CHEMICAL EQUATIONS:
MORE WRITING AND
BALANCING PRACTICE
Go through this PowerPoint for
extra tips and extra practice! 
CHEMICAL EQUATIONS
Two basic rules apply in writing chemical equations:
 We cannot write an equation for a reaction unless we know how the substances
react and what new substances they form
 Every chemical equation must be balanced (the number of atoms of each
element on the left side must be the same as the number of atoms of that
element on the right side of the equation)
Chemical equations are balanced according to the law of conservation of mass
 nothing is created from nowhere or destroyed to nowhere.
 Mass of matter present after the reaction is the same as before the reaction
 A rearrangement of entities at the molecular level has occurred
COMMUNICATING CHEMICAL
REACTIONS
Chemical Equations from Word Equations
 Hydrogen gas burns in the presence of oxygen gas to produce water vapour
H2(g) + O2(g)  H2O(g)
(unbalanced)
2H2(g) + O2(g)  2H2O(g) (balanced)
 Sodium metal reacts with chlorine gas to form sodium chloride
Na(s) + Cl2(g)  NaCl(s) (unbalanced)
2Na(s) + Cl2(g)  2NaCl(s) (balanced)
 A solution of silver nitrate reacts with a solution of potassium chloride to form
a precipitate of silver chloride and a solution of potassium nitrate
AgNO3(aq) + KCl(aq)  AgCl(s) + KNO3(aq)
COMMUNICATING CHEMICAL
REACTIONS
Word Equations from Chemical Equations

 NaOH(aq) + HCl(aq)
NaCl(aq) + H2O(l)
A sodium hydroxide solution and a hydrochloric acid solution will
react to produce a solution of sodium chloride and liquid water
 2Na(s) + 2H2O(l)

H2(g) +
2NaOH(aq)
Solid sodium metal and water will react to produce hydrogen gas
and a solution of sodium hydroxide
 C(s) +
O2(g) 
CO2(g)
Solid carbon and oxygen gas will react (burn) to produce carbon
dioxide gas
 CaCO3(s)

CaO(s) + CO2(g)
Solid calcium carbonate (when heated) will decompose to form
solid calcium oxide and carbon dioxide gas
PRACTICE
Rewrite the following word equations as formula equation and balance them:
a)
Solid potassium metal reacts with oxygen gas to produce solid
potassium oxide
b)
Methane gas reacts with oxygen gas to produce carbon dioxide gas
and water vapour
c)
Mercury(II) sulfide is decomposed into liquid mercury and solid sulfur
d)
Aqueous cobalt(III) nitrate reacts with solid zinc to produce aqueous
zinc nitrate and solid cobalt.
PRACTICE
Answers: (remember: when there is no coefficient in front of a compound, there is an invisible “1” there)
a)
Solid potassium metal reacts with oxygen gas to produce solid potassium
oxide
4K(s) + O2(g)  2K2O(s)
b)
Methane gas reacts with oxygen gas to produce carbon dioxide gas and water
vapour
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
c)
Mercury(II) sulfide is decomposed into liquid mercury and solid sulfur
8HgS(s)  8Hg(l) + S8(s)
d)
Aqueous cobalt(III) nitrate reacts with solid zinc to produce aqueous zinc
nitrate and solid cobalt
2Co(NO3)3(aq) + 3Zn(s)  3Zn(NO3)2(aq) + 2Co(s)
CHEMICAL EQUATIONS
A balanced chemical equation has the total atoms/ions on reactant side equal
to the total number of the same kind of atom/ions on the product side
Coefficients are used as necessary in front of the chemical formulas of the
skeletal equation to produce a balanced equation
States of matter are provide after each chemical formula
Summary and Practice on Pages 49 – 50 of Text
BALANCING CHEMICAL
REACTION EQUATIONS
General steps for balancing chemical equations
 Write an equation using the correct chemical formulas
 Check the formulas of the reactants and products
 Determine if the equation is balanced
 Count the number of atoms of each element on the left side and the right side. Count polyatomic ion as a
group if its formula is unchanged
 Balance the equation one element at a time
 Pick an element and balance the equation for that element by placing coefficients in front of the formulas to
equalize the number of that element on both sides of the equation
 A typical starting point is with the most complex formula
 Balance diatomic elements last
 Check to see if the equation is balanced
 Sometimes balancing one element will put another element out of balance. If that happens, repeat steps 3
and 4 until the equation is balanced. Make sure the coefficients you use are the smallest possible whole
numbers.
Never try to balance an equation by changing the subscripts in a formula – that would make the formula incorrect!
EXAMPLES
Balance the following chemical equations
1.
Fe(s) + HCl(aq) → FeCl2(aq) + H2(g)
2.
Ca OH
2(s)
+ H3 PO4(aq) → Ca3 PO4
2(s)
+ HOH(l)
Note: if polyatomic ions appear on both sides, keep them as a whole thing
Also it might be easier to write water as HOH instead of H2O if hydroxide ions
(𝑂𝐻− ) are present on the other side of the equation
3.
C4 H10(g) + O2(g) → CO2(g) + H2 O(g)
Note: fractions are not used for balancing equations – get rid of them by
multiplying all coefficients in the equation by the denominator of the fraction
EXAMPLES
Answers:
1.
𝟐Fe(s) + 𝟐HCl(aq) → 𝟏FeCl2(aq) + 𝟏H2(g)
2.
3Ca OH
2(s)
+ 𝟐H3 PO4(aq) → 𝟏Ca3 PO4
2(s)
+ 𝟔HOH(l)
Note: if polyatomic ions appear on both sides, keep them as a whole thing
Also it might be easier to write water as HOH instead of H2O if hydroxide ions
(𝑂𝐻− ) are present on the other side of the equation
3.
𝟐C4 H10(g) + 𝟏𝟑O2(g) → 𝟖CO2(g) + 𝟏𝟎H2 O(g)
Note: fractions are not used for balancing equations – get rid of them by
multiplying all coefficients in the equation by the denominator of the fraction
DO NOT
change correct chemical formulas and/or subscripts to make atoms of an
element balance
add some real or imaginary chemical species to either side of the equation just
to make atoms of an element balance
DO
perform a check to make sure all atoms/ions balance
 most chemical equations are balanced by what is called the inspection method
 This is somewhat a trial and error method which succeeds in most instances
Practice is necessary to become successful 
Useful strategies develop as practice continues.
PRACTICE
Balance the following chemical equations
1.
__Na(s) + __O2(g)  __Na2O(s)
2.
__HI(g)  __H2(g) + __I2(g)
3.
__SnF4(aq) + __Cr(s)  __CrF3(aq) + __Sn(s)
4.
__(NH4)3PO4(aq) + __CaBr2(aq)  __Ca3(PO4)2(s) + __NH4Br(aq)
5.
__C3H8(g) + __O2(g)  __CO2(g) + __H2O(g)
6.
__Al(NO3)3(aq) + __NaOH(aq)  __NaNO3(aq) + __Al(OH)3(s)
PRACTICE
Answers: (Remember: it is not necessary to write down the coefficient when it is “1,” I put it here to
make it easier to understand. It is more conventionally correct to NOT put it.)
1.
4Na(s) + 1O2(g)  2Na2O(s)
2.
2HI(g)  1H2(g) + 1I2(g)
3.
3SnF4(aq) + 4Cr(s)  4CrF3(aq) + 3Sn(s)
4.
2(NH4)3PO4(aq) + 3CaBr2(aq)  1Ca3(PO4)2(s) + 6NH4Br(aq)
5.
1C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)
6.
1Al(NO3)3(aq) + 3NaOH(aq)  3NaNO3(aq) + 1Al(OH)3(s)