nuclear-power.com/nuclea...-conditioning/ (accessed Oct 27, 2022).
2. Vapor-compression refrigeration.
https://en.wikipedia.org/wiki/Vapor-..._of_the_system (accessed Oct 27, 2022).
3. Thermodynamics I: Air conditioners and refrigerators.
https://www.pa.uky.edu/~brill/PHY120...April%2017.pdf (accessed Oct 27, 2022).
4. Air Conditioning.
https://www.energy.gov/energysaver/air-conditioning (accessed Dec 6, 2022).
5. Thermodynamic properties - freon.
https://www.freon.com/en/-/media/files/freon/freon-134a-eng-thermodynamic-properties.pdf (accessed Oct 28, 2022).
Further Reading:
Balmer, R. T. Modern Engineering Thermodynamics; Academic Press: Burlington, Mass. u.a, 2011.
Contributors and Attributions:
- Luke Rinaldi (Duke University), Ismael M. Alvarez (Duke University).