1. Making sense of chemical equations
Reactions
Making sense of chemical equations

2. Types of Reactions
There are millions of reactions, and we cannot remember them all. Luckily they fall into several categories.
By looking at the reactants, we will:
learn the 5 major types
predict the products
predict if they happen at all

3. Types of Reactions

4. Type 1: Combination
Combination reactions happen when 2 elements combine to make a compound (also called synthesis).
Examples:
Ca + O2  CaO
SO3 + H2O  H2SO4

5. Type 2: Decomposition
Decomposition reactions occur when a compound breaks apart into two or more elements or compounds.
Examples:
NaCl electricity Na + Cl2
CaCO ∆ CaO + CO2
**Note that energy is usually required.

6. Type 3: Single Replacement
In a single replacement reaction, one element replaces another.
Reactants must be one element and one compound.
Products will be a different element and a different compound.

7. Type 3: Single Replacement
Examples:
Na + KCl  K + NaCl
cation replaces cation
F2 + LiCl  LiF + Cl2
anion replaces anion

8. Type 4: Double Replacement
Double replacement reactions occur when two elements replace each other in two different compounds.
Reactants must be ionic compounds in aqueous solution.
NaOH + FeCl3  Fe(OH)3 + NaCl
The positive ions change places.

9. How to recognize each type:
Look at the reactants:
E + E = Combination
C = Decomposition
E + C = Single Replacement
C + C = Double Replacement
E = element; C = compound

10. Type 5: Combustion Combustion means “add oxygen”.
Combustion reacts occur when compounds containing C, H, and O react with oxygen – usually called “burning”.
If the combustion reaction is complete, the products will be CO2 and H2O.
If the combustion reaction is incomplete, the products will be CO (possibly just C) and H2O.

11. Predicting Products

12. Type 1: Combination
We can predict the products, especially if the reactants are two elements:
Mg + N2 
Al + Cl2 
Mg3N2
AlCl3

13. Type 1: Combination Practice: Ca + Cl2 
Fe + O2  (assume Fe (II) in the product)
Al + O2 

14. Type 1: Combination Watch out for:
Some nonmetal oxides react with water to form acids:
SO2 + H2O  H2SO4
**This is what happens to make “acid rain”
Some metallic oxides react with water to for bases:
CaO + H2O  Ca(OH)2
**The hydroxide polyatomic ion indicates that the compound is a base.

15. Type 2: Decomposition
We can predict the products if it is binary compound (made up of two elements) – it breaks apart into the elements.
Examples:
H2O electricity
HgO ∆

16. Type 2: Decomposition
If the compound has more than two elements, you must be given at least one of the products. The other product will be made from the missing pieces.
Examples:
NiCO ∆ CO2 + _______
H2CO3(aq) heat CO2 + _______

17. Type 3: Single Replacement
Metals will also replace other metals (and also hydrogen)
Examples:
K + AlN 
Zn + HCl 
Think of water at HOH:
Metals replace the first H and then combine with OH-.
Na + HOH 

18. Type 3: Single Replacement
We can even tell whether or not a single replacement reaction will happen:
Because some metals are more “active” than others.
More active replaces less active.
There is a list on page 217 in the textbook.
Called the Activity Series of Metals.
Higher on the list replaces lower on the list.

19. Type 3: Single Replacement
Lithium
Potassium
Calcium
Sodium
Magnesium
Aluminum
Zinc
Chromium
Iron
Nickel
Lead
Hydrogen
Bismuth
Copper
Mercury
Silver
Platinum
Gold
Higher activity
Lower activity
Rules:
1) Metals can replace other metals provided they are above the metal they are trying to replace. For example, zinc will replace lead.
2) Metals above hydrogen will replace hydrogen in acids.
3) Metals from sodium upward can replace hydrogen in water (HOH).

20. Type 3: Single Replacement
Practice:
Fe + CuSO4 
Pb + KCl 
Al + HCl 

21. Type 3: Single Replacement
Halogens have a similar activity series.
Halogens can replace other halogens in compounds if they are above the halogen they are trying to replace.
Examples:
2NaCl + F2 
MgCl2 + Br2 
Fluorine
Chlorine
Bromine
Iodine
Higher Activity
Lower Activity

22. Type 4: Double Replacement
Double replacement reactions occur because of certain driving forces (or reasons). The reaction will only happen if one of the products:
doesn’t dissolve in water and forms a solid (a precipitate), or
is a gas that bubbles out, or
is a molecular compound (which will usually be water).

23. Complete and Balance CaCl2 + NaOH  CuCl2 + K2S  KOH + Fe(NO3)3 
assume all of the reactions take place
CaCl2 + NaOH 
CuCl2 + K2S 
KOH + Fe(NO3)3 
(NH4)2SO4 + BaF2 

24. How to recognize each type:
Look at the reactants:
E + E = Combination
C = Decomposition
E + C = Single Replacement
C + C = Double Replacement
E = element; C = compound

25. Practice H2 + O2  H2O  Zn + H2SO4  HgO  KBr + Cl2  AgNO3 + NaCl 
Mg(OH)2 + H2SO3 

26. Type 5: Combustion C4H10 + O2  (assume complete)
C4H10 + O2  (incomplete)
C6H12O6 + O2  (complete)
C8H8 + O2  (incomplete)

27. SUMMARY Reactions come in 5 types.
We can tell what type of reaction by looking at the reactants.
Single replacement reactions occur based on activity series.
Double replacement reactions occur if one product is: 1) a precipitate, 2) a gas, or 3) water (a molecular compound).