CHEMICAL REACTIONS
Chemical reaction, a process in which
one or more substances, the reactants, are converted to one or more different
substances, the products. Substances are either chemical elements or compounds. A chemical reaction rearranges the
constituent atoms of the reactants to create different substances as
products.
Chemical reactions are an integral part of
technology, of culture, and indeed of life itself. Burning fuels, smelting iron, making glass and pottery, brewing beer, and making wine and cheese are among many examples of activities incorporating
chemical reactions that have been known and used for thousands of years.
Chemical reactions abound in the geology of Earth, in the atmosphere and oceans, and in a vast array of complicated processes that occur
in all living systems.
Chemical reactions must be distinguished from
physical changes. Physical changes include changes of state, such as ice melting to water and water evaporating to vapour. If a physical change
occurs, the physical properties of a substance will change, but its chemical
identity will remain the same. No matter what its physical state, water (H2O)
is the same compound, with each molecule composed of two atoms of hydrogen and one atom of oxygen. However, if water, as ice, liquid, or vapour,
encounters sodium metal (Na), the atoms will be redistributed to give the
new substances molecular hydrogen (H2) and sodium hydroxide (NaOH).
By this, we know that a chemical change or reaction has occurred.
The concept of a chemical reaction dates back
about 250 years. It had its origins in early experiments that classified
substances as elements and compounds and in theories that explained these
processes. Development of the concept of a chemical reaction had a primary role
in defining the science of chemistry as it is known today.
The first substantive studies in this area
were on gases. The identification of oxygen in the 18th century by
Swedish chemist Carl Wilhelm Scheele and English clergyman Joseph Priestley had particular significance. The influence
of French chemist Antoine-Laurent Lavoisier was especially notable, in
that his insights confirmed the importance of quantitative measurements of
chemical processes. In his book Traité élémentaire de chimie (1789; Elementary
Treatise on Chemistry), Lavoisier identified 33 “elements”—substances not
broken down into simpler entities. Among his many discoveries, Lavoisier
accurately measured the weight gained when elements were oxidized, and he
ascribed the result to the combining of the element with oxygen. The concept of chemical reactions involving the
combination of elements clearly emerged from his writing, and his approach led
others to pursue experimental chemistry as a quantitative science.
The other occurrence of historical
significance concerning chemical reactions was the development of atomic theory. For this, much credit goes to English
chemist John Dalton, who postulated his atomic theory early in
the 19th century. Dalton maintained that matter is composed of small,
indivisible particles, that the particles, or atoms, of each element were unique, and that chemical
reactions were involved in rearranging atoms to form new substances. This view
of chemical reactions accurately defines the current subject. Dalton’s theory
provided a basis for understanding the results of earlier experimentalists,
including the law of conservation of matter (matter is neither created nor
destroyed) and the law of constant composition (all samples of a substance have
identical elemental compositions).
Thus, experiment and theory, the two
cornerstones of chemical science in the modern world, together defined the
concept of chemical reactions. Today experimental chemistry provides
innumerable examples, and theoretical chemistry allows an understanding of
their meaning.
Basic concepts of chemical reactions
When making a new substance from other
substances, chemists say either that they carry out a synthesis or that they synthesize the new material.
Reactants are converted to products, and the process is symbolized by a chemical equation. For example, iron (Fe) and sulfur (S) combine to form iron sulfide (FeS).
Fe(s)
+ S(s) → FeS(s)
The plus sign indicates that iron reacts with
sulfur. The arrow signifies that the reaction “forms” or “yields” iron sulfide,
the product. The state of matter of reactants and products is designated with
the symbols (s) for solids, (l) for liquids, and (g) for gases
Chemists ordinarily work with
weighable quantities of elements and compounds.
For example, in the iron-sulfur equation the symbol Fe represents 55.845 grams
of iron, S represents 32.066 grams of sulfur, and FeS represents 87.911 grams
of iron sulfide. Because matter is not created or destroyed in a chemical
reaction, the total mass of reactants is the same as the total mass of
products. If some other amount of iron is used, say, one-tenth as much (5.585
grams), only one-tenth as much sulfur can be consumed (3.207 grams), and only
one-tenth as much iron sulfide is produced (8.791 grams). If 32.066 grams of
sulfur were initially present with 5.585 grams of iron, then 28.859 grams of
sulfur would be left over when the reaction was complete.
The reaction of methane (CH4,
a major component of natural gas)
with molecular oxygen (O2)
to produce carbon dioxide
(CO2) and water can be
depicted by the chemical equation
CH4(g) + 2O2(g)
→ CO2(g) + 2H2O(l)
Here another feature of chemical equations
appears. The number 2 preceding O2 and H2O is a stoichiometric
factor. (The number 1 preceding CH4 and CO2 is implied.)
This indicates that one molecule of methane reacts with two molecules of
oxygen to produce one molecule of carbon dioxide and two molecules of water.
The equation is balanced because the same number of atoms of each element
appears on both sides of the equation (here one carbon, four hydrogen, and four
oxygen atoms). Analogously with the iron-sulfur example, we can say that 16
grams of methane and 64 grams of oxygen will produce 44 grams of carbon dioxide
and 36 grams of water. That is, 80 grams of reactants will lead to 80 grams of
products.
The ratio of reactants and products in a
chemical reaction is called chemical stoichiometry. Stoichiometry depends on the fact that
matter is conserved in chemical processes, and calculations giving mass
relationships are based on the concept of the mole. One mole of any element or compound contains the same
number of atoms or molecules, respectively, as one mole of any other element or
compound. By international agreement, one mole of the most common isotope of carbon (carbon-12) has a mass of exactly 12 grams (this is
called the molar mass) and represents 6.02214179 × 1023 atoms (Avogadro’s number). One mole of iron contains 55.847 grams;
one mole of methane contains 16.043 grams; one mole of molecular oxygen is
equivalent to 31.999 grams; and one mole of water is 18.015 grams. Each of
these masses represents 6.0221 × 1023 molecules.
Energy plays a
key role in chemical processes. According to the modern view of chemical
reactions, bonds between
atoms in the reactants must be broken, and the atoms or pieces of molecules are
reassembled into products by forming new bonds. Energy is absorbed to break
bonds, and energy is evolved as bonds are made. In some reactions the energy
required to break bonds is larger than the energy evolved on making new bonds,
and the net result is the absorption of energy. Such a reaction is said to be
endothermic if the energy is in the form of heat. The opposite of endothermic is
exothermic; in an exothermic reaction, energy as heat is evolved. The more general terms exoergic
(energy evolved) and endoergic (energy required) are used when forms
of energy other than heat are involved.
The formation of slaked lime (calcium hydroxide, Ca(OH)2) when
water is added to lime (CaO) is exothermic.
CaO(s) + H2O (l) → Ca(OH)2(s)
This
reaction occurs when water is added to dry portland cement
to make concrete, and heat evolution
of energy as heat is evident because the mixture becomes warm.
Not
all reactions are exothermic (or exoergic). A few compounds, such as nitric oxide
(NO) and hydrazine (N2H4),
require energy input when they are formed from the elements. The decomposition
of limestone (CaCO3)
to make lime (CaO) is also an endothermic process; it is necessary to heat
limestone to a high temperature for this reaction to occur.
CaCO3(s) → CaO(s)
+ CO2(g)
The decomposition of water into its elements by the process of electrolysis
is another endoergic process. Electrical energy is used rather than heat energy
to carry out this reaction.
2 H2O(g) → 2 H2(g)
+ O2(g)
Classifying chemical reactions
Chemists classify reactions in a number of ways:
(a)
by the type of product,
in a chemical reaction, how to balance the number of elements in the reactants and products?
BalasHapusAn element-reaction-product table is used to find coefficients while balancing an equation representing a chemical reaction. Coefficients represent moles of a substance so that the amount of atoms produced is equal to the amount of atoms being reacted with. This is the common setup:
BalasHapusElement: it is all the elements that are in the reaction equation. Beneath the "element" section you list them all.
Reactant: is the numbers of each of the elements on the reactants side of the reaction equation.
Product: is the number of each element on the product side of the reaction equation.
The layout should eventually look like this, for a balanced reaction of baking soda and vinegar (HC2H3O2 + NaHCO3 = NaC2H3O2 + H2CO3)
Element Number of reactants Number of products
Hydrogen 5 5
Carbon 3 3
Oxygen 5 5
Sodium 1 1
From this, since the number of reactants for each element equals the number of products for each element, we can tell that each side is balanced in the equation.
Balancing
When a reaction equation is not balanced, it needs coefficients to show equality. Here is an example with the separation of natural gas from hydrochloric acid using magnesium.
Mg + HCl = MgCl2 + H2
Here is the element-reaction-product table:
Element Number of reactants Number of products zx,nZ<xn,AXn,an,AN,NAX<nA<
Hydrogen k 2
Chlorine 1 2
Magnesium 1 1
From this table we see that the number of hydrogen and chlorine atoms on the product's side are twice the amount of atoms on the reactant's side. Therefore, we add the coefficient "2" in front of the HCl on the products side, to get our equation to look like this:
Mg + 2HCl = MgCl2 + H2
and our table looks like this:
Element Number of reactants Number of products
Hydrogen 1 2 2
Chlorine 1 2 2
Magnesium 1 1
Because of the coefficients, the equation is balanced.
thank's for your coment,, i will try to understand it.
BalasHapusfrom the article I read, I get an explanation that to balance the chemical reaction, calculate the number of atoms of each element in both the reactants and products. Then determine the number, when multiplied by the number of atoms in the reactants or products, will make the number of atoms on both sides of the same arrow. These numbers are known as coefficients. Next check the equation by counting the number of atoms in the reactants and products. If the equation is balanced, the number of atoms on each side will be the same.
BalasHapus~ Element is all the elements in the equation.
BalasHapus~ Reactant is the amount of each element on the reactant side of the equation.
~ The product is the amount of each element on the product side of the equation.
If the equation is not balanced, the coefficients required to demonstrate equivalence.