Chapter 7 Chemical Reactions

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Chapter 7ChemicalReactions
2
Experiencing Chemical Change

• chemical reactions are happening both around you
  and in you all the time
• some are very simple, others are complex
• in terms of the pieces even the simple ones
  have a lot of interesting principles to learn
  from
• chemical reactions involve changes in the
  structures of the molecules, and many times we
  can experience the effects of those changes
• What are some examples of chemical reactions you
  experience?

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Chemical Reactions

• Reactions involve chemical changes in matter
  resulting in new substances
• Reactions involve rearrangement and exchange of
  atoms to produce new molecules
• Elements are not transmuted during a reaction

4
Evidence of Chemical Reactions

• look for evidence of a new substance
• visual clues (permanent)
• color change
• precipitate formation
• solid that forms when liquid solutions are mixed
• gas bubbles
• large energy changes
• container becomes very hot or cold
• emission of light
• other clues
• new odor
• whooshing sound from a tube
• permanent new state

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Evidence of Chemical Change
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Evidence of Chemical Change

• in order to be absolutely sure that a chemical
  reaction has taken place, you need to go down to
  the molecular level and analyze the structures of
  the molecules at the beginning and end

Is boiling water a chemical change?
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Chemical Equations

• Shorthand way of describing a reaction
• Provides information about the reaction
• Formulas of reactants and products
• States of reactants and products
• Relative numbers of reactant and product
  molecules that are required
• Can be used to determine weights of reactants
  used and products that can be made

8
Conservation of Mass

• Matter cannot be created or destroyed
• Therefore the total mass cannot change
• and the total mass of the reactants will be the
  same as the total mass of the products
• In a chemical reaction, all the atoms present at
  the beginning are still present at the end
• if all the atoms are still there, then the mass
  will not change

9
The Combustion of Methane

• methane gas burns to produce carbon dioxide gas
  and gaseous water
• whenever something burns it combines with O2(g)

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Combustion of Methane

• methane gas burns to produce carbon dioxide gas
  and gaseous water
• whenever something burns it combines with O2(g)
• CH4(g) O2(g) CO2(g) H2O(g)

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Combustion of MethaneBalanced

• to show the reaction obeys the Law of
  Conservation of Mass it must be balanced
• CH4(g) 2 O2(g) CO2(g) 2 H2O(g)

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Chemical Equations
CH4(g) 2 O2(g) CO2(g) 2 H2O(g)

• CH4 and O2 are the reactants, and CO2 and H2O are
  the products
• the (g) after the formulas tells us the state of
  the chemical
• the number in front of each substance tells us
  the numbers of those molecules in the reaction
• called the coefficients

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Chemical Equations
CH4(g) 2 O2(g) CO2(g) 2 H2O(g)

• this equation is balanced, meaning that there are
  equal numbers of atoms of each element on the
  reactant and product sides
• to obtain the number of atoms of an element,
  multiply the subscript by the coefficient
• 1 ? C ? 1
• 4 ? H ? 4
• 4 ? O ? 2 2

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Symbols Used in Equations

• symbols used to indicate state after chemical
• (g) gas (l) liquid (s) solid
• (aq) aqueous dissolved in water
• energy symbols used above the arrow for
  decomposition reactions
• D heat
• hn light
• shock mechanical
• elec electrical

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Writing Balanced Chemical Equations

• Write a skeletal equation by writing the formula
  of each reactant and product
• Count the number of atoms of each element on each
  side of the equation
• polyatomic ions may often be counted as if they
  are one element
• Pick an element to balance
• if an element is found in only one compound on
  both sides, balance it first
• leave elements that are free elements somewhere
  in the equation until last

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Writing Balanced Chemical Equations

• Find the Least Common Multiple of the number of
  atoms on each side
• the LCM of 3 and 2 is 6
• Multiply each count by a factor to make it equal
  to the LCM
• Use this factor as a coefficient in the equation
• if there is already a coefficient there, multiply
  it by the factor
• it must go in front of entire molecules, not
  between atoms within a molecule
• Recount and Repeat until Balanced

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Examples

• when magnesium metal burns in air it produces a
  white, powdery compound magnesium oxide
• Mg(s) O2(g) MgO(s)

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Examples

• Under appropriate conditions at 1000C ammonia
  gas reacts with oxygen gas to produce gaseous
  nitrogen monoxide and gaseous water
• NH3(g) O2(g) NO(g) H2O(g)

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Aqueous Solutions

• Many times, the chemicals we are reacting
  together are dissolved in water
• mixtures of a chemical dissolved in water are
  called aqueous solutions
• Dissolving the chemicals in water helps them to
  react together faster
• the water separates the chemicals into individual
  molecules or ions
• the separate, free floating particles come in
  contact more frequently so the reaction speeds up

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Predicting Whether a Reaction Will Occur in
Aqueous Solution

• forces that drive a reaction
• formation of a solid
• formation of water
• formation of a gas
• transfer of electrons
• when chemicals (dissolved in water) are mixed and
  one of these 4 things can occur, the reaction
  will generally happen

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Dissociation

• when ionic compounds dissolve in water, the
  anions and cations are separated from each other
  - this is called dissociation
• however not all ionic compounds are soluble in
  water!
• when compounds containing polyatomic ions
  dissociate, the polyatomic group stays together
  as one ion

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Dissociation

• potassium iodide dissociates in water into
  potassium cations and iodide anions
• KI(aq) ? K1(aq) I-1(aq)
• copper(II) sulfate dissociates in water into
  copper(II) cations and sulfate anions
• CuSO4(aq) ? Cu2(aq) SO4-2(aq)

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Dissociation

• potassium sulfate dissociates in water into
  potassium cations and sulfate anions
• K2SO4(aq) ? 2 K1(aq) SO4-2(aq)

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Electrolytes

• electrolytes are substances whose water solution
  is a conductor of electricity
• all electrolyte have ions dissolved in water

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Electrolytes

• in strong electrolytes, all the electrolyte
  molecules or formula units are separated into
  ions
• in weak electrolytes, a small percentage of the
  molecules are separated into ions
• in nonelectrolytes, none of the molecules are
  separated into ions

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Types of Electrolytes

• salts water soluble ionic compounds
• all strong electrolytes
• acids form H1 ions in water solution
• sour taste
• react and dissolve many metals
• strong acid strong electrolyte, weak acid
  weak electrolyte
• bases water soluble metal hydroxides
• bitter taste, slippery (soapy) feeling solutions
• increases the OH-1 concentration

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When will a Salt Dissolve?

• a compound is soluble in a liquid if it dissolves
  in that liquid
• NaCl is soluble in water, but AgCl is not
• a compound is insoluble if a significant amount
  does not dissolve in that liquid
• AgCl is insoluble in water
• though there is a very small amount dissolved,
  but not enough to be significant

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When will a Salt Dissolve?

• Predicting whether a compound will dissolve in
  water is not easy
• The best way to do it is to do some experiments
  to test whether a compound will dissolve in
  water, then develop some rules based on those
  experimental results
• we call this method the empirical method

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Solubility RulesCompounds that are Generally
Soluble in Water
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Solubility RulesCompounds that are Generally
Insoluble
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Using the Solubility Rules to Predict an Ionic
Compounds Solubility in Water

• first check the cation, if it is Li, Na, K, or
  NH4 then the compound will be soluble in water
• regardless of the anion!
• if the cation is not Li, Na, K, or NH4 then
  follow the rule for the anion
• if a rule says the compounds are mostly soluble,
  then the exceptions are insoluble
• but if a rule says the compounds are mostly
  insoluble, then the exceptions are soluble
• note slightly soluble ? insoluble

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Determine if Each of the Following is Soluble in
Water

• KOH
• AgBr
• CaCl2
• Pb(NO3)2
• PbSO4

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Determine if Each of the Following is Soluble in
Water

• KOH Soluble, because the cation is K
• AgBr Insoluble, even though most compounds with
  Br- are soluble, this is an exception
• CaCl2 Soluble, most compounds with Cl- are
  soluble
• Pb(NO3)2 Soluble, because the anion is NO3-
• PbSO4 Insoluble, even though most compounds with
  SO42- are soluble, this is an exception

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Precipitation Reactions

• many reactions are done by mixing aqueous
  solutions of electrolytes together
• when this is done, often a reaction will take
  place from the cations and anions in the two
  solutions exchanging
• if the ion exchange results in forming a compound
  that is insoluble in water, it will come out of
  solution as a precipitate

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Precipitation Reactions
Pb(NO3)2(aq) 2 KI(aq) ? 2 KNO3(aq) PbI2(s)
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Precipitation Reactions
Pb(NO3)2(aq) 2 KI(aq) ? 2 KNO3(aq) PbI2(s)
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No Precipitate Formation No Reaction
KI(aq) NaCl(aq) ? KCl(aq) NaI(aq) all ions
still present, ? no reaction
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Process for Predicting the Products ofa
Precipitation Reaction

• Determine what ions each aqueous reactant has
• Exchange Ions
• () ion from one reactant with (-) ion from other
• Balance Charges of combined ions to get formula
  of each product
• Balance the Equation
• count atoms
• Determine Solubility of Each Product in Water
• use the Solubility Rules
• if product is insoluble or slightly soluble, it
  will precipitate
• if neither product will precipitate, no reaction

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Example 7.7 - When an aqueous solution of sodium
carbonate is added to an aqueous solution of
copper(II) chloride, a white solid forms

• Write the formulas of the reactants
• Na2CO3(aq) CuCl2(aq) ?
• Determine the ions present when each reactant
  dissociates
• (Na CO32-) (Cu2 Cl-) ?
• Exchange the Ions
• (Na CO32-) (Cu2 Cl-) ? (Na Cl-)
  (Cu2 CO32-)

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Example 7.7 - When an aqueous solution of sodium
carbonate is added to an aqueous solution of
copper(II) chloride, a white solid forms

• Write the formulas of the products
• cross charges and reduce
• Na2CO3(aq) CuCl2(aq) ? NaCl CuCO3
• Balance the Equation
• Na2CO3(aq) CuCl2(aq) ? 2 NaCl CuCO3

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Example 7.7 - When an aqueous solution of sodium
carbonate is added to an aqueous solution of
copper(II) chloride, a white solid forms

• Determine the solubility of each product
• NaCl is soluble
• CuCO3 is insoluble
• Write an (s) after the insoluble products and a
  (aq) after the soluble products
• Na2CO3(aq) CuCl2(aq) ? 2 NaCl(aq) CuCO3(s)

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Ionic Equations

• equations which describe the chemicals put into
  the water and the product molecules are called
  molecular equations
• 2 KOH(aq) Mg(NO3)2(aq) 2 KNO3(aq)
  Mg(OH)2(s)
• equations which describe the actual dissolved
  species are called ionic equations
• aqueous electrolytes are written as ions
• soluble salts, strong acids, strong bases
• insoluble substances and nonelectrolytes written
  in molecule form
• solids, liquids and gases are not dissolved,
  therefore molecule form
• 2K1(aq) 2OH-1(aq) Mg2(aq) 2NO3-1(aq)
  2K1(aq) 2NO3-1(aq) Mg(OH)2(s)

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Ionic Equations

• ions that are both reactants and products are
  called spectator ions
• 2K1(aq) 2OH-1(aq) Mg2(aq) 2NO3-1(aq)
  2K1(aq) 2NO3-1(aq) Mg(OH)2(s)

• an ionic equation in which the spectator ions
  are
• removed is called a net ionic equation
• 2OH-1(aq) Mg2(aq) Mg(OH)2(s)

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Acid-Base Reactions

• also called neutralization reactions because the
  acid and base neutralize each others properties
• in the reaction of an acid with a base, the H1
  from the acid combines with the OH-1 from the
  base to make water
• the cation from the base combines with the anion
  from the acid to make the salt
• acid base ???salt water
• 2 HNO3(aq) Ca(OH)2(aq) ? Ca(NO3)2(aq) 2
  H2O(l)
• the net ionic equation for an Acid-Base reaction
  is
• H1(aq) OH-1(aq) ? H2O(l)
• as long as the salt that forms is soluble in water

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Example 7.11 - Write the molecular, ionic and
net-ionic equation for the reaction of aqueous
nitric acid with aqueous calcium hydroxide

• Write the formulas of the reactants
• HNO3(aq) Ca(OH)2(aq) ?
• Determine the ions present when each reactant
  dissociates
• (H NO3-) (Ca2 OH-) ?
• Exchange the ions, H1 combines with OH-1 to make
  H2O(l)
• (H NO3-) (Ca2 OH-) ? (Ca2 NO3-)
  H2O(l)

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Example 7.11 - Write the molecular, ionic and
net-ionic equation for the reaction of aqueous
nitric acid with aqueous calcium hydroxide

• Write the formulas of the products
• cross charges and reduce
• HNO3(aq) Ca(OH)2(aq) ? Ca(NO3)2 H2O(l)
• Balance the Equation
• may be quickly balanced by matching the numbers
  of H and OH to make H2O
• coefficient of the salt is always 1
• 2 HNO3(aq) Ca(OH)2(aq) ? Ca(NO3)2 2 H2O(l)

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Example 7.11 - Write the molecular, ionic and
net-ionic equation for the reaction of aqueous
nitric acid with aqueous calcium hydroxide

• Determine the solubility of the salt
• Ca(NO3)2 is soluble
• Write an (s) after the insoluble products and a
  (aq) after the soluble products
• 2 HNO3(aq) Ca(OH)2(aq) ? Ca(NO3)2(aq) 2
  H2O(l)

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Example 7.11 - Write the molecular, ionic and
net-ionic equation for the reaction of aqueous
nitric acid with aqueous calcium hydroxide

• Dissociate all aqueous materials to get complete
  ionic equation
• not H2O
• 2 H(aq) 2 NO3-(aq) Ca2(aq) 2 OH-(aq) ?
  Ca2(aq) 2 NO3-(aq) H2O(l)
• Eliminate spectator ions to get net-ionic
  equation
• 2 H1(aq) 2 OH-1(aq) ? 2 H2O(l)
• H1(aq) OH-1(aq) ? H2O(l)

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Gas Evolving Reactions

• Some reactions form a gas directly from the ion
  exchange
• K2S(aq) H2SO4(aq) ? K2SO4(aq) H2S(g)
• Other reactions form a gas by the decomposition
  of one of the ion exchange products into a gas
  and water
• K2SO3(aq) H2SO4(aq) ? K2SO4(aq) H2SO3(aq)
• H2SO3 ? H2O(l) SO2(g)

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Compounds that UndergoGas Evolving Reactions
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Process for Predicting the Products ofa Gas
Evolving Reaction

• Determine what ions each aqueous reactant has
• Exchange Ions
• () ion from one reactant with (-) ion from other
• Balance Charges of combined ions to get formula
  of each product
• Check to see if either product is H2S
• Check to see if either product decomposes, if so
  rewrite as H2O(l) and a gas
• see table
• Balance the Equation
• Determine Solubility of Other Product in Water

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Example - When an aqueous solution of sodium
sulfite is added to an aqueous solution of nitric
acid, a gas evolves

• Write the formulas of the reactants
• Na2SO3(aq) HNO3(aq) ?
• Determine the ions present when each reactant
  dissociates
• (Na1 SO3-2) (H1 NO3-1) ?
• Exchange the Ions
• (Na1 SO3-2) (H1 NO3-1) ? (Na1 NO3-1)
  (H1 SO3-2)

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Example - When an aqueous solution of sodium
sulfite is added to an aqueous solution of nitric
acid, a gas evolves

• Write the formulas of the products
• cross charges and reduce
• Na2SO3(aq) HNO3(aq) ? NaNO3 H2SO3
• Check to see either product H2S - No
• Check to see of either product decomposes Yes
• H2SO3 decomposes into SO2(g) H2O(l)
• Na2SO3(aq) HNO3(aq) ? NaNO3 SO2(g) H2O(l)

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Example - When an aqueous solution of sodium
sulfite is added to an aqueous solution of nitric
acid, a gas evolves

• Balance the Equation
• Na2SO3(aq) 2 HNO3(aq) ? 2 NaNO3 SO2(g)
  H2O(l)
• Determine the solubility of other product
• NaNO3 is soluble
• Write an (s) after the insoluble products and a
  (aq) after the soluble products
• Na2SO3(aq) 2 HNO3(aq) ? 2 NaNO3(aq) SO2(g)
  H2O(l)

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Other Patterns in Reactions

• the precipitation, acid-base, and gas evolving
  reactions all involved exchanging the ions in the
  solution
• other kinds of reactions involve transferring
  electrons from one atom to another these are
  called oxidation-reduction reactions
• also known as redox reactions
• unlike the others, many of these reactions are
  not done by dissolving the reactants in water

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Oxidation-Reduction Reactions

• We say that the element that loses electrons in
  the reaction is oxidized
• and the substance that gains electrons in the
  reaction is reduced
• you cannot have one without the other

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Reactions of Metals with Nonmetals(Oxidation-Redu
ction)

• metals react with nonmetals to form ionic
  compounds
• ionic compounds are solids at room temperature
• the metal loses electrons and becomes a cation
• the metal undergoes oxidation
• the nonmetal gains electrons and becomes an anion
• the nonmetal undergoes reduction
• In the reaction, electrons are transferred from
  the metal to the nonmetal
• 2 Na(s) Cl2(g) ? NaCl(s)

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Oxidation-Reduction Reactions

• any reaction that has an element that is
  uncombined on one side and combined on the other
  is a redox reaction
• uncombined free element
• 2 CO O2 ? 2 CO2
• 2 N2O5 ? 4 NO2 O2
• 3 C Fe2O3 ? 3 CO 2 Fe
• Mg Cl2 ? MgCl2
• any reaction where a cation changes charge is
  redox
• CuCl FeCl3 ? FeCl2 CuCl2
• SnCl2 F2 ? SnCl2F2

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Combustion Reactions

• Reactions in which O2(g) is a reactant are called
  Combustion Reactions
• Combustion reactions release lots of energy
• Combustion reactions are a subclass of
  Oxidation-Reduction reactions

2 C8H18(g) 25 O2(g) ? 16 CO2(g) 18 H2O(g)
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Combustion Products

• to predict the products of a combustion reaction,
  combine each element in the other reactant with
  oxygen

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Classifying Reactions

• one way is based on the process that happens
• precipitation, neutralization, formation of a
  gas, or transfer of electrons

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Classifying Reactions

• another scheme classifies reactions by what the
  atoms do

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Synthesis Reactions

• also known as Composition or Combination
  reactions
• two (or more) reactants combine together to make
  one product
• simpler substances combining together
• 2 CO O2 2 CO2
• 2 Mg O2 2 MgO
• HgI2 2 KI K2HgI4

64
Decomposition Reactions

• a large molecule is broken apart into smaller
  molecules or its elements
• caused by addition of energy into the molecule
• have only one reactant, make 2 or more products

65
Decomposition of Water
66
Displacement Reactions

• also known as single-displacement reactions
• Reactions that involve one anion being
  transferred from one cation to another
• X ÅYq A X AÅYq
• Zn(s) 2 HCl(aq) ? ZnCl2(aq) H2(g)
• Fe2O3(s) Al(s) ? Fe(s) Al2O3(s)
• 2 Na(s) 2 H2O(aq) ? 2 NaOH(aq) H2(g)

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Displacement of Copper by Zinc
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Double Displacement Reactions

• two ionic compounds exchange ions
• may be followed by decomposition of one of the
  products to make a gas
• X ÅYq (aq) A ÅBq (aq) XB AY
• precipitation, acid-base and gas-evolving
  reactions are also double displacement reactions