Four Classes of Chemical Reaction

1) Combination: A combination reaction is when two or more simple compounds combine to form one compound. These reactions come in the general form of:
A + B ---> AB

An example of a combination reaction is the combination of iron and sulfur to form iron (II) sulfide:

 

8 Fe (S) + S8 (S) ---> 8 FeS (S)

2) Decomposition: A decomposition reaction is the opposite of a synthesis reaction - a complex molecule breaks down to make simpler ones. These reactions come in the general form:

 

AB ---> A + B

One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas:

2 H2O (l) ---> 2 H2 (g) + O2 (g)

3)  Displacement Reactions

a) Single displacement: This is when one element trades places with another element in a compound. These reactions come in the general form of:

A + BC ---> AC + B

One example of a single displacement reaction is when magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas:

Mg + 2 H2O ---> Mg(OH)2 + H2

 

*To determine whether a single displacement reaction will occur  -- Consult the "Activity Series."

 

b) Double displacement: This is when the anions and cations of two different molecules switch places, forming two or more entirely different compounds. These reactions are in the general form:

 

AB + CD ---> AD + CB

 

A double displacement reaction occurs when a product is either insoluble in water, is a gas, is water.

Formation of a Solid (water insoluble substance)

One example of a double displacement reaction is the reaction of lead (II) nitrate with potassium iodide to form solid lead (II) iodide, a solid, and potassium nitrate:

 

Pb(NO3)2 (aq) + 2 KI  (aq) ---> PbI2 (s) + 2 KNO3 (aq)

*To determine whether a double displacement reaction will form a solid  -- Consult a "Solubility Table."

 

Gas formation

In this type of double displacement reaction, one of the reactants is an acid and the other is a carbonate, a sulfite, or hydrogen sulfide.

Na2CO3 (aq) + HCl (aq) --->  NaCl + H2CO3 (aq)

H2CO3 (aq) ---> H2O (l) + CO2 (g)

 

Neutralization (formation of water from acid and base)

This is a  kind 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. Generally, the product of this reaction is some ionic salt and water:

HA + BOH ---> H2O + BA

One example of an acid-base reaction is the reaction of hydrobromic acid (HBr) with sodium hydroxide:

 

HBr (aq)  + NaOH (aq)  ---> NaBr (aq) + H2O (l)

 

In addition, neutralization occurs when an acid is added to a carbonate or a sulfite. See above.

4) Combustion:

A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide. These reactions are exothermic, meaning they produce heat. An example of this kind of reaction is the burning of napthalene:

C10H8 (s) + 12 O2 (g) ---> 10 CO2 (g) + 4 H2O (g)

 

Some elements are combustible --- in other words, oxygen can be added to them:

2H2 (g)  +  O2 (g) --> 2H2O (g)

 
 

Sample Problems (the solutions are in the next section)

Determine if the following reactions can happen and name the type of reaction that occurs:

 

1)  CH4 (g) + 2 O2 (g) ---> CO2 (g) + 2 H2O (l)

2)  Cu (s) + ZnCl2 (aq) ---> CuCl2 (aq) + Zn (s)

3)  NaOH (aq) + KNO3 (aq) ---> NaNO3 (aq) + KOH (aq)

4)  NH4OH (aq) + HBr (aq) ---> H2O (l) + NH4Br (aq)

5)  Pb (s) + O2 (g) ---> PbO2 (s)

6)  Na2CO3 (s)  ---> Na2O (s) + CO2 (g)

7)  Na2S (aq)  +  H2SO4 (aq) ---> Na2SO4 (aq)  +  H2S (s)

8)  Ni (s) + CuCl2 (aq) ---> Cu + NiCl2
 

Solutions to the Sample Problems


1)  combustion of a hydrocarbon
2)  single displacement - consult Activity Series
3)  No reaction. All products are water soluble, not gaseous,  not water.
4)  double displacement - neutralization between an acid and a base
5)   combustion of an element is a type of combination reaction
6)  decomposition
7)  double displacement -gas formation - H2S
8)  No Reaction.  See Activity Series!