3. Precipitation or Exchange (Metathesis) Reactions
•
Precipitation
! The “key event” is formation of an insoluble, ionic precipitate when two
!
!
•
solutions containing dissolved ions are mixed.
A precipitate has the phase symbol (s).
These are sometimes called “double replacement reactions”.
You must know the solubility guidelines to predict products.
Examples
a) AgNO3(aq) + NaCl(aq) —> AgCl(s) + NaNO3(aq)
b) Fe(NO3)3(aq) + 3NaOH(aq) —> Fe(OH)3(s) + 3NaNO3(aq)
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Will precipitation occur when the following solutions are mixed? If so, write a
balanced molecular chemical equation for the reaction, include phase labels. Then
identify the spectator ions and write the balanced net-ionic equation.
a) Pb(NO3)2(aq) and Na2S(aq)
b) Na2CO3 and AgNO3
c) NaNO3 and NiSO4
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Examples: Precipitation Reactions
1. When aqueous solutions of potassium chromate and lead (II) nitrate are
mixed, a precipitate forms.
What is the identity of the precipitate?
Write the net-ionic equation for the reaction.
2. Suggest two soluble salts that when mixed together in aqueous solution
will produce calcium phosphate.
Write the balanced net-ionic equation for the reaction.
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Precipitation Reaction Titration Example
What is the molarity of a barium chloride solution that requires 14.5 mL of
0.412 M Al2(SO4)3 to titrate 20.0 mL of the solution to the equivalence point?
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4. Oxidation and Reduction (redox) Reactions
The “key event” is the transfer of electrons from one species to
another. We will watch video: OxidationReductionPart1
Reducing Agent
Oxidizing Agent
• Oxidation: The loss of electrons
• Reduction: The gain of electrons
One cannot occur without the other!
Identify the following for this
reaction:
Oxidized species
Reduced species
Oxidizing agent
Reducing agent
http://www.youtube.com/watch?v=Lpw_cR-sbks
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Recognizing Redox Reactions: Assigning
Oxidation Numbers to Atoms
I.
For an atom in its elemental form the oxidation number is
zero.
II. For any monatomic ion the oxidation number equals the
charge of the ion.
III. For nonmetals the oxidation number is usually negative.
a) Oxygen is usually -2 in all compounds.
b) Fluorine is -1 in all compounds.
c) Hydrogen is +1 when bonded to nonmetals and -1 when bonded
to metals (metal hydrides).
IV. The sum of the oxidation numbers for all atoms is zero for
neutral compounds or equals the charge for polyatomic ions.
Assign oxidation numbers to each atom in the following:
(a) Na3N
(b) CO2
(c) CH4
(d) Mn(NO3)2
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We will study four types of redox reactions:
1. Combination Reactions
Two or more substances, at least one
of which is an element, react to form
one product.
Examples: 2 Na(s) + Cl2(g) —> 2 NaCl(s)
N2(g) + 3 H2(g) —> 2 NH3(g)
2 H2(g) + O2(g) —> 2 H2O(g)
Write the balanced chemical equation for the
combination reaction between aluminum and liquid
bromine. Include phase labels.
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Stoichiometry
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We will study four types of redox reactions:
2. Decomposition Reactions
One substance breaks down into two or
more simpler substances.
Examples:
CaCO3(s) —> CaO(s) + CO2(g)
2 KClO3(s) —> 2 KCl(s) + 3 O2(g)
2 NaN3(s) —> 2 Na(s) + 3 N2(g)
Chemical reaction to inflate an airbag
with N2 gas.
Write the balanced chemical equation for the following decomposition reaction:
When heated, solid sodium bicarbonate decomposes into sodium carbonate, water
vapor and gaseous carbon dioxide. (Include phase labels.)
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Stoichiometry
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We will study four types of redox reactions:
3. Simple Organic Combustion Reactions
When a simple organic compound (contains only C, H and O) burns in air, what
reactant besides the organic compound is involved in the reaction?
What products are formed?
Write a balanced chemical equation for the combustion of benzene, C6H6(l), in
air. What reduced in the process? What is the reducing agent?
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Stoichiometry
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We will study four types of redox reactions:
4: Displacement Reactions
• Elemental metals can be oxidized in the presence of a salt or acid.
The general form is:
M + AX —> MX + A where M is a metal and AX is a salt or acid.
• (Often called “single-displacement reactions”)
Examples:
Zn(s) + 2 HCl(aq) —> ZnCl2(aq) + H2(g)
Zn(s) + CuSO4(aq) —> ZnSO4(aq) + Cu(s)
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Activity Series of Metals/Ions
Reactivity of Metal Ion Increases
Reactivity of Metal Increases
(Used to predict if a single replacement redox reaction will take place.)
Any metal on the left will
be oxidized (will react)
with any ion on the right
that is below the metal in
the table.
Noble Metals
(don’t react with acids!
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Examples: Oxidation Reduction Reactions
• Predict the products (if any) of the following reactions. If a
reaction occurs, write the net-ionic equation.
a) Mg(s) + FeCl2(aq) —>
b) HCl(aq) + Cu(s) —>
c)
chromium with hydrobromic acid
d) aluminum with formic acid, HCOOH
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Additional Practice Problems
•
You know that an unlabeled bottle contains a solution of one of the following: AgNO3,
CaCl2, or Al2(SO4)3. A friend suggests that you test a portion of the solution with
Ba(NO3)2 and then with NaCl solutions. Explain how these two tests together would be
sufficient to determine which salt is present in the solution.
•
Which of the following solutions has the largest concentration of solvated protons: (a)
0.1 M LiOH, (b) 0.1 M HI, (c) 0.5 M methyl alcohol (CH3OH)? Explain.
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Additional Practice Problems
•
A solution of 100.0 mL of 0.200 M KOH is mixed with a solution of 200.0
mL of 0.150 M NiSO4.
a) What precipitate forms?
b)
Write the balanced chemical equation for the reaction that occurs.
c)
d)
e)
What is the limiting reactant?
How many grams of the precipitate form?
What is the concentration of each ion that remains in solution?
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Additional Practice Problems
•
A 0.5895-g sample of impure magnesium hydroxide is dissolved in 100.0
mL of 0.2050 M HCl solution. The excess acid then needs 19.85 mL of
0.1020 M NaOH for neutralization. Calculate the percent by mass of
magnesium hydroxide in the sample, assuming that it is the only
substance reacting with the HCl solution.
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Additional Practice Problems
• To find the mass percent of dolomite, CaMg(CO3)2, in a soil sample, a
geochemist titrate 13.36 g of the soil wirh 33.56 mL of 0.2516 M HCl. What is
the mass percent of dolomite in the soil?
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Reaction Types Review
Acid Base Reactions:
• Must be able to predict products for proton transfer reactions for weak and strong acids and bases.
• Must be able to predict products for reactions involving acids and carbonates or hydrogen carbonates.
Precipitation Reactions:
• Must be able to predict if a reaction will occur or not and the products for precipitation reactions.
Combination:
• Easy to predict product formulas and balance when a metal and a nonmetal combine to form a
binary ionic compound. You must be able to do this for exams!
• Can be hard to predict products for reactions yielding molecular compounds; more familiarity with
chemistry is needed; you will not be ask to predict these on exams.
Decomposition:
• Easy to predict product formulas and balance when a binary ionic compound decomposes into its
elements. You must be able to do this for for exams!
• Harder to predict products for the decomposition of ionic compounds containing polyatomic ions or
the decomposition of molecular compounds; more familiarity with chemistry is still needed; you will
not be ask to predict these on exams.
Simple Organic Combustion:
• Easy to predict products and balance. You must be able to do this for for exams!
Single Displacement:
• Must be able to use the activity series to predict if a reaction will occur or not and the products for
single replacement reactions.
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Stoichiometry
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