Chemical Reactions
Chemical reactions are processes by which atoms
or molecules are redistributed, resulting in different substances with unique
properties. Many industries rely on large-scale chemical reactions to make
products, such as alloys, fertilizers, and building materials (including glass
and concrete), that are vital to modern life.
Chemical reactions are classified
into different categories according to the mechanics of the reactions. The
original elements or compounds involved in a chemical reaction are called
reactants, and the chemicals that result are called products.
Combination and Decomposition Reactions
Combination reactions follow the simple formula:
A + B = C, where A and B are elements or compounds that combine to form the new
compound C. For example, aluminum (Al) combines rapidly with molecular oxygen (O2)
to form aluminum oxide (Al2O3): 4Al + 3O2 → 2Al2O3
The numbers before each formula refer to the relative amounts of each element or
compound in the reaction. Aluminum oxide forms a protective layer over aluminum
products such as soda cans and foil, preventing further oxidation of the metal.
In contrast, iron is poorly protected by its oxide layer and subsequently rusts
with exposure to air. Another combination reaction occurs when sulfur (S) reacts
with molecular oxygen to form gaseous sulfur dioxide (SO2): S(s)
+ O2(g) → SO2(g). The letter subscripts in the equation
refer to the physical states of the reactants and product�s stands for
solid, g stands for gas, and an l would stand for liquid.
A decomposition reaction is the reverse of a
combination reaction. For example, when solid potassium chlorate (KClO3)
is heated, it decomposes into molecular oxygen and potassium chloride (KCl):
2KClO3(s) → 3O2(g) + 2KCl(s). The oxygen
product of this decomposition reaction is useful for applications such as
medical emergencies. Another decomposition reaction is the rapid decay when
exposed to light of silver iodide (AgI) into silver (Ag) and iodine (I): 2AgI →
2Ag + I2.
Oxidation-Reduction Reactions
Oxidation-reduction, or redox, reactions combine
chemicals wanting to gain electrons (or be reduced) with chemicals that are
willing to give up electrons (or be oxidized). For example, sodium (Na), with
its single loosely held valence electron, gives up its outer electron (or is
oxidized) by sulfur (S) to form sodium sulfide (Na2S): 2Na + S → Na2S.
In this redox reaction, two sodium atoms each give up an electron to fill the
sulfur atom�s outer shell. Each of the sodium atoms are subsequently oxidized to
form positive ions (Na+), while the sulfur atom is reduced to a
negative ion (S-2). These oppositely charged ions combine to form
sodium sulfide.
Acid-Base Reactions
An acid-base reaction occurs when a strong acid�a
substance capable of donating hydrogen ions (H+)�reacts with a strong
base�a substance capable of accepting hydrogen ions . Acid-base reactions produce water and a
salt. Salts are defined in chemistry as ionic compounds where the cation
(positive ion) is not H+ and the anion (negative ion) is not O2-
or OH-. For example, hydrochloric acid (HCl) reacts with sodium
hydroxide (NaOH) to produce sodium chloride (NaCl), which is a salt compound,
and water: HCl + NaOH → NaCl + H2O.
Displacement Reactions
Displacement reactions cause elements to displace
each other from a compound. For example, magnesium (Mg) displaces titanium (Ti)
in the following reaction: 2Mg + TiCl4 → Ti + 2MgCl2.
Manufacturers use this particular displacement reaction to extract titanium,
valued for its strength and light weight by the aerospace and other industries,
from the compound titanium tetrachloride (TiCl4).
Exchange Reactions
Exchange reactions are driven by compounds
wanting to exchange ions in order to form more stable products, namely acids or
salts. For example, titanium tetrachloride (TiCl4) reacts violently
with water as TiCl4 exchanges one titanium ion (Ti+4) for
every four hydrogen ions (H+) in the water: TiCl4(g)+ 2H2O(g)
→ 4HCl(g) + TiO2(s). This reaction produces hydrochloric
acid (HCl) and titanium dioxide (TiO2). Titanium dioxide typically
occurs in mineral deposits as an impure black substance. Chlorinating titanium
oxides from mineral compounds produces TiCl4. Because titanium
dioxide is needed as the major pigment in white paint, manufacturers use the
above exchange reaction to isolate titanium dioxide from TiCl4.
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