heme4

Table 7.3	Some Symbols Used in Writtng Eąuations
Symbol	Meaning
+	Separates two or morę formulas
	Reaets to form products
A	The reactants are heated
CO	Solid
(0	Liąuid
(8)	Gas
<*?)	Aąueous

§ +	• -	A ' J
2H,(g) +	1 w fN O	2H,0(«)
n	•	• A H w
m #	•	# A # ^w
H	0	H 0

Reactant atoms - Product atoms

Chemical Eąuations

Hydrogen and nitrogen react to form ammonia, NH₃, 3H₂(g) + N₂(g) -

Sometimes, as in the charcoal case, the formulas in an eąuation may include letters, in parentheses, that give the physical State of the substanees as solid (5-), liąuid (I), or gas (g). If a substance is dissolved in water, it is an aqueous {aą) solution. Table 7.3 summarizes some of the symbols used in eąuations.

When a reaction takes place, the bonds between the atoms of the reactants are broken and new bonds are formed to give the products. In any Chemical reaction, there must be the same number of atoms of each element present in the original substanees as there are in the new substanees. Atoms cannot be gained, lost, or changed into other types of atoms during a reaction. There-fore, a reaction must be written as a balanced eąuation, which shows the same number of atoms for each element on both sides of the arrow. Let’s see if the preceding eąuation we wrote for burning carbon is balanced:

C(s) + 0,(g) -	—- co2(g)
• :	^w •
c 0	c 0

Reactant atoms = Prodact atoms

The answer is yes: this eąuation is balanced because there is one carbon atom and two oxygen atoms on each side of the arrow.

Now consider the reaction in which hydrogen reaets with oxygen to form water. First we write the formulas of the reactants and products:

H₂(g) + 0₂(g)-* H₂0(g)

Is the eąuation balanced? The answer is no; the eąuation is not balanced. There are two oxygen atoms to the left of the arrow, but only one to the right. Thus, the atoms on the left side do not match the atoms on the right side. To balance this eąuation, we place whole numbers called coefficients in front of the formulas. First we write a coefficient of 2 in front of the H₂0 formula in order to have two oxygen atoms. Now the product has four hydrogen atoms, which means we must also write a coefficient of 2 in front of the formula H₂ in the reactants. Now the number of hydrogen atoms and oxygen atoms is the same in the reactants as in the products. The eąuation is balanced.

Sannple Problem 7.3

- 2NH₃(g)

Ammonia

a.    What are the coefficients in the eąuation?

b.    How many atoms of each element are in the reactants and products of the eąuation?

?.3 Balancing a Chemical Equation

We have seen that a Chemical eąuation must be balanced. In many cases, we can use a method of trial and error to balance an eąuation. To demonstrate the process, let us balance the reaction of the gas methane, CH4, with oxygen to produce carbon dioxide and water. This is the principal reaction that occurs in the flame of a laboratory burner or a gas stove.