1. Types of Reactions Notes
Section 9.2

2. Reaction Type Notes
Most chemical reactions can be categorized 
into one of five types.
1. Synthesis/Composition Reactions
2. Decomposition Reactions
3. Single Replacement Reactions
4. Double Replacement Reactions (Precipitates)
5. Combustion Reactions

3. You can usually identify the reaction type by looking at the reactants.
Once the type of reaction is identified, we can predict the products.

4. 1. Synthesis/Composition Reactions
A synthesis reaction is a reaction in which two or more substances react 
to produce a single product.
General form:
A + B  AB or
A + BC  ABC
AB + CD  ABCD

5. When only two elements react, the 
reaction is always a synthesis 
reaction.

6. Example: 2 H2 + O2  2 H2O
Draw the reaction.

7. Generally exothermic (release energy).
1. Synthesis/Composition Reactions
These reactions generally happen spontaneously (automatically).
Generally exothermic (release energy).

8. 2. Decomposition Reactions
A decomposition reaction is one in which a single compound breaks down into two or more elements or new compounds.
General form: AB  A + B

9. Example: 2H2O2  2 H2O + O2
(it needs a catalyst—Potassium Iodide)
More Typical Example:

10. To predict the products:
2. Decomposition Reactions
To predict the products:
Binary compounds separate into their two elements
Check back of yellow periodic table handout for other decomposition predictions.

11. Endothermic reactions (need energy to start, do not release energy).
2. Decomposition Reactions
These reactions are not spontaneous. Need a form of energy or catalyst to make the reaction occur.
Endothermic reactions (need energy to start, do not release energy).

12. Synthesis or Decomposition?
1. KClO3    KCl  +  
O2                                     1.
Zn +   S  ZnS   2.
3.  Al2S3  Al S   3.
Fe +   O2   Fe2O3  4.
Na Cl2  NaCl  5.
H2O  H O2  6.
Synthesis or Decomposition?
1. KClO3    KCl  +  O2 
2. Zn S  ZnS
3. Al2S3  Al S
4. Fe O2  Fe2O3
5. Na Cl2  NaCl
6. H2O  H O2

13. Reaction Type Notes Practice pg . 292 18. 2Al2O3(s)  4 Al(s) + 3O2(g)
19. Ni(OH)2(s)  NiO(s) + H2O(l)
NaHCO3(s)  Na2CO3(aq) + CO2(g) + H2O(l)

14. Practice: Pg. 313 Numbers 85 and 87
85. a. 2 B (s) + 3 F2 (g) --> 2BF3 (g)
b. Ge (s) + 2 S (s) --> GeS2 (s)
c. 3 Zr (s) + 2 N2 (g) --> Zr3N4 (s)
d. P4O10 (s) + 6 H2O (l) --> 4 H3PO4 (aq)
87. a. MgBr2 (S) --> Mg (s) + Br2 (l)
b. 2 CoO (s) --> 2 Co (s) + O2 (g)
c. Ti(OH)4 (s) --> TiO2 (s) + 2 H2O (l)
d. BaCO3 (s) --> BaO (s) + CO2 (g)

15. General form: A + BX → AX + B
3. Replacement Reactions
A reaction in which the atoms of one element replace the atoms of another element in a compound is called a single replacement reaction.
General form: A + BX → AX + B

16. 3. Replacement Reactions (cont.)
A metal will not always 
replace a metal in a 
compound dissolved in 
water because of differing 
reactivities.
An activity series can 
be used to predict if 
reactions will occur.

17. Halogens frequently replace other halogens in replacement reactions.
3. Replacement Reactions (cont.)
Halogens frequently replace other halogens in replacement reactions.
Thus, halogens (and other anions) are a part of the activity series along with metal (cations).

18. Example:
2 Na + 2 H(OH)  2 Na(OH)+ H2

19. To predict the products: The element switches places with the element of similar charge within the compound.
If the element is more reactive then the one it attempts to replace, then the reaction will be spontaneous and exothermic. Otherwise, it will not be spontaneous and will be endothermic (require the input of energy).

20. Activity Series: Most Active Cations  Least
Lithium
Rubidium
Potassium
Barium
Calcium
Sodium
Hydrogen (in water)
Magnesium
Aluminum
Manganese
Zinc
Chromium
Iron
Nickel
Tin
Lead
Hydrogen (in acids)
Copper
Mercury
Silver
Platinum
Gold
*Red not included in book values

21. Activity Series: Most Active Anions  Least
Fluorine
Oxygen
Chlorine
Nitrogen
Bromine
Iodine
Sulfur
Selenium
Astatine
Phosphorus
Tellurium
Arsenic
Polonium
*Red not included in book values

22. Reaction Type Notes Practice p. 295
21. 2K(s) + ZnCl2(aq)  Zn(s) + 2KCl(aq)
22. No Reaction—Cl is below F in the activity 
series
23. No Reaction—Fe is below Na in the activity 
series
24. 2Al(s) + 3Pb(NO3)2(s)  3Pb(s) + 2Al(NO3)3(aq)

23. 4. Double Replacement Reactions
Double replacement reactions 
occur when ions exchange 
between two compounds.
General Form:

24. To predict the products: Like charged particles switch places
4. Double Replacement Reactions
To predict the products: Like charged particles switch places
Both compounds are initially stable and so reactions will usually not be spontaneous as solids. If compounds can be dissolved and dissociated 
(broken apart) then reaction will usually occur spontaneously to some degree, but not be highly exothermic.

25. 4. Double Replacement Reactions (cont.)
A solid product produced during a chemical reaction in a solution is called a precipitate.
All double replacement reactions produce either water, a precipitate, or a gas.

26. Example: 2 NaI(aq) + Pb(NO3)2(aq) PbI2(s) + 2NaNO3(aq)

27. Reaction Type Notes Practice p. 297
25. LiI(aq) + AgNO3(aq)  AgI(s) + LiNO3(aq)
26. BaCl2(aq) + K2CO3(aq)  BaCO3(s) + 2KCl(aq)
27. Na2C2O4(aq) + Pb(NO3)2(aq)  PbC2O4(s) + 2NaNO3(aq)

28. 5. Combustion Reactions
In a combustion reaction, 
oxygen combines with a substance 
and releases energy in the form of 
heat and light.

29. 5. Combustion Reactions
Hydrocarbon Combustion—Involves a specific case of combustion in which a 
hydrocarbon (a substance containing carbon and hydrogen) reacts with oxygen to form carbon dioxide and water
General form:
CxHy + O2  CO2 + H2O

30. 5. Combustion Reactions
Example:
2C2H6 + 7O2  4CO2 + 6H2O

31. To predict products: Will always include carbon dioxide and water.
5. Combustion Reactions
To predict products: Will always include carbon dioxide and water.
Always exothermic, but often requires a large activation energy (not truly spontaneous).

32. Reaction Type Notes Practice p. 291
Al(s) + 3 S(s)  Al2S3; synthesis
15. H2O(l) + N2O5(g)  2HNO3(aq); synthesis
16. 4 NO2(g) + O2(g)  2N2O5(g); synthesis and combustion

33. Reaction Type Notes
Most reactions which do not appear to fit any of these five patterns are usually a combination of two or more of them happening simultaneously

34. PRACTICE - Identify the type of reaction.
1. ___Fe + ___ HCl  ___ FeCl2 + ___H2
2. ___Al + __ H2SO4  __ Al2(SO4)3 + __ H2
3. ___Pb(NO3)2 + __ H2S  __ PbS + __ HNO3
4. ___Na2CO3 + __ HCl  __ NaCl + __H2CO3
5. ___C2H2 + ___O2  ___ CO2 + ___H2O
6. ___C6H14 + ____ O2  ___ CO2 + ___H2O

35. Replacement Reactions (cont.)
This table summarizes different 
ways to predict the products of a 
chemical reaction.