Using Chemical Equations in Environmental Chemistry and Green Chemistry - ChemPRIME

Using Chemical Equations in Environmental Chemistry and Green Chemistry

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In this chapter we deal with topics central to both environmental chemistry and green chemistry. In both areas, the amounts of substances which participate in chemical reactions, the quantities of heat given off or absorbed when reactions occur, and the volumes of solutions which react exactly with one another are central.

For example, calculating the amount of carbon dioxide released into the atmosphere when known quantities of fuel are burned is a first step in calculating "carbon footprints" in Environmental Chemistry. We find that the "emission factor" for coal used to generate electricity is about 1000 g of CO2 per kilowatt hour (kWh) of electricity, while it's only about 100 g of CO2 per kWh for photovoltaics [1].

Green Chemistry is concerned with reduction of waste in chemical production (for example, reducing the emission factor for photovoltaics and other sources where production causes larger "carbon footprints".

Four of the "Twelve Principles of Green Chemistry", shown bold are closely related to topics of this chapter:

  1. It is better to prevent waste than to treat or clean up waste after it is formed.
  2. Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
  3. Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
  4. Chemical products should be designed to preserve efficacy of function while reducing toxicity.
  5. The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible and, innocuous when used.
  6. Energy requirements should be recognized for their environmental and economic impacts and should be minimized. Synthetic methods should be conducted at ambient temperature and pressure.
  7. A raw material or feedstock should be renewable rather than depleting wherever technically and economically practicable.
  8. Reduce derivatives - Unnecessary derivatization (blocking group, protection/ deprotection, temporary modification) should be avoided whenever possible.
  9. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
  10. Chemical products should be designed so that at the end of their function they do not persist in the environment and break down into innocuous degradation products.
  11. Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
  12. Substances and the form of a substance used in a chemical process should be chosen to minimize potential for chemical accidents, including releases, explosions, and fires.


These seemingly unrelated subjects are discussed together because many of the calculations involving them are almost identical in form. The same is true of the density calculations and of the calculations involving molar mass and the Avogadro constant. In each case one quantity is defined as the ratio of two others.


TABLE 1 Summary of Related Quantities and Conversion Factors.

Related Quantities Conversion Factor Definition Road Map
Volume ↔ mass Density, ρ \rho=\frac{m}{V} V\text{ }\overset{\rho }{\longleftrightarrow}\text{ }m
Amount of substance ↔ mass Molar Mass, M M=\frac{m}{n} n\text{ }\overset{M}{\longleftrightarrow}\text{ }m
Amount of substance ↔ number of particles Avogadro constant, NA N_{\text{A}}=\frac{N}{n} n\text{ }\overset{N_{\text{A}}}{\longleftrightarrow}\text{ }N
Amount of X consumed or produced ↔ amount of Y consumed or produced Stoichiometric ratio, S(Y/X) S\text{(Y/X)}=\frac{n_{\text{Y}}}{n_{\text{X}}} n_{\text{X}}\text{ }\overset{S\text{(Y/X)}}{\longleftrightarrow}\text{ }n_{\text{Y}}
Amount of X consumed or produced ↔ quantity of heat absorbed during reaction ΔHm for thermochemical equation \Delta H_{\text{m}}=\frac{q}{n_{\text{X}}} n_{\text{X}}\text{ }\overset{\Delta H_{m}}{\longleftrightarrow}\text{ }q
Volume of solution ↔ amount of solute Concentration of solute, cX c_{\text{X}}=\frac{n_{\text{X}}}{V} V\text{ }\overset{c_{\text{X}}}{\longleftrightarrow}\text{ }n_{\text{X}}


The first quantity serves as a conversion factor relating the other two. A summary of the relationships and conversion factors we have encountered so far is given in Table 1.

An incredible variety of problems can he solved using the conversion factors in Table 1. Sometimes only one factor is needed, but quite often several are applied in sequence, as in Example 3 in Titrations. In solving such problems, it is necessary first to think your way through, perhaps by writing down a road map showing the relationships among the quantities given in the problem. Then you can apply conversion factors, making sure that the units cancel, and calculate the result.

The examples in these sections should give you some indication of the broad applications of the problem-solving techniques we have developed here. Once you have mastered these techniques, you will be able to do a great many useful computations which are related to problems in the chemical laboratory, in everyday life, and in the general environment. You will find that the same type of calculations, or more complicated problems based on them, will be encountered again and again throughout your study of chemistry and other sciences.


\underset{\text{Gasoline}}{\mathop \text{2C}_{\text{8}}\text{H}_{\text{12}}}\,\text{ + }\underset{\text{Air}}{\mathop \text{25O}_{\text{2}}}\,\text{ }\to \text{ }\underset{\begin{smallmatrix} 
 \text{Carbon} \\ 
 \text{dioxide} 
\end{smallmatrix}}{\mathop \text{16CO}_{\text{2}}}\,\text{ + }\underset{\text{Water}}{\mathop \text{18H}_{\text{2}}\text{O}}\,

There are a great many circumstances in which you may need to use a balanced equation. For example, you might want to know how much air pollution would occur when 100 metric tons of coal were burned in an electric power plant, how much heat could be obtained from a kilogram of natural gas, or how much vitamin C is really present in a 300-mg tablet. In each instance someone else would probably have determined what reaction takes place, but you would need to use the balanced equation to get the desired information.

References

  1. http://en.wikipedia.org/wiki/Carbon_footprint
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