Conservation of Mass and Balancing Equations

Law of Conservation of Mass

During chemical reactions, matter is neither created nor destroyed. It is only transformed from one form to another. Mass ALWAYS remains the same.

During a chemical change or a chemical reaction....

• The atoms and molecules that interact with each other are called reactants.

• The atoms and molecules that are produced are called products.

• In a chemical reaction, the bonds are broken and the atoms and molecules are rearranged.

• No new atoms are created. No atoms are destroyed.

• They are simply rearranged.

Chemical Reactions

A chemical equation is a combination of chemical symbols and formulas that explain what happened during a chemical reaction.

Parts of a chemical reaction

Reactants - these are the elements that are interacting. In this experiment, the reactants would be baking soda and vinegar.

Parts of a chemical reaction

Products - this is what is produced. In this experiment, sodium acetate, water, and carbon dioxide are produced.

Multiple Choice

What are the reactants in the following chemical equation?

• CH₄ and 2O₂

• CO₂ and 2H₂O

Multiple Choice

What are the products in the following chemical equation?

• CH₄ and 2O₂

• CO₂ and 2H₂O

Balancing Chemical Equations

For an equation to be balanced, there needs to be the same number of atoms on both sides of the equation.

Multiple Choice

How many carbon atoms are on both sides of the equation?

• 1

• 2

• 3

Multiple Choice

How many hydrogen atoms are on both sides of the equation?

• 2

• 4

• 1

Multiple Choice

How many oxygen atoms are on both sides of the equation?

• 2

• 4

• 1

In this example, we want the number of Mg (Magnesium) on the LEFT of the arrow to be the same on the RIGHT of the arrow.

We also want the number of O (Oxygen) to be the same number on both the LEFT and RIGHT of the arrow.

Once we count up the Mg and O on both sides, we can see what needs to change.

We CANNOT TAKE ANYTHING AWAY.

We can ONLY ADD using coefficients.

You'll see this done in the next slide...

Multiple Choice

Is the equation balanced?: 2H₂O + O₂ --> 4MgO + 3Fe

• yes

• no

Multiple Choice

Is the following equation balanced?

Al + O₂ ---> 2Al₂O₃

• YES

• NO

Multiple Choice

Balance this equation:

___H₂ +___ Cl₂ --->___ HCl

• It is balanced.

• H₂ + Cl₂ ---> 2HCl

• 3H₂ + Cl₂ ---> 6HCl

• H₂ + 3Cl₂ ---> 6HCl

Multiple Choice

How many HCl molecules do you need to balance this equation? 
Mg +  __HCl --->  MgCl₂ + H₂

• 1
• 2
• 3
• 4