Chemical Formulas and Equations

8.5D recognize that chemical formulas are used to identify substances and determine the number of atoms of each element in chemical formulas containing substances; 8.5F

recognize whether a chemical equation containing coefficients is balanced or not and how that relates to the law of conservation of mass

Chemical Compounds

Chemists have identified over 10 million chemical compounds

Some are molecular – like the proteins and hormones in your body

Some are ionic compounds – such as the salts in your body fluids

A chemical formula shows the kinds and numbers of atoms in the smallest representative unit of the substance (recall this from the last lesson?)

Formulas

H2O shows that a water molecule has 2 hydrogen and 1 oxygen atom

CO2 – carbon dioxide 1 carbon and 2 oxygen atoms

C2H6 – ethane (natural gas) 2 carbon and 6 hydrogen atoms

What they don’t show: If we look at NH3, it does show how many atoms

and elements and the ratio that make up that molecule.

What a formula doesn’t show is the structure You had a bit of practice with this idea during the

bonding practice

So why the formula?

There are many reasons, but the best reason is to demonstrate on paper, chemical reactions!

Chemical reactions are shown with chemical equations.

Learning how to read one is very important in chemistry

Chemical formulas practice:

Make sure to complete the front page and ask questions for now. When everyone is done, we will continue on with equations.

Chemical Equations

A chemical equation is a symbolic representation of a chemical

reaction

2 2

Equation Example:

The burning of methane gas in oxygen is:

CH4 + 2 O2 → CO2 + 2 H2O

Reading Chemical Equations

Each side of an equation represents a combination of chemicals

The combination is written as a set of chemical formulas, separated by +symbols.

CH4 + 2 O2 → CO2 + 2 H2O

Reading Chemical Equations

The two sides of the equation are separated by an arrow

•The combination of chemicals before the reaction are on the left side of the arrow

•The right side indicates the combination of chemicals after the reaction

Language of Chemical Equations

In this reaction, sodium (Na) and oxygen (O2) react to produce a single molecule, Na2O (this together is known as “sodium oxide”

4Na + O2 → 2Na2O

Reactants Products

Yields

Reactants

Arrow (yields)

Products

Conservation of Mass and Balancing Equations

8.5F recognize whether a chemical equation containing coefficients is balanced or not and how that relates to

the law of conservation of mass

Law of Conservation of Matter

Mass is neither created nor

destroyed (during a chemical

reaction)

Conservation of Mass and/or Matter

What this means: 1. Matter is conserved type of atoms does

not change

•Nothing is created or destroyed

2. Mass is conserved amount of atoms cannot change

•Nothing is created or destroyed

So…IN A CHEMICAL REACTION:

Elements can combine to form new substances

Substances can be broken down into simpler substances (down to the elemental level)

Atoms are re-arranged; NOT created or destroyed

Reactants Products

Mass of Reactants = Mass of Products

In a chemical formula this law is demonstrated

Balancing Equations

To show conservation of mass We Balance equations

•Make sure there are the same number of each type of atom in the products (right side) and on the reactants side (left)

Take a look at this reaction. How many

hydrogen atoms are there on each side of the reaction?

• Now, look at oxygen.

Balancing Equations

If there is not a subscript or coefficient number shown, then it is understood to be 1

CH4 + 2O2 → CO2 + 2H2O =C1H4 + 2O2 → C1O2 + 2H2O1

1 C C 4 H 4 H4 O 4 O

Complete (and correct) equations signifying reactions are always balanced

Are the following balanced?

Let’s look at the number of atoms on each side of the reaction and then decide if the equation is balanced.

Counting Atoms – remember the rules from the chemical formulas practice?

2CuO2

2NH4

3S2O2

Copper - 2Oxygen - 4

Nitrogen - 2Hydrogen - 8

Sulfur - 6Oxygen - 6

Counting Atoms – remember the rules from the chemical formulas practice?

4Mg2O4H2

2NaN3HO Sodium – 2Nitrogen – 6Hydrogen – 2Oxygen - 2

Magnesium – 8Oxygen – 16Hydrogen - 8

BALANCED or NOT BALANCED?

Cu + O2 CuO

2Cu + O 2CuO

S + O2 SO2

Mg + O2 MgO

2Mg + O2 2MgO

Not Balanced

Not Balanced

Balanced

Balanced

Not Balanced

Now, in your notes, there is a space to write examples and to explain why the examples are balanced or not.

Now let’s practice! Steps: 1. Work with a

partner to fill out this page

2. When done, check your answers with another group and then with the teacher