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Lecture Presentation
Chapter 4
Reactions in
Aqueous Solution
© 2012 Pearson Education, Inc.
Solutions
• Solutions are defined
as homogeneous
mixtures of two or more
pure substances.
• The solvent is present
in greatest abundance.
• All other substances are
solutes.
Aqueous
Reactions
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Dissociation
• When an ionic
substance dissolves
in water, the solvent
pulls the individual
ions from the crystal
and solvates them.
• This process is
called dissociation.
Aqueous
Reactions
© 2012 Pearson Education, Inc.
Solutions
• An electrolyte is a
substance that
dissociates into ions
when dissolved in water.
• A nonelectrolyte may
dissolve in water, but it
does not dissociate into
ions when it does so.
Aqueous
Reactions
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Electrolytes
• A strong electrolyte dissociates completely when
dissolved in water and conducts electricity
• A weak electrolyte only dissociates partially when
dissolved in water and partially conducts electricity
Aqueous
Reactions
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Conduct electricity in solution?
Cations (+) and Anions (-)
Strong Electrolyte – 100% dissociation
NaCl (s)
H 2O
Na+ (aq) + Cl- (aq)
Weak Electrolyte – not completely dissociated
CH3COOH
CH3COO- (aq) + H+ (aq)
Aqueous
Reactions
4.1
Ionization of acetic acid
CH3COOH
CH3COO- (aq) + H+ (aq)
A reversible reaction. The reaction can
occur in both directions.
Acetic acid is a weak electrolyte because its
ionization in water is incomplete.
Aqueous
Reactions
4.1
Strong Electrolytes Are…
• Strong acids
• Strong bases
• Soluble ionic salts
Aqueous
Reactions
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Double-Displacement Reactions
• Is a reaction that involves the exchange
of parts between two compounds
• Precipitation Reactions
• Acid-base Reactions
Aqueous
Reactions
Precipitation Reactions
• Two soluble ionic compounds react to form an
insoluble product called a precipitate
• 2NaI(aq) + Pb(NO3)2(aq)
• AgNO3(aq) +KBr(aq)
PbI2(s) + 2NaNO3(aq)
AgBr(s) + KNO3(aq)
• These reactions are also called double displacement
reactions – swapping partners
Aqueous
Reactions
Writing Equations for Aqueous Ionic Reactions
The molecular equation
shows all of the reactants and products as intact, undissociated compounds.
The total ionic equation
shows all of the soluble ionic substances dissociated into ions.
The net ionic equation
eliminates the spectator ions and shows the actual
chemical change taking place.
Aqueous
Reactions
Precipitation Reactions
Precipitate – insoluble solid that separates from solution
precipitate
Pb(NO3)2 (aq) + 2NaI (aq)
PbI2 (s) + 2NaNO3 (aq)
molecular equation
Pb2+ + 2NO3- + 2Na+ + 2I-
PbI2 (s) + 2Na+ + 2NO3-
ionic equation
Pb2+ + 2IPbI2
PbI2 (s)
net ionic equation
Na+ and NO3- are spectator ions
Aqueous
Reactions
Solubility is the maximum amount of solute that will dissolve
in a given quantity of solvent at a specific temperature.
Aqueous
Reactions
4.2
Predicting Whether a Precipitation Reaction
Occurs; Writing Ionic Equations
PROBLEM: 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)
SOLUTION:
(a) Na2SO4(aq) + Sr(NO3)2 (aq)
2NaNO3(aq) + SrSO4(s)
2Na+(aq) +SO42-(aq)+ Sr2+(aq)+2NO3-(aq)
2Na+(aq) +2NO3-(aq)+ SrSO4(s)
SO42-(aq)+ Sr2+(aq)
(b) NH4ClO4(aq) + NaBr (aq)
SrSO4(s)
NH4Br (aq) + NaClO4(aq)
All reactants and products are soluble so no reaction
occurs.
Aqueous
Reactions
Arrhenius acid is a substance that produces H+ (H3O+) in water
Arrhenius base is a substance that produces OH- in water
Aqueous
Reactions
4.3
A Brønsted acid is a proton donor
A Brønsted base is a proton acceptor
base
acid
acid
base
A Brønsted acid must contain at least one
ionizable proton!
Aqueous
Reactions
4.3
Aqueous
Reactions
© 2012 Pearson Education, Inc.
Identify each of the following species as a Brønsted acid,
base, or both. (a) HI, (b) CH3COO-, (c) H2PO4HI (aq)
H+ (aq) + I- (aq)
CH3COO- (aq) + H+ (aq)
H2PO4- (aq)
Brønsted acid
CH3COOH (aq)
H+ (aq) + HPO42- (aq)
H2PO4- (aq) + H+ (aq)
H3PO4 (aq)
Brønsted base
Brønsted acid
Brønsted base
Aqueous
Reactions
4.3
Aqueous acid-base neutralization reaction
• An acid and a base produce a salt
solution and water
– HX(aq) + MOH(aq)
ACID
BASE
MX(aq) + H2O(l)
metal salt
• They are also double displacement
reactions like precipitation reactions
Aqueous
Reactions
The key event in an acid-base
(neutralization) reaction
• The formation of water from H+ and OH- ions
– 2HCl(aq) + Ba(OH)2(aq)
BaCl2(aq) + 2H20(l)
– 2H+(aq) + 2Cl-(aq) + Ba2+(aq) + 2OH-(aq)
(aq) +2H2O(l)
– 2H+(aq) + 2OH-(aq)
Ba2+(aq) + 2Cl-
2H2O(l)
Aqueous
Reactions
Gas-Forming Reactions
When a carbonate or bicarbonate reacts with
an acid, the products are a salt, carbon
dioxide, and water:
CaCO3(s) + HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)
NaHCO3(aq) + HBr(aq) NaBr(aq) + CO2(g) + H2O(l)
Aqueous
Reactions
© 2012 Pearson Education, Inc.
Important Redox definitions
• Oxidation – loss of electrons
• Reduction – gain of electrons
• Oxidizing agent – the species doing
the oxidation
• Reducing agent – the species doing
the reduction
• Therefore,
– The oxidizing agent is reduced
– The reducing agent is oxidized
Aqueous
Reactions
Example
2Mg(s) + O2(g)
2MgO(s)
Oxidation (electron loss by Mg)
Mg
Reduction (electron gain by O2)
½O2 + 2e-
O2 oxidizes Mg
O2 is the oxidizing agent
Mg2+ + 2eO2-
Mg reduces O2
Mg is the reducing agent
The oxidation state of O starts as 0 and becomes 2The oxidation state of Mg starts as 0 and becomes 2+
Aqueous
Reactions
Determining the Oxidation Number of an Element
PROBLEM:
PLAN:
Determine the oxidation number (O.N.) of each element in these
compounds:
(a) zinc chloride (b) sulfur trioxide (c) nitric acid
The O.N.s of the ions in a polyatomic ion add up to the charge of the
ion and the O.N.s of the ions in the compound add up to zero.
SOLUTION:
(a) ZnCl2. The O.N. for zinc is +2 and that for chloride is -1.
(b) SO3. Each oxygen is an oxide with an O.N. of -2. Therefore the
O.N. of sulfur must be +6.
(c) HNO3. H has an O.N. of +1 and each oxygen is -2. Therefore
the N must have an O.N. of +5.
Aqueous
Reactions
Recognizing Oxidizing and Reducing Agents
PROBLEM: Identify the oxidizing agent and reducing agent in each of the following:
(a) 2Al(s) + 3H2SO4(aq)
Al2(SO4)3(aq) + 3H2(g)
(b) PbO(s) + CO(g)
(c) 2H2(g) + O2(g)
Pb(s) + CO2(g)
2H2O(g)
PLAN: Assign an O.N. for each atom and see which atom gained and which atom
lost electrons in going from reactants to products.
An increase in O.N. means the species was oxidized (and is the reducing
agent) and a decrease in O.N. means the species was reduced (is the
oxidizing agent).
SOLUTION:
0
+1 +6 -2
(a) 2Al(s) + 3H2SO4(aq)
+3 +6 -2
0
Al2(SO4)3(aq) + 3H2(g)
The O.N. of Al increases; it is oxidized; it is the reducing agent.
The O.N. of H decreases; it is reduced; H2SO4 is the oxidizing
Aqueous
agent. Reactions
Recognizing Oxidizing and Reducing Agents
continued
+2 -2
+2 -2
(b) PbO(s) + CO(g)
0
+4 -2
Pb(s) + CO2(g)
The O.N. of C increases; it is oxidized; CO is the reducing agent.
The O.N. of Pb decreases; it is reduced; PbO is the oxidizing agent.
0
0
(c) 2H2(g) + O2(g)
+1 -2
2H2O(g)
The O.N. of H increases; it is oxidized; it is the reducing agent.
The O.N. of O decreases; it is reduced; it is the oxidizing agent.
Aqueous
Reactions
Solution Chemistry
• Concentration – number of moles
present in a certain volume of solution
• Molarity (M) – The most used and
convenient way to express
concentration
Aqueous
Reactions
Molarity
moles of solute
Molarity 
volume of solution in liters
Aqueous
Reactions
Calculating the Molarity of a Solution
PROBLEM:
PLAN:
Glycine (H2NCH2COOH) is the simplest amino acid. What is the
molarity of an aqueous solution that contains 0.715 mol of
glycine in 495 mL?
Molarity is the number of moles of solute per liter of solution.
SOLUTION:
0.715 mol glycine
1000mL
= 1.44 M glycine
495 mL soln
1L
Aqueous
Reactions
Dilution
The molarity of the new solution can be determined
from the equation
Mc  Vc = Md  Vd,
where Mc and Md are the molarity of the concentrated
and dilute solutions, respectively, and Vc and Vd are
the volumes of the two solutions.
Aqueous
Reactions
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Titration
Titration is an
analytical
technique in
which one can
calculate the
concentration
of a solute in
a solution.
Aqueous
Reactions
© 2012 Pearson Education, Inc.
Sample Problem 4.5
PROBLEM:
Finding the Concentration of Acid from an
Acid-Base Titration
You perform an acid-base titration to standardize an HCl solution by
placing 50.00 mL of HCl in a flask with a few drops of indicator
solution. You put 0.1524 M NaOH into the buret, and the initial
reading is 0.55 mL. At the end point, the buret reading is 33.87 mL.
What is the concentration of the HCl solution?
NaOH(aq) + HCl(aq)
SOLUTION:
(33.87-0.55) mL x
NaCl(aq) + H2O(l)
1L
= 0.03332 L
103 mL
0.03332 L X 0.1524 M = 5.078x10-3 mol
NaOH
Molar ratio is 1:1
5.078x10-3 mol HCl
0.05000 L
= 0.1016 M HCl
Aqueous
Reactions
Calculating Mass of Solute in a Given Volume
of Solution
PROBLEM:
How many grams of solute are in 1.75 L of 0.460 M sodium
monohydrogen phosphate?
SOLUTION:
1.75 L 0.460 moles
1L
= 0.805 mol Na2HPO4
0.805 mol Na2HPO4 141.96 g Na2HPO4
mol Na2HPO4
= 114 g Na2HPO4
Aqueous
Reactions
Preparing a Dilute Solution from a Concentrated
Solution
PROBLEM:
SOLUTION:
“Isotonic saline” is a 0.15 M aqueous solution of NaCl that
simulates the total concentration of ions found in many cellular
fluids. How would you prepare 0.80 L of isotomic saline from a
6.0 M stock solution?
C1V1 = C2V2
0.80 L soln 0.15 mol NaCl = 0.12 mol NaCl
L soln
0.12 mol NaCl L solnconc = 0.020 L soln
6 mol
Aqueous
Reactions
Sample Problem 4.2
Determining the Molarity of H+ Ions in Aqueous
Solutions of Acids
PROBLEM: 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?
PLAN:
Use the formula to find the molarity of H+.
SOLUTION: One mole of H+(aq) is released per mole of nitric acid (HNO3)
H2O
HNO3(l)
H+(aq) + NO3-(aq)
1.4M HNO3(aq) should have 1.4M H+(aq).
Aqueous
Reactions
The oxidation numbers of elements in their compounds
Aqueous
Reactions
4.4
Table 4.3 Rules for Assigning an Oxidation Number (O.N.)
General rules
1. For an atom in its elemental form (Na, O2, Cl2, etc.): O.N. = 0
2. For a monoatomic ion: O.N. = ion charge
3. The sum of O.N. values for the atoms in a compound equals zero. The
sum of O.N. values for the atoms in a polyatomic ion equals the ion’s charge.
Rules for specific atoms or periodic table groups
1. For Group 1A(1):
O.N. = +1 in all compounds
2. For Group 2A(2):
O.N. = +2 in all compounds
3. For hydrogen:
O.N. = +1 in combination with nonmetals
4. For fluorine:
O.N. = -1 in combination with metals and boron
5. For oxygen:
O.N. = -1 in peroxides
O.N. = -2 in all other compounds(except with F)
O.N. = -1 in combination with metals, nonmetals
(except O), and other halogens lower in the group
Aqueous
6. For Group 7A(17):
Reactions