1. The Rearranging of Atoms
Chemical Reactions:
The Rearranging of Atoms

2. Rearranging Atoms
When two or more atoms combine and bond to form a new substance, a chemical change occurs.

3. Evidences for a Chemical Change
A change in color
A solid (precipitate) forms
Gas is produced
A change in Energy

4. The Rearrangement of Atoms
Chemical reactions always involve the rearrangement of the way atoms are grouped.
For example: When methane, CH4, combines with oxygen, O2, in the air and burns, carbon dioxide, CO2, and water, H2O, are produced.

5. Chemical Reaction Between O2 & CH4
We represent this chemical reaction by writing a chemical equation.
methane + oxygen  carbon dioxide + water
CH O  CO H2O

6. Chemical Equations
The chemicals represented before the reaction (to the left of the arrow), are called reactants. The chemicals produced by the reaction (to the right of the arrow), are called products. The arrow indicates the direction of the change and is read as “yields” or “produces”.
reactants produce products
CH O  CO H2O

7. Reactants & Products CH4 + O2  CO2 + H2O
Notice, there are the same KINDS of atoms on both sides of the chemical equation. We begin with carbon, hydrogen and oxygen, (C, O & H), and end with carbon, hydrogen and oxygen.

8. The Law of Conservation of Mass
In chemical reactions, atoms are neither created nor destroyed. ALL atoms present in the reactants must be accounted for in the products. The mass of the reactants equal the mass of the products.
The same number of each type of atom on the reactant side must appear on the product side.
Obeying the law of conservation of mass in chemical equations is called balancing the chemical equation.

9. Balancing Chemical Equations
The equation for the reaction between methane and oxygen, is not balanced.
CH O  CO H2O
one carbon atom one carbon atom
four hydrogen atoms two hydrogen atoms
two oxygen atoms three oxygen atoms
As written, this equation states that one oxygen atom was created and two hydrogen atoms were destroyed.

10. Balancing Chemical Equations
CH O  CO H2O
We can fix this imbalance by involving another O2 molecule on the left (reactants) and displaying the production of more H2O molecules on the right (products).
CH O O2  CO H2O H2O
one carbon atom one carbon atom
four hydrogen atoms four hydrogen atoms
four oxygen atoms four oxygen atoms
This chemical equation is now balanced!

11. A Balanced Chemical Equation
methane oxygen  carbon dioxide water
The rearrangement below leaves no leftovers.

12. Coefficients CH4 + O2 + O2  CO2 + H2O + H2O
When we write balanced chemical equations we group like molecules together. The finished equation is below.
CH O  CO H2O
The numbers in front of the O2 and the H2O molecules are called coefficients. Coefficients are written in front of atoms and molecules. They multiply all atoms to their right and tell us how many of that particular molecule or atom is needed to balance the chemical equation.

13. Subscripts vs. Coefficients
A subscript is a small number written to the right of an atom in a formula indicating how many atoms of that element are present in the formula.
A coefficient is a number written in front of a reactant or product that indicates the smallest number of particles of the substance that are involved in the reaction.

14. Balancing Chemical Equations
CH O  CO H2O
Notice we did not change any of the actual atoms.
We did not change any subscripts.
We did not take any molecules away.
When balancing chemical equations, you can only add more of the same molecules or atoms.

15. Another Example Oxygen combines with hydrogen to produce water.
O H  H2O
First count the number of atoms on each side of the chemical equation.
___ oxygen atoms ___ oxygen atom
___ hydrogen atoms ___ hydrogen atoms
We need more _________ atoms on the __________ side.

16. Water
Oxygen is only present in H2O on the product side. The only way to add more oxygen atoms to the product side of the equation is to add more H2O molecules.
O H  H2O
We add the coefficient 2 in front of water and count again. Coefficients multiply everything to their right.
____ oxygen atoms ____ oxygen atoms
____ hydrogen atoms ____ hydrogen atoms

17. Water O2 + H2  2H2O This equation is not yet balanced.
We need more _______ atoms on the __________side.
We add the coefficient ___ in front of the ________ molecule on the __________ side and count again.
O __H  H2O
The chemical equation is now balanced!

18. 2H2 + O2  2H2O

19. Diatomics
There are seven elements that are not found in nature as single atoms. The molecules formed are more stable than the individual atoms. If they are not in a chemical compound with other atoms, they pair up with another atom of their own kind.
Br2 I2 N2 Cl2 H2 O2 F2
Notice how the diatomics were used in the chemical equations we looked at earlier.