Chapter 7
Predicting Whether a Reaction Will Occur
Why does a reaction occur? What causes a reaction to “want” to form products?
Scientists have recognized several tendencies in reactants the DRIVE them to form
products. There are several driving forces:
Formation of a solid
Formation of water
Formation of a gas
Transfer of electrons
When two or more chemicals are brought together, if any of these things can occur,
a chemical change is likely to take place.
*IF NONE OF THESE FACTORS TAKE PLACE NO REACTION WILL TAKE PLACE
Our goal is to determine whether a reaction will occur and what products might form.
All chemical reactions can be placed into one of six categories:
1) Combustion: A combustion reaction is when oxygen combines with a hydrocarbon (a combination
of C and H) 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 + 12 O2 ---> 10 CO2 + 4 H2O
2) Synthesis: A synthesis reaction is when two or more simple compounds combine to form a more
complicated one. These reactions come in the general form of:
A + B ---> AB
One example of a synthesis reaction is the combination of iron and sulfur to form iron (II) sulfide:
8 Fe + S8 ---> 8 FeS
3) 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 ---> 2 H2 + O2
4) 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
5) Double displacement: This is when the anions and cations of two different molecules switch
places, forming two entirely different compounds. These reactions are in the general form:
AB + CD ---> AD + CB
One example of a double displacement reaction is the reaction of lead (II) nitrate with potassium
iodide to form lead (II) iodide and potassium nitrate:
Pb(NO3)2 + 2 KI ---> PbI2 + 2 KNO3
6) Acid-base: This is a special 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: Term “salt”
is used to mean ionic compound
HA + BOH ---> H2O + BA
One example of an acid-base reaction is the reaction of hydrobromic acid (HBr) with sodium
hydroxide:
HBr + NaOH ---> NaBr + H2O
7*) Special Reaction Type: Redox: or oxidation-reduction reactions, primarily involve the transfer of
electrons between two chemical species. The compound that loses an electron is said to be oxidized
(OIL = oxidization is loss), the one that gains an electron is said to be reduced (RIG = reduced is
gained).
*If the element stands alone its net charge is ZERO.
*If the element is contained in a compound then you must look at their individual oxidation numbers
(periodic table)
*If it is a transition metal you must split the compound apart and figure out the charge on the transition
metal based off of its partner (like you did in chapter 5 when naming)
H2 + F2 = 2HF
H2
+0
+
F2
+0
=
H +1 F
-1
hydrogen lost an electron to become positively charged ………..OIL
Fluorine gained an electron to become negatively charged……RIG
Substance oxidized: Hydrogen
Substance reduced: Fluorine
Oxidation Reaction: H2 → 2H+ + 2eReduction Reaction: F2 + 2e- → 2F-
****A lot of reactions are redox and another type of reaction from the above list.
***YOU MUST FIRST FIGURE OUT THE REACTION TYPE 1-6 AND
THEN DECIDE IF IT IS ALSO A REDOX REACTION
Questions to ask yourself if you cannot figure out 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 it's 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) Does your reaction have water as one of the products? If yes, then it's an acid-base reaction
6) If you haven't answered "yes" to any of the questions above, then you've got a double
displacement reaction
Examples to figure out:
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) NH4OH + HBr --> H2O + NH4Br
6) Pb + O2 --> PbO2
7) Na2CO3 --> Na2O + CO2
Answers:
1) double displacement
2) combustion
3) single displacement
4) double displacement
5) acid-base
6) synthesis
7) decomposition
Solubility Tables
A Solubility Table summarizes the solubility behavior of a large group of ionic substances. How to
interpret a Solubility Table?
Example 1:
Example 2:
We can use the Solubility Table to determine whether an ionic compound exist as ions in aqueous
solution (soluble) or as a solid (insoluble). Once we know the compound we use the Solubility
Table to determine its solubility.
For example, consider the following compounds; NaCl, BaSO4, NaC2H3O2, and CaS. Determine
the solubility in water for these ionic substances.
NaCl (all chlorides are soluble except...) SOLUBLE = aqueous
BaSO4 (all sulfates are soluble except...) INSOLUBLE = solid
NaC2H3O2 (all sodium compounds are soluble) SOLUBLE = aqueous
CaS (all sulfides are insoluble...) INSOLUBLE = solid
We'll also use the information in a Solubility Table to help identify the phase of ionic substance in
a chemical equation. The chemical reaction types where the Solubility Table is important are;
 Double Replacement reactions
 Neutralization reactions
 Single Replacement reactions
Example 1: double displacement reaction problem;
Write the formula and identify the phase for the product(s) and balance the following reaction.
Na2SO4(aq) + CaCl2(aq) --->
Since this is a double replacement reaction we can write the formulas of the products by
exchanging the cations and anions.
Na2SO4(aq) + CaCl2(aq) ---> CaSO4(?) + 2NaCl(?)
Now we'll use the Solubility Table to predict the phases of the products. According to the table
CaSO4 is INSOLUBLE and NaCl is SOLUBLE.
Na2SO4(aq) + CaCl2(aq) ---> CaSO4(s) + 2NaCl(aq)
Example.2: double displacement reaction problem
Write the formula and identify the phase for the product(s) and balance the following reaction.
AgNO3(aq) + Na2CO3(aq) --->
Since this is a double replacement reaction we can write the formulas of the products by
exchanging the cations and anions.
2AgNO3(aq) + Na2CO3(aq) --->Ag2CO3(?) + 2NaNO3(?)
Now we'll use the Solubility Table to predict the phases of the products. According to the table
Ag2CO3 is INSOLUBLE and NaNO3 is SOLUBLE.
2AgNO3(aq) + Na2CO3(aq) ---> Ag2CO3(s) + 2NaNO3(aq)
Example 3:
KNO3(aq) + BaCl2 (aq) ) --->
Example 4:
Na2SO 4(aq) + Pb(NO3)2(aq) ) --->
Example 5:
KOH (aq) + Fe(NO3) 3(aq)
)
--->