Writing Chemical Equations
SECTION 3.2
Objectives
 At the end of this lesson, you will be able to:
 Translate chemical word equations into formula
equations
 Balance simple chemical equations
Chemical Equations
 Chemical equations represent the process of a
chemical reaction
 To write one, must know the reactants and products
 Need to know if a chemical change has occurred

Use our evidence of chemical changes- what are they?
 Takes everything we have learned so far and puts it
into a condensed equation

i.e. compound names and formulas, states, ionic vs. molecular
etc
Writing Word Equations
 A plus sign (+) groups the reactants together
 It does not matter which order the reactants are written in
 An arrow () separates the reactants from the
products and is read “produces”
 A plus sign (+) also joins the products
Word Equations
 Taken from descriptions of a chemical reaction
 Ex. Solid magnesium metal reacts with aqueous hydrochloric
acid to produce aqueous magnesium chloride and hydrogen
gas
 Evidence of a chemical change: gas, heat is given off
 How do we write this as a chemical equation? Using
words?

magnesium + hydrochloric acid  magnesium chloride + hydrogen gas
Word Equations
 Another example: aluminium foil reacts with blue
copper II sulphate solution and produced solid
copper and colourless (grey?) aluminium sulphate
solution
 As a word equation:
To Do:
 Writing Formula Equations from Word Equations
Objectives
 At the end of this lesson, you will be able to:
 Translate chemical word equations into formula
equations
 Balance simple chemical equations
Formula equations
 Uses chemical formulas of reactants and products in
a chemical equations to represent a reaction



Ex. hydrogen + oxygen  water
How do we write this as a formula equation?
H(2)g + O(2)g  H2O(l)
 This is called a skeleton equation (just shows what is
involved with the reaction)

Doesn’t show correct proportions of reactants and products
Formula Equations - example
 Aluminium and copper II sulphate example:
 Aluminium (s) + copper II sulphate (aq)  copper (s) +
aluminium sulphate (aq)
 As a formula equation:
Balancing Equations
 We know what the correct proportions are through:
 Law of Conservation of Mass
 Developed by Antoine Lavoisier
 Total mass of the reactants equals the total
mass of the products
 Using this, we can deduce that:

** total # of atoms present before a reaction equals the total #
of atoms after a reaction
Balancing Equations
 Let’s look at this example:
 Water decomposes (with electrical energy) to
produce oxygen gas and hydrogen gas
 As a formula equation:
 Does this equation follow the conservation of mass?
Why or why not?
Balancing Equations
 To get the number of atoms to stay the same, before
and after the equation, more than one of a molecule
may be involved in the reaction
 The number of molecules is represented by a number
in front of the formula called the coefficient
Balancing Equations
 Look at the following reaction:
 4 AlCl3 + 3 PbO2  2 Al2O3 + 3 PbCl4
 An equation is balanced if the # of each type of atom
on reactant side = # of each type of atom on product
side – is it?
 The numbers in front are coefficients
 Cannot change formulas of any substances; you can
only add coefficients to balance
 How is this related to the Law of Conservation of
Mass?
Practice
Keep a tally of the # atoms as you choose coefficients – you may
change your mind, so change tally as you go

___Zn + ___HCl  ___ZnCl2 + ___H2

___Li2O + ___Mg3P2  ___Li3P + ___MgO

___K + ___O2  ___K2O

___CH4 +___ O2  ___CO2 +___ H2O
Polyatomic Ions
 When dealing with polyatomic ions (i.e. SO42-), treat
them as single units to be balanced
 Treat any number outside of the brackets as
another coefficient
 Fe(NO3)2(aq) + Na3PO4(aq)  NaNO3(aq) + Fe3(PO4)2(s)
 How do we approach this problem? Which atoms
should we start with first?
Practice
 Fe(NO3)2 + Na3PO4  NaNO3 + Fe3(PO4)2
 H2SO4 + NaOH  Na2SO4 + H2O
Practice
 N2(g) + H2(g)  NH3(g)
 CaC2(s) + H2O(l)  Ca(OH)2(s) + C2H2(g)
 SiCl4(s) + H2O(l)  SiO2(s) + HCl(aq)
 H3PO4(aq) + CaSO4(s)  Ca3(PO4)2(s) + H2SO4(aq)
 Methane + oxygen  carbon dioxide + water vapor
 Sodium chloride  sodium + chlorine
 Calcium nitrate + sodium sulfate  sodium nitrate +
calcium sulfate
 Sulfur + oxygen  sulfur dioxide
Practice
Try the following for more practice:
 P. 90 # 7-9
 Chem Workbook:


“Balancing Formula Equations 1”
“Balancing Formula Equations 2”