Chemical Reactions:

The Rearranging of Atoms

Rearranging Atoms

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

Evidences for a Chemical Change

A change in colorA solid (precipitate) forms Gas is producedA change in Energy

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.

Chemical Reaction Between O2 & CH4

We represent this chemical reaction by writing a chemical equation.

methane + oxygen carbon dioxide + water

CH4 + O2 CO2 + H2O

Chemical EquationsThe 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 productsCH4 + O2 CO2 +

H2O

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.

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.

Balancing Chemical Equations

The equation for the reaction between methane and oxygen, is not balanced.

CH4 + O2 CO2 + H2Oone carbon atom one carbon atomfour hydrogen atoms two hydrogen atomstwo oxygen atoms three oxygen atoms

As written, this equation states that one oxygen atom was created and two hydrogen atoms were destroyed.

Balancing Chemical Equations

CH4 + O2 CO2 + H2OWe 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).

CH4 + O2 + O2 CO2 + H2O + H2O

one carbon atom one carbon atomfour hydrogen atoms four hydrogen atomsfour oxygen atoms four oxygen atomsThis chemical equation is now balanced!

A Balanced Chemical Equation

methane + oxygen carbon dioxide + water

The rearrangement below leaves no leftovers.

Coefficients

CH4 + O2 + O2 CO2 + H2O + H2O

When we write balanced chemical equations we group like molecules together. The finished equation is below.

CH4 + 2O2 CO2 + 2H2O

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.

Subscripts vs. CoefficientsA 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.

Balancing Chemical Equations

CH4 + 2O2 CO2 +

2H2O

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.

Another ExampleOxygen combines with hydrogen to produce water.O2 + H2 H2O

First count the number of atoms on each side of the chemical equation.

O2 + H2 H2O___ oxygen atoms ___ oxygen atom___ hydrogen atoms ___ hydrogen atoms

We need more _________ atoms on the __________ side.

WaterOxygen 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.

O2 + H2 2H2OWe 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

WaterO2 + H2 2H2OThis 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.

O2 + __H2 2H2OThe chemical equation is now balanced!

2H2 + O2 2H2O

DiatomicsThere 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.