The Major Classes of Chemical Reactions
Chapter 4
Learning Goals: Chapter 4
4.1 The Role of Water as a Solvent
4.2 Writing Equations for Aqueous Ionic Reactions
4.3 Precipitation Reactions
4.4 Acid-Base Reactions
4.5 Oxidation-Reduction (Redox) Reactions
4.6 Elements in Redox Reactions
4.7 Reversible Reactions: An Introduction to Chemical
Equilibrium
There are three classes of chemical reactions
that occur in aqueous solution.
1. Precipitation Reactions-– an insoluble solid is
formed from specific cation-anion combinations.
2. Acid-Base Reactions-– a protons donor
substance reacts with a hydroxide donor substance
forming a salt and water.
3. Oxidation Reduction Reactions-electron donor
substances react with react with substances that accept
electrons.
In a precipitation reactions a metal cation and nonmetal anion combine spontaneously in solution
forming an insoluble solid that “precipitates”.
Precipitation occurs
when ionic attractive
forces overcome
water’s tendency to
hydrate and dissolve.
Any species that
remains soluble--is
called spectator
ions.
AgNO3(aq) + Na2CrO4(aq) => Ag2CrO4 (s) + NaNO3(aq)
How Do We Predict If A Precipitate Reaction Will
Occur?
Solubility Rules
1. Note the ions present in the reactants.
2. Consider the possible cation-anion combinations.
3. Refer to the table of solubility rules and decide
whether any of the ion combinations is insoluble.
4. If a candidate is insoluble, that reaction will occur.
5. Write the molecular, ionic and net ionic equation
for the reaction.
Table of Solubility Rules
Example Precipitation Reaction: The reaction of
the salts: Pb(NO3)2(aq) + NaI(aq) ==> PbI2(s)
1. Recognize that the reactants are
ionic and will dissociate into ions in
aqueous solution (not always).
2. Consult the solubility table to see
if PbI2 or NaNO3 are insoluble.
3. PbI2 is insoluble so that reaction will
occur. We write a complete molecular
equation, ionic and net equation as:
2NaI(aq) + Pb(NO3)2(aq)
2Na+ + 2I- + Pb2+ + 2NO32I-(aq) + Pb2+(aq)
PbI2(s) + 2NaNO3(aq)
Pb2+ + 2I- + 2Na+ + NO3PbI2(s)
There are 3-types of equations that are written
for precipitation, acid-base and redox reactions
1. The molecular equation
shows all reactants and products as undissociated compounds.
precipitate
2NaI(aq) + Pb(NO3)2(aq)
PbI2(s) + 2NaNO3(aq)
2. The total ionic equation
shows all of the soluble ionic substances dissociated into ions.
Pb2+ + 2NO3- + 2Na+ + 2I-
PbI2 (s) + 2Na+ + 2NO3-
Na+ and NO3- are spectator ions---they don’t do much but sit there!
3. The net ionic equation
eliminates the spectator ions and shows the actual chemical
change taking place.
Pb2+ + 2IPbI2 (s)
Learning Check: Precipitation Reactions
Predict whether a reaction occurs when each of the following
pairs of solutions are mixed. If a reaction does occur, write
balanced molecular, total ionic, and net ionic equations, and
identify the spectator ions.
(a) sodium sulfate(aq) + strontium nitrate(aq)
(b) ammonium perchlorate(aq) + sodium bromide(aq)
(c) silver nitrate(aq) + sodium chromate(aq)
Table of Solubility Rules
1. Convert names formulas, write a balanced equation
showing intact compounds (not dissociated).
Molecular Equation
Na2SO4(aq) + Sr(NO3)2 (aq)
2NaNO3(aq) + SrSO4(s)
2. Dissoicate the formula equation to an ionic equation
showing cations and anions with charge and phase.
Ionic Equation
2Na+(aq) +SO42-(aq)+ Sr2+(aq)+2NO3-(aq)
2Na+(aq) +2NO3-(aq)+ SrSO4(s)
3. Cancel the spectator ions to obtain the net ionic equation
Net Ionic Equation
SO42-(aq)+ Sr2+(aq)
SrSO4(s)
Molecular Equation
NH4ClO4(aq) + NaBr (aq)
NH4Br (aq) + NaClO4(aq)
Examing the solubility table shows that none of the possible
combinations of ions would result in an insoluble precipitate
(NH4ClO4 is soluble as is NH4Br, NaBr and NaClO4)
In this case all ions are spectator ions.
Ionic Equation
NH4+ + ClO4+(aq) + Na+ + Br- (aq)
NH4+ + Br -(aq) +
Na+ + ClO4 -(aq)
Learning Check: Write the net ionic equation
for the reaction of silver nitrate with sodium
chromate.
1. Write the balanced molecular equation.
2. Write the ionic equation showing the strong
electrolytes completely dissociated into cations and
anions.
3. Cancel the spectator ions on both sides of the ionic
equation to obtain the net ionic equation
Table of Solubility Rules
Write the net ionic equation for the reaction of
silver nitrate with sodium chromate.
A precipitation reaction will occur if the reaction
can produce an insoluble product.
Will a precipitate form when Mg(NO3)2 is added to
NaOH?
1. Write a balanced equation
2. Ask whether the ions will interact by looking at the
solubility table. Sometimes you won’t know.
3. Write the molecular, ionic and net ionic equations
Mg(NO3)2 + 2NaOH ==> Mg(OH)2 + 2NaNO3
Mg2+ + 2NO32- + 2Na+ + 2OH- ==> Mg
Mg2+ + 2OH- ==> Mg(OH)2 (s)
Note ionic species exchanged---called a metathesis reaction
Will a precipitate form when Mg(NO3)2 is added to NaOH?
1. Write a balanced equation
2. Ask whether any combination of ions will be insoluble
from a solubity table. If so, the reaction will occur.
3. Write the molecular, ionic and net ionic equations
Note ionic species exchanged---called a metathesis reaction
Molecular Equation
Mg(NO3)2 + 2NaOH ==> Mg(OH)2 + 2NaNO3
Ionic Equation
Mg2+ + 2NO32- + 2Na+ + 2OH- ==> Mg
Net Ionic Equation
Mg2+ + 2OH- ==> Mg(OH)2 (s)
2. Acid-Base Reactions
The effects of acid rain on a statue of George Washington
taken in 1935 (left) and 2001 (right) marble.
Acids and bases react in a characteristic chemical
reaction called a “neutralization reaction” to form a
salt and water.
NaOH (aq) + HCl (aq) ==> H2O + Na+ + Clbase
+ acid
==> Water + salt
A salt is an ionic compound whose cation comes
from a parent base and whose anion comes from
a parent acid:
Properties of Acids and Bases
Acids: 200 Million Tons H2SO4
•
•
•
•
Acrid sour taste
React with metals giving ion + H2 gas
Changes plant dye litmus from blue to red
Acid
React with carbonates and bicarbonates to
produce CO2 gas
Base
Bases:50 Million Tons NaOH
•
•
•
•
Bitter taste
Slippery feel
Changes plant dye litmus from red to blue
React and neutralizes the effects of acids
Acids are substances that produce H+ when
dissolved in water.
H 2O
HA(g) ==> H+ + A-
Acid donate H+
Bases are substances that produce OH- when
dissolved in water.
H 2O
MOH(s) ==>
OH-
+
M+
Bases donate OH-
Both are strong electrolytes!
Acids can have and donate one, two or three acidic
protons depending on their structure.
Monoprotic acids--only a single H+ available
HCl
H+ + ClHNO3
H+ + NO3CH3COOH
H+ + CH3COO-
Strong electrolyte, strong acid
Strong electrolyte, strong acid
Weak electrolyte, weak acid
Diprotic acids--two acidic p+ available for reaction
H2SO4
H+ + HSO4-
Strong electrolyte, strong acid
HSO4-
H+ + SO42-
Weak electrolyte, weak acid
Triprotic acids
H3PO4
H2PO4HPO42-
H+ + H2PO4H+ + HPO42H+ + PO43-
Weak electrolyte, weak acid
There are many common strong acids and strong
bases that we must memorize. All are strong
electrolytes that dissociate completely in solution.
All are strong electrolytes!
All that are not strong are weak acids and weak
bases = weak electrolytes. They do not dissociate
to a large extent in solution.
HX ←−→ H + Cl
+
−
Weakly ionized in water (not 100% ionized)
Learning check:
1. Identify the following as a strong or weak acid or base
a salt. If a salt what is the parent acid and base?
HF, HI, LiOH, Ca(OH)2, Na2SO4 CH3COO-, NH4+
2. Classify the following as strong, weak acid or base?
HClO4, Sr(OH)2, HClO2, NH3, H3PO4, H2SO4
3. What is the correct formula of the salt formed in the
neutralization reaction of hydrochloric acid with calcium
hydroxide?
4. What is the chemical formula of the salt produced by the
neutralization of sodium hydroxide with sulfurous acid?
Learning check: write the molecular, ionic and net
equation for the neutralization between calcium
hydroxide and sulfuric acid.
1. The molecular equation
shows all reactants and products as undissociated compounds.
Ca(OH)2 (aq) + H2SO4 (aq) ===> 2H2O + CaSO4
2. The total ionic equation
shows all of the soluble ionic substances dissociated into ions.
Ca2+ + 2OH- + 2H+ + SO42- ===> 2H2O + Ca2+ + SO42Ca2+ and SO42- are spectator ions---they just watch!
3. The net ionic equation
eliminate spectator ions and show actual chemical change
H+(aq) + OH- (aq) ===> H2O(l)
Learning check: Write molecular, ionic and net
equations for the following acid base neutralization
reactions.
strontium hydroxide(aq) + perchloric acid(aq)
barium hydroxide(aq) + sulfuric acid(aq)
Acetic acid(aq) + potassium hydroxide
Nitric acid(aq) + barium hydroxide(aq)
Writing Ionic Equations for Acid-Base Reactions
strontium hydroxide(aq) + perchloric acid(aq)
(a) Sr(OH)2(aq)+2HClO4(aq)
2H2O(l)+Sr(ClO4)2(aq)
(b) Sr2+(aq)+2OH-(aq)+ 2H+(aq)+2ClO4-(aq)
2H2O(l)+Sr2+(aq)+2ClO4-(aq)
(c) 2OH-(aq)+ 2H+(aq)
2H2O(l)
barium hydroxide(aq) + sulfuric acid(aq)
(b) Ba(OH)2(aq) + H2SO4(aq)
2H2O(l) + BaSO4(aq)
Ba2+(aq) + 2OH-(aq)+ 2H+(aq)+ SO42-(aq)
2H2O(l)+Ba2+(aq)+SO42-(aq)
(c) 2OH-(aq)+ 2H+(aq)
2H2O(l)
Determining the Molarity of H+ Ions in Aqueous
Solutions of Acids
Nitric acid is a major chemical in the fertilizer and
explosives industries. In aqueous solution, each
molecule dissociates and the H becomes a
solvated H+ ion. What is the molarity of H+(aq) in
1.4M nitric acid?
What is the molarity of 0.70 M H2SO4?
Of .466 M H3PO4?
Determining the Molarity of H+ Ions in Aqueous
Solutions of Acids
Nitric acid is a major chemical in the fertilizer and
explosives industries. In aqueous solution, each
molecule dissociates and the H becomes a solvated H+
ion. What is the molarity of H+(aq) in 1.4M nitric acid?
One mole of H+(aq) is released per mole of nitric acid (HNO3)
HNO3(l)
H 2O
H+(aq) + NO3-(aq)
1.4M HNO3(aq) is 1.4M H+(aq).
What is the molarity of H+ in a 0.70 M H2SO4?
What is the molarity of H+ in a 0.466 M H3PO4?
A titration is the lab technique in which a solution of
known molarity is added to another solution of
unknown concentration until the chemical reaction
between the two solutions is stoichiometric.
Start of
titration
Equivalence
Point
Slight excess
of base
There are many different types of indicator dyes
used in titrations.
1. Pipet 20.0 mL
of an acid
solution of
unknown
molarity
2. Titrate with
a solution of
known [C]
3. When just the right
volume of titrant is
added, the solution
changes color =
equivalence pt
Buzzwords To Understand & Know
• Equivalence point:
– The point in the reaction at which both acid and base have been
consumed. Neither acid nor base is present in excess.
• End point:
– The point at which the pH indicator changes color. It is not
identical to the equivalence point---but close to it.
• Titrant:
– The solution of known concentration (often standardized) and
added to the solution of unknown concentration.
• Titration Curve:
– Terminology for a graph or plot of Solution pH vs. volume of
titrant added to the unknown solution.
What volume of a 1.420 M NaOH solution is
Required to titrate 25.00 mL of a 4.50 M H2SO4
solution?
What volume of a 1.420 M NaOH solution is
Required to titrate 25.00 mL of a 4.50 M H2SO4
solution?
H2SO4 + 2NaOH
volume acid
25.00 mL x
M
acid
moles acid
4.50 mol H2SO4
1000 mL soln
x
2H2O + Na2SO4
rx
coef.
moles base
2 mol NaOH
1 mol H2SO4
x
M
base
volume base
1000 ml soln
1.420 mol NaOH
= 158 mL