Chemical Reactions & the Conservation of Mass

Chemical Reactions & the Conservation of Mass

A puddle of water evaporating and rust forming on metal are both examples of changes in matter. Water evaporating is a physical change and rust forming is a chemical change. In both types of changes, the total amount of matter remains the same. Matter is neither created nor destroyed, even when a new substance forms.

Chemical Equations

Remember that a chemical reaction is a process in which elements and compounds combine to form new substances. For example, iron oxide (Fe₂O₃) is a compound you know as rust. The formation of iron oxide is an example of a chemical change. Iron oxide forms when iron (Fe) reacts with oxygen gas (O₂) in the air. This reaction can be shown as a word equation. The arrow in the equation is read as "produces" or "yields."

Iron + oxygen ➡️ iron oxide

Multiple Select

How do you read the arrow in a chemical equation?

(choose all that apply)

• "produces"

• "yields"

• "product"

• "right"

Iron + oxygen ➡️ iron oxide

In this reaction, iron and oxygen are the reactants. A reactant is a substance that is present at the beginning of a chemical reaction. Reactants are always shown to the left of the arrow. Iron oxide is the product of this reaction. A product is a substance that is produced during a chemical reaction. Products are always written to the right of the arrow.

The word equation above can be rewritten as a chemical equation. A chemical equation describes a chemical reactions using chemical formulas, subscripts, and coefficients. A coefficient is a number written before a chemical formula to show how many atoms or molecules of that substance are involved in the reaction. The equation for rust formation is:

4Fe + 3O₂ ➡️ 2Fe₂O₃

Multiple Choice

True or False: A reactant is a substance present at the beginning of a chemical reaction.

• True

• False

Multiple Choice

In a chemical reaction what is the product?

• a substance that is present at the beginning of a chemical reaction

• a substance that is produced during a chemical reaction

• a number written before a chemical formula to show how many atoms or molecules of that substance are involved in a chemical reaction

4Fe + 3O₂ ➡️ 2Fe₂O₃

The reactants are four iron atoms & three molecules of oxygen gas (O₂). Each molecule of oxygen gas is made up of two oxygen atoms. Since the reactants include three oxygen molecules, there are a total of six oxygen atoms in the reactants. The product of this reaction is iron oxide. Two molecules of iron oxide form. Each molecule has the formula Fe₂O₃.

The formula Fe₂O₃ shows that a molecule of iron oxide contains two iron atoms and three oxygen atoms. Since there are two molecules of iron oxide, the product contains a total of 4 iron atoms and 6 oxygen atoms. If you compare this to the number of iron atoms and oxygen atoms in the reactant, you will find that the numbers are the same. This means that the chemical equation is balanced.

Balanced Chemical Equations

To find out if an equation is balanced, follow these steps:

1. Make sure that the reactants and the products contain the same elements.

2. Count the number of atoms of each element in the reactants. Remember to multiply the coefficient by the subscript (if there is one).

3. Count the number of atoms of each element in the products.

4. Check that these number are equal. If they are, the equation is balanced.

Multiple Choice

True or False: A balanced equation refers to a chemical equation in which the number of atoms in the product are the same as in the reactant.

• True

• False

4Fe + 3O₂ ➡️ 2Fe₂O₃

You may need to make a chart to compare the numbers.

This chart show that the equation for rust formation is balanced.

Multiple Choice

Examine the following chemical equation, which shows only the reactants and one product.

Zn + 2HCl ➡️ ZnCl₂ + ?

How many H atoms must be present in the second product that is formed?

• 1

• 2

• 3

• 4

The Law of Conservation of Mass

When a log on a campfire burns, most of the log seems to disappear. You see only a small amount of ash left behind. You might think that most of the matter in a log is destroyed when it burns. However, in a chemical reaction, matter is not created or destroyed. In the 1700s, a French chemist named Antoine Lavoisier conducted experiments to prove this concept.

Lavoisier measured the masses of the reactants and products in reactions. He discovered that in any reaction, the total mass of the reactants is always the same as the total mass of the products. This discovery is expressed as the law of the conservation of mass. According to this law, matter is neither created nor destroyed during a chemical change.

Multiple Choice

According to the law of conservation of mass, which of the following is a TRUE statement about any chemical reaction?

• The total mass of the reactants is greater than the total mass of the products.

• The total mass of the reactants is less than the total mass of the products.

• The total mass of the reactants equals the total mass of the products.

• In a chemical reaction, matter can be created but not destroyed.

Closed System

• A closed system supports the law of conservation of mass.

• In a closed system, a chemical reaction occurs in a closed container.

• In a closed system, no matter can enter or leave the system.

• The example of the log burning on a fire is not a closed system. The mass of the gases released by the burning log cannot be measured, so the conservation of mass cannot be observed.

Multiple Choice

Which of the following equations does not demonstrate the law of conservation of mass?

• 2Na + Cl₂ ➡️ 2NaCl

• NaOH + HCl ➡️ NaCl + H₂0

• P₄ + 5O₂ ➡️ 2P₄O₁₀