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Reactions
On the A.P. Test there will be one question (question #4) that will say:
Give the formulas to show the reactants and the products for the following chemical
reactions. Each occurs in aqueous solution unless otherwise indicated. Represent
substances in solution as ions if the substance is extensively ionized. Omit formulas for
any ions or molecules that are unchanged by the reaction. In all cases a reaction occurs.
You need to balance the equation and answer the question that follows it..
Some suggestions:
Identify what type of reaction it is: redox, combustion, single replacement, acid-base,
precipitation, etc.
Use your knowledge of these types of rxns to determine where to go from there.
Remember that when things form they make common substances, not complex ones. If
your answer looks complicated, it is probably wrong.
Only do net ionic reactions. Identify those species that are actually going to react, the
rest you can ignore.
Make sure you know the solubility rules. What substances will precipitate?
Make sure you know the strong acids and bases.
Types of Reactions:
Reactions with no changes in Oxidation Numbers
I. Dissociation Reactions of Salts in Aqueous Solutions: (3,4,5,6)
dissociation: When ionic compounds dissolve in water the ions which compose
these compounds separate from each other and enter the solution as more or less
independent particles (ions): Cations or anions
NaCl ----> Na+1(aq) +
Cl-(aq)
H2SO4(l)
----> Na+(aq)
+
HSO4-(aq)
II. Nonmetallic oxides react with H2O to form acids
This is called an acid anhydride
+ H2O
----> H2SO4 +
H2O ----> H33O+(aq) + HSO4N2O5(g) + H2O ----> 2 HNO3 + H2O ----> 2 H3O+ + 2 NO3-
SO3(g)
III. Metallic Oxides react with H2O to form bases.
Called a basic anhydride
Na2O(s) + H2O ----> 2 NaOH + H2O (l) ----> 2 Na+(aq) + 2 OH-(aq)
IV. Some Decomposition Reactions
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A. Base ----> metal oxide + water
Ca(OH)2 ----> CaO + H2O
B. Salt containing oxygen ----> metal oxide + nonmetal oxide
CaCO3 ---->CaO + CO2
C. Acid containing oxygen ----> water + nonmetal oxide
H2CO3 ----> H2O + CO2
V. Metallic oxides + nonmetallic oxides ----> Salt
CaO(s) + SO2 (g) ----> CaSO3 (s)
VI. Reactions of Coordination compounds
These are called “complex” ions. (Though an ion is not always made). It
involves adding a ligand to a 4th row transition metal, and a few other selected metals.
The most common ligands include: ammonia (NH3), water, hydroxide (OH-), thiocynate
(SCN-), and cyanide (CN-). The number of ligands that attach to the metal will almost
always be double the amount of the charge on the metal. To determine the final charge,
add the total charge from the metal to the sum of the charges from all of the ligands.
Cu2+ + OH- ----> Cu(OH)42VI. Ionization Reactions of Molecular Compounds
Ionization: One example of ionization is when a molecular substance dissolves in water
and produces ions.
HCl(g) + H2O ---> H3O+(aq) + Cl-(aq)
HF(aq) + H2O ---> H3O+(aq) + F- (aq)
Molecular Equations
Complete formulas of all reactants and products are written as if they were molecules.
No explicit indication has been made that the soluble compounds are actually dissociated
into ions, so “molecular” in this context means only that complete formulas are used.
Cd(NO3)2(aq) + Na2S(aq) ---> CdS(s) + 2 NaNO3(aq)
Ionic Equations: All strong electrolytes are shown ionized
Cd2+(aq) + 2 NO3-(aq) + 2 Na+(aq) + S2-(aq) ---> CdS(s) + 2 Na+(aq) + 2NO3-(aq)
Net Ionic Equations:
Cd2+(aq) + S2-(aq) ---> CdS(s)
YOU MUST LEARN THE SOLUBILITY RULES: THEY MUST BE
MEMORIZED!!!
What to do when a nonsoluble reactant is used:
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Molecular Equation
Mg(OH)2(s) + 2 HCl(aq) ---> MgCl2(aq) + 2H2O
Ionic Equation
Mg(OH)2(s) + 2 H+(aq) + 2 Cl-(aq) ---> Mg2+(aq) + 2 Cl-(aq) + 2H2O
Net Ionic Equation
Mg(OH)2(s) + 2 H+(aq) Mg2+(aq) + 2 H2O
When a Weak Electrolyte is the Reactant
Molecular Equation
NaC2H3O2(aq) + HCl(aq) ---> HC2H3O2(aq) + NaCl (aq)
Ionic Equation
Na+(aq) + C2H3O2-(aq) + H+(aq) + Cl-(aq) ---> HC2H3O2(aq) + Na+(aq) + Cl-(aq)
Net Ionic Equation
C2H3O2-(aq) + H+(aq) ---> HC2H3O2
Reactions in Which a Gas Forms
Molecular Equation
2 HCl(aq) + Na2S(aq) ---> 2NaCl(aq) + H2S(g)
Ionic Equation
2H+(aq) + 2 Cl-(aq) + 2Na+(aq) + S2-(aq) --> 2H2S(g) + 2Na+(aq) + 2Cl-(aq)
Net Ionic Equation
2H+(aq) + S2-(aq) ---> H2S(g)
Reactions when Oxidation Numbers do change
I. Rules for Oxidation Numbers
The oxidation number of any free element (an element not combined chemically
with a different element) is zero, regardless of how complex its molecules might
be.
The oxidation number for any simple, monoatomic ion is equal to the charge on the ion.
The sum of all the oxidation numbers of the atoms in a molecule or ion must be equal the
charge of the particle.
In its compound, fluorine has an oxidation of -1
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In its compounds, hydrogen has an oxidation number of +, except hen combined with
metals when it is -1
In its compounds, oxygen has an oxidation number of -2, except in forming peroxides
then it is -1.
All Group I metals in compounds have a charge of +1
All Group II metals in compounds have a charge of +2
All Halogens have a charge of -1 when combined with metals in binary compounds
II. Balancing Redox Equations
A. Half-Reaction (Acidic Solution)
1.
Divide the skeleton equation into half-reactions
2.
Balance atoms other than H & O
3.
Balance oxygen by adding H2O to the side that needs O
4.
Balance hydrogen by adding H+ to the side that needs H
5.
Balance the charge by adding electrons
6.
Make the number of electrons gained equal to the number lost and then
add the two half-reactions
7.
Cancel anything that is the same on both sides.
B. Half-Reactions Method (Basic Solution)
1-7 Same as A above
8.
Add to both sides of the equation the same number of OH- as there are H+
9.
Combine H+ and OH- to form H2O
10.
Cancel any H2O that you can
III. Types of Redox Reactions
A. Simple Redox
1. Hydrogen Displacement
Ca(s) + 2H2O(l) --> Ca(OH)2(s) + H2
2. Metal Displacement
Zn(s) + CuSO4(aq) ---> ZnSO4(aq) + Cu(s)
3. Halogen Displacement
Cl2(g) + KBr(aq) ----> 2KCl(aq) + Br2(l)
4. Combustion
CH4(g) + 2O2(g)
--->
CO2(g) + 2H2O(g)
B. Disportionation
This is where one substance both oxidizes and reduces
Cl2(g) + 2OH-(aq) ------> OCl-(aq) + Cl-(aq) + H2O(l)
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C. Reactions involving oxoanions such as Cr2O7214H+(aq) + Cr2O72- + 6 Fe2+ ---> Cr3+ + 7 H2O + 6 Fe3+
IV. Redox Reaction Prediction
Important Oxidizers
MnO4- (acid solution)
MnO4- (basic solution)
MnO2 (acid solution)
Cr2O72- (acid)
CrO42HNO3, conc
HNO3, dilute
H2SO4, hot conc
Metallic Ions
Free Halogens
HClO4
Na2O2
H2O2
Formed in reaction
Mn2+
MnO2
Mn2+
Cr3+
Cr3+
NO2
NO
SO2
Metallous Ions
Halide ions
ClOHO2
Important Reducers
Halide Ions
Free Metals
Metalous Ions
Nitrite Ions
Sulfite Ions
Free Halogens (dil, basic, sol)
Free Halogens (conc, basic sol)
Formed in Reaction
Halogens
Metal Ions
Metallic ions
Nitrate Ions
SO42Hypohalite ions
Halite ions
Redox reactions involve the transfer of electrons. The oxidation numbers of at
least two elements must change. Single replacement, some combination and some
decomposition reactions are redox reactions.
To predict the products of a redox reaction, look at the reagents given to see if
there is both an oxidizing agent and a reducing agent. When a problem mentions an
acidic or basic solution, it is probably is redox.
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#1:
(a)
(b)
(c)
#2:
(a)
(b)
(c)
#3:
(a)
(b)
(c)
#4:
(a)
(b)
(c)
#5:
(a)
(b)
(c)
(i)
(ii)
(i)
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a solution containing iodide ions is added to a solution containing iron
(III) ion.
Which halogen is the best oxidizing agent? Explain your answer.
lithium metal is burned in air
What is the change in oxidization station of the Li?
small chunks of solid sodium are added to water
What might one observe in this reaction?
an excess of sodium hydroxide solution is added to a solution of
magnesium nitrate
What is the oxidizing agent in this reaction?
solutions of ammonia and hydrofluoric acid are mixed.
What is the ammonia acting as in this reaction?
solutions of manganese (II) sulfate and ammonium sulfide are mixed
What type of reaction is this?
a piece of aluminum metal is added to a solution of silver nitrate.
Identify the oxidizing agent in this reaction.
tetraphosphorus decoxide powder is sprinkled over distilled water
Would you predict the pH of the resulting solution to be 7, above 7, or
below 7 for the resulting solution? Support your answer.
Solid ammonium carbonate is heated.
What type of reaction is this?
(ii)
(i)
(ii)
(i)
(ii)
carbon dioxide gas is bubbled through a concentrated solution of
potassium hydroxide
What would be your prediction for pH for the resulting solution?
hydrogen sulfide gas is bubbled into a solution of mercury (II) chloride
What would you expect to see as a result of this reaction?
A bar of zinc metal is immersed in a solution of cupper(II) sulfate.
Identify the reducing agent in this reaction.
(i)
(ii)
(i)
(ii)
(i)
(ii)
Solid barium oxide is added to distilled water.
What type of reaction is this?
Excess concentrated sulfuric acid is added to solid calcium phosphate
What is the oxidation state of sulfur on the product side?
hydrogen peroxide is added to an acidified solution of sodium bromide
Identify the oxidizing agent.
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REACTIONS ANSWERS
#1 Answers:
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#2 Answers:
(a)
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#3 Answers:
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#4 Answers:
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#5 Answers:
(a)
(i)
(ii)
(b)
(i)
(ii)
(c)
(i)
2I- + 2Fe3+ --- 2Fe2+ + I2
fluorine since it most readily accepts electrons
4Li + O2 --- 2Li2O
Li0 --- Li+
2Na + 2H2O --- 2Na+ + OH- + H2
The sodium would float on the surface with effervescence. The gas may
ignite.
OH- + Mg2+ --- Ng(OH)2
None – this isn’t a redox reaction
NH3 + HF - NH4+ + Fweak base
Mn2+ + S2- ----- MnS
double replacement
Al + Ag+ ---
Al3+ + Ag
P4O10 + H2O --- H3PO4
Less than 7 because H3PO4 is an acid
(NH4)2CO3 ----- 2NH3 + CO2
decomposition
+ H2O
CO2 + OH- --- CO32- + H2O
pH would be above 7 because carbonate is basic
H2S + Hg2+ --- HgS + H+
a solid precipitate would form (HgS)
Zn + Cu2+ --- Zn2+ + Cu
BaO + H2O ---- Ba(OH)2
synthesis
H+ + HSO4- + Ca3(PO4)2 -- CaSO4 + H3PO4
+6
H2O2 + Br- --- Br2 + H2O