Name __________________________________________________ Date __________________________ Hour _______
Balancing Chemical Equations
Physical States:
Besides specifying the compounds involved in a reaction, we often indicate the physical states of the reactants
and the products by using the following state symbols.
Symbol (subscript used)
State (phase)
(s)
(l)
(g)
(aq)
Solid
Liquid (a pure solution)
Gas
Aqueous solution (a metal salt dissolved in water or an acid or base).
The word solution is your clue that you are to use the subscript (aq).
Example:
When solid potassium reacts with liquid water, the products are hydrogen gas and potassium hydroxide
solution.
potassium
solid
2K(s)
water
liquid
+
2H2O(l)
hydrogen
gas

H2(g)
potassium hydroxide
solution
+
2KOH(aq)
When writing chemical equations from written chemical reactions, use these CLUES and HINTS:
 Products usually follow words like produces, yields, forms, decomposes
 Watch for our diatomic elements (Br2, I2, N2, Cl2, H2, O2, F2) which are often (but not always) gases
 Include ‘state subscripts’ behind each substance [(s), (l), (g) (aq)] when the state (phase) is given
 Remember air is a mixture of (primarily) two gases, O2 and N2. Oxygen is the most likely to
participate in a reaction.
 Elemental metals exist as single, unbonded atoms. (Ex: formula for copper metal is Cu)
 Watch for ionic vs. molecular compounds. Use nomenclature rules and your periodic table to figure
out the formulas for these.
PART I:
Balance the following equations by inserting the proper coefficients. You must keep track of the total
number of elements on each side of the equation by keeping a tally beneath each reaction. For selected
reactions, draw Before and After particle diagrams to show the particles involved in the reaction instead of
doing the tally.
1.
____C + ____H2O  ____CO + ____H2
2.
____MgO  ____Mg + ____O2
3.
____Al
+
#3
____O2

____Al2O3
Before
4.
____Zn + ____H2SO4  ____ZnSO4 + ____H2
5.
____Cl2 + ____KI  ____KCl + ____I2
6.
____CuCl  ____Cu + ____Cl2
#3
After
7.
____Li + ____Cl2  ____NaCl
8.
____Al +
____HCl

____AlCl3
+
#8 Before
9. ____ZnCl2 + ____(NH4)2S  ____ZnS + ____ NH4Cl
a) Find the molar mass of the reactants in # 9.
b) How many moles of ZnCl2 would be in 25.0 g of ZnCl2?
c) How much mass would 0.55 moles of (NH4)2S have?
____H2
#8 After
Part II:
Write the formulas of the reactants and products, then balance the equations. (See Clues and Hints.)
1.
Nitrogen monoxide gas (NO) reacts with ozone gas (O3) to produce nitrogen dioxide gas and oxygen gas.
2.
Solid iron burns in air to form a black solid, Fe3O4.
3.
When hydrogen gas reacts with bromine gas, hydrogen bromide gas is produced.
4.
Acetylene gas, C2H2, burns in air to form carbon dioxide gas and liquid water.
5.
Liquid hydrogen peroxide (H2O2) easily decomposes into liquid water and oxygen gas.
6.
Solid hydrazine (N2H4) and liquid hydrogen peroxide (H2O2) are used together as rocket fuel. The products
are nitrogen gas and gaseous water.
7.
If solid potassium chlorate is strongly heated, it decomposes to yield oxygen gas and solid potassium
chloride.
8.
When sodium hydroxide is added to sulfuric acid (H2SO4), the products are liquid water and sodium sulfate
solution.
9.
In the Haber process, hydrogen gas and nitrogen gas react to form ammonia, NH3 gas.