Ch 7: Reactions
• 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
• 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
Oxidation Reaction: H2 → 2H+ + 2eSubstance reduced: Fluorine
Reduction Reaction: F2 + 2e- → 2F• ****A lot of reactions are redox and another
type of reaction from the above list.
• 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)
• ***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
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•
•
•
•
•
•
•
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)
)
--->