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Chemistry for Instrumentation

Chemistry is the study of how atoms join with and separate from one another. Like so many other areas of physical science, the patterns and laws we see in chemical reactions are dominated by two fundamental laws of physics: the Conservation of Mass and the Conservation of Energy. The particles of matter comprising atoms have the ability to store energy in potential form, and their tendency is to “seek” the most stable energy state1. The arrangement of electrons around the nucleus of an atom is largely dictated by the tendency of electrons to “prefer” stable energy states, and so is the formation of molecules (atoms bonded together): electrons seeking energy states least liable to disturbance. The rest, as they say, is all detail.

We exploit this property of energy storage in the fuels we use. Atoms bound together to form molecules are in a lower energy state than when they exist as separate atoms. Therefore, an investment of energy is required to force molecules apart (into separate atoms), and energy is returned (released) when atoms join together to form molecules. The combustion of a fuel, for example, is nothing more than a process of the atoms in relatively unstable (high-energy) fuel molecules joining with oxygen atoms in air to form stable (low-energy) molecules such as water (H2O) and carbon dioxide (CO2).

Natural gas, for example, is a relatively stable combination of hydrogen (H) and carbon (C) atoms, mostly in the form of molecules with a 4:1 hydrogen-to-carbon ratio (CH4). However, when placed in the vicinity of free oxygen (O) atoms, and given enough energy (a spark) to cause the hydrogen and carbon atoms to separate from each other, the hydrogen atoms strongly bond with oxygen atoms to form water molecules (H2O), while the carbon atoms also strongly bond with oxygen atoms to form carbon dioxide molecules (CO2). These strong bonds formed between hydrogen, carbon, and oxygen in the water and carbon dioxide molecules are the result of electrons within those atoms seeking lower energy states than they possessed while forming molecules of natural gas (CH4). In other words, the energy states of the electrons in the hydrogen and carbon atoms were higher when they were joined to form natural gas than they are when joined with oxygen to form water and carbon dioxide. As those electrons attain lower energy states, they difference of energy must go somewhere (since energy cannot be created or destroyed), and so the chemical reaction releases that energy in the forms of heat and light. This is what you see and feel in the presence of a natural gas flame: the heat and light emitted by hydrogen and carbon atoms joining with oxygen atoms.

 

The Law of Mass Conservation plays an important role in chemistry as well. When atoms join to form molecules, their masses add. That is, the mass of a molecule is precisely equal2 to the mass of its constituent atoms. When chemical engineers design manufacturing processes, they must pay close attention to a principle called mass balance, where the sum total of all masses for chemical feeds into a process precisely equals the sum total of all masses exiting the process.

Too many other practical applications of chemistry exist to summarize in these pages, but this chapter aims to give you an foundation to understand certain chemistry concepts necessary to comprehend the function of certain instruments (notably analyzers) and processes.

 

Terms and Definitions - Learn the words frequently used in chemistry... Read more...

Atomic Theory and Chemical Symbols - The three “elementary” particles of matter comprising all atoms are electrons, protons, and neutrons. Combinations of these three particle types in various whole-number quantities constitute every type of atom. These fundamental particles are absolutely miniscule in comparison to the macroscopic existence of human beings... Read more...

Periodic Table of Elements - A Periodic Table of the Elements is a table listing the elements in order of their atomic numbers... Read more...

Electronic Structure - Somewhere in your education, you were probably shown a model of the atom showing a dense nucleus (comprised of protons and neutrons) surrounded by electrons whirling around like satellites around a planet. While there are some useful features of this model, it is largely in error... Read more...

Spectroscopy - Much of our knowledge about atomic structure comes from experimental data relating the interaction between light and atoms of the different elements. Light may be modeled as an electromagnetic wave, consisting of an oscillating electric field and an oscillating magnetic field... Read more...

  • Emission Spectroscopy - If we take a sample of atoms, all of the same element and at a low density9 (e.g. a gas or vapor), and “excite” them with a source of energy such as an electric arc, we will notice those atoms emit colors of light that are characteristically unique to that element. The unique electron configurations of each element creates a unique set of energy values between which atomic electrons of that element may “jump.” Since no two elements have the exact same electron configurations, no two elements will have the same exact set of available energy levels for their electrons... Read more...
  • Absorption Spectroscopy - If we take a sample of atoms, all of the same element and at a low density (e.g. a gas or vapor), and pass a continuous (“white”) spectrum of light wavelengths through that sample, we will notice certain colors of light missing from the light exiting the sample. Not only are these missing wavelengths characteristically unique to that element, but they are the exact same wavelengths of light found in the emission spectrum for that element!... Read more...

Formulae for Common Chemical Compounds - Most of these formulae appear in molecular chemical form rather than structural form. For example, ethanol appears here as C2H6O rather than C2H5OH. Also, the entries for fructose and glucose are identical (C6H12O6) despite the two compounds having different structures. This means most of the formulae shown in this section merely represent the ratios of each element in a compound, making little or no attempt to convey the structure of the molecule... Read more...

Molecular Quantities - Sample sizes of chemical substances are often measured in moles. One mole of a substance is defined as a sample having 6.022 × 1023 (Avogadro’s number) molecules. An elemental sample’s mass is equal to its molecular quantity in moles multiplied by the element’s atomic mass in amu (atomic mass units, otherwise known as Daltons)... Read more...

Stoichiometry - Stoichiometry is the accounting of atoms in a chemical equation. It is an expression of the Law of Mass Conservation, in that elements are neither created nor destroyed in a chemical reaction, and that mass is an intrinsic property of every element... Read more...

  • Balancing Chemical Equations by Trial and Error - Balancing a chemical equation is a task that may be done by trial-and-error. For example, let us consider the case of complete combustion for the hydrocarbon fuel ethane (C2H6) with oxygen (O2)...Read more...
  • Balancing Chemical Equations Using Algebra - A more mathematically sophisticated approach to stoichiometry involves the use of simultaneous systems of linear equations. The fundamental problem chemists must solve when balancing reaction equations is to determine the ratios of reactant and product molecules. If we assign a variable to each molecular quantity, we may then write a mathematical equation for each element represented by the reaction, and use algebra to solve for the variable values... Read more...
  • Stoichiometric Ratios - Regardless of the technique used to balance the equation for a chemical reaction, the most practical purpose of balancing the equation is to be able to relate the reactant and reaction product quantities to each other. For instance, we may wish to know how much oxygen will be required to completely combust with a given quantity of fuel, so that we will be able to engineer a burner system capable of handling the necessary flow rates of fuel and oxygen. Balancing the chemical reaction is just the first part of the solution. Once we have a balanced equation, we need to consider the ratios of the substances to each other... Read more...

Energy in Chemical Reactions - A chemical reaction resulting in a net release of energy is called exothermic. Conversely, a chemical reaction requiring a net input of energy to occur is called endothermic. The relationship between chemical reactions and energy exchange is correlated with the breaking or making of chemical bonds. Atoms bonded together represent a lower state of total energy than those same atoms existing separately, all other factors being equal. Thus, when separate atoms join together to form a molecule, they go from a high state of energy to a low state of energy, releasing the difference in energy in some form (heat, light, etc.). Conversely, an input of energy is required to break that chemical bond and force the atoms to separate... Read more...

Periodic Table of the Ions -Click here to see the elements' periodic table of the ions... 

Ions in Liquid Solutions - Many liquid substances undergo a process whereby their constituent molecules split into positively and negatively charged ion pairs, the positively-charge ion called a cation and the negatively-charged ion called an anion. Liquid ionic compounds split into ions completely or nearly completely, while only a small percentage of the molecules in a liquid covalent compound split into ions. The process of neutral molecules separating into ion pairs is called dissociation when it happens to ionic compounds, and ionization when it happens to covalent compounds... Read more...

pH -  Hydrogen ion activity in aqueous (water-solvent) solutions is a very important parameter for a wide variety of industrial processes. A number of reactions important to chemical processing are inhibited or significantly slowed if the hydrogen ion activity of a solution does not fall within a narrow range. Some additives used in water treatment processes (e.g. flocculants) will fail to function efficiently if the hydrogen ion activity in the water is not kept within a certain range. Alcohol and other fermentation processes strongly depend on tight control of hydrogen ion activity, as an incorrect level of ion activity will not only slow production, but may also spoil the product. Hydrogen ions are always measured on a logarithmic scale, and referred to as pH... Read more...



1This generally means to seek the lowest available energy state, but there are important exceptions where chemical reactions actually proceed in the opposite direction (with atoms seeking higher energy states and absorbing energy from the surrounding environment to achieve those higher states). A more general and consistent understanding of matter and energy interactions involves a more complex concept called entropy.

2This statement is not perfectly honest. When atoms join to form molecules, the subsequent release of energy is translated into an incredibly small loss of mass for the molecule, as described by Albert Einstein’s famous mass-energy equation E = mc2. However, this mass discrepancy is so small (typically less than one part per billion of the original mass!), we may safely ignore it for the purposes of understanding chemical reactions in industrial processes.

 

References

“Fundamental Physical Constants – Extensive Listing”, from http://physics.nist.gov/constants, National Institute of Standards and Technology (NIST), 2006.

Giancoli, Douglas C., Physics for Scientists & Engineers, Third Edition, Prentice Hall, Upper Saddle River, NJ, 2000.

Haug, Roger Tim, The Practical Handbook of Compost Engineering, CRC Press, LLC, Boca Raton, FL, 1993.

Mills, Ian; Cvita˘s, Tomislav; Homann, Klaus; Kallay, Nikola; Kuchitsu, Kozo, Quantities, Units and Symbols in Physical Chemistry (the “Green Book”), Second Edition, International Union of Pure and Applied Chemistry (IUPAC), Blackwell Science Ltd., Oxford, England, 1993.

“NIOSH Pocket Guide to Chemical Hazards”, DHHS (NIOSH) publication # 2005-149, Department of Health and Human Services (DHHS), Centers for Disease Control and Prevention (CDC), National Institute for Occupational Safety and Health (NIOSH), Cincinnati, OH, September 2005.

Pauling, Linus, General Chemistry, Dover Publications, Inc., Mineola, NY, 1988.

Rosman, K.J.R. and Taylor, P.D.P, Isotopic Compositions of the Elements 1997, International Union of Pure and Applied Chemistry (IUPAC), 1997.

Scerri, Eric R., How Good Is the Quantum Mechanical Explanation of the Periodic System?, Journal of Chemical Education, Volume 75, Number 11, pages 1384-1385, 1998.

Weast, Robert C.; Astel, Melvin J.; and Beyer, William H., CRC Handbook of Chemistry and Physics, 64th Edition, CRC Press, Inc., Boca Raton, FL, 1984.

Whitten, Kenneth W.; Gailey, Kenneth D.; and Davis, Raymond E., General Chemistry, Third Edition, Saunders College Publishing, Philadelphia, PA, 1988.

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