Chapter 11 – Chemical Reactions
Section 11.1 Describing Chemical
Reactions (Rxns)
• OBJECTIVES:
– Describe how to write a word equation.
– Describe how to write a skeleton
equation.
– Describe the steps for writing a balanced
chemical equation.
Chemical Rxns
• Have two parts:


Reactants = the starting substances
Products = the ending substances
• The reactants turn into the products
• Reactants  Products
• Atoms aren’t created or destroyed (according to
the Law of Conservation of Mass)
• A reaction can be described several ways:
 In
a sentence every item is a word
Copper reacts with chlorine to form copper (II) chloride.
 In a
word equation some symbols used
Copper + chlorine  copper (II) chloride
Symbols in
Equations
(Text page 323)
•  used after a
product indicates
a gas has been
produced: H2↑
•  used after a
product indicates
a solid has been
produced: PbI2↓
• A catalyst is a substance that speeds up a reaction, without
being changed or used up by the rxn.
• Enzymes are biological or protein catalysts in your body.
The Skeleton Equation
• Uses formulas and symbols to describe a rxn, but it doesn’t
indicate how many, so they are usually NOT balanced
• Chemical equations are a description of the rxn
• Write a skeleton equation for:
1. Solid iron (III) sulfide reacts with gaseous hydrogen chloride to
form iron (III) chloride and hydrogen sulfide gas.
2. Nitric acid dissolved in water reacts with solid sodium
carbonate to form liquid water and carbon dioxide gas and
sodium nitrate dissolved in water.
• Now, read these equations:
Fe(s) + O2(g)  Fe2O3(s)
Cu(s) + AgNO3(aq)  Ag(s) + Cu(NO3)2(aq)
NO2(g)
N2(g) + O2(g)
Balanced Chemical Equations
• Atoms can’t be created or destroyed in an
ordinary rxn:
– All the atoms we start with we must end up with
• A balanced equation has the same number of each
element on both sides of the equation.
• Rules for balancing:
1) Assemble the correct formulas for all the reactants
and products, using “+” and “→”
2) Count the number of atoms of each type appearing
on both sides
3) Balance the elements one at a time by adding
coefficients (the numbers in front) – save balancing
the H and O until LAST!
4) Double-Check to make sure it is balanced.
Never, ever…
• Never change a subscript to balance an equation (only
change coefficients)
– When the subscript (formula) is changed, the chemical is
changed; H2O is a different compound than H2O2
• Never put a coefficient in the middle of a formula; they can
only go in the front; 2NaCl, NOT Na2Cl; practice balancing…
• ___AgNO3 + ___Cu  ___Cu(NO3)2 + ___Ag
• ___Mg + ___N2  ___Mg3N2
• ___P + ___O2  ___P4O10
• ___Na + ___H2O  ___H2 + ___NaOH
• ___CH4 + ___O2  ___CO2 + ___H2O
Section 11.2 Types of Chemical Rxns
• OBJECTIVES:
– Describe the five general types of rxns.
– Predict the products of 5 general types of rxns.
Types of Reactions
• There are 5 major types of rxns.
– Combination, or synthesis (2 or more reactants yield 1 product)
• Element or cmpd + element or cmpd  Compound
– Decomposition (1 reactant yields 2 or more products)
• Compound  element or cmpd + element or cmpd
– Single replacement (swap once)
• Compound + element  (different) compound + (different) element
– Double replacement (swap twice; a double date that swaps dates)
• Compound + compound  (different) compound + (different) compound
– Combustion (when reacting a hydrocarbon)
• Hydrocarbon + oxygen  carbon dioxide + water
Combination Reactions
• Combine = put together = synthesize, or
synthesis
• Ca + O2  CaO; SO3 + H2O  H2SO4
• Complete and balance the following:
– Mg + N2 
– Ca + Cl2 
– Fe + O2  (assume iron (II) oxide is the product)
– Al + O2 
Decomposition Reactions
• Decompose = fall apart; 1 reactant breaks
apart into two or more elements or cmpds
electricity

• NaCl   Na + Cl2


• CaCO3  CaO + CO2
• We can predict the products if the reactant is
a binary cmpd (made up of only 2 elements)
–It breaks apart into the elements:
electricity
• H2O   
• HgO  
Single Replacement Reactions
• One element replaces another
• Reactants must be an element and a compound.
• Products will be a different element and a
different compound.
• Na + KCl  K + NaCl (Cations switched)
• F2 + LiCl  LiF + Cl2 (Anions switched)
• Metals will replace other metals (and they can
also replace hydrogen)
• K + AlN 
• Zn + HCl 
Single Replacement Reactions
• Think of water as: HOH
– Metals replace the first H, and then combines
with the hydroxide (OH).
• Na + HOH 
• We can even tell whether or not a single
replacement reaction will happen:
– Because some chemicals are more “active”
than others
– More active replaces less active
“Activity Series”
Higher
activity
Lower
activity
1) Metals from sodium upward can
Lithium
Potassium
replace hydrogen in water.
Calcium
2) Metals can replace other metals,
Sodium
Magnesium
provided they are above the metal
Aluminum
they are trying to replace (for
Zinc
Chromium
example, zinc will replace lead)
Iron
3) Metals above hydrogen can
Nickel
Lead
replace hydrogen in acids.
Hydrogen
Bismuth
Higher Activity
Halogens
can
replace
other
Copper
halogens in cmpds, provided
Mercury
Silver
they are above the halogen
Platinum
they are trying to replace.
Gold
2NaCl(s) + F2(g)  2NaF(s) + Cl2(g)
MgCl2(s) + Br2(g)  No Reaction!
Fluorine
Chlorine
Bromine
Iodine
Lower Activity
Double Replacement Reactions
• Two things replace each other.
– Reactants must be two ionic compounds, in
aqueous solution
• NaOH + FeCl3 
– The positive ions change place.
• NaOH + FeCl3  Fe+3 OH- + Na+1 Cl-1
= NaOH + FeCl3  Fe(OH)3 + NaCl
Double Replacement Reactions
• Have certain “driving forces”, or reasons
–Will only happen if one of the products:
a) doesn’t dissolve in water and forms a
solid (a “precipitate”), or
b) is a gas that bubbles out, or
c) is a molecular compound (which will
usually be water).
Complete and balance:
CaCl2 + NaOH 
CuCl2 + K2S 
KOH + Fe(NO3)3 
(NH4)2SO4 + BaF2 
Fe + CuSO4 
Pb + KCl 
Al + HCl 
Practice Examples:
•
•
•
•
•
•
•
H2 + O2 
H 2O 
Zn + H2SO4 
HgO 
KBr + Cl2 
AgNO3 + NaCl 
Mg(OH)2 + H2SO3 
Combustion Reactions
• Combustion means “add oxygen”
• Normally, a compound composed of only C, H,
(and maybe O) is reacted with oxygen – usually
called “burning”
• If the combustion is complete, the products will
be CO2 and H2O.
• C4H10 + O2 
• C5H12 + O2 
• C6H12O6 + O2 
• C8H8 + O2 
SUMMARY: Equations and Reactions
• Equations describe a reaction
– Must be balanced in order to follow the Law of Conservation
of Mass
– Can only be balanced by changing the coefficients.
– Has special symbols to indicate the physical state, if a catalyst
or energy is required, etc.
• Reactions come in 5 major types.
– We can tell what type they are by looking at the reactants.
– Single Replacement happens based on the Activity Series
– Double Replacement happens if one product is: 1) a precipitate
(an insoluble solid), 2) water, or 3) a gas.