Unit 6
Chemical Reactions
Writing Chemical Equations
• All chemical reactions have two parts:
• Reactants - the substances you start with (on
left side of arrow)
• Products - the substances you end up with (on
right side of arrow)
• The reactants turn into the products.
Reactants  Products
In a chemical reaction…
• The way atoms are joined is changed.
• Atoms aren’t created or destroyed; they just
combine together in new ways.
– Law of Conservation of Mass
• Can be described using sentences, word
equations, or skeleton equations (symbols):
#1. In a sentence every item is a word
Copper reacts with chlorine to form
copper (II) chloride.
#2. In a word equation symbols used w/
element or compound names
Copper + chlorine  copper (II) chloride
#3. In a skeleton equation formulas and
symbols are used to describe a reaction
Cu + Cl2  CuCl2
– These are NOT balanced (# of atoms on each side is
not necessarily equal)
Symbols used in Equations
• The arrow () separates the reactants from the
products; means “reacts” or “yields”
• The plus sign (+) = between products or
between reactants; means “and”
• Subscripts are used to describe the number of
atoms in a FORMULA.
• Coefficients are used to describe the number of
molecules in the REACTION. They are the only
things changed when balancing a reaction.
Subscripts vs. Coefficients
3 CuCl2 + 2 Al  2 AlCl3 + 3 Cu
On the reactant side of the equation:
How many copper (II) chloride molecules react?
How many total chlorine atoms are there?
How many aluminum atoms are there?
States of Matter
• Solid--(s) after the formula
– Precipitate -- a solid formed in a reaction
• Gas--(g) after the formula
• Liquid—(l) after the formula
• Aqueous-- (aq) after the formula dissolved in water.
Other Symbols used in Equations
•
•
indicates a reversible reaction
(More later)

heat
  ,    show that heat is
supplied to the reaction
Pt
•   is used to indicate a catalyst used
or supplied, in this case, platinum.
Must Memorize Elements THAT
EXIST AS DIATOMIC
MOLECULES
Remember
HON17 !!!
Write a skeleton equation for:
Solid iron (III) sulfide reacts with gaseous hydrogen
chloride to form solid iron (III) chloride and
hydrogen sulfide gas.
Types of Reactions
• There are probably millions of reactions.
• We can’t remember them all, but luckily
they will fall into several categories.
• We will learn the 5 major types.
– We will then be able to predict the
products of a reaction when given just the
reactants!
Types of Equations
Synthesis (or Combination)
Equation in Symbols: A + B  AB
Sample Equation:
2Cu (s) + O2 (g)  2 CuO (s)
2 or more elements combine to form
only 1 product
Types of Equations
Decomposition
Equation in Symbols: AB  A + B
Sample Equation:
2 NH3 (g)  N2 (g) + 3 H2 (g)
1 reactant breaks down into more
than 1 product
Types of Equations
Single Replacement
Equation in Symbols: A + BC  AB + C
Sample Equation:
Mg (s) + CuCl2 (aq)  Cu (s) + MgCl2 (aq)
• Metal replaces metal; Nonmetal replaces
nonmetal
– Positive ion replaces positive ion; negative ion
replaces negative ion!
Types of Equations
Double Replacement –
Equation in Symbols: AX + BY  BX + AY
Sample Equation:
2AgNO3(aq) + CuCl2 (aq)  Cu(NO3)2 (aq) + 2AgCl (s)
2 ionic compounds switching “partners”
Types of Equations
Combustion
Equation in Symbols:
CxHy + O2 CO2 + H2O
Sample Equation:
CH4(g) + O2 (g)  CO2 (g) + H2O (l)
Hydrocarbons react to form CO2 and H2O
Examples: Identify the Type of Rxn.
1. 2 AgNO3 + Ni  Ni(NO3)2 + 2 Ag
2. BaCO3  BaO + CO2
3. 4 Cr + 3 O2  2 Cr2O3
4. Ca + 2 HCl  CaCl2 + H2
5. Cu(OH)2 + 2 HC2H3O2  Cu(C2H3O2)2 + 2 H2O
6. C8H18 + O2  CO2 + H2O
Balancing Chemical Equations
• Atoms can’t be created or destroyed in
an ordinary reaction:
– All the atoms we start with we must end
up with (meaning: balanced!)
• A balanced equation has the same
number of atoms of each element on
both sides of the equation.
Balancing Equations
Chemical “bookkeeping” of atoms
involved in the reaction:
H2(g) + O2(g)  H2O(g)
H–2
O–2
Reactants
H–2 O–1
Products
Note the imbalance in oxygen atoms
COEFFICIENTS must be added so reactant
atoms EQUAL product atoms!
Rules for Balancing Chemical Equations
1) Count the number of atoms of each type
of element appearing on both sides
2) Balance the elements one at a time by
adding coefficients (the numbers in
front) where you need more - save
balancing the H and O until LAST!
(HINT: save O until the very last)
3) Double-Check to make sure it is balanced.
• Never change a subscript to balance an equation
(You can only change coefficients)
– If you change the subscript (formula) you are
describing a different chemical.
– H2O is a different compound than H2O2
• Never put a coefficient in the middle of a formula;
they must go only in the front
2NaCl is okay, but Na2Cl is not.
Hints & Tips for Balancing Equations
• Take one element at a time, working from left to right
• Save H for next to last and O for last.
• (Shortcut) polyatomic ions that appear on both sides of
the equation should be balanced as independent units
• IF EVERYTHING BALANCES EXCEPT FOR O, and there is
no way to balance O with a whole number, double all
the coefficients and try again. (Because O is a diatomic
element)
– Same goes for HON17
Must Memorize Elements THAT
EXIST AS DIATOMIC
MOLECULES
Remember
HON17 !!!
Balancing Equations Practice
Balance the following chemical equations using the
appropriate coefficients:
____ Al(s) + _____ Br2 (l)  _____ Al2Br6 (s)
Balancing Equations Practice
Balance the following chemical equations using the
appropriate coefficients:
____ Na3PO4 + ____ Fe2O3  ____ Na2O + ____ FePO4
Predicting Products - Single Replacement Rxns.
• One element replaces another
• Reactants must be 1 element and 1 ionic
compound.
• Products will be a different element and a
different compound.
• Na + KCl  K + NaCl
• F2 + LiCl  LiF + Cl2
(Cations switched)
(Anions switched)
Predicting Products - Single Replacement Rxns.
• Metals will replace other metals (and they
can also replace hydrogen)
• K + AlN 
• Zn + HCl 
• Think of water as: HOH
– Metals replace the first H, and then combines
with the hydroxide (OH).
• Na + HOH 
Single Replacement Rxns. Practice:
• Fe + CuSO4 
• Pb + KCl 
• Al + HCl 
Predicting Products - Double Replacement Rxns.
• Two things replace each other.
– Reactants must be two ionic compounds, in
aqueous solution
• NaOH + FeCl3 
– The positive ions change place.
NaOH + FeCl3 Na+1 Cl-1 + Fe+3 OH-1
NaOH + FeCl3 Fe(OH)3 + NaCl
Complete and balance:
__CaCl2 + __NaOH 
__CuCl2 + __K2S 
__KOH + __Fe(NO3)3 
__(NH4)2SO4 + __BaF2 
Predicting Products – Combustion Rxns.
• Combustion means “add oxygen”
• Normally, a compound composed of only
C, H, (and maybe O) is reacted with
oxygen (O2) – usually called “burning”
• In a combustion reaction, the products
will ALWAYS be CO2 and H2O.
C3H8 + O2  CO2 + H2O
Mixed Practice Examples:
• H2 + O2 
• H2O 
• Zn + H2SO4 
• HgO 
• KBr + Cl2 
• AgNO3 + NaCl 
• Mg(OH)2 + H2SO3 