Matter & its Changes

Chemical Reactions

&

Chemical Formulas

Chemical Reactions • A change of matter that produces new

substance is called a chemical change or a chemical reaction.

• A chemical reaction is like scrambling words to make new words!

• Evidence for chemical reactions:– Bubble formation (gas production)– Appearance of a solid (precipitate)– Color change– Energy change ( in temperature)

Chemical Reactions• The key characteristic of a chemical reaction is

the production of new materials that are chemically different from the starting ones.

• Chemical reactions occur when chemical bonds are broken or formed.

• The starting materials of a reaction are called reactants.

• The newly formed materials are called products.

Chemical Reactions• The formulas for all the reactants are written

on the left side of the equation.• The formulas for all the products are written

on the right side of the equation.• An arrow [ ] separates the two sides of

the equation.

• Mg + O2 MgO (magnesium oxide)

Conservation of Mass• At the end of a reaction, the same atoms exist,

but they are grouped together in different molecules.

• The amount of matter in a chemical reaction does not change, so the total mass of the reactants must be equal the total mass of the products.

• This principle is called conservation of mass and it states that matter is not created or destroyed during a chemical reaction.

Chemical Equations• A chemical equation is a shorter, easier way to

show chemical reactions.• Chemical symbols and numbers are used

instead of words.

What Is a Chemical Formula?• A chemical formula indicates the types, number,

and the ratio of the atoms combined.• A chemical formula consists of the elements’

symbols and subscript numbers to the right of symbols.

• The subscript indicates the number of atoms making up a molecule.

• Examples: H2 , O2, F2, Cl2…

Rules for Writing Formulas• Represent each kind of element with the correct

symbol• Use subscript to indicate the number of atoms• If there is only one atom of an element, no

subscript needed• Write the symbol for more metallic first

• H2O, CO2, SO2…

Naming Formulas • Name the elements in the same order as they

appear in the formula• Drop the last syllable in the name of the final

element and add -ide• Example: chlorine chloride

oxygen oxide

Practice…Write the formula for these ionic compounds.

Build each compound using the molecular models. Draw the diagram of the model and indicate the atoms on it:

1. Magnesium(yellow) & oxygen(red)

2. Aluminum(orange) & bromine(green)

3. Sodium(white) & sulfur(yellow)

4. Potassium(white) & nitrogen(orange)