WRITING AP EQUATIONS AP equation sets are found in the freeresponse section of the AP test' You are

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WRITING AP EQUATIONSAP equation sets are found
in the free-response section of the AP test. You
are given three sets of reactants and you must
write balanced net ionic equations for the
reaction that would occur. The equations are of
mixed types. You will also answer a short
question about each reaction. The section is
worth 15 points and is 15 of the free response
grade. Free response is 50 of the total AP test
grade.
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All AP equations "work". In each case, a
reaction will occur. These equations need to be
written in net ionic form. All spectator ions
must be left out and all ions must be written in
ionic form. All molecular substances and
insoluble compounds must be written together (not
ionized!). Know your solubility rules!!!
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Ca(OH)2 and Sr(OH)2 are moderately soluble and
can be written together or as ions. Ba(OH)2 is
soluble and Mg(OH)2 is insoluble. CaSO4 and SrSO4
are moderately soluble and can be written
together or as ions. BaSO4 is insoluble and MgSO4
is soluble.
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Weak electrolytes, such as acetic acid, are not
ionized. Solids and pure liquids are written
together, also. A saturated solution is written
in ionic form while a suspension is written
together.
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• Each reaction will usually be worth a total of 5
  points.
• General guidelines (may not always be true!)
  One point is given for the correct reactants and
  two points for all correct products. If a
  reaction has three products, one point is given
  for two correct products and two points for all
  correct products. One point will be given for
  correct balancing and the final point will be
  given for correctly and completely answering the
  question concerning the reaction.

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The best way to prepare for the equation section
of the AP test is to practice lots of equations.
The equation sets are similar and some equations
show up year after year. Save the reactions that
you write and practice them again before the AP
test in May.
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When you are reading an equation, first try to
classify it by type. If it says anything about
acidic or basic solution, it is redox. If you
are totally stuck, look up the compounds in the
index of your book or other reference books and
try to find information that will help you with
the equation. All reactions do not fit neatly
into the five types of reactions that you learned
in Chemistry I.
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Double Replacement (metathesis)
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Two compounds react to form two new compounds.
No changes in oxidation numbers occur. All
double replacement reactions must have a "driving
force" that removes a pair of ions from solution.
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Formation of a precipitate A precipitate is an
insoluble substance formed by the reaction of two
aqueous substances. Two ions bond together so
strongly that water can not pull them apart. You
must know your solubility rules to write these
net ionic equations!
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Ex. Solutions of silver nitrate and lithium
bromide are mixed.

• AgNO3(aq) LiBr(aq) ? AgBr(s) LiNO3(aq)
• Ag NO3- Li Br- ? AgBr Li NO3-
• Ag NO3- Li Br- ? AgBr Li NO3-
• Ag Br- ? AgBr

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Formation of a gas Gases may form directly in a
double replacement reaction or can form from the
decomposition of a product such as H2CO3 or
H2SO3.
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Ex. Excess hydrochloric acid solution is added
to a solution of potassium sulfite.

• 2HCl(aq) K2SO3(aq) ? H2SO3 H2O(l) SO2(g)
  2KCl(aq)
• 2H 2Cl- 2K SO32- ? H2O SO2 2K 2Cl-
• 2H 2Cl- 2K SO32- ? H2O SO2 2K 2Cl-
• 2H SO32- ? H2O SO2

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Ex. A solution of sodium hydroxide is added to a
solution of ammonium chloride.

• NaOH(aq) NH4Cl(aq) ? NaCl(aq) NH4OH NH3(g)
  H2O(l)
• Na OH- NH4 Cl- ?
• Na Cl- NH3 H2O
• Na OH- NH4 Cl- ?
• Na Cl- NH3 H2O
• OH- NH4 ? NH3 H2O

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Formation of a molecular substanceWhen a
molecular substance such as water or acetic acid
is formed, ions are removed from solution and the
reaction "works".
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Ex. Dilute solutions of lithium hydroxide and
hydrobromic acid are mixed.

• LiOH(aq) HBr(aq) ? LiBr(aq) H2O(l)
• Li OH- H Br- ? Li Br- H2O
• Li OH- H Br- ? Li Br- H2O
• OH- H ? H2O (HBr, HCl, and HI are strong
  acids)

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Ex. Gaseous hydrofluoric acid reacts with solid
silicon dioxide.

• 4HF(g) SiO2(s) ? SiF4(g) 2H2O(l)
• 4HF SiO2 ? SiF4 2H2O
• 4HF SiO2 ? SiF4 2H2O

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Single Replacement
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Reaction where one element displaces another in a
compound. One element is oxidized and another is
reduced. A BC ? B AC
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Active metals replace less active metals or
hydrogen from their compounds in aqueous
solution. Use an activity series or a reduction
potential table to determine activity. The more
easily oxidized metal replaces the less easily
oxidized metal. The metal with the most negative
reduction potential will be the most active.
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Ex. Magnesium turnings are added to a solution
of iron(III) chloride.

• 3Mg(s) 2FeCl3(aq) ?2Fe(s)3MgCl2(aq)
• 3Mg 2Fe3 6Cl- ? 2Fe 3Mg2 6Cl-
• 3Mg 2Fe3 6Cl- ? 2Fe 3Mg2 6Cl-
• 3Mg 2Fe3 ? 2Fe 3Mg2

Always make sure that charge is balanced, as well
as mass (atoms)!
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Ex. Sodium is added to water.

• 2Na(s) 2H2O(l) ? 2NaOH(aq) H2(g)
• 2Na 2H2O ? 2Na 2OH- H2
• 2Na 2H2O ? 2Na 2OH- H2
• Alkali metal demo

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Active nonmetals replace less active nonmetals
from their compounds in aqueous solution. Each
halogen will displace less electronegative
(heavier) halogens from their binary salts.
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Ex. Chlorine gas is bubbled into a solution of
potassium iodide.

• Cl2(g) 2KI(aq) ?2KCl(aq) I2(s)
• Cl2 2K 2I- ? 2K 2Cl- I2
• Cl2 2K 2I- ? 2K 2Cl- I2
• Cl2 2I- ? I2 2Cl-

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Anhydrides
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Anhydride means "without water".Water is a
reactant in each of these equations.
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Nonmetallic oxides (acidic anhydrides) plus water
yield acids.
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Ex. Carbon dioxide is bubbled into water.

• CO2 H2O ? H2CO3

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Metallic oxides (basic anhydrides) plus water
yield bases.
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Ex. Solid sodium oxide is added to water.

• Na2O H2O ? 2Na 2OH-

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Metallic hydrides (ionic hydrides) plus water
yield metallic hydroxides and hydrogen gas.
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Ex. Solid sodium hydride is added to water.

• NaH H2O ?
• Na OH- H2

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Phosphorus halides react with water to produce
an acid of phosphorus (phosphorous acid or
phosphoric acid) and a hydrohalic acid. The
oxidation number of the phosphorus remains the
same in both compounds. Phosphorus
oxytrichloride reacts with water to make the same
products.
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Ex. Phosphorus tribromide is added to water.

• PBr3 3H2O ? H3PO3 3H 3Br-

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Group III nitrides react with water to produce
the metallic hydroxide and ammonia.
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Amines react with water to produce alkylammonium
ions and hydroxide ions.
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Ex. Methylamine gas is bubbled into distilled
water.

• CH3NH2 H2O ? CH3NH3 OH-

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OXIDATION-REDUCTION REACTIONS
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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.
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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 redox.
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Common oxidizing agents Products

formed?MnO4- in acidic solution
Mn2 MnO2 in acidic solution
Mn2 MnO4- in neutral or basic solution
MnO2(s)?Cr2O72- in acidic solution
Cr3 ?HNO3, concentrated
NO2 HNO3, dilute
NO H2SO4, hot, concentrated
SO2
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Common oxidizing agents Products

formed ?metal-ic ions
metal-ous ions?free halogens
halide ions Na2O2
NaOH HClO4
Cl- ? H2O2
H2O
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Common Products
reducing agents formed ?
halide ions free halogen? free
metals metal ions sulfite
ions or SO2 sulfate ions
nitrite ions nitrate
ions ? free halogens, dilute basic solution
hypohalite ions free halogens, conc. basic
solution halate ions ? metal-ous ions
metal-ic ions ? H2O2
O2 C2O42 -
CO2
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Ex. A solution of tin(II) chloride is added to an
acidified solution of potassium permanganate.

• 5Sn2 16H 2MnO4- ? 5Sn4 2Mn2 8H2O

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Ex. A solution of potassium iodide is added to
an acidified solution of potassium dichromate.

• 6I- 14H Cr2O72- ? 2Cr3 3I2 7H2O

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Ex. Hydrogen peroxide solution is added to a
solution of iron(II) sulfate.

• H2O2 2Fe2 2H?
• 2Fe3 2H2O

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Ex. A piece of iron is added to a solution of
iron(III) sulfate.

• Fe 2Fe3 ? 3Fe2

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Tricky redox reactions that appear to be ordinary
single replacement reactionsHydrogen reacts
with a hot metallic oxide to produce the
elemental metal and water.
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Ex. Hydrogen gas is passed over hot copper(II)
oxide.

• H2(g) CuO(s) ? Cu(s) H2O(l)

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A metal sulfide reacts with oxygen to produce the
metallic oxide and sulfur dioxide.
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Chlorine gas reacts with dilute sodium hydroxide
to produce sodium hypochlorite, sodium chloride
and water.(disproportionation reaction)
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Copper reacts with concentrated sulfuric acid to
produce copper(II) sulfate, sulfur dioxide, and
water.(Cu doesnt react with dilute sulfuric
acid!)
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Copper reacts with dilute nitric acid to produce
copper(II) nitrate, nitrogen monoxide and water.
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Copper reacts with concentrated nitric acid to
produce copper(II) nitrate, nitrogen dioxide and
water.
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COMPLEX ION REACTIONS
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Complex ion- the combination of a central metal
ion and its ligandsLigand- group bonded to a
metal ion
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Coordination compound- a neutral compound
containing complex ions
Co(NH3)6Cl3 (NH3 is the ligand,
Co(NH3)63is the complex ion)
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Common complex ions formed in AP equations
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Complex ion Name
Formed fromAl(OH)4- tetrahydroxoaluminate
ion (Al or Al(OH)3 or
Al3 OH-)

Ag(NH3)2 diamminesilver(I) ion (Ag
NH3)Zn(OH)4 2- tetrahydroxozincate ion
(Zn(OH)2 OH-)Zn(NH3)4 2 tetramminezinc
ion (Zn2 NH3)Cu(NH3)42
tetramminecopper(II) ion (Cu2
NH3)Cd(NH3)4 2 tetramminecadmium(II) ion
(Cd2 NH3)FeSCN 2 thiocyanoiron(III)
ion (Fe3 SCN-)Ag(CN)2-
dicyanoargentate(I) ion (Ag and
CN-)Ag(Cl)2- dichloroargentate(I) ion
(AgCl and Cl-)
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Adding an acid to a complex ion will break it up.
If HCl is added to a silver complex, AgCl(s) is
formed. If an acid is added to an ammonia
complex, NH4 is formed.
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Ex. Excess ammonia is added to a solution of
zinc nitrate.

• 4NH3 Zn2 ? Zn(NH3)42
• (Other coordination numbers are
  acceptable, as long as correct charge is given.)

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Ex. A solution of diamminesilver(I) chloride is
treated with dilute nitric acid.

• Ag(NH3)2 Cl- 2H ?
• AgCl 2NH4

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DECOMPOSITION REACTIONS
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Reaction where a compound breaks down into two or
more elements or compounds. Heat, electrolysis,
or a catalyst is usually necessary.
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A compound may break down to produce two
elements.Ex. Molten sodium chloride is
electrolyzed.
2NaCl(l)? 2Na Cl2
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A compound may break down to produce an element
and a compound.Ex. A solution of hydrogen
peroxide is decomposed catalytically.2H2O2 ?
2H2O O2
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A compound may break down to produce two
compounds.Ex. Solid magnesium carbonate is
heated. MgCO3 ? MgO CO2
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Metallic carbonates break down to yield metallic
oxides and carbon dioxide.
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Metallic chlorates break down to yield metallic
chlorides and oxygen.
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Hydrogen peroxide decomposes into water and
oxygen.
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Ammonium carbonate decomposes into ammonia, water
and carbon dioxide.
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Sulfurous acid decomposes into water and sulfur
dioxide.
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Carbonic acid decomposes into water and carbon
dioxide.
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ADDITION REACTIONS
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Two or more elements or compounds combine to form
a single product.
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A Group IA or IIA metal may combine with a
nonmetal to make a salt.
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A piece of lithium metal is dropped into a
container of nitrogen gas.

• 6Li N2 ? 2Li3N

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Two nonmetals may combine to form a molecular
compound. The oxidation number of the less
electronegative element is often variable
depending upon conditions. Generally, a higher
oxidation state of one nonmetal is obtained when
reacting with an excess of the other nonmetal.
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P4 6Cl2 ? 4PCl3 limited ClP4 10Cl2 ?
4PCl5 excess Cl
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When an element combines with a compound, you can
usually sum up all of the elements on the product
side.Ex. PCl3 Cl2 ? PCl5
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Two compounds combine to form a single
product.Ex. Sulfur dioxide gas is passed over
solid calcium oxide.SO2 CaO ? CaSO3
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Ex. The gases boron trifluoride and ammonia are
mixed. Lewis Acid-Base Reaction BF3 NH3 ?
H3NBF3 F H H F
F-B N-H ? H-N-B-F F H
H F
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A metallic oxide plus carbon dioxide yields a
metallic carbonate. (Carbon keeps the same
oxidation state)
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A metallic oxide plus sulfur dioxide yields a
metallic sulfite. (Sulfur keeps the same
oxidation state)
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A metallic oxide plus water yields a metallic
hydroxide.A nonmetallic oxide plus water yields
an acid.
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ACID-BASE NEUTRALIZATION REACTIONS
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Acids react with bases to produce salts and
water. One mole of hydrogen ions react with one
mole of hydroxide ions to produce one mole of
water.
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Watch out for information about quantities of
each reactant! Remember which acids are strong
(ionize completely) and which are weak (write as
molecule).
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Sulfuric acid (strong acid) can be written as H
and SO42- or as H and HSO4-.Concentrated
sulfuric acid is left together because there is
not enough water present for it to ionize.
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Ex. A solution of sulfuric acid is added to a
solution of barium hydroxide until the same
number of moles of each compound has been added.

• H HSO42 - Ba2 2OH- ? BaSO4 2H2O

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Ex. Hydrogen sulfide gas is bubbled through
excess potassium hydroxide solution.

• H2S OH- ? H2O HS-
• (S2- does not exist in water)

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Watch out for substances that react with water
before reacting with an acid or a base. These
are two step reactions.
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Sulfur dioxide gas is bubbled into an excess of a
saturated solution of calcium hydroxide.
2-step reaction!

• SO2 H2O ? H2SO3
• H2SO3 Ca2 2OH-?2H2O CaSO3
• H2SO3 and 1 H2O cancel.
• SO2 Ca2 2OH- ? CaSO3 H2O

Add steps together!
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COMBUSTION REACTIONS
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-Elements or compounds combine with oxygen to
produce oxides of each element.
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Hydrocarbons or alcohols combine with oxygen to
form carbon dioxide and water.
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Nonmetallic hydrides combine with oxygen to form
oxides and water.
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Nonmetallic sulfides combine with oxygen to form
oxides and sulfur dioxide.
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Ex. Carbon disulfide vapor is burned in excess
oxygen.

• CS2 3O2 ? CO2 2SO2

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Ex. Ethanol is burned completely in air.

• C2H5OH 3O2 ? 2CO2 3H2O

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Ammonia combines with limited oxygen to produce
NO and water and with excess oxygen to produce
NO2 and water.