Chemistry for nonchemists—part II

Chemistry for N o n c h e m i s t s - - P a r t II by Leslie W. Flott

"From the pyramid-builders of the Old Kingdom--to Nefertiti and her son-in-law Tutankhamen--the ancient Egyptians pampered their bodies--so much so, it now seems, that they went to the trouble of inventing chemistry." Robert Kunzig, "Discovery," Sept, 1999

great m a n y otherwise intelligent people seem to be completely stymied by two fields of s t u d y - - m a t h e m a t i c s , and chemistry. Some of these people find the terminology beyond t h e m and others seem to feel t h a t there are j us t too m a n y rules to memorize. Like math, chemistry is largely a m a t t e r of logic. Chemistry may be thought of in the same way m a n y of us th in k of foreign languages. Remember, however, t h a t even little babies learn all of the languages t h a t baffle us as adults. A good place to begin to u n d e r s t a n d chemistry is to begin to grasp something of basic chemical nomenclature. U n d e r s t a n d i n g the writing of chemical symbols and chemical formulas puts us firmly on the road to comprehending the n a t u r e of chemical reactions. All substances are made up of some combination of chemical elements. Chemists have developed a sort of s h o r t h a n d code to identify these elements to other scientists and knowledgeable laypersons.

A

NAMING

CHEMICALS

R a t h e r t h a n use the entire name of an e]ement,

unique one or two letter symbols or abbreviations are used to identify them. These symbols or abbreviations are often ta ke n from the German, French, Latin, and Russian nam e or most commonly from English. Some common symbols are shown in the Table I. The first letter in each symbol is capitalized and the second, when there is one, is lower case. All formulas t h a t are multiples of simpler ratios can be assumed to r e p r e s e n t molecules. The formulas N2, H2, H202, and C2H 6 r e p r e s e n t nitrogen gas, hydrogen gas, hydrogen peroxide, and ethane; however, formulas th a t show the simplest possible atomic ratios mu s t be assumed to be empirical unless evidence exists to the contrary. The formulas NaC1 and Fe203, for example, are empirical; the Les Flott is a consultant with Summit Process Consultants Inc., Wabash, Ind. E-mail, [email protected]. June 2002

Table I. The Elements

Element Symbol Hydrogen H Carbon C Nitrogen N Oxygen O Sodium Na Magnesium Mg Aluminum A1 Sulphur S Chlorine C1 Potassium K Calcium Ca Manganese Mn Iron Fe Nickel Ni Copper Cu Zinc Zn Cadmium Cd Silver Ag Tin Sn Gold A GAW = gram atomic weight.

GAW 1.0 12.0 14.0 16.0 23.0 24.3 27.0 32.0 35.0 39.0 40.0 54.9 55.9 58.7 63.6 65.4 112.4 108.0 118.7 197.2

former represents sodium chloride (table salt) and the latter iron oxide (rust), but no single molecules of NaC1 or Fe203 are present. All organic and inorganic compounds can be given systematic names based on the e l e m e n t a r y composition and often the structure of the substance. Bin a r y inorganic compounds contain two different elements and are w ri t t en with the more metallic (more electrically positive) element first. Such compounds are nam ed by taking the n a m e of the first element followed by the main part of the nam e of the second, more negative, element combined with the suffix -ide: NaC1, sodium chloride; CaS, calcium sulfide; MgO, magnesium oxide; SiN, silicon nitride. When the atomic ratio differs from 1:1, a prefix to the name often makes this clear: CS 2 carbon disulfide; GeC14, g e r m a n m m tetrachloride; SF6, sulfur hexafluoride; NO2, nitrogen dioxide; N204, dinitrogen tetraoxide. Many groups of elements occur so often as ions 127

Table II. Names from Alchemy

Chemical Name Potassium chloride Lead acetate Sodium hydroxide Hydrochloric acid Nitrocellulose Ferrous oxide Ferric oxide Sodium chloride

Alchemist's Name Muriate of potash Sugar of lead Caustic soda Muriatic acid Gun cotton Magnetite Hematite Table salt

Formula KC1 Pb(C2H302)2 NaOH HC1 C12H17(ONO2)Os F%O 4 Fe20~ NaC1

t h a t they are given names: nitrate, NO3 ; sulfate, SO42-; and phosphate, PO43-. The suffix -ate usually indicates the presence of oxygen. The positive ion, NH4 +, is called ammonium, as in NH4C1, ammonium chloride, or (NH4)3PO4, ammonium phosphate. Rules for naming more complicated compounds exist, but m a n y compounds have been given trivial names--for example, Na2B4OT.10H20, borax--or proprietary names--F(CF2)nF, Teflon. These nonsystematic names may be convenient in some usages but they are often difficult to interpret. Unfortunately, there are also m a n y names for chemicals t h a t remain in common usage and date back hundreds of years to alchemy. A few of these you might be familiar with (see Table II). CHEMICAL REACTIONS

Chemical formulas are easy to understand once the relationships between Gram Atomic Weight (GAW) and Gram Molecular Weight (GMW) for compounds is grasped. The GAW of several common elements is shown in Table I. The formula weight of a compound is found by adding the atomic weights (expressed in grams) of the elements t h a t make up the compound; for example, table salt is sodium chloride and has the formula NaC1. The molecular weight of sodium chloride is the sum of the gram atomic weight of sodium and the gram atomic weight of chlorine. The molecular weight of NaC1 equals 23.0 grams plus 35.0 grams for a total of 58.0 grams. The same is true of any simple compound, thus: • Sulfuric acid = H2SO 4 = 2 (H) + 1 (S) + 4 (O) = 2 grams ÷ 32 grams + 4 times 16 grams = 98 grams. • Caustic soda = NaOH = 1 (Na) + 1 (O) + 1 (H) = 23 grams plus 16 grams plus 1 gram = 36 grams. • Hydrochloric acid = HC1 -- 1 (H) + 1 (CL) = 1 gram plus 35 grams = 36 grams. • Nitric acid = HNO 3 = 1 (H) + 1 (N) + 3 (O) = 1 gram plus 14 grams plus 3 times 16 grams = 63.0 grams. 128

Note t h a t sulfuric acid has two hydrogen atoms while hydrochloric and nitric acids have only one each. This leads to another basic chemical principle. Simple compounds combine with each other in simple whole number multiples called the Gram Equivalent Weight (GEW). For the sake of simplicity the GEW may be found by dividing the GMW by the number of hydrogen atoms or the number of hydroxyl radicals in the formula; for example, NaOH is the formula for caustic soda, otherwise called sodium hydroxide. As noted above the GMW is 36 grams. Since there is only one hydroxyl radical the GEW is the GMW divided by 1. Sulfuric acid has a GMW of 98 but since there are two hydrogen atoms in the molecule, the GEW is the GMW divided by two. These types of chemical reactions are called redox, for reduction-oxidation, reactions. The key to balancing red-ox equations is to note how m a n y hydrogen atoms and hydroxyl radicals are on each side of the equation. Since in the reaction H + plus OH- yields H + OH- two things are obvious. First, the electrical charges are equal and opposite and neutralize each other. Second, the reaction forms water, H20. This is true of all red-ox reactions. Here are some additional examples: • NaOH + HCI --~ NaCI + H20 • H2SO 4 + 2 NaOH ~ NaOH2 + 2H20 • H2SO 4 + MgOH2 --~ MgSO4 + 2H20 Note t h a t there are the same numbers of atoms on both sides of each equation. This is an illustration of the Law of Conservation of Matter. The process by which individual atoms, groups of atoms, or chemicals redistribute themselves relative to one another is called a "chemical reaction." Chemical reactions result in a change in the molecular composition of substances. A simple example of a chemical reaction is the oxidation of iron, which produces rust (iron oxide). The exact products obtained by a given set of reactants or starting materials depends on the conditions under which the chemical reaction takes place. For example, under differing conditions, various oxides of iron including FeO (ferrous oxide), Fe304 (magnetite), or Fe203 (hematite) might form. Each of these compounds has different physical and/or chemical properties. While the resultant compounds formed during a chemical reaction may vary due to conditions, some things remain constant. These constants include the number of each kind of atom present, the total electrical charge, and the total mass. The commonly quoted but simple statement, "Matter cannot be created or destroyed," was one of the very first and Metal Finishing

most p r o f o u n d steps t a k e n by e a r l y c h e m i s t s tow a r d s a b e t t e r u n d e r s t a n d i n g of chemistry. The more m o d e r n scientific w a y of p r e s e n t i n g the above s t a t e m e n t is: W h e n a reaction t a k e s place, the total m a s s of r e a c t a n t s is e q u a l to the total m a s s of products f o r m e d plus the mass of r e a c t a n t r e m a i n i n g . TITRATION O1= ACIDS AND BASES

T i t r a t i o n is t h e process, operation, or m e t h o d of d e t e r m i n i n g the c o n c e n t r a t i o n of a s u b s t a n c e in solution by a d d i n g to it a s t a n d a r d r e a g e n t of k n o w n c o n c e n t r a t i o n in carefully m e a s u r e d a m o u n t s until a r e a c t i o n of definite a n d k n o w n proportion is completed, as s h o w n by a color c h a n g e or by electrical m e a s u r e m e n t , a n d t h e n calculating the u n k n o w n concentration. T h e t i t r a t i o n of acids a n d bases m e a n s n e u t r a l i z ing t h e m . T h e specific case for the n e u t r a l i z a t i o n of acids a n d bases is the g e n e r a l case for all red-ox equations. In o t h e r words, the milliliters of acid t i m e s the n o r m a l i t y of t h e acid equals the milliliters of base t i m e s t h e n o r m a l i t y of t h e base. This titration m a y be e x p r e s s e d in g e n e r a l t e r m s as:

ml A X N A = mlB X NB where: ml A = milliliters of acid N A = n o r m a l i t y of the acid ml B = milliliters of base N B = n o r m a l i t y of base. Example: How m u c h 0.1 N acid would be r e q u i r e d to n e u t r a l i z e 25 ml of 0.53 N N a O H ? Solution: ml A x N A = ml B x N B ml A = ml B x NB]N A 25 x 0.53/0.1 = 132.5 ml Note the c o m m o n f e a t u r e in the r e a c t i o n s for sulfuric and hydrochloric acid. It t a k e s one G E W of each to n e u t r a l i z e one G E W of caustic soda. One GAW of sulfuric acid contains two G E W s b u t one GAW of hydrochloric acid contains only one GEW. O f course, this b r i e f discussion of some of t h e principles of c h e m i s t r y won't m a k e the r e a d e r a chemist. B u t I've w o r k e d w i t h people over the y e a r s t h a t claimed to be chemists t h a t didn't know a n acid from a base. MF

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