Chemistry 112
CHEMICAL REACTIONS
Writing Chemical Reactions
 In any chemical reaction, reactants are converted to
products
reactants → products
 There are different ways we can communicate
chemical reactions: by word, by mass, and by
equation
Word Equations
 Reactants are always written first – these are your
starting materials
 The arrow indicates that a change has taken place –
another way to describe it is “produces” or “yields”
 The products are written to the right of the arrow – they
are your ending materials
 Reactants (as well as products) are separated by plus
signs
Example
iron + oxygen → iron (III) oxide
 A shortcoming of word equations is the uncertainty
of how much is present of both reactants and
products
 Because of this, chemical equations are the
preferred method of writing chemical
reactions
Chemical Equations
 Replace the words with chemical formulas.
Fe(s) + O2(g) → Fe2O3(s)
 The states of matter are included in the formulas
of the reactants and products. When a solid has
been dissolved in water, it is given the state of
matter (aq)
 Only writing the formulas of reactants and products
is called a skeleton equation
Presence of a Catalyst
 Sometimes, a catalyst is added to a chemical reaction
to speed it up
 It is not used up in the reaction
 As a result, it is written on top of the arrow
Ex. C6H12O6 + 6O2 𝑙𝑖𝑔ℎ𝑡 6CO2 + 6H2O
Balancing Chemical Equations
 From a skeleton equation, coefficients can be
added to balance the equation
 According to the law of conservation of mass, matter
can neither be created or destroyed

The total mass of the reactants in an equation has to equal the
total mass of the products
 A balanced chemical reaction has an equal
number of atoms on both sides of the equation
4Fe(s) + 3O2(g) → 2Fe2O3(s)
Practice
 Questions 3, 4 page 327
 Rewrite the following word equations as a balanced
chemical equation: aluminum sulfate + calcium
hydroxide → aluminum hydroxide + calcium sulfate
 Rewrite the following word equation as a balanced
chemical equation: phosphoric acid + sodium
hydroxide → sodium phosphate + water
For the remainder of class…
Complete the handout on writing word equations,
skeleton equations and balancing equations.