Transcript Chapter 4 Aqueous Reactions and Solution Stoichiometry

Chemistry, The Central Science, 11th edition
Theodore L. Brown; H. Eugene LeMay, Jr.;
and Bruce E. Bursten
Chapter 4
Aqueous Reactions and
Solution Stoichiometry
Mrs. Debbie Amuso
Camden High School
Camden, NY
Aqueous
Reactions
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Aqueous Solutions (aq)
1. Solutions consist of two or more substances
physically combined in a homogeneous
mixture.
2. Aqueous solutions consist of solute(s)
dissolved in a solvent (water).
3. Solute = substance dissolved
4. Solvent = substance of greater
quantity (usually water)
Aqueous
Reactions
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Aqueous Solutions (aq)
5. Properties: Clear or transparent but may be
colored, Solute doesn’t settle on standing,
Can’t be separated by filtering, Components are
not in a definite ratio
6. Example: NaCl(aq)
Aqueous
Reactions
Water:
The Universal Solvent
1. Due to its polar nature, water will dissolve
ionic substances and most polar substances.
2. “Like dissolves like.”
3. Example: NaCl(s)  Na 1+ (aq) + Cl 1= (aq)

+
Aqueous
Reactions
Dissociation
Dissociation is the process
that occurs when an ionic
substance dissolves in
water and the solvent pulls
the individual ions from the
crystal and solvates them.
Recall: Ionic substances
contain a metal & nonmetal
or polyatomic ion.
Examples: NH4Cl or KBr
Aqueous
Reactions
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Electrolytes vs Nonelectrolytes
• An electrolyte is a
substance that
dissociates into ions
when dissolved in
water.
• A nonelectrolyte may
dissolve in water, but it
remains a molecule and
does not dissociate into
ions.
Aqueous
Reactions
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Electrolytes
• A strong electrolyte
dissociates completely
when dissolved in
water & forms lots of
ions.
• A weak electrolyte
only dissociates
partially when
dissolved in water &
only forms a few ions.
Aqueous
Reactions
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Electrolytes vs Nonelectrolytes
1.
2.
3.
4.
Memorize the Chart
Note:
Ionic compounds contain a metal and nonmetal and/or
polyatomic ion.
Molecular (also called covalent) compounds contain only
nonmetals.
There is a separate list of strong acids you need to
memorize.
The compound must also be soluble in water – Be able to
use the Solubility Table.
Note: Strong bases are ionic & also strong electrolytes!!
Aqueous
Reactions
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Strong Acids & Bases
Memorize!
Aqueous
Reactions
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Solubility Guidelines
Know how to use this Chart.
Aqueous
Reactions
How to Show
Strong Electrolyte:
KCl (s)  K 1+ (aq) + Cl 1- (aq)
Weak Electrolyte:
HF (g)  H 1+ (aq) + F 1- (aq)
Nonelectrolyte:
C12H22O11 (s)  C12H22O11 (aq)
Aqueous
Reactions
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Check Yourself:
1. Electrolyte or Nonelectrolyte?
a) C6H12O6
b) MgSO4
c) LiBr
d) NH3
2. Soluble or Insoluble?
a) AgCl
b) BaCO3
c) NaC2H3O2
d) Li2S
Answers: 1 a) NE b) E c) E d) E
2 a) Ins b) Ins c) Sol d) Sol
Aqueous
Reactions
Solution Chemistry
• It is helpful to pay attention to exactly
what species are present in a reaction
mixture (i.e., solid, liquid, gas, or an
aqueous solution made of ions).
• If we are to understand reactivity, we
must be aware of just what is changing
during the course of a reaction.
Aqueous
Reactions
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•
•
•
•
Metathesis (Exchange)
Reactions
Metathesis comes from a Greek word that means
“to transpose.”
It appears the ions in the reactant compounds
exchange, or transpose, ions.
Metathesis is another name for a Double
Replacement Reaction.
Recall the General form: AB + CD  AD + CB
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)
Aqueous
Reactions
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Precipitation Reactions
A Precipitation Reaction
is a metathesis reaction
that results in the
formation of an insoluble
solid (precipitate).
Aqueous
Reactions
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Precipitates
1. Precipitates are insoluble solids.
2. Practice: Name the precipitate formed when solutions
of barium nitrate and sodium sulfate are mixed.
Aqueous
Reactions
Answer: Barium sulfate
Writing Net Ionic Equations
1. Write the balanced Molecular Equation.
2. Identify the product that does not ionize.
It will be either:
Water - H2O
A gas - CO2 or H2 or O2 , etc.
A precipitate – one of the insoluble ionic solids on
the Solubility Table
3. Write the Ionic Equation by ionizing everything else.
4. Cross-out Spectator Ions (Those ions that don’t
change.)
Aqueous
5. What’s left is the Net Ionic Reaction.
Reactions
Note
1. It is said that a reaction “takes place” or “is
spontaneous” or “goes to completion” only if
one of the products is water, a gas, or a
precipitate.
2. For metathesis reactions, if the products
formed are also aqueous, we say the
reaction is “not spontaneous” or doesn’t go
to competition.
Aqueous
Reactions
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Molecular Equation
The molecular equation lists the reactants
and products in their molecular form. Be
sure to include phases.
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)
This rxn is spontaneous because a precipitate forms.
Aqueous
Reactions
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Ionic Equation
• In the ionic equation all strong electrolytes (strong
acids, strong bases, and soluble ionic salts) are
dissociated into their ions.
• This more accurately reflects the species that are
found in the reaction mixture.
Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq) 
AgCl (s) + K+ (aq) + NO3- (aq)
Aqueous
Reactions
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Net Ionic Equation
• To form the net ionic equation, cross out anything
that does not change from the left side of the
equation to the right.
• The only things left in the equation are those things
that change (i.e., react) during the course of the
reaction.
• Those things that didn’t change (and were deleted
from the net ionic equation) are called spectator
ions.
Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) 
Aqueous
AgCl (s) + K+(aq) + NO3-(aq)
Reactions
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Practice Completing & Balancing
Metathesis Reactions
BaCl2 (aq) + K2SO4 (aq) 
Answer: BaCl2(aq) + K2SO4(aq)  BaSO4(s) + 2KCl(aq)
Fe2(SO4 )3(aq) + LiOH (aq) 
Answer: Fe2(SO4 )3(aq) + 6LiOH(aq)  2Fe(OH)3(s) + 3Li2SO4(aq)
Aqueous
Reactions
Write Ionic & Net Ionic Equations
too
BaCl2 (aq) + K2SO4 (aq) 
Answer: Ba2+(aq) + 2Cl- (aq) + 2K+(aq) + SO4 2-(aq)  BaSO4(s) + 2K+(aq) + 2Cl- (aq)
= spectator ions
Net Ionic Rx = Ba2+(aq) + SO4 2-(aq)  BaSO4(s)
Fe2(SO4 )3(aq) + LiOH (aq) 
Answer: 2Fe3+(aq) + 3SO4 2-(aq) + 6Li+(aq) + 6OH-(aq)  2Fe(OH)3(s) + 6Li+(aq) + SO42= spectator ions
Net Ionic Rx =
2Fe3+(aq) +
6OH-(aq)

(aq)
Aqueous
2Fe(OH)3(s) Reactions
Acids
• Arrhenius defined acids
as substances that
increase the
concentration of H+
when dissolved in water.
• Brønsted and Lowry
defined them as proton
donors.
Aqueous
Reactions
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Acids
There are only seven
strong acids:
• Hydrochloric (HCl)
• Hydrobromic (HBr)
• Hydroiodic (HI)
• Nitric (HNO3)
• Sulfuric (H2SO4)
• Chloric (HClO3)
• Perchloric (HClO4)
A common weak acid:
– Acetic (HC2H3O2)
Aqueous
Reactions
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Bases
• Arrhenius defined
bases as substances
that increase the
concentration of OH−
when dissolved in water.
• Brønsted and Lowry
defined them as proton
acceptors.
Aqueous
Reactions
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Bases
The strong bases
are the soluble metal
salts of hydroxide
ion:
• Alkali metals
• Calcium
• Strontium
• Barium
A common weak base:
– Ammonia (NH3)
They are:
LiOH, NaOH,
KOH, RbOH,
CsOH, Ca(OH)2,
Sr(OH)2, Ba(OH)2
Aqueous
Reactions
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Acid-Base Reactions
In an acid-base
reaction, the acid
donates a proton
(H+) to the base.
Aqueous
Reactions
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Neutralization Reactions
Generally, when solutions of an acid and a base are
combined, the products are a salt and water.
Neutralization
Acid + Base  Salt + Water
Example:
CH3COOH (aq) + NaOH (aq) CH3COONa (aq) + H2O (l)
Aqueous
Reactions
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Neutralization Reactions
When a strong acid reacts with a strong base, the net
ionic equation is…
HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)
H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq) 
Na+ (aq) + Cl- (aq) + H2O (l)
H+ (aq) + OH- (aq)  H2O (l)
Aqueous
Reactions
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Gas-Forming Reactions
Memorize:
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
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Gas-Forming Reactions
Memorize: Similarly, when a sulfite reacts
with an acid, the products are a salt, sulfur
dioxide, and water.
NO  SrSO3 (s) + 2 HI (aq) SrI2 (aq) + H2SO3 (aq)
YES SrSO3 (s) + 2 HI (aq) SrI2 (aq) + SO2 (g) + H2O (l)
Aqueous
Reactions
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Gas-Forming Reactions
• This reaction gives the predicted product, but
you had better carry it out in the hood, or you
will be very unpopular!
• But just as in the previous examples, a gas is
formed as a product of this reaction.
Na2S (aq) + H2SO4 (aq)  Na2SO4 (aq) + H2S (g)
Aqueous
Reactions
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Oxidation-Reduction Reactions
• An oxidation occurs
when an atom or ion
loses electrons.
• A reduction occurs
when an atom or ion
gains electrons.
• One cannot occur
without the other.
Aqueous
Reactions
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Oxidation-Reduction Reactions
• The substance
oxidized is called
the reducing agent.
• The substance
reduced is called the
oxidizing agent.
Aqueous
Reactions
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Oxidation-Reduction Reactions
• The substance
oxidized
experiences an
increase in
oxidation number.
• The substance
reduced
experiences a
decrease in
oxidation number.
Aqueous
Reactions
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Oxidation Numbers
To determine if an oxidation-reduction
reaction has occurred, we assign an
oxidation number to each element in a
neutral compound or charged entity.
If the oxidation numbers change, a redox
rxn has occurred.
Aqueous
Reactions
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Rules for Assigning
Oxidation Numbers
1. Elements in their elemental form have an oxidation
number of zero.
Mg0(s) + 2HCl(aq)  MgCl2 (aq) + H20 (g)
2. The oxidation number of a monatomic ion is the same as
its charge.
Mg0(s) + 2H+1Cl-1(aq)  Mg+2Cl-12 (aq) + H20 (g)
Note: Mg0 is oxidized (its charge increased)
2H1+ is reduced (its charge decreased)
Aqueous
Reactions
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Rules for Assigning
Oxidation Numbers
3. The sum of the oxidation numbers in a neutral
compound is 0.
FeCl3 (Fe = +3 & each Cl = -1)
FeCl2 (Fe = +2 & each Cl = -1)
4. The sum of the oxidation numbers in a polyatomic
ion is the charge on the ion.
Sulfate Ion = -2, so for SO4 2(S = +6 and each O = -2 which adds up to -2)
Aqueous
Reactions
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Rules for Assigning
Oxidation Numbers
5. Nonmetals tend to have negative oxidation
numbers, although there are some exceptions.
 Oxygen has an oxidation number of −2, except in
the peroxide ion in which it has an oxidation
number of −1.
O2 (Here oxygen is 0)
MgO
(Here Mg is +2 and O is -2)
Aqueous
Na2O2 (In peroxides, Na is +1 and O is -1)Reactions
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Rules for Assigning
Oxidation Numbers
5. Nonmetals tend to have negative oxidation numbers,
although there are some exceptions.
 Hydrogen is −1 when bonded to a metal, +1 when
bonded to a nonmetal.
HCl
NH3
NaH
(Here H is +1 and Cl is -1)
(Here N is -3 and H is still +1)
(Here Na is +1 and H is -1)
Aqueous
Reactions
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Rules for Assigning
Oxidation Numbers
5. Nonmetals tend to have negative oxidation numbers,
although there are some exceptions.
 Fluorine always has an oxidation number of −1.
NaF (Na is +1 and F is -1)
 The other halogens have an oxidation number of
−1 when they are negative; they can have
positive oxidation numbers, however, most
notably in oxyanions.
KClO3 (K is +1, Cl is +5 and each O is -2)
Aqueous
Reactions
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Redox Reactions
1. Be able to identify which substance is oxidized and
which substance is reduced.
Example:
0
0
H2
+ O2
+1 -2
 H2O
(H2 is oxidized because its charge increases)
(O2 is reduced because its charge decreases)
2. Metathesis Rxn are not redox because the charges
don’t change.
Aqueous
Reactions
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Displacement Reactions
• In displacement reactions,
ions oxidize an element.
• The ions, then, are
reduced.
Aqueous
Reactions
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Displacement Reactions
In this reaction,
silver ions oxidize
copper metal.
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Net Ionic Rx: Cu(s) + 2Ag+(aq)  Cu2+(aq) + 2Ag(s)
NO31- is the spectator ion
Aqueous
Reactions
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Displacement Reactions
The reverse reaction,
however, does not
occur.
x Cu (s) + 2 Ag+ (aq)
Cu2+ (aq) + 2 Ag (s) 
Aqueous
Reactions
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Activity Series
Rule:
A metal will replace (reduce)
any metal below it on the
table.
Given:
A + BC  AC + B
The reaction will take place
only if A is more reactive
(above) B on the table!
If so, we say the reaction isAqueous
Reactions
spontaneous.
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Molarity
• Two solutions can contain the same compounds but
be quite different because the proportions of those
compounds are different.
• Molarity is one way to measure the concentration of a
solution.
Molarity (M) =
moles of solute
volume of solution in liters
M = mol/L
(Also helpful: mol = mass/gfm)
Aqueous
Reactions
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Mixing a Solution
• To create a solution of a known
molarity, one weighs out a known
mass (and, therefore, number of
moles) of the solute.
• The solute is added to a volumetric
flask, and solvent is added to the
line on the neck of the flask.
mass = (Molarity) (gfm) (Vliters )
mass = (M) (gfm) (VL)
Aqueous
Reactions
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Solution Dilution
• One can also dilute a more concentrated
solution by
– Using a pipet to deliver a volume of the solution to
a new volumetric flask, and
– Adding solvent to the line on the neck of the new
flask.
Aqueous
Reactions
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Solution Dilution
The molarity of the new solution can be determined
from the equation
M1  V1 = M2  V2,
where M1 and M2 are the molarity of the concentrated and dilute
solutions, respectively, and V2 and V2 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
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Acid-Base Titration
Use:
(Ma)(Va) = (Mb)(Vb)
Shortcut:
Diprotic Acid Like H2SO4 :
2 (Ma)(Va) = (Mb)(Vb)
Dihydroxyl Base Like Ca(OH)2 :
(Ma)(Va) = 2 (Mb)(Vb)
Aqueous
Reactions
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Using Molarities in
Stoichiometric Calculations
Aqueous
Reactions
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Stoichiometric Calculation Steps
Step 1) Complete & balance equation using coefficients.
Step 2) Identify Given & Unknown.
Step 3) Convert Given into moles using either
#moles = mass / GFM -or- #moles = (Molarity)(VL)
Step 4) Use mole ratio to find moles of Unknown.
#molesgiven
Coeffgiven
=
#molesunk
Coeffunk
Step 5) Convert moles of ‘unknown’ to answer using either
mass = (#moles) (GFM) –or- Molarity = #moles / VL
Aqueous
Reactions
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Example: How many grams of NaOH(s) are needed to
neutralize exactly 50.0 mL of 2.00 M H2SO4 ?
Step 1) 2 NaOH(s) + H2SO4(aq)  Na2SO4(aq) + 2H2O(l)
Step 2) Given = 50.0mL of 2.00M H2SO4 Unknown = mass of NaOH
Step 3) Convert Given into moles using #moles = (Molarity)(VL)
#moles H2SO4 = (2.00M)(0.0500L) = 0.100 moles H2SO4
Step 4) Use mole ratio to find moles of Unknown.
#molesgiven = #molesunk
0.100
Coeffgiven
Coeffunk
1
= x
2
x = 0.200 mol
Step 5) Convert moles of ‘unk’ to answer using mass = (#moles) (GFM)
Aqueous
mass = (0.200moles)(40.0g/mol) = 8.00 grams NaOHReactions
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