Download How to balance chemical equations.

How to balance chemical
equations.
•When pieces of matter come together or break
apart, it is called a reaction.
•The law of conservation of matter states that
matter can not be created or destroyed.
•In any reaction, you must have the same
amount of each part before and after the
reaction happens.
•We show how reactions become balanced by
using chemical equations.
How to balance chemical
equations.
2Na + Cl2 2NaCl
This is a balanced
chemical equation.
How to balance chemical
equations.
2Na + Cl2  2NaCl
The arrow is called the yields sign.
It separates what you start with on
the left from what you end up with
on the right.
How to balance chemical
equations.
2Na + Cl2  2NaCl
To the left of the yields sign are
the parts that react. They are
called the reactants.
How to balance chemical
equations.
2Na + Cl2  2NaCl
To the right of the yields sign is
what you end up with after the
reaction. These are called the
products.
How to balance chemical
equations.
2Na + Cl2  2NaCl
The little number next to and
below an element is called a
subscript. We have been using
them all along. We never change
subscripts in a chemical equation.
How to balance chemical
equations.
2Na + Cl2  2NaCl
The big numbers next to the
elements or compounds are called
coefficients. These are the only
things we can change in a chemical
equation.
How to balance chemical
equations.
2Na + Cl2  2NaCl
Coefficients tell us the amount of each
compound or element we have. You
multiply the subscript for each
element to the right of a coefficient
until you are stopped by a + or 
How to balance chemical
equations.
2Na + Cl2  2NaCl
So in this equation you have 2 x 1
sodium atoms being added to 2
chlorine atoms yielding 2 x 1 sodium
chloride molecules.
How to balance chemical
equations.
___H2 + ___O2 ___H2O
When we balance chemical equations,
we need to figure out what
coefficients to use.
How to balance chemical
equations.
___H2 + ___O2 ___H2O
To do that, we list how many atoms of
each element we start with, then use
math to figure out the coefficients.
How to balance chemical
equations.
___H2 + ___O2 ___H2O
In this equation, we start with:
Reactants
Products
H=2
H=2
O=2
O=1
How to balance chemical
equations.
___H2 + ___O2 ___H2O
Our hydrogen is balanced, but we don’t
have enough oxygen in the product.
Reactants
Products
H=2
H=2
O=2
O=1
How to balance chemical
equations.
___H2 + ___O2 ___H2O
We need to make our best mathematical
guess as to which coefficient will bring
balance to the equation.
Reactants
Products
H=2
H=2
O=2
O=1
How to balance chemical
equations.
2 H2O
___H2 + ___O2  ___
We need two oxygen atoms, so lets try a
coefficient of 2 in front of the product.
Reactants
Products
H=2
H=2
O=2
O=1
How to balance chemical
equations.
2 H2O
___H2 + ___O2  ___
Multiply that 2 by the subscripts for each
atom in the product.
Reactants
Products
H=2
H= 4
O=2
O= 2
How to balance chemical
equations.
2 H2O
___H2 + ___O2  ___
Now we have enough oxygen, but too much
hydrogen. How do we balance it?
Reactants
Products
H=2
H= 4
O=2
O= 2
How to balance chemical
equations.
2___H2 + ___O2  ___
2 H2O
If we put a coefficient of 2 in front of the
H2 on the reactants side, we can balance
the equation.
Reactants
Products
H= 4
H= 4
O-2
O= 2
How to balance chemical
equations.
•Multiply the number of atoms you have by
coefficients to balance equations.
•Multiply coefficients by every subscript until
you hit a + or a .
•Never, ever, EVER change a subscript.
•Make a list of how much of each atom you have
and change it as you add coefficients to your
equation.
•If there are poly-atomic ions on both sides of
your equation, don’t separate them!
Quick Check
___Al(OH)3 + ___H2SO4 ___Al2(SO4)3 + ___ H2O