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The chemical equation is simply a representation of a chemical reaction in the chemist's shorthand. In a chemical equation, the substances originally present are called the reactants and the new substances being formed are called the products. In the chemical reaction, the reactants are on the left of the arrow and the products are on the right of the arrow. For example, when looking at the combination of hydrogen and oxygen into water, the water molecule would be the product on the right.
If looking at the decomposition of water, the hydrogen and oxygen would be the products and the water is the reactant. The arrow means yields and is used to separate the reactants from the products. It also indicates the direction of the reaction as illustrated below.
The single arrow in the above equation implies that the reaction will proceed in only one direction until it subsides or stops. If the reaction is reversible, that is, it can proceed left to right and right to left until it reaches an equilibrium, then a double arrow is used.
When writing an equation, always place the reactant on the left and the products on the right even in the case of a reversible reaction. A chemical equation represents not only the reaction, but also shows the number of atoms or molecules entering into and produced by the reaction. The formulas must be balanced correctly based on the valences of the constituent elements. Balancing Chemical
Equations The number of atoms or molecules of each substance is shown by the coefficients in the equation. Because atoms cannot be created or destroyed in a chemical reaction, a chemical equation must be balanced so that there are exactly the same number of atoms of each element on each side of the equation. Example: Explain the following
chemical equation.
Solution: This chemical equation
shows that iron reacts with water to form ferric oxide and hydrogen gas (the
vertical arrow t indicates a gas). This chemical equation also shows that for
every two atoms of iron that react, three molecules of water are used to form
one molecule of ferric oxide and three molecules of hydrogen gas. This is a
balanced chemical equation. There are two iron atoms on each side of the
equation; there are six hydrogen atoms on each side; and there are three oxygen
atoms on each side. There are no fixed rules
for balancing chemical equations. Learning how is a matter of practice. The
balancing of most equations can be accomplished by following the guidelines
explained below. Guidelines: a.Once the correct chemical formula for a compound is
written in an equation, do not modify it. b.Select the compound with the greatest number of atoms.
Then begin by balancing the element in that compound with the most atoms. There
must be the same number of atoms of an element on each side of the equation. As
a rule of thumb, this first element should not be hydrogen, oxygen, or a
polyatomic ion. c.Balance the atoms of each element in the compound by
placing the appropriate coefficient in front of
the chemical symbol or formula. d.Next, balance the polyatomic ions. In some cases, the
coefficient assigned in guideline 2 may have to be changed to balance the
polyatomic ion. e.Balance the hydrogen atoms next, then the oxygen atoms.
If these elements appear in the polyatomic ion it should not be necessary to
balance them again. f. All coefficients will be whole numbers. The
coefficients should be reduced to the lowest possible ratios. g. As simple as it sounds,
check off each element as it is accounted for since this will prevent double
inclusion or a missed atom. Example 1:
Solution 1: Starting with Fe203 (see guideline b),
write 2FeS2 + O2
Now there are 4 S atoms on the left side so balance the S
atoms by writing 2FeS2 + O2
Everything is balanced except the O. There are now 2 O atoms on the left and 110 atoms on the right. To get 110 atoms on the left write 2FeS2 + 5.502
This makes both sides of the equation balanced except the coefficients must be whole numbers (guideline f). To meet guideline f, multiply both sides by two which will bring the left side to a whole number of 02 molecules. Thus, the solution is 4FeS2 + 1102
Example 2: NH3 + CuO
Solution 2: Start with NH3 since there are two N atoms on the
right of the equation. To balance the N atoms write 2NH3 + CuO
Since the H appears in only the NH3 and H2O
and the NH3 has been balanced, the H2O will be balanced.
Write 2NH3 + CuO
Oxygen appears only in CuO and in H2O, and since
the H20 has been already been balanced write 2NH3 + 3CuO
That leaves the Cu to be balanced. Thus, the solution is 2NH3
+ 3CuO
Example 3: Na2CO3
+ Ca(OH)2
Solution 3: There are two Na atoms on
the left so start with the Na by writing Na2CO3
+ Ca(OH)2
By adding the 2, the
equation is now completely balanced. This equation illustrates that not all
equations are that hard to balance. Most chemical equations do
not indicate a number of important facts about the chemical reactions they
represent. Chemical equations do not necessarily describe the path by which the
substances reacting are converted to products. 2H2 + 02
- 2H20 The equation would seem to
imply that two molecules of hydrogen collide with one molecule of oxygen, and
two molecules of water are produced. The actual mechanism by which this reaction
takes place is much more complicated and involves a series of processes.
Chemical equations do not indicate the rate at which the reaction proceeds, or
even whether the reaction will occur in a finite time. In many cases, reactions
will occur only under a particular set of circumstances and then only at a
definite rate. Chemical equations do not show whether the reaction proceeds to
completion or, if incomplete, the extent of reaction. In most cases, the
substances that react never completely disappear; however, their concentration
may be exceedingly small. Reactions that do not go to completion are usually
represented in chemical equations by using double horizontal arrows
In the discussion of
chemical equations, emphasis is normally placed on the number of atoms or
molecules involved in the reaction. However, chemical equations are very
effective in representing chemical reactions on a macroscopic scale. Practical
chemical calculations involve very large numbers of atoms and molecules. The equation weight in
grams of a compound or element is defined as the gram molecular weight times
the number of molecules of the compound, as shown by the coefficients of the
chemical equation for the reaction. The sums of the equation weights on each
side of a chemical equation must be equal. Chemical calculations are based on
the fact that every fraction or multiple of the equation weights of substances
that react gives a corresponding fraction or multiple of the equation weights
of the products of the reaction. In other words, if 30 grams of a substance
that has an equation weight of 15 grams reacts with some amount of another
substance to form a product with an equation weight of 20 grams, then 40 grams
of that product will be formed. Example: How many grams of ferric
oxide will be formed if 27.9 grams of iron reacts with water according to the
following equation.
Solution: The equation weight of
iron equals the gram atomic weight of iron times the number of atoms shown
reacting in the equation, which is two. Using Table 2:
Because 27.9 g of iron
react, the fraction of the equation weight that reacts is:
Thus, 1/4 of the equation
weight of ferric oxide will be formed. The equation weight of
ferric oxide equals the gram molecular weight of ferric oxide times the number
of molecules shown formed in the equation, which is one. Using Table 2:
Thus, the amount of ferric
oxide formed is:
Summary The important information
from this chapter is summarized below.
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. In
general, a reaction will go to completion only if one or more of the products
is removed from the field of the reaction. This is often accomplished if one of
the products is a gas or is insoluble in the reaction mixture.
