# chemical equations

## coefficients

Lets return to the chemical reaction in which iron (Fe) combines with oxygen (O2 ) to form rust, or iron oxide (Fe2 O3 ). The equation for this reaction is: 4Fe+ 3O2 2Fe2 O3 This equation illustrates the use of coefficients to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula that shows how many atoms or molecules of the substance are involved in the reaction. From the equation for rusting, you can see that four atoms of iron combine with three molecules of oxygen to form two molecules of iron oxide. Q: Is the equation for the rusting reaction balanced? How can you tell? A: Yes, the equation is balanced. You can tell because there is the same number of each type of atom on both sides of the arrow. First count the iron atoms. There are four iron atoms in the reactants. There are also four iron atoms in the products (two in each of the two iron oxide molecules). Now count the oxygen atoms. There are six on each side of the arrow, confirming that the equation is balanced in terms of oxygen as well as iron.

## is it balanced

All chemical equations, like equations in math, must balance. This means that there must be the same number of each type of atom on both sides of the arrow. Thats because matter is always conserved in a chemical reaction. This is the law of conservation of mass. Look at the equation above for the reaction between carbon and oxygen in the formation of carbon dioxide. Count the number of atoms of each type. Are the numbers the same on both sides of the arrow? The answer is yes, so the equation is balanced.

## using chemical symbols and formulas

When scientists write chemical equations, they use chemical symbols and chemical formulas instead of names to represent reactants and products. Look at the chemical reaction illustrated in the Figure 1.1. In this reaction, carbon reacts with oxygen to produce carbon dioxide. Carbon is represented by the chemical symbol C. The chemical symbol for oxygen is O, but pure oxygen exists as diatomic (two-atom) molecules, represented by the chemical formula O2 . A molecule of the compound carbon dioxide consists of one atom of carbon and two atoms of oxygen, so carbon dioxide is represented by the chemical formula CO2 . Q: What is the chemical equation for this reaction? A: The chemical equation is: C + O2 CO2 Q: How have the atoms of the reactants been rearranged in the products of the reaction? What bonds have been broken, and what new bonds have formed? A: Bonds between the oxygen atoms in the oxygen molecule have been broken, and new bonds have formed between the carbon atom and the two oxygen atoms.

## what is a chemical equation

A chemical equation is a shorthand way to sum up what occurs in a chemical reaction. The general form of a chemical equation is: Reactants Products The reactants in a chemical equation are the substances that begin the reaction, and the products are the substances that are produced in the reaction. The reactants are always written on the left side of the equation and the products on the right. The arrow pointing from left to right shows that the reactants change into the products during the reaction. This happens when chemical bonds break in the reactants and new bonds form in the products. As a result, the products are different chemical substances than the reactants that started the reaction. Q: What is the general equation for the reaction in which iron rusts? A: Iron combines with oxygen to produce rust, which is the compound named iron oxide. This reaction could be represented by the general chemical equation below. Note that when there is more than one reactant, they are separated by plus signs (+). If more than one product were produced, plus signs would be used between them as well. Iron + Oxygen Iron Oxide

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

the substances that form in a chemical reaction are completely different from the substances that start the reaction.

``````-->  a. true

b. false
``````

a chemical equation is balanced when

``````a) the reactants are the same as the products.

b) the same atoms and molecules appear on each side of the equation.

-->  c) the same number of each type of atom appears on both sides of the arrow.

d) none of the above
``````

which of the following chemical equations is balanced?

``````a) 3fe + 3o2  3fe2o3

b) 2fe + o2  fe2o3

c) 4fe + 2o2  3fe2o3

-->  d) 4fe + 3o2  2fe2o3
``````

## diagram questions

No diagram questions associated with this lesson