REACTIONS: A TUTORIAL
By: Jennifer Kocan & Hunter Edwards
What is a Chemical Reaction?
A chemical reaction is a change in which two or
more substances combine to form a new substance
The two or more products you begin with are
The new substance(s) formed are called products.
For example, when the acetic acid in vinegar and
baking soda (reactants) are combined, the
products are bubbles of carbon dioxide gas,
water, and sodium acetate.
The Law of Conservation of Mass
“A relation stating that in a chemical reaction, the
mass of the products equals the mass of the
This means when a chemical reaction occurs, the
products and reactants will have the same mass.
Why is it Important to Balance
Balanced chemical equations obey the Law of
Conservation of Matter and are true
representations of what actually occurs in nature.
Remember- A chemical equation is a shorthand
method for describing a chemical change.
To start, place reactants on the left-hand side of the
Products go on the right hand side of the equation.
Remember- The law of conservation of mass states
that matter is neither created or destroyed, so all
the atoms in the reactants must end up somewhere
among the products.
Example (Step 1)
Here you have an unbalanced equation.
K + Br2 KBr
Count the number of atoms of each element,
compound or ion in the reactants and products.
Example (Step 2)
Count the number of atoms of each element.
K + Br2 KBr
1 1 1
*Make sure you understand that you must have 2 atoms of
Bromine on the right side of the equation, since there are 2 atoms
of Bromine on the left side.
Remember- numbers appearing in the formula
below an element are known as subscripts. These
can never be changed when balancing the equation
or you will change the entire equation.
Balance the atoms one at a time by putting coefficients
(simple, whole numbers written in front of chemical
formulas) in front of the formula, so that the numbers of
atoms of each element are equal on both sides of the
This means that if you have atoms A2 and B2 on one
side of the equation, you must have A2 and B2 on the
other side (equal number of atoms).
Example (Step 4)
Balance: Two 2’s were placed in the underlined spots
so the number of atoms of Bromine are equal.
2K + Br2 2KBr
•2 times K is 2 K (Two atoms of Potassium) •2 atoms of Bromine •2 times K and Br is two atoms of Potassium and two atoms of Bromine
Make sure all the coefficients are in the smallest
possible whole number ratio (simplify).
Example: 4 and 6 are simplified to 2 and 3.
Basically all you’re doing when your balancing a
chemical reaction equation is making sure that each
side of the equation has the same number of atoms
as the other side does.
Example (Step 5)
Here is the complete balanced equation.
2K + Br2 2KBr
C5H12 + O2 -----> CO2 + H2O
Chemistrty Ciriculum center- http://dev-school.discoveryeducation.com/curriculumcenter/chemistry/glossary.html
Chemistry 1: Balancing Equations http://chemistrygeek.com/balance1.htmconservation
Conservation of matter and balancing equations