Aqueous Solutions

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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 nonelectrolytes, none of the molecules are
  separated into ions.
• In weak electrolytes, a small percentage 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

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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

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Decomposition of Water
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Displacement Reactions

• Also known as single-displacement reactions.
• Reactions that involve one anion being
  transferred from one cation to another.
• AX B ? BX A
• 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 Y (aq) A B (aq) ? XB AY
• Precipitation, acid-base and gas-evolving
  reactions are also double displacement reactions.