1. Forming New Substances
Chemical Reactions
Forming New Substances

2. Chemical Equation
A chemical equation is a shorter, easier way to show chemical reactions, using symbols instead of words.
We use chemical equations because they follow a common structure that all chemists understand.

3. Chemical Formulas Remember – all elements are represented by a symbol.
We use these symbols to represent compounds too…by combining them.
This combination is called a formula.
A formula also shows the ratio of the different atoms that make up that substance by using subscripts.
Subscripts are the very small numbers in a formula that are written beside and below the element symbol.

4. Chemical Formula cont. For example:
Water has the ratio of 2 hydrogen atoms to 1 oxygen atom.
The chemical formula would look like this – H2O
If there is no subscript, the number next to the symbol is assumed to be 1.
CO2
What is the ratio of atoms in Iron oxide (rust) if its formula is Fe2O3?
The formula tells you that there are 2 atoms of iron for every 3 atoms of oxygen.

5. Structure of the Equation
A chemical equation summarizes a reaction.
It tells you the substances you start with and end with.
The materials that are on the left are those that you start with and are called the reactants.
The materials that are on the right are those that you end with and are called the products.

6. Structure of an Equation continued…
The formulas for the reactants are written on the left, followed by an arrow and the product on the right.
Example:
Reactant + Reactant Product
H O H2O

7. Conservation of Mass
No matter how many reactants and products are involved, all the atoms present at the start of a reaction are present at the end.
Think about what happens with students in school…you have a set in one class then when the bell rings, there is a new set in a different class…there are still the same students and number of students in the building, just in different locations.
The amount of matter involved in a chemical reaction does not change. The total mass of the reactants must equal the total mass of the products. This is called the conservation of mass.

8. Conservation of Mass
Matter is neither created or destroyed. It can only be changed from one form to another.
Where does the log go when it is burned?

9. Balancing Chemical Equations
Remember, the principle of Conservation of mass indicates that the same number of atoms exist in the products as were present in the reactants.
To balance an equation, begin by looking at the formulas:
H2 + O2 H2O
How many atoms does the hydrogen molecule have? 2
How about oxygen?
How many of each kind of atom are present in one water molecule?
2 Hydrogen and 1 Oxygen

10. Balancing Chemical Equation for water…
Since we started with 2 Hydrogen and 2 Oxygen reactants, according to the conservation of mass, shouldn’t water have the chemical formula of H2O2?
No, that would be the formula for hydrogen peroxide (H2O2)To balance this equation, we must use a coefficient.
A coefficient is a number placed in front of a chemical formula in an equation. It tells you how many atoms or molecules of each reactant and product take part in the reaction.

11. Clues to Chemical Reactions
How can you tell a chemical reaction is taking place?
Color Change
Gas bubbles
A precipitate- a solid formed in a solution
Energy is given off: light, thermal or electrical