If chemistry were compared to a sport, then the study of atomic and molecular properties, along with learning about the elements and how they relate on the periodic table, would be like going to practice. Learning about chemical reactions, which includes observing them and sometimes producing them in a laboratory situation, is like stepping out onto the field for the game itself. Just as every sport has its "vocabulary"—the concepts of offense and defense, as well as various rules and strategies—the study of chemical reactions involves a large set of terms. Some aspects of reactions may seem rather abstract, but the effects are not. Every day, we witness evidence of chemical reactions—for instance, when a fire burns, or metal rusts. To an even greater extent, we are surrounded by the products of chemical reactions: the colors in the clothes we wear, or artificial materials such as polymers, used in everything from nylon running jackets to plastic milk containers.
HOW IT WORKS
What Is a Chemical Reaction?
If liquid water is boiled, it is still water; likewise frozen water, or ice, is still water. Melting, boiling, or freezing simply by the application of a change in temperature are examples of physical changes, because they do not affect the internal composition of the item or items involved. A chemical change, on the other hand, occurs when the actual composition changes—that is, when one substance is transformed into another. Water can be chemically changed, for instance, when an electric current is run through a sample, separating it into oxygen and hydrogen gas.
Chemical change requires a chemical reaction, a process whereby the chemical properties of a substance are altered by a rearrangement of the atoms in the substance. Of course we cannot see atoms with the naked eye, but fortunately, there are a number of clues that tell us when a chemical reaction has occurred. In many chemical reactions, for instance, the substance may experience a change of state or phase—as for instance when liquid water turns into gaseous oxygen and hydrogen as a result of electrolysis.
HOW DO WE KNOW WHEN A CHEMICAL REACTION HAS OCCURRED?
Changes of state may of course be merely physical—as for example when liquid water is boiled to form a vapor. (These and other examples of physical changes resulting from temperature changes are discussed in the essays on Properties of Matter; Temperature and Heat.) The vapor produced by boiling water, as noted above, is still water; on the other hand, when liquid water is turned into the elemental gases hydrogen and oxygen, a more profound change has occurred.
Likewise the addition of liquid potassium chromate (K2CrO4) to a solution of barium nitrate (Ba[NO3]2 forms solid barium chromate (BaCrO4). In the reaction described, a solution is also formed, but the fact remains that the mixture of two solids has resulted in the formation of a solid in a different solution. Again, this is a far more complex phenomenon than the mere freezing of water to form ice: here the fundamental properties of the materials involved have changed.
The physical change of water to ice or steam, of course, involves changes in temperature; likewise, chemical changes are often accompanied by changes in temperature, the crucial difference being that these changes are the result of alterations in the chemical properties of the substances involved. Such is the case, for instance, when wood burns in the presence of oxygen: once wood is turned to ash, it has become an entirely different mixture than it was before. Obviously, the ashes cannot be simply frozen to turn them back into wood again. This is an example of an irreversible chemical reaction.
Chemical reactions may also involve changes in color. In specific proportions and under the right conditions, carbon—which is black—can be combined with colorless hydrogen and oxygen to produce white sugar. This suggests another kind of change: a change in taste. (Of course, not every product of a chemical reaction should be tasted—some of the compounds produced may be toxic, or at the very least, extremely unpleasant to the taste buds.) Smell, too, can change. Sulfur is odorless in its elemental form, but when combined with hydrogen to form hydrogen sulfide (H2S), it becomes an evil-smelling, highly toxic gas.
The bubbling of a substance is yet another clue that a chemical reaction has occurred. Though water bubbles when it boils, this is merely because heat has been added to the water, increasing the kinetic energy of its molecules. But when hydrogen peroxide bubbles when exposed to oxygen, no heat has been added. As with many of the characteristics of a chemical reaction described above, bubbling does not always occur when two chemicals react; however, when one of these clues is present, it tells us that a chemical reaction may have taken place.
In every chemical reaction, there are participants known as reactants, which, by chemically reacting to one another, result in the creation of a product or products. As stated earlier, a chemical reaction involves changes in the arrangement of atoms. The atoms in the reactants (or, if the reactant is a compound, the atoms in its molecules) are rearranged. The atomic or molecular structure of the product is different from that of either reactant.
Note, however, that the number of atoms does not change. Atoms themselves are neither created nor destroyed, and in a chemical reaction, they merely change partners, or lose partners altogether as they return to their elemental form. This is a critical principle in chemistry, one that proves that medieval alchemists' dream of turning lead into gold was based on a fallacy. Lead and gold are both elements, meaning that each has different atoms. To imagine a chemical reaction in which one becomes the other is like saying "one plus one equals one."
SYMBOLS IN A CHEMICAL EQUATION.
In a mathematical equation, the sums of the numbers on one side of the equals sign must be the same as the sum of the numbers on the other side. The same is true of a chemical equation, a representation of a chemical reaction in which the chemical symbols on the left stand for the reactants, and those on the right are the product or products. Instead of an equals sign separating them, an arrow, pointing to the right to indicate the direction of the reaction, is used.
Chemical equations usually include notation indicating the state or phase of matter for the reactants and products. These symbols are as follows:
- (s) : solid
- (l) : liquid
- (g) : gas
- (aq) : dissolved in water (an aqueous solution)
The fourth symbol, of course, does not indicate a phase of matter per se (though obviously it appears to be a liquid); but as we shall see, aqueous solutions play a role in so many chemical reactions that these have their own symbol. At any rate, using this notation, we begin to symbolize the reaction of hydrogen and oxygen to form water thus: H(g) + O(g) →H2O(l).
This equation as written, however, needs to be modified in several ways. First of all, neither hydrogen nor oxygen is monatomic. In other words, in their elemental form, neither appears as a single atom; rather, these form diatomic (two-atom) molecules. Therefore, the equation must be rewritten as H2(g) + O2(g) →H2O(l). But this is still not correct, as a little rudimentary analysis will show.
Balancing Chemical Equations
When checking a chemical equation, one should always break it down into its constituent elements, to determine whether all the atoms on the left side reappear on the right side; otherwise, the result may be an incorrect equation, along the lines of "1 + 1 = 1." That is exactly what has happened here. On the left side, we have two hydrogen atoms and two oxygen atoms; on the right side, however, there is only one oxygen atom to go with the two hydrogens.
Obviously, this equation needs to be corrected to account for the second oxygen atom, and the best way to do that is to show a second water molecule on the right side. This will be represented by a 2 before the H2O, indicating that two water molecules now have been created. The 2, or any other number used for showing more than one of a particular chemical species in a chemical equation, is called a coefficient. Now we have H2(g) + O2(g) →2H2O(l).
Is this right? Once again, it is time to analyze the equation, to see if the number of atoms on the left equals the number on the right. Such analysis can be done in a number of ways: for instance, by symbolizing each chemical species as a circle with chemical symbols for each element in it. Thus a single water molecule would be shown as a circle containing two H's and one O.
Whatever the method used, analysis will reveal that the problem of the oxygen imbalance has been solved: now there are two oxygens on the left, and two on the right. But solving that problem has created another, because now there are four hydrogen atoms on the right, as compared with two on the left. Obviously, another coefficient of 2 is needed, this time in front of the hydrogen molecule on the left. The changed equation is thus written as: 2H2(g) + O2(g) → 2H2O(l). Now, finally, the equation is correct.
THE PROCESS OF BALANCING CHEMICAL EQUATIONS.
What we have done is to balance an unbalanced equation. An unbalanced equation is one in which the numbers of atoms on the left are not the same as the number of atoms on the right. Though an unbalanced equation is incorrect, it is sometimes a necessary step in the process of finding the balanced equation—one in which the number of atoms in the reactants and those in the product are equal.
In writing and balancing a chemical equation, the first step is to ascertain the identities, by formula, of the chemical species involved, as well as their states of matter. After identifying the reactants and product, the next step is to write an unbalanced equation. After that, the unbalanced equation should be subjected to analysis, as demonstrated above.
The example used, of course, involves a fairly simple substance, but often, much more complex molecules will be part of the equation. In performing analysis to balance the equation, it is best to start with the most complex molecule, and determine whether the same numbers and proportions of elements appear in the product or products. After the most complicated molecule has been dealt with, the second-most complex can then be addressed, and so on.
Assuming the numbers of atoms in the reactant and product do not match, it will be necessary to place coefficients before one or more chemical species. After this has been done, the equation should again be checked, because as we have seen, the use of a coefficient to straighten out one discrepancy may create another. Note that only coefficients can be changed; the formulas of the species themselves (assuming they were correct to begin with) should not be changed.
After the equation has been fully balanced, one final step is necessary. The coefficients must be checked to ensure that the smallest integers possible have been used. Suppose, in the above exercise, we had ended up with an equation that looked like this: 12H2(g) + 6O2(g) →12H2O(l). This is correct, but not very "clean." Just as a fraction such as 12/24 needs to be reduced to its simplest form, 1/2, the same is true of a chemical equation. The coefficients should thus always be the smallest number that can be used to yield a correct result.
Types of Chemical Reactions
Note that in chemical equations, one of the symbols used is (aq), which indicates a chemical species that has been dissolved in water—that is, an aqueous solution. The fact that this has its own special symbol indicates that aqueous solutions are an important part of chemistry. Examples of reactions in aqueous solutions are discussed, for instance, in the essays on Acid-Base Reactions; Chemical Equilibrium; Solutions.
Another extremely important type of reaction is an oxidation-reduction reaction. Sometimes called a redox reaction, an oxidation-reduction reaction occurs during the transfer of electrons. The rusting of iron is an example of an oxidation-reduction reaction; so too is combustion. Indeed, combustion reactions—in which oxygen produces energy so rapidly that a flame or even an explosion results—are an important subset of oxidation-reduction reactions.
REACTIONS THAT FORM WATER, SOLIDS, OR GASES.
Another type of reaction is an acid-base reaction, in which an acid is mixed with a base, resulting in the formation of water along with a salt.
Other reactions form gases, as for instance when water is separated into hydrogen and oxygen. Similarly, heating calcium carbonate (lime-stone) to make calcium oxide or lime for cement also yields gaseous carbon dioxide: CaCO3(s) + heat →CaO(s) + CO2(g).
There are also reactions that form a solid, such as the one mentioned much earlier, in which solid BaCrO4(s) is formed. Such reactions are called precipitation reactions. But this is also a reaction in an aqueous solution, and there is another product: 2KNO3(aq), or potassium nitrate dissolved in water.
SINGLE AND DOUBLE DISPLACEMENT.
The reaction referred to in the preceding paragraph also happens to be an example of another type of reaction, because two anions (negatively charged ions) have been exchanged. Initially K+ and CrO42− were together, and these reacted with a compound in which Ba2+ and NO3− were combined. The anions changed places, an instance of a double-displacement reaction, which is symbolized thus: AB + CD →AD + CB.
It is also possible to have a single-displacement reaction, in which an element reacts with a compound, and one of the elements in the compound is released as a free element. This can be represented symbolically as A + BC →B + AC. Single-displacement reactions often occur with metals and with halogens. For instance, a metal(A) reacts with an acid (BC) to produce hydrogen (B) and a salt (AC).
COMBINATION AND DECOMPOSITION.
A synthesis, or combination, reaction is one in which a compound is formed from simpler materials—whether those materials be elements or simple compounds. A basic example of this is the reaction described earlier in relation to chemical equations, when hydrogen and oxygen combine to form water. On the other hand, some extremely complex substances, such as the polymers in plastics and synthetic fabrics such as nylon, also involve synthesis reactions.
When iron rusts (in other words, it oxidizes in the presence of air), this is both an oxidation-reduction and a synthesis reaction. This also represents one of many instances in which the language of science is quite different from everyday language. If a piece of iron—say, a railing on a balcony—rusts due to the fact that the paint has peeled off, it would seem from an unscientific standpoint that the iron has "decomposed." However, rust (or rather, metal oxide) is a more complex substance than the iron, so this is actually a synthesis or combination reaction.
A true decomposition reaction occurs when a compound is broken down into simpler compounds, or even into elements. When water is subjected to electrolysis such that the hydrogen and oxygen are separated, this is a decomposition reaction. The fermentation of grapes to make wine is also a form of decomposition.
And then, of course, there are the processes that normally come to mind when we think of "decomposition": the decay or rotting of a formerly living thing. This could also include the decay of something, such as an item of food, made from a formerly living thing. In such instances, an organic substance is eventually broken down through a number of processes, most notably the activity of bacteria, until it ultimately becomes carbon, nitrogen, oxygen, and other elements that are returned to the environment.
SOME OTHER PARAMETERS.
Obviously, there are numerous ways to classify chemical reactions. Just to complicate things a little more, they can also be identified as to whether they produce heat (exothermic) or absorb heat (endothermic). Combustion is clearly an example of an exothermic reaction, while an endothermic reaction can be exemplified by the process that takes place in a cold pack. Used for instance to prevent swelling on an injured ankle, a cold pack contains an ampule that absorbs heat when broken.
Still another way to identify chemical reactions is in terms of the phases of matter involved. We have already seen that some reactions form gases, some solids, and some yield water as one of the products. If reactants in one phase of matter produce a substance or substances in the same phase (liquid, solid, or gas), this is called a homogeneous reaction. On the other hand, if the reactants are in different phases of matter, or if they produce a substance or substances that are in a different phase, this is called a heterogeneous reaction.
An example of a homogeneous reaction occurs when gaseous nitrogen combines with oxygen, also a gas, to produce nitrous oxide, or "laughing gas." Similarly, nitrogen and hydrogen combine to form ammonia, also a gas. But when hydrogen and oxygen form water, this is a heterogeneous reaction. Likewise, when a metal undergoes an oxidation-reduction reaction, a gas and a solid react, resulting in a changed form of the metal, along with the production of new gases.
Finally, a chemical reaction can be either reversible or irreversible. Much earlier, we described how wood experiences combustion, resulting in the production of ash. This is clearly an example of an irreversible reaction. The atoms in the wood and the air that oxidized it have not been destroyed, but it would be impossible to put the ash back together to make a piece of wood. By contrast, the formation of water by hydrogen and oxygen is reversible by means of electrolysis.
KEEPING IT ALL STRAIGHT.
The different classifications of reactions discussed above are clearly not mutually exclusive; they simply identify specific aspects of the same thing. This is rather like the many physical characteristics that describe a person: gender, height, weight, eye color, hair color, race, and so on. Just because someone is blonde, for instance, does not mean that the person cannot also be brown-eyed; these are two different parameters that are more or less independent.
On the other hand, there is some relation between these parameters in specific instances: for example, females over six feet tall are rare, simply because women tend to be shorter than men. But there are women who are six feet tall, or even considerably taller. In the same way, it is unlikely that a reaction in an aqueous solution will be a combustion reaction—yet it does happen, as for instance when potassium reacts with water.
Studying Chemical Reactions
Several aspects or subdisciplines of chemistry are brought to bear in the study of chemical reactions. One is stoichiometry (stoy-kee-AH-muh-tree), which is concerned with the relationships among the amounts of reactants and products in a chemical reaction. The balancing of the chemical equation for water earlier in this essay is an example of basic stoichiometry.
Chemical thermodynamics is the area of chemistry that addresses the amounts of heat and other forms of energy associated with chemical reactions. Thermodynamics is also a branch of physics, but in that realm, it is concerned purely with physical processes involving heat and energy. Likewise physicists study kinetics, associated with the movement of objects. Chemical kinetics, on the other hand, involves the study of the collisions between molecules that produce a chemical reaction, and is specifically concerned with the rates and mechanisms of reaction.
SPEEDING UP A CHEMICAL REACTION.
Essentially, a chemical reaction is the result of collisions between molecules. According to this collision model, if the collision is strong enough, it can break the chemical bonds in the reactants, resulting in a rearrangement of the atoms to form products. The more the molecules collide, the faster the reaction. Increase in the numbers of collisions can be produced in two ways: either the concentrations of the reactants are increased, or the temperature is increased. In either case, more molecules are colliding.
Increases of concentration and temperature can be applied together to produce an even faster reaction, but rates of reaction can also be increased by use of a catalyst, a substance that speeds up the reaction without participating in it either as a reactant or product. Catalysts are thus not consumed in the reaction. One very important example of a catalyst is an enzyme, which speeds up complex reactions in the human body. At ordinary body temperatures, these reactions are too slow, but the enzyme hastens them along. Thus human life can be said to depend on chemical reactions aided by a wondrous form of catalyst.
WHERE TO LEARN MORE
Bender, Hal. "Chemical Reactions." Clackamas Community College (Web site). <http://dl.clackamas.cc.or.us/ch104-01/chemical.htm> (June 3, 2001).
"Catalysis, Separations, and Reactions." Accelrys (Web site). <http://www.accelrys.com/chemicals/catalysis/> (June 3, 2001).
Goo, Edward. "Chemical Reactions" (Web site). <http://www-classes.usc.edu/engr/ms/125/MDA125/reactions/> (June 3, 2001).
Knapp, Brian J. Oxidation and Reduction. Illustrated by David Woodroffe. Danbury, CT: Grolier Educational, 1998.
Knapp, Brian J. Energy and Chemical Change. Illustrated by David Woodroffe. Danbury, CT: Grolier Educational, 1998.
Newmark, Ann. Chemistry. New York: Dorling Kindersley, 1993.
"Periodic Table: Chemical Reaction Data." WebElements (Web site). <http://www.webelements.com/webelements/elements/text/periodic-table/chem.html>(June 3, 2001).
Richards, Jon. Chemicals and Reactions. Brookfield, CT: Copper Beech Books, 2000.
"Types of Chemical Reactions" (Web site). <http://www.usoe.k12.ut.us/curr/science/sciber00/8th/matter/sciber/chemtype.htm> (June 3, 2001).
Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.
A chemical reaction in which an acid is mixed with a base, resulting in the formation of water along with a salt.
A mixture of water and any substance that is solvent in it.
A chemical equation in which the numbers of atoms in the reactants and those in the product areequal. In the course of balancing an equation, coefficients may need to be applied to one or more of the chemical species involved; however, the actual formulas of the species cannot be changed.
A substance that speeds upa chemical reaction without participating in it either as a reactant or product. Catalysts are thus not consumed in the reaction.
A representation of a chemical reaction in which the chemical symbols on the left stand for the reactants, and those on the right for the product or products. On paper, a chemical equation looks much like a mathematical one; however, instead of an equals sign, a chemical equation uses an arrow to show the direction of the reaction.
the study of the rate at which chemical reactions occur.
A process whereby the chemical properties of a substance are changed by a rearrangement of the atoms in the substance.
A generic term used for any substance studied in chemistry—whether it be an element, compound, mixture, atom, molecule, ion, and so forth.
The study of the amounts of heat and other forms of energy associated with chemical reactions.
A number used to indicate the presence of more than one unit—typically, more than one molecule—of a chemical species in a chemical equation. For instance, 2H2O indicates two water molecules. (Note that 1 is never used as a coefficient.)
The theory that chemical reactions are the result of collisions between molecules that are strong enough to break bonds in the reactants, resulting in a rearrangement of atoms to form a product or products.
A chemical reaction in which a compound is broken down into simpler compounds, or even into elements. This is the opposite of a synthesis or combination reaction.
A chemical reaction in which the partners in two compounds changeplaces. This can be symbolized as AB + CD →AD + CB. Compare single-displacement reaction.
A term describing a chemical reaction in which heat is absorbed or consumed.
A term describing a chemical reaction in which heat is produced.
A term describing a chemical reaction in which the reactants are in different phases of matter (liquid, solid, or gas), or one in which the product is in a different phase from that of the reactants.
A term describing a chemical reaction in which the reactants and the product are all in the same phase of matter (liquid, solid, or gas).
A chemical reaction involving the transfer of electrons.
A chemical reaction in which a solid isformed.
The substance or substances that result from a chemical reaction.
A substance that interacts with another substance in a chemical reaction, resulting in the formation of aproduct.
A chemical reaction in which an element reacts with a compound, and one of the elements in the compound is released as a free element. This can be represented symbolically as A + BC →B + AC. Compare double-displacement reaction.
The study of the relationships among the amounts of reactants and products in a chemical reaction. Producing a balanced equation requires application of stoichiometry (pronounced "stoy-kee-AH-muh-tree").
SYNTHESIS OR COMBINATIONREACTION:
A chemical reaction in which a compound is formed from simpler materials—either elements or simple compounds. It is the opposite of a decomposition reaction.
A chemical equation in which the sum of atoms in the product or products does not equal the sum of atoms in the reactants. Initial observations of a chemical reaction usually produce an unbalanced equation, which needs to be analyzed and corrected (by the use of coefficients) to yield a balancedequation.
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A chemical reaction is a process in which one set of chemical substances (reactants) is converted into another (products). It involves making and breaking chemical bonds and the rearrangement of atoms. Chemical reactions are represented by balanced chemical equations, with chemical formulas symbolizing reactants and products. For specific chemical reactants, two questions may be posed about a possible chemical reaction. First, will a reaction occur? Second, what are the possible products if a reaction occurs? This
entry will focus only on the second question. The most reliable answer is obtained by conducting an experiment—mixing the reactants and then isolating and identifying the products. We can also use periodicity, since elements within the same group in the Periodic Table undergo similar reactions. Finally, we can use rules to help predict the products of reactions, based on the classification of inorganic chemical reactions into four general categories: combination, decomposition, single-displacement, and double-displacement reactions.
Reactions may also be classified according to whether the oxidation number of one or more elements changes. Those reactions in which a change in oxidation number occurs are called oxidation–reduction reactions . One element increases its oxidation number (is oxidized), while the other decreases its oxidation number (is reduced).
In combination reactions, two substances, either elements or compounds, react to produce a single compound. One type of combination reaction involves two elements. Most metals react with most nonmetals to form ionic compounds. The products can be predicted from the charges expected for cations of the metal and anions of the nonmetal. For example, the product of the reaction between aluminum and bromine can be predicted from the following charges: 3+ for aluminum ion and 1− for bromide ion. Since there is a change in the oxidation numbers of the elements, this type of reaction is an oxidation–reduction reaction:
2Al (s ) + 3Br2 (g ) → 2AlBr3 (s )
Similarly, a nonmetal may react with a more reactive nonmetal to form a covalent compound. The composition of the product is predicted from the common oxidation numbers of the elements, positive for the less reactive and negative for the more reactive nonmetal (usually located closer to the upper right side of the Periodic Table). For example, sulfur reacts with oxygen gas to form gaseous sulfur dioxide:
S8 (s ) + 8O2 (g ) → 8SO2 (g )
A compound and an element may unite to form another compound if in the original compound, the element with a positive oxidation number has an accessible higher oxidation number. Carbon monoxide, formed by the burning of hydrocarbons under conditions of oxygen deficiency, reacts with oxygen to form carbon dioxide:
2CO (g ) + O2 (g ) → 2CO2 (g )
The oxidation number of carbon changes from +2 to +4 so this reaction is an oxidation–reduction reaction.
Two compounds may react to form a new compound. For example, calcium oxide (or lime) reacts with carbon dioxide to form calcium carbonate (limestone):
CaO (s ) + CO2 (g ) → CaCO3 (s )
When a compound undergoes a decomposition reaction, usually when heated, it breaks down into its component elements or simpler compounds. The products of a decomposition reaction are determined largely by the identity of the anion in the compound. The ammonium ion also has characteristic decomposition reactions.
A few binary compounds decompose to their constituent elements upon heating. This is an oxidation–reduction reaction since the elements undergo a change in oxidation number. For example, the oxides and halides of noble metals (primarily Au, Pt, and Hg) decompose when heated. When red solid mercury(II) oxide is heated, it decomposes to liquid metallic mercury and oxygen gas:
2HgO (s ) → 2Hg (l ) + O2 (g )
Some nonmetal oxides, such as the halogen oxides, also decompose upon heating:
2Cl2O5 (g ) → 2Cl2 (g ) + 5O2 (g )
Other nonmetal oxides, such as dinitrogen pentoxide, decompose to an element and a compound:
2N2O5 (g ) → O2 (g ) + 4NO2 (g )
Many metal salts containing oxoanions decompose upon heating. These salts either give off oxygen gas, forming a metal salt with a different nonmetal anion, or they give off a nonmetal oxide, forming a metal oxide. For example, metal nitrates containing Group 1A or 2A metals or aluminum decompose to metal nitrites and oxygen gas:
Mg(NO3)2 (s ) → Mg(NO2)2 (s ) + O2 (g )
All other metal nitrates decompose to metal oxides, along with nitrogen dioxide and oxygen:
2Cu(NO3)2 (s ) → 2CuO (s ) + 4NO2 (g ) + O2 (g )
Salts of the halogen oxoanions decompose to halides and oxygen upon heating:
2KBrO3 (s ) → 2KBr (s ) + 3O2 (g )
Carbonates, except for those of the alkali metals, decompose to oxides and carbon dioxide.
CaCO3 (s ) → CaO (s ) + CO2 (g )
A number of compounds—hydrates, hydroxides, and oxoacids—that contain water or its components lose water when heated. Hydrates, compounds that contain water molecules, lose water to form anhydrous compounds, free of molecular water.
CaSO4 · 2H2O (s ) → CaSO4 (s ) + 2H2O (g )
Metal hydroxides are converted to metal oxides by heating:
2Fe(OH)3 (s ) → Fe2O3 (s ) + 3H2O (g )
Most oxoacids lose water until no hydrogen remains, leaving a nonmetal oxide:
H2SO4 (l ) → H2O (g ) + SO3 (g )
Oxoanion salts that contain hydrogen ions break down into the corresponding oxoanion salts and oxoacids:
Ca(HSO4)2 (s ) → CaSO4 (s ) + H2SO4 (l )
Finally, some ammonium salts undergo an oxidation–reduction reaction when heated. Common salts of this type are ammonium dichromate, ammonium permanganate, ammonium nitrate, and ammonium nitrite. When these salts decompose, they give off nitrogen gas and water.
(NH4)2Cr2O7 (s ) → Cr2O3 (s ) + 4H2O (g ) + N2 (g )
2NH4NO3 (s ) → 2N2 (g ) + 4H2O (g ) + O2 (g )
Ammonium salts, which do not contain an oxidizing agent, lose ammonia gas upon heating:
(NH4)2SO4 (s ) → 2NH3 (g ) + H2SO4 (l )
In a single-displacement reaction, a free element displaces another element from a compound to produce a different compound and a different free element. A more active element displaces a less active element from its compounds. These are all oxidation–reduction reactions. An example is the thermite reaction between aluminum and iron(III) oxide:
2Al (s ) + Fe2O3 (s ) → Al2O3 (s ) + 2Fe (l )
The element displaced from the compound is always the more metallic element—the one nearer the bottom left of the Periodic Table. The displaced element need not always be a metal, however. Consider a common type of single-displacement reaction, the displacement of hydrogen from water or from acids by metals.
The very active metals react with water. For example, calcium reacts with water to form calcium hydroxide and hydrogen gas. Calcium metal has an oxidation number of 0, whereas Ca2+ in Ca(OH)2 has an oxidation number of +2, so calcium is oxidized. Hydrogen's oxidation number changes from +1 to 0, so it is reduced.
Ca (s ) + 2H2O (l ) → Ca(OH)2 (aq ) + H2 (g )
Some metals, such as magnesium, do not react with cold water, but react slowly with steam:
Mg (s ) + 2H2O (g ) → Mg(OH)2 (aq ) + H2 (g )
Still less active metals, such as iron, do not react with water at all, but react with acids.
Fe (s ) + 2HCl (aq ) → FeCl2 (aq ) + H2 (g )
Metals that are even less active, such as copper, generally do not react with acids.
To determine which metals react with water or with acids, we can use an activity series (see Figure 1), a list of metals in order of decreasing activity. Elements at the top of the series react with cold water. Elements above hydrogen in the series react with acids; elements below hydrogen do not react to release hydrogen gas.
The displacement of hydrogen from water or acids is just one type of single-displacement reaction. Other elements can also be displaced from their compounds. For example, copper metal reduces aqueous solutions of ionic silver compounds, such as silver nitrate, to deposit silver metal. The copper is oxidized.
Cu (s ) + 2AgNO3 (aq ) → Cu(NO3)2 (aq ) + 2Ag (s )
The activity series can be used to predict which single-displacement reactions will take place. The elemental metal produced is always lower in the activity series than the displacing element. Thus, iron could be displaced from FeCl2 by zinc metal but not by tin.
|K||These metals will displace hydrogen gas from water|
|Zn||These metals will displace hydrogen gas from acids|
|Hg||These metals will not displace hydrogen gas from water or acids|
Aqueous barium chloride reacts with sulfuric acid to form solid barium sulfate and hydrochloric acid:
BaCl2 (aq ) + H2SO4 (aq ) → BaSO4 (s ) + 2HCl (aq )
Sodium sulfide reacts with hydrochloric acid to form sodium chloride and hydrogen sulfide gas:
Na2S (aq ) + 2HCl (aq ) → 2NaCl (aq ) + H2S (g )
Potassium hydroxide reacts with nitric acid to form water and potassium nitrate:
KOH (aq ) + HNO3 (aq ) → H2O (l ) + KNO3 (aq )
These double-displacement reactions have two major features in common. First, two compounds exchange ions or elements to form new compounds. Second, one of the products is either a compound that will separate from the reaction mixture in some way (commonly as a solid or gas) or a stable covalent compound, often water.
Double-displacement reactions can be further classified as precipitation, gas formation, and acid–base neutralization reactions.
Precipitation reactions are those in which the reactants exchange ions to form an insoluble salt—one which does not dissolve in water. Reaction occurs when two ions combine to form an insoluble solid or precipitate. We predict whether such a compound can be formed by consulting solubility rules (see Table 1). If a possible product is insoluble, a precipitation reaction should occur.
A mixture of aqueous solutions of barium chloride and sodium sulfate contains the following ions: Ba2+ (aq ), Cl− (aq ), Na+ (aq ), and SO42− (aq ). According to solubility rules, most sulfate, sodium, and chloride salts are soluble. However, barium sulfate is insoluble. Since a barium ion and sulfate ion could combine to form insoluble barium sulfate, a reaction occurs.
|SOME SOLUBILITY RULES FOR INORGANIC SALTS IN WATER|
|Na+, K+, NH4+||Most salts of sodium, potassium, and ammonium ions are soluble.|
|NO3−||All nitrates are soluble.|
|SO42−||Most sulfates are soluble. Exceptions: BaSO4, SrSO4, PbSO4, CaSO4, Hg2SO4, and Ag2SO4.|
|Cl−, Br−, I−,||Most chlorides, bromides, and iodides are soluble. Exceptions: AgX, Hg2X2, PbX2, and HgI2.|
|Ag+||Silver salts, except AgNO3, are insoluble.|
|O2−, OH−||Oxides and hydroxides are insoluble. Exceptions: NaOH, KOH, NH4OH, Ba(OH)2, and Ca(OH)2 (somewhat soluble).|
|S2−||Sulfides are insoluble. Exceptions: salts of Na+, K+, NH4+ and the alkaline earth metal ions.|
|CrO42−||Most chromates are insoluble. Exceptions: salts of K+, Na+, NH4+, Mg2+, Ca2+, Al3+, and Ni2+.|
|CO32−, PO43−, SO32−, SiO32−||Most carbonates, phosphates, sulfites, and silicates are insoluble. Exceptions: salts of K+, Na+, and NH4+.|
BaCl2 (aq ) + Na2SO4 (aq ) → BaSO4 (s ) + 2NaCl (aq )
A double-displacement reaction should also occur if an insoluble gas is formed. All gases are soluble in water to some extent, but only a few gases [HCl (g ) and NH3 (g )] are highly soluble. All other gases, generally binary covalent compounds, are sufficiently insoluble to provide a driving force if they are formed as a reaction product. For example, many sulfide salts will react with acids to form gaseous hydrogen sulfide:
ZnS (s ) + 2HCl (aq ) → ZnCl2 (aq ) + H2S (g )
Insoluble gases are often formed by the breakdown of an unstable double-displacement reaction product. For example, carbonates react with acids to form carbonic acid (H2CO3), an unstable substance that readily decomposes into water and carbon dioxide. Calcium carbonate reacts with hydrochloric acid to form calcium chloride and carbonic acid:
CaCO3 (s ) + 2HCl (aq ) → CaCl2 (aq ) + H2CO3 (aq )
Carbonic acid decomposes into water and carbon dioxide:
H2CO3 (aq ) → H2O (l ) + CO2 (g )
The net reaction is:
CaCO3 (s ) + 2HCl (aq) → CaCl2 (aq ) + H2O (l ) + CO2 (g )
Sulfites react with acids in a similar manner to release sulfur dioxide.
Acid-Base Neutralization Reactions
A neutralization reaction is a double-displacement reaction of an acid and a base. Acids are compounds that can release hydrogen ions; bases are compounds that can neutralize acids by reacting with hydrogen ions. The most common bases are hydroxide and oxide compounds of the metals. Normally, an acid reacts with a base to form a salt and water. Neutralization reactions occur because of the formation of the very stable covalent water molecule, H2O, from hydrogen and hydroxide ions.
HCl (aq ) + NaOH (aq ) → NaCl (aq ) + H2O (l )
Recognizing the pattern of reactants (element or compound, and the number of each) allows us to assign a possible reaction to one of the described classes. Recognizing the class of reaction allows us to predict possible products with some reliability.
see also Acid-Base Chemistry; Solution Chemistry; Thermodynamics.
James P. Birk
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When a chemical reaction occurs, at least one product is formed that is different from the substances present before the change occurred. As an example, it is possible to pass an electric current through a sample of water and obtain a mixture of oxygen and hydrogen gases. That change is a chemical reaction because neither oxygen nor hydrogen were present as elements before the change took place.
Any chemical change involves two sets of substances: reactants and products. A reactant is an element or compound present before a chemical change takes place. In the example above, only one reactant was present: water. A product is an element or compound formed as a result of the chemical reaction. In the preceding example, both hydrogen and oxygen are products of the reaction.
Chemical reactions are represented by means of chemical equations. A chemical equation is a symbolic statement that represents the changes that occur during a chemical reaction. The statement consists of the symbols of the elements and the formulas of the products and reactants, along with other symbols that represent certain conditions present in the reaction. For example, the arrow (or yields) sign, *, separates the reactants from the products in a reaction. The chemical equation that represents the electrolysis of water is 2 H2O → 2 H2 + O2.
Types of chemical reactions
Most chemical reactions can be categorized into one of about five general types: synthesis, decomposition, single replacement, double replacement, and oxidation-reduction. A miscellaneous category is also needed for reactions that do not fit into one of these five categories.
Characteristics of each type.
Synthesis: Two substances combine to form one new substance:
In general: A + B → AB
2 Na + Cl2 → 2 NaCl or CaO + H2O → Ca(OH)2
Decomposition: One substance breaks down to form two new substances:
In general: AB → A + B
For example: 2 H2O → 2 H2 + O2
Single Replacement: An element and a compound react such that the element replaces one other element in the compound:
In general: A + BC → AC + B
For example: Mg + 2 HCl → MgCl2 + H2
Double Replacement: Two compounds react with each other in such a way that they exchange partners with each other:
In general: AB + CD → AD + CB
NaBr + HCl → NaCl + HBr
Oxidation-reduction: One or more elements in the reaction changes its oxidation state during the reaction: In general: A3+ → A6+
For example: Cr3+ → Cr6+
Energy changes and chemical kinetics
Chemical reactions are typically accompanied by energy changes. The equation for the synthesis of ammonia from its elements is N2 + 3 H2 → 2 NH3, but that reaction takes place only under very special conditions—namely at a high temperature and pressure and in the presence of a catalyst. Energy changes that occur during chemical reactions are the subject of a field of science known as thermodynamics.
In addition, chemical reactions are often a good deal more complex than a chemical equation might lead one to believe. For example, one can write the equation for the synthesis of hydrogen iodide from its elements, as follows: H2 + I2 → 2 HI. In fact, chemists know that this reaction does not take place in a single step. Instead, it occurs in a series of reactions in which hydrogen and iodine atoms react with each other one at a time. The final equation, H2 + I2 → 2 HI, is actually no more than a summary of the net result of all those reactions. The field of chemistry that deals with the details of chemical reactions is known as chemical kinetics.
"Reaction, Chemical." UXL Encyclopedia of Science. . Encyclopedia.com. (February 24, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/reaction-chemical
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chemical reaction, process by which one or more substances may be transformed into one or more new substances. Energy is released or is absorbed, but no loss in total molecular weight occurs. When, for example, water is decomposed, its molecules, each of which consists of one atom of oxygen and two of hydrogen, are broken down; the hydrogen atoms then combine in pairs to form hydrogen molecules and the oxygen atoms to form oxygen molecules. In a chemical reaction, substances lose their characteristic properties. Water, for example, a liquid which neither burns nor supports combustion, is decomposed to yield flammable hydrogen and combustion-supporting oxygen. In some reactions heat is given off (exothermic reactions), and in others heat is absorbed (endothermic reactions). Furthermore, the new substances formed differ from the original substances in the energy they contain. Chemical reactions are classified according to the kind of change that takes place. When a compound, which consists of two or more elements or groups of elements, is broken down into its constituents, the reaction is called simple decomposition. When two compounds react with one another to form two new compounds, the reaction is called double decomposition. In so-called replacement reactions the place of one of the elements in a compound is taken by another element reacting with the compound. When elements combine to form a compound, the reaction is termed chemical combination. Oxidation and reduction reactions are extremely important. Reversible reactions are those in which the chemical change taking place may be paralleled by another change back to the original substances. The rates at which chemical reactions proceed depend upon various factors, e.g., upon temperature, pressure, and the concentration of the substances involved and, sometimes, upon the use of a chemical called a catalyst. In some chemical reactions, such as that of photographic film, light is an important factor. The changes taking place in a chemical reaction are represented by a chemical equation. An element's activity, i.e., its tendency to enter into compounds, varies from one element to another.
"chemical reaction." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (February 24, 2018). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/chemical-reaction
"chemical reaction." The Columbia Encyclopedia, 6th ed.. . Retrieved February 24, 2018 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/chemical-reaction
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"chemical reaction." A Dictionary of Biology. . Encyclopedia.com. (February 24, 2018). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/chemical-reaction
"chemical reaction." A Dictionary of Biology. . Retrieved February 24, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/chemical-reaction
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"chemical reaction." World Encyclopedia. . Encyclopedia.com. (February 24, 2018). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/chemical-reaction
"chemical reaction." World Encyclopedia. . Retrieved February 24, 2018 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/chemical-reaction
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chemical reaction: see chemical reaction.
"reaction, chemical." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (February 24, 2018). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/reaction-chemical
"reaction, chemical." The Columbia Encyclopedia, 6th ed.. . Retrieved February 24, 2018 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/reaction-chemical