Unit 8 - Chemical
Reactions
Student Outcomes:
• I can give 5 indications that a chemical reaction has occurred.
• I can use the activity series to predict the reactivity of chemical
reactions.
•
•
•
•
I can describe the five basic types of chemical reactions,
including single displacement/replacement, double
displacement/replacement, synthesis, decomposition, and
combustion.
I can explain how to identify the type of chemical reactions.
I can balance chemical equations when given the reactants and
products.
I can predict products in chemical reactions.
Indications that a chemical
reaction has occurred:
•
Unexpected change in temperature (it
gets hotter or colder but you did not
intentionally add or remove heat)
•
•
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Unexpected color change
Unexpected odor change
Unexpected solid formation (precipitate)
or unexpected gas formation (bubbles)
Watch these chemical
reactions. Notice the evidence
of a chemical reaction taking
place.

https://www.youtube.com/watch?v=lJjfb2
ydVzU
Types of Reactions
•
There are five types of chemical
reactions we will talk about:
1.
2.
3.
4.
5.
•
Synthesis reactions
Decomposition reactions
Single displacement reactions
Double displacement reactions
Combustion reactions
You need to be able to identify the type
of reaction and predict the product(s)
Steps to Writing Reactions
Main steps for doing reactions:
1.
2.
3.
4.
Identify the type of reaction
Predict the product(s) using the type of
reaction as a model.
Use subscripts to write the compounds.
Use coefficients to balance the whole reaction
so that there are the same number of each
element on each side of the reaction equation.
Don’t forget about the diatomic elements!
(BrINClHOF)
If any of these are NOT part of a compound, they
must be diatomic.
Bromine = Br2
Nitrogen = N2
Hydrogen = H2
Fluorine = F2
Iodine = I2
Chlorine = Cl2
Oxygen = O2
The word problem will simply state the name but
you will write the diatomic in the reaction.
If it is part of a compound, do not add a suffix
unless the criss-cross requires it.
Example: magnesium oxide = MgO
Indicators of state (these are written in
italics and parenthesis after the reactants and products)




(s) = solid state
(l) = liquid state
(g) = gas state
(aq) = dissolved in water (aqueous)
Note: Hg(l) means liquid mercury
Hg+2(aq) or HgO(aq) means the mercury
ions are dissolved in water.
1. Synthesis reactions
•
•
Synthesis reactions occur when two
substances (usually elements) combine and
form a compound. (Sometimes these are
called “combination” or “addition” reactions.)
reactant + reactant  1 product
Basically: A + B  AB
•
•
Example: 2H2 + O2  2H2O
Example: C + O2  CO2
Synthesis Reaction Example
1. Predict the products. (Criss-Cross for subscripts.)
2. Balance the reactions. (Use only coefficients.)
•
•
•
Sodium metal reacts with chlorine gas
Na(s)
+
Cl

2Na
Cl
→
2NaCl
2(g)
(s) +
2(g)
Solid Magnesium reacts with fluorine gas
Mg(s)
 MgF2
Mg
(s)++FF
2(g)→
2(g)
Aluminum metal reacts with fluorine gas
Al(s)
+ F 2(g)
2Al
→ 2AlF3
(s) + 3F2(g)
2. Decomposition Reactions
•
•
•
•
•
Decomposition reactions occur when a
compound breaks up into the elements or
in a few to simpler compounds
1 Reactant  Product + Product
In general: AB  A + B
Example: 2 H2O  2H2 + O2
Example: 2 HgO  2Hg + O2
Decomposition Reaction:
Decomposition Exceptions
•
Carbonates and chlorates are special case
decomposition reactions that do not go to
the elements.
•
Carbonates (CO32-) decompose to carbon
dioxide and a metal oxide
•
•
Chlorates (ClO3-) decompose to oxygen gas
and a metal chloride
•
•
Example: CaCO3  CO2 + CaO
Example: 2 Al(ClO3)3  2 AlCl3 + 9 O2
There are other special cases, but we will not
explore those in high school Chemistry.
1. Predict the products. (Criss-Cross for subscripts.)
2. Balance the reactions. (Use only coefficients.)
•
•
Solid Lead (IV) oxide decomposes
PbO2(s)PbO
→2(s)
Pb
(s) + O2(g)
Aluminum nitride decomposes
2AlN(s)
→(s)2Al
AlN
 (s) + N2(g)
Practice
Identify the type of reaction for each of the
following synthesis or decomposition
reactions, and write the balanced equation:
N
++O2(g)
→
monoxide
N2(g)
O
2NO (g)
synthesis
2(g)
2(g) Nitrogen
BaCO3(s)
 CO2(g) + BaO
3(s) →
decomposition
Co
S(s)3S
 (s)
2Co
→ Cobalt
Co2S3 (III) synthesis
(use
in product)
(s)+
(s) +
NI
→ N2(g) + 3I2(g)
2NI
3(s)
3(s)
decomposition
3. Single Replacement Reactions
•
•
•
Single Replacement Reactions occur when one
element replaces another in a compound.
A metal can replace a metal (+) OR
a nonmetal can replace a nonmetal (-).
element + compound element + compound
A + BC  AC + B (if A is a metal) OR
A + BC  BA + C (if A is a nonmetal)
(remember the cation always goes first!)
When H2O splits into ions, it splits into
H+ and OH- (not H+ and O-2 !!)
Single Replacement Reaction
Single Replacement Reactions
Write and balance the following single
replacement reaction equation:
• Zinc metal reacts with aqueous
hydrochloric acid (HCl)
Zn(s) + 2 HCl(aq)  ZnCl2 + H2(g)
Note: Zinc (cation) replaces the hydrogen
(cation) in the reaction
•
Single Replacement Reactions
•
Sodium chloride solid reacts with fluorine gas
2 NaCl(s) + F2(g)  2 NaF(s) + Cl2(g)
Note that fluorine (anion) replaces chlorine (anion) in the compound
Aluminum metal reacts with aqueous
copper (II) nitrate
2 Al(s)+ 3 Cu(NO3)2(aq) 3 Cu(s) + 2 Al(NO3)3
•
4. Double Replacement Reactions
•
•
•
Double Replacement Reactions occur
when a metal replaces a metal in a compound
and a nonmetal replaces a nonmetal in a
compound
Compound + compound  compound+
compound
AB + CD  AD + CB
Double Replacement Reactions
•
•
•
Think about it like trading square dance
partners- cations are always paired with anions.
Example:
AgNO3(aq) + NaCl(s)  AgCl(s) + NaNO3(aq)
Pretend Ag and Na are guys partnered with
different girls in the dance. Ag switches
partners with Na. NO3 is a polyatomic, so it acts
like one partner. Remember the Ag and Na are
still written first in the products (not NO3Na).
Double Replacement Reactions
•
Another example:
K2SO4(aq) + Ba(NO3)2(aq)  2 KNO3(aq) + BaSO4(s)
Pretend K and Ba are guys partnered with different girls in the
dance. K switches partners with Ba. Both SO4 and NO3 are
polyatomics, so they act like one partner each.
Notice the subscripts are not the same on each side because they
depend on the criss-cross of the compound itself and are not
carried over to the other side of the equation.
Predict the products by “trading partners.”
Use subscripts to write the compounds.
Use coefficients to balance the equation.
5.
HCl(aq) + AgNO3(aq) 
CaCl2(aq) + Na3PO4(aq) 
Pb(NO3)2(aq) + BaCl2(aq) 
FeCl3(aq) + NaOH(aq) 
H2SO4(aq) + NaOH(aq) 
6.
KOH(aq) + CuSO4(aq) 
1.
2.
3.
4.
See next slide for answers.
Predict the products by “trading partners.”
Use subscripts to write the compounds.
Use coefficients to balance the equation.
5.
HCl(aq) + AgNO3(aq)  HNO3 + AgCl
3 CaCl2(aq) + 2 Na3PO4(aq)  Ca3(PO4)2 + 6 NaCl
Pb(NO3)2(aq) + BaCl2(aq)  PbCl2 +Ba(NO3)2
FeCl3(aq) + 3 NaOH(aq)  Fe(OH)3 + 3 NaCl
H2SO4(aq) + 2 NaOH(aq)  2 H2O + Na2SO4
6.
2 KOH(aq) + CuSO4(aq)  K2SO4 + Cu(OH)2
1.
2.
3.
4.
5. Combustion Reactions
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•
Combustion reactions
occur when a hydrocarbon
reacts with oxygen gas.
This is also called
burning!!! In order to burn
something you need the 3
things in the “fire
triangle”:
1) A Fuel (hydrocarbon)
2) Oxygen to burn it with
3) Something to ignite the
reaction (spark)
Combustion Reactions
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In general:
CxHy + O2  CO2 + H2O
Products in combustion are
ALWAYS carbon dioxide and
water. (although incomplete
burning does cause some byproducts like carbon monoxide)
Combustion is used to heat
homes and run automobiles
(octane, as in gasoline, is C8H18)
Combustion
Reactions
Edgar Allen Poe’s
drooping eyes and
mouth are potential
signs of CO
poisoning.
Combustion
•
Example
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C5H12 + 8 O2  5 CO2 + 6 H2O
Write the products and balance the
following combustion reaction:
•
2 C10H22 + 31 O2  20 CO2 + 22H2O
Mixed Practice
•
1.
2.
3.
4.
5.
State the type, predict the products, and
balance the following reactions:
BaCl2 + H2SO4  BaSO4 + 2 HCl
C6H12 + 9 O2  6 CO2 + 6 H2O
Zn + CuSO4  ZnSO4 + Cu
2Cs + Br2  2 CsBr
FeCO3  FeO + CO2