Chemical Equations
Putting chemical reactions into words
A chemical equation is a
symbolic representation of a
chemical reaction.
Reactants  Products
Reactants yield products
Reactants produce products
Equation Example:
The burning of methane gas in
oxygen is:
CH4 + 2 O2 → CO2 + 2 H2O
Review:
Element Symbols
All elements are represented by a 1 or 2 letter
symbol
For example
C = Carbon
Ne = Neon
O = Oxygen
The first letter is capitalized, the second is not
Chemical Formulas
Shows the elements & number of atoms of each element
in a molecule/compound
H2SO4
Elements
Subscript
Hydrogen; 2 atoms
Sulfur: 1 atom
Oxygen: 4 atoms
7 atoms total
The subscript describes the number of atoms of an element
If there is not a subscript listed, it is understood to be 1.
Coefficients
A formula may begin with a number.
If there is no number, then “1” is understood to be in
front of the formula.
This number is called the coefficient.
The coefficient represents the number of molecules of
that compound or atom needed in the reaction.
For example:
2H2SO4 – 2 molecules of Sulfuric Acid
2H2SO4 – 2 molecules of Sulfuric Acid
A coefficient is distributed to ALL elements in a
compound
2 – H2 (for a total of 4 H atoms)
2 – S (for a total of 2 S atoms)
2 – O4 (for a total of 8 O atoms)
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.
Reactants: The combination of chemicals before the
reaction are on the left side of the arrow
Products: The right side indicates the combination of
chemicals after the reaction.
For Example:
4Na + O2 → 2Na2O
In this reaction, sodium (Na) and oxygen (O2) react to
make a single molecule, Na2O
Label: Reactants, products, subscripts, coefficients
Atom Counts
1. List the element symbols present (under the arrow).
2. Count the number of atoms for EACH element on the
reactants side and also on the products side.
3. If both sides are equal for EVERY element, the
equation is balanced.
List all element symbols and the
number of atoms of each
Na + Cl

Is it balanced?
NaCl
List the element symbols and the number
of atoms of each
2K + Cl2

Is it balanced?
2KCl
Chemical Equations and
Heat
Exothermic Reactions – chemical equations that
produce heat. Heat is a product.
Endothermic Reactions – chemical equations that use
heat. Heat is a reactant.