Classifying Chemical
Reactions
Chemistry
Classifying Chemical reactions
Essential Questions
1.
2.
Is there a way to organize chemical
reactions to make them easier to
understand?
And maybe easier to balance?
Types of Reactions


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We classify reactions to make predicting the
products easier
Each type of reaction follows a certain pattern
which make balancing easier:)
But…The 5 reactions we talk about are not
the only types
And …each reaction does not have to fit into
just one type

Maybe more than one maybe none
5 types of reactions
1)
2)
3)
4)
5)
Synthesis
Decomposition
Single Replacement
Double Replacement
Combustion
Video: http://www.youtube.com/watch?v=iHHvx1VC_8 (1:29 min)
(1) Synthesis Reactions
Synthesis means “to put together”
A + B  AB
Two or more reactants react to form
one product
Examples:


CaO (s) + H2O (l)  Ca(OH)2 (s)
2SO2(g) + O2 (g)  2SO3 (g)
Example of Synthesis
Let’s practice synthesis
Predict the products. Write and balance the
following synthesis reaction equations.
• Sodium metal reacts with chlorine gas
Na(s) + Cl2(g) 
• Solid Magnesium reacts with fluorine gas
Mg(s) + F2(g) 
• Aluminum metal reacts with fluorine gas
Al(s) + F2(g) 
(2) Decomposition Reactions
Opposite of synthesis reactions
A single compound breaks down into two or
more elements or simpler compounds
AB  A + B
Often need an input of energy (such as heat,
light, or electricity) to make the reaction happen
Decomposition Reactions
Example: Car air bags
Sodium azide pellets
decompose to produce
sodium and nitrogen gas
hat inflates the airbag!
2NaN3(s)  2Na(s) + 3N2(g)
Let’s practice decomposition
Predict the products. Then, write and balance
the following decomposition reaction
equations:
• Solid Lead (IV) oxide decomposes
PbO2(s) 
• Aluminum nitride decomposes
AlN(s) 
Replacement Reactions
Aka displacement reactions
Two types:
Single replacement and Double replacement
(3) Single Replacement



A single element reacts with a compound and
displaces another less reactive element from
the compound
Like they switch places
Example:
Mg + Zn(NO3)2 --> Mg(NO3)2 + Zn
Single replacement reaction
For example:
Zinc metal reacts with aqueous hydrochloric
acid
Zn(s) + HCl(aq)  ZnCl2 + H2(g)
Zinc replaces the hydrogen ion in the reaction
Single replacement
•
A metal can replace hydrogen or another
metal (+) OR a nonmetal can replace a
nonmetal (-).
Single Replacement
Do single replacement reactions always
occur?
 NO! A metal will only replace a less reactive
metal.
 The activity series is a way to predict whether
or not certain reactions will occur.
 Any specific metal can replace any metal
listed below it that is in a compound.
 It cannot replace any metal listed above it!
What is the Activity Series?



Activity series a series of elements that
have similar properties and that are arranged
in descending order of chemical activity
(meaning most reactive at the top)
Elements can only “switch” places with
elements below it on the activity series
Activity Series for Metals
Activity series for Halogens
Do single replacement reactions only occur
for metals?
 No – non metals or halogens can also
replace less reactive halogens in single
replacement reactions.
Activity Series for
Halogens

Each replaces
halogens listed below it.

For example, fluorine
will replace chlorine,
chlorine will replace
bromine, etc.
Step for doing single
replacement reactions
1) Identify the reactants
Determine if element is metal or halogen
Determine the element that might be displaced from
compound
2) Check the activity series
Determine which element is more active
3) Write the products and balance the equation
If the more active element is already part of the compound
then NO reaction will happen
4) Check your work
Make sure the equation is balanced (just like always)
Check for Understanding
Will the following react?
If yes, write the balanced equation.
If no, write NR for no reaction!
1. Cu + ZnSO4 (compare copper and zinc)
2. Fe + CuNO3 (compare iron and copper)
(iron takes +2 charge)
1. Al + Fe2O3 (compare aluminum and iron)
Check for Understanding
Will the following react?
If yes, write the balanced equation.
If no, write no reaction!
1. Cu + ZnSO4  no reaction
2. Fe + 2CuNO3  Fe(NO3)2 + 2Cu
3. 2Al + Fe2O3  Al2O3 + 2Fe
STOP!

Work on identifying and writing the first three
types of reactions on the next worksheet both
sides.
(4) Double- replacement
Reactions
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When a metal replaces a metal in a compound
and a nonmetal replaces a nonmetal in a
compound
A reaction in which a gas, a solid precipitate, or
a molecular compound forms from the apparent
exchange of atoms or ions between two
compounds
Happens in aqueous solutions
Must end up with a solid precipitate, gas, or
molecular compound (like water)
Double replacement
Double Replacement
•
Think about it like “foil”ing in algebra, first
and last ions go together + inside ions go
together
•
AgNO3(aq) + NaCl(s)  AgCl(s) + NaNO3(aq)
Special DD – Acid/Base
Reaction

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Type of double displacement reaction that
takes place when an acid and base react with
each other.
The H+ ion in the acid reacts with the OH- ion
in the base, causing the formation of water.
Product of this reaction is typically an ionic
salt and water:
HA + BOH ---> H2O + BA
Special DD – Acid/Base
Reaction
•
•
In double- displacement reactions water, as a
product, is often written as HOH so you can
more easily see the exchange of
atoms/compounds
HCl(aq) + NaOH(aq) -> HOH(l) + NaCl(aq)
This is a double replacement reaction
•
•
The H and the Na changed partners
You formed water (that’s one of the signs!!)
Practice double replacement
1. HCl(aq) + AgNO3(aq) 
(hint: silver takes a +1 charge)
2. CaCl2(aq) + Na3PO4(aq) 
(hint: You can tell that phosphate takes a -3 charge)
3. Pb(NO3)2(aq) + BaCl2(aq) 
(hint: You can tell that lead takes a +2 charge)
4. HBr + NaOH 
(5) Combustion Reactions
Combustion reaction
Reaction of a hydrocarbon with oxygen
AKA “burning!”
Products are carbon dioxide and water vapor
Releases energy in the form of heat & light


Ex: the lighting in here is because of the
combustion of coal
Do you have any others?
Hydrocarbons

Just a fancy way to say the compound only
has hydrogen and carbon in it
Let’s try some:
___C3H8 + ___O2  ___CO2 + ___H2O
___C5H12 + ___O2 
___ C10H22 + ___ O2 
How to identify the type of
reaction!
Follow this series of questions. When you can
answer "yes" to a question, then stop!
1) Does your reaction have oxygen as one of
its reactants and carbon dioxide and water as
products? If yes, then it's a combustion
reaction
2) Does your reaction have two (or more)
chemicals combining to form one chemical? If
yes, then it's a synthesis reaction
3) Does your reaction have one large molecule
falling apart to make several small ones? If
yes, then it's a decomposition reaction
4) Does your reaction have any molecules that
contain only one element? If yes, then it's a
single displacement reaction
5) If you haven't answered "yes" to any of the
questions above, then you've got a double
displacement reaction
Check for understanding
List what type the following reactions are:
1) NaOH + KNO3 --> NaNO3 + KOH
2) CH4 + 2 O2 --> CO2 + 2 H2O
3) 2 Fe + 6 NaBr --> 2 FeBr3 + 6 Na
4) CaSO4 + Mg(OH)2 --> Ca(OH)2 + MgSO4
5) Pb + O2 --> PbO2
6) Na2CO3 --> Na2O + CO2
1)
2)
3)
4)
5)
6)
double displacement
combustion
single displacement
double displacement
synthesis
decomposition
Pharaoh's Snakes
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Now for something fun

Check this out…
http://www.youtube.com/watch?v=ritaljhhk7s
Pharaoh's Snakes
Igniting mercury(II) thiocyanate Hg(SCN)2
causes it to decompose into an insoluble brown
mass that is primarily tricarbon tetranitride,
mercury(II) sulfide and carbon disulfide.
Write the decomposition reaction and balance
it!
2Hg(SCN)2 → 2HgS + CS2 + C3N4
Pharaoh's Snakes
Then flammable carbon disulfide combusts in
oxygen to produce carbon dioxide and sulfur
dioxide:
Write the combustion reaction and balance it!
CS2 + 3O2 → CO2 + 2SO2
Stop!
Let’s practice what we have learned with some
worksheets!
“Writing Net Ionic Equations”
Goals
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Write total ionic equations for reactions in
aqueous solutions
Identify spectator ions and write ionic
equations for reactions in aqueous solutions
Ionic Equations
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When ionic compounds dissolve in water, the
ions separate form each other and spread
throughout the solution
So when you see KI(aq) what it really means is
K+(aq) + I-(aq)
Or if you see Pb(NO3)2(aq) it means Pb+2(aq) +
2NO3-(aq)
More Correct
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When we react the two compounds on the
last slide we would do this:
KI(aq)+ Pb(NO3)2(aq) -> PbI2(s) + 2KNO3(aq)
But we should put the charges on them and
then balance the charges and masses
K+(aq) + I-(aq) + Pb+2(aq) + 2NO3-(aq) -> PbI2(s) +
2K+ (aq) + 2NO3-(aq)
Most Correct
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When you mix two solutions all of the ions are
present in the combined solution
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But not all of the ions will react with each other
Spectator ions=> ions that are present in a solution
in which a reaction is taking place but that do not
participate in the reaction
These spectator ions can be taken out of the total
ionic equation
Example

Bottom of page 286
Writing Net Ionic Equations

These equations only show the compounds
that change
Displacement Net Ionic

Happens in displacement reactions too

Zn(s)+Cu+2(aq)+SO4-2(aq)-> Cu(s)+Zn+2 (aq)+ SO4-2(aq)

Which is the spectator ion?
How would we correctly write this?

Skills Toolkit Page 288
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1) List what you know
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2) Write a balanced equation
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Predict products
Include states (next slide shows how to know!!!)
3) Write total ionic equation
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ID each chemical described
ID type of reaction
Write separated aqueous ions for each aqueous ionic
substance
DO NOT split up anything that is a solid, liquid, or gas
4) Find the net ionic equation
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Cancel out spectator ions
Double check numbers and charges
Solubility
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All group 1 compounds and NH4
All nitrates
All halides (but Ag+ and Pb+2)
All sulfates (but Ca+2, Sr+2, Ba+2, Hg+2,
and Pb+2)
All carbonates (except group 1 and NH4) are
INSOLUBLE!
Charges
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Make sure charges are equal on both sides
SO4-2 (aq) + Ba+2 (aq) -> BaSO4(s)
Notice no charge on reactant side so no
charge on product side
Practice
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Br2(l) + NaI(aq) -> NaBr(aq) + I(s)
Br2(l) + 2Na+(aq) + 2I-(aq) -> 2Na+(aq) +
2Br-(aq) + I2(s)