Counting Atoms and Balancing Chemical

Equations

Elements, Compounds, Mixtures

Hydrogen is an element. Oxygen is an element. When 2 hydrogen atoms and

one oxygen atom bond they make the compound water.

The formula for water is H2O

Chemical FormulasEach element in a chemical formula is

identified by a capital letter. If an element’s symbol contains two letters, the second letter is lower case.

Example: H2SO4

The elements in Sulfuric Acid HydrogenSulfurOxygen

SubscriptsSubscripts tell us how many atoms of each

element are present.C12H22O11

There are 12 atoms of CarbonThere are 22 atoms of HydrogenThere are 11 atoms of Oxygen

If there is not a subscript listed, it is understood to be 1.

Example: NaClThere is one atom of SodiumThere is one atom of Chlorine

You Practice!

NaHCO3

Sodium – 1Hydrogen – 1Carbon – 1Oxygen -3

HClHydrogen – 1Chlorine - 1

There are times you will see a chemical formula with parenthesis. The subscript after the parenthesis affects everything inside the parenthesis.

Pb(NO3)2

So, in counting the atoms, you would have the following:

Lead – 1 Oxygen - 6Nitrogen -2

You Practice!!

(NH4)3PO4

Nitrogen – 3Hydrogen – 12Phosphorus – 1Oxygen – 4

Mg(OH)2

Magnesium – 1Oxygen – 2Hydrogen – 2

CoefficientA coefficient identifies how many molecules are present.

2H2SO4This means there are 2 molecules of H2SO4.

Counting the atoms:Hydrogen – 4Sulfur – 2Oxygen - 8

CoefficientA coefficient can also be used to tell us how many atoms

are present.

2MgThis means there are 2 atoms of Mg.

Counting the atoms: Mg = 2

You Practice!!

3H3PO4

Hydrogen – 9Phosphorus – 3Oxygen - 12

2H2O

Hydrogen – 4Oxygen - 2

Got It????

Chemical Equations

Chemical equations express what is happening in a chemical reaction using symbols.

2K + Cl2 2KClReactant Product

yields

Law of Conservation of Mass

In a chemical reaction, matter cannot be created or destroyed. It can only be changed.

Therefore, the amount of matter in the reactants must equal the amount of matter in the products.

What does that mean??

The atoms contained in the reactants must be the same as the atoms in the products. Atoms can not be created or destroyed.

The atoms CAN be rearranged into different molecules.

Is the Law of Conservation of Mass supported by this chemical equation?

H2 + O2 H2O

Reactants ProductsH = 2 atoms H = 2

atomsO = 2 atoms O = 1 atom

NO!Oxygen is not equal on each side of the equation.

So what do we do?The chemical equation must be written according to

the law.

To do this we change the coefficients. We NEVER change a subscript or a formula!

2H2 + O2 2H2O

Reactants: Products: H = 4 H = 4O = 2 O = 2

Do I Need to Know How to Balance?

NOT YET. You need to be able to recognize if the chemical reaction follows the law of conservation of mass.

Na + O2 Na2O

Mg + 2HCl MgCl2 + H2

NOT BALANCED

BALANCED!

You Practice!!!

HgO Hg + O2

Hg = Hg = O = O =

N2 + 3H2 2NH3

Zn + 2HCl ZnCl2 + H2

NOT BALANCED

BALANCED!

BALANCED

More Practice

K + Br2 2KBr

2Fe + O2 6Fe2O3

2Na + 2H2O 2NaOH + H2

NOT BALANCED

NOT BALANCED

BALANCED

Got It???

Balancing Chemical Equations

H2 + O2 H2O

H = 2 atoms H = 2 atomsO = 2 atoms O = 1 atoms

How can we use coefficients to balance this equation?

H2 + O2 2H2O

Reactants: H = 2 Products: H = 4 O = 2 O = 2

Now the Oxygen is balanced but Hydrogen is not. What can be done to balance Hydrogen?

Using a coefficient of 2 will balance oxygen.

Adding a coefficient of 2 here will balance hydrogen.

2H2 + O2 2H2O

H = 4 atoms H = 4 atomsO = 2 atoms O = 2 atoms

Now it is balanced!!!

Now You Practice

___HgO ___Hg + ___O2

Hg = Hg = O = O =

___K + ___Br2 ___KBrK = K = Br = Br =

Balance two more…___N2 + ___H2 ___NH3

N = N = H = H =

___C2H6 + ___O2 ___CO2 +___H2O C = C= H = H = O = O =